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Main Menu
*********

This is Edition 0.12, last updated 2007-10-27, of `The GNU C Library
Reference Manual', for Version 2.8 of the GNU C Library.

* Menu:

* Introduction::                 Purpose of the GNU C Library.
* Error Reporting::              How library functions report errors.
* Memory::                       Allocating virtual memory and controlling
                                   paging.
* Character Handling::           Character testing and conversion functions.
* String and Array Utilities::   Utilities for copying and comparing strings
                                   and arrays.
* Character Set Handling::       Support for extended character sets.
* Locales::                      The country and language can affect the
                                   behavior of library functions.
* Message Translation::          How to make the program speak the user's
                                   language.
* Searching and Sorting::        General searching and sorting functions.
* Pattern Matching::             Matching shell ``globs'' and regular
                                   expressions.
* I/O Overview::                 Introduction to the I/O facilities.
* I/O on Streams::               High-level, portable I/O facilities.
* Low-Level I/O::                Low-level, less portable I/O.
* File System Interface::        Functions for manipulating files.
* Pipes and FIFOs::              A simple interprocess communication
                                   mechanism.
* Sockets::                      A more complicated IPC mechanism, with
                                   networking support.
* Low-Level Terminal Interface:: How to change the characteristics of a
                                   terminal device.
* Syslog::                       System logging and messaging.
* Mathematics::                  Math functions, useful constants, random
                                   numbers.
* Arithmetic::                   Low level arithmetic functions.
* Date and Time::                Functions for getting the date and time and
                                   formatting them nicely.
* Resource Usage And Limitation:: Functions for examining resource usage and
                                   getting and setting limits.
* Non-Local Exits::              Jumping out of nested function calls.
* Signal Handling::              How to send, block, and handle signals.
* Program Basics::               Writing the beginning and end of your
                                   program.
* Processes::                    How to create processes and run other
                                   programs.
* Job Control::                  All about process groups and sessions.
* Name Service Switch::          Accessing system databases.
* Users and Groups::             How users are identified and classified.
* System Management::            Controlling the system and getting
                                   information about it.
* System Configuration::         Parameters describing operating system
                                   limits.
* Cryptographic Functions::      DES encryption and password handling.
* Debugging Support::            Functions to help debugging applications.

Appendices

* Language Features::            C language features provided by the library.
* Library Summary::              A summary showing the syntax, header file,
                                   and derivation of each library feature.
* Installation::                 How to install the GNU C library.
* Maintenance::                  How to enhance and port the GNU C Library.
* Contributors::                 Who wrote what parts of the GNU C library.
* Free Manuals::		 Free Software Needs Free Documentation.
* Copying::                      The GNU Lesser General Public License says
                                  how you can copy and share the GNU C Library.
* Documentation License::        This manual is under the GNU Free
                                  Documentation License.

Indices

* Concept Index::                Index of concepts and names.
* Type Index::                   Index of types and type qualifiers.
* Function Index::               Index of functions and function-like macros.
* Variable Index::               Index of variables and variable-like macros.
* File Index::                   Index of programs and files.

 --- The Detailed Node Listing ---

Introduction

* Getting Started::             What this manual is for and how to use it.
* Standards and Portability::   Standards and sources upon which the GNU
                                 C library is based.
* Using the Library::           Some practical uses for the library.
* Roadmap to the Manual::       Overview of the remaining chapters in
                                 this manual.

Standards and Portability

* ISO C::                       The international standard for the C
                                 programming language.
* POSIX::                       The ISO/IEC 9945 (aka IEEE 1003) standards
                                 for operating systems.
* Berkeley Unix::               BSD and SunOS.
* SVID::                        The System V Interface Description.
* XPG::                         The X/Open Portability Guide.

Using the Library

* Header Files::                How to include the header files in your
                                 programs.
* Macro Definitions::           Some functions in the library may really
                                 be implemented as macros.
* Reserved Names::              The C standard reserves some names for
                                 the library, and some for users.
* Feature Test Macros::         How to control what names are defined.

Error Reporting

* Checking for Errors::         How errors are reported by library functions.
* Error Codes::                 Error code macros; all of these expand
                                 into integer constant values.
* Error Messages::              Mapping error codes onto error messages.

Memory

* Memory Concepts::             An introduction to concepts and terminology.
* Memory Allocation::           Allocating storage for your program data
* Locking Pages::               Preventing page faults
* Resizing the Data Segment::   `brk', `sbrk'

Memory Allocation

* Memory Allocation and C::     How to get different kinds of allocation in C.
* Unconstrained Allocation::    The `malloc' facility allows fully general
		 		 dynamic allocation.
* Allocation Debugging::        Finding memory leaks and not freed memory.
* Obstacks::                    Obstacks are less general than malloc
				 but more efficient and convenient.
* Variable Size Automatic::     Allocation of variable-sized blocks
				 of automatic storage that are freed when the
				 calling function returns.

Unconstrained Allocation

* Basic Allocation::            Simple use of `malloc'.
* Malloc Examples::             Examples of `malloc'.  `xmalloc'.
* Freeing after Malloc::        Use `free' to free a block you
				 got with `malloc'.
* Changing Block Size::         Use `realloc' to make a block
				 bigger or smaller.
* Allocating Cleared Space::    Use `calloc' to allocate a
				 block and clear it.
* Efficiency and Malloc::       Efficiency considerations in use of
				 these functions.
* Aligned Memory Blocks::       Allocating specially aligned memory.
* Malloc Tunable Parameters::   Use `mallopt' to adjust allocation
                                 parameters.
* Heap Consistency Checking::   Automatic checking for errors.
* Hooks for Malloc::            You can use these hooks for debugging
				 programs that use `malloc'.
* Statistics of Malloc::        Getting information about how much
				 memory your program is using.
* Summary of Malloc::           Summary of `malloc' and related functions.

Allocation Debugging

* Tracing malloc::               How to install the tracing functionality.
* Using the Memory Debugger::    Example programs excerpts.
* Tips for the Memory Debugger:: Some more or less clever ideas.
* Interpreting the traces::      What do all these lines mean?

Obstacks

* Creating Obstacks::		How to declare an obstack in your program.
* Preparing for Obstacks::	Preparations needed before you can
				 use obstacks.
* Allocation in an Obstack::    Allocating objects in an obstack.
* Freeing Obstack Objects::     Freeing objects in an obstack.
* Obstack Functions::		The obstack functions are both
				 functions and macros.
* Growing Objects::             Making an object bigger by stages.
* Extra Fast Growing::		Extra-high-efficiency (though more
				 complicated) growing objects.
* Status of an Obstack::        Inquiries about the status of an obstack.
* Obstacks Data Alignment::     Controlling alignment of objects in obstacks.
* Obstack Chunks::              How obstacks obtain and release chunks;
				 efficiency considerations.
* Summary of Obstacks::

Variable Size Automatic

* Alloca Example::              Example of using `alloca'.
* Advantages of Alloca::        Reasons to use `alloca'.
* Disadvantages of Alloca::     Reasons to avoid `alloca'.
* GNU C Variable-Size Arrays::  Only in GNU C, here is an alternative
				 method of allocating dynamically and
				 freeing automatically.

Locking Pages

* Why Lock Pages::                Reasons to read this section.
* Locked Memory Details::         Everything you need to know locked
                                    memory
* Page Lock Functions::           Here's how to do it.

Character Handling

* Classification of Characters::       Testing whether characters are
			                letters, digits, punctuation, etc.

* Case Conversion::                    Case mapping, and the like.
* Classification of Wide Characters::  Character class determination for
                                        wide characters.
* Using Wide Char Classes::            Notes on using the wide character
                                        classes.
* Wide Character Case Conversion::     Mapping of wide characters.

String and Array Utilities

* Representation of Strings::   Introduction to basic concepts.
* String/Array Conventions::    Whether to use a string function or an
				 arbitrary array function.
* String Length::               Determining the length of a string.
* Copying and Concatenation::   Functions to copy the contents of strings
				 and arrays.
* String/Array Comparison::     Functions for byte-wise and character-wise
				 comparison.
* Collation Functions::         Functions for collating strings.
* Search Functions::            Searching for a specific element or substring.
* Finding Tokens in a String::  Splitting a string into tokens by looking
				 for delimiters.
* strfry::                      Function for flash-cooking a string.
* Trivial Encryption::          Obscuring data.
* Encode Binary Data::          Encoding and Decoding of Binary Data.
* Argz and Envz Vectors::       Null-separated string vectors.

Argz and Envz Vectors

* Argz Functions::              Operations on argz vectors.
* Envz Functions::              Additional operations on environment vectors.

Character Set Handling

* Extended Char Intro::              Introduction to Extended Characters.
* Charset Function Overview::        Overview about Character Handling
                                      Functions.
* Restartable multibyte conversion:: Restartable multibyte conversion
                                      Functions.
* Non-reentrant Conversion::         Non-reentrant Conversion Function.
* Generic Charset Conversion::       Generic Charset Conversion.

Restartable multibyte conversion

* Selecting the Conversion::     Selecting the conversion and its properties.
* Keeping the state::            Representing the state of the conversion.
* Converting a Character::       Converting Single Characters.
* Converting Strings::           Converting Multibyte and Wide Character
                                  Strings.
* Multibyte Conversion Example:: A Complete Multibyte Conversion Example.

Non-reentrant Conversion

* Non-reentrant Character Conversion::  Non-reentrant Conversion of Single
                                         Characters.
* Non-reentrant String Conversion::     Non-reentrant Conversion of Strings.
* Shift State::                         States in Non-reentrant Functions.

Generic Charset Conversion

* Generic Conversion Interface::    Generic Character Set Conversion Interface.
* iconv Examples::                  A complete `iconv' example.
* Other iconv Implementations::     Some Details about other `iconv'
                                     Implementations.
* glibc iconv Implementation::      The `iconv' Implementation in the GNU C
                                     library.

Locales

* Effects of Locale::           Actions affected by the choice of
                                 locale.
* Choosing Locale::             How the user specifies a locale.
* Locale Categories::           Different purposes for which you can
                                 select a locale.
* Setting the Locale::          How a program specifies the locale
                                 with library functions.
* Standard Locales::            Locale names available on all systems.
* Locale Names::                Format of system-specific locale names.
* Locale Information::          How to access the information for the locale.
* Formatting Numbers::          A dedicated function to format numbers.
* Yes-or-No Questions::         Check a Response against the locale.

Locale Information

* The Lame Way to Locale Data::   ISO C's `localeconv'.
* The Elegant and Fast Way::      X/Open's `nl_langinfo'.

The Lame Way to Locale Data

* General Numeric::             Parameters for formatting numbers and
                                 currency amounts.
* Currency Symbol::             How to print the symbol that identifies an
                                 amount of money (e.g. `$').
* Sign of Money Amount::        How to print the (positive or negative) sign
                                 for a monetary amount, if one exists.

Message Translation

* Message catalogs a la X/Open::  The `catgets' family of functions.
* The Uniforum approach::         The `gettext' family of functions.

Message catalogs a la X/Open

* The catgets Functions::      The `catgets' function family.
* The message catalog files::  Format of the message catalog files.
* The gencat program::         How to generate message catalogs files which
                                can be used by the functions.
* Common Usage::               How to use the `catgets' interface.

The Uniforum approach

* Message catalogs with gettext::  The `gettext' family of functions.
* Helper programs for gettext::    Programs to handle message catalogs
                                    for `gettext'.

Message catalogs with gettext

* Translation with gettext::       What has to be done to translate a message.
* Locating gettext catalog::       How to determine which catalog to be used.
* Advanced gettext functions::     Additional functions for more complicated
                                    situations.
* Charset conversion in gettext::  How to specify the output character set
                                    `gettext' uses.
* GUI program problems::           How to use `gettext' in GUI programs.
* Using gettextized software::     The possibilities of the user to influence
                                    the way `gettext' works.

Searching and Sorting

* Comparison Functions::        Defining how to compare two objects.
				 Since the sort and search facilities
                                 are general, you have to specify the
                                 ordering.
* Array Search Function::       The `bsearch' function.
* Array Sort Function::         The `qsort' function.
* Search/Sort Example::         An example program.
* Hash Search Function::        The `hsearch' function.
* Tree Search Function::        The `tsearch' function.

Pattern Matching

* Wildcard Matching::    Matching a wildcard pattern against a single string.
* Globbing::             Finding the files that match a wildcard pattern.
* Regular Expressions::  Matching regular expressions against strings.
* Word Expansion::       Expanding shell variables, nested commands,
			    arithmetic, and wildcards.
			    This is what the shell does with shell commands.

Globbing

* Calling Glob::             Basic use of `glob'.
* Flags for Globbing::       Flags that enable various options in `glob'.
* More Flags for Globbing::  GNU specific extensions to `glob'.

Regular Expressions

* POSIX Regexp Compilation::    Using `regcomp' to prepare to match.
* Flags for POSIX Regexps::     Syntax variations for `regcomp'.
* Matching POSIX Regexps::      Using `regexec' to match the compiled
				   pattern that you get from `regcomp'.
* Regexp Subexpressions::       Finding which parts of the string were matched.
* Subexpression Complications:: Find points of which parts were matched.
* Regexp Cleanup::		Freeing storage; reporting errors.

Word Expansion

* Expansion Stages::            What word expansion does to a string.
* Calling Wordexp::             How to call `wordexp'.
* Flags for Wordexp::           Options you can enable in `wordexp'.
* Wordexp Example::             A sample program that does word expansion.
* Tilde Expansion::             Details of how tilde expansion works.
* Variable Substitution::       Different types of variable substitution.

I/O Overview

* I/O Concepts::       Some basic information and terminology.
* File Names::         How to refer to a file.

I/O Concepts

* Streams and File Descriptors::    The GNU Library provides two ways
			             to access the contents of files.
* File Position::                   The number of bytes from the
                                     beginning of the file.

File Names

* Directories::                 Directories contain entries for files.
* File Name Resolution::        A file name specifies how to look up a file.
* File Name Errors::            Error conditions relating to file names.
* File Name Portability::       File name portability and syntax issues.

I/O on Streams

* Streams::                     About the data type representing a stream.
* Standard Streams::            Streams to the standard input and output
                                 devices are created for you.
* Opening Streams::             How to create a stream to talk to a file.
* Closing Streams::             Close a stream when you are finished with it.
* Streams and Threads::         Issues with streams in threaded programs.
* Streams and I18N::            Streams in internationalized applications.
* Simple Output::               Unformatted output by characters and lines.
* Character Input::             Unformatted input by characters and words.
* Line Input::                  Reading a line or a record from a stream.
* Unreading::                   Peeking ahead/pushing back input just read.
* Block Input/Output::          Input and output operations on blocks of data.
* Formatted Output::            `printf' and related functions.
* Customizing Printf::          You can define new conversion specifiers for
                                 `printf' and friends.
* Formatted Input::             `scanf' and related functions.
* EOF and Errors::              How you can tell if an I/O error happens.
* Error Recovery::		What you can do about errors.
* Binary Streams::              Some systems distinguish between text files
                                 and binary files.
* File Positioning::            About random-access streams.
* Portable Positioning::        Random access on peculiar ISO C systems.
* Stream Buffering::            How to control buffering of streams.
* Other Kinds of Streams::      Streams that do not necessarily correspond
                                 to an open file.
* Formatted Messages::          Print strictly formatted messages.

Unreading

* Unreading Idea::              An explanation of unreading with pictures.
* How Unread::                  How to call `ungetc' to do unreading.

Formatted Output

* Formatted Output Basics::     Some examples to get you started.
* Output Conversion Syntax::    General syntax of conversion
                                 specifications.
* Table of Output Conversions:: Summary of output conversions and
                                 what they do.
* Integer Conversions::         Details about formatting of integers.
* Floating-Point Conversions::  Details about formatting of
                                 floating-point numbers.
* Other Output Conversions::    Details about formatting of strings,
                                 characters, pointers, and the like.
* Formatted Output Functions::  Descriptions of the actual functions.
* Dynamic Output::		Functions that allocate memory for the output.
* Variable Arguments Output::   `vprintf' and friends.
* Parsing a Template String::   What kinds of args does a given template
                                 call for?
* Example of Parsing::          Sample program using `parse_printf_format'.

Customizing Printf

* Registering New Conversions::         Using `register_printf_function'
                                         to register a new output conversion.
* Conversion Specifier Options::        The handler must be able to get
                                         the options specified in the
                                         template when it is called.
* Defining the Output Handler::         Defining the handler and arginfo
                                         functions that are passed as arguments
                                         to `register_printf_function'.
* Printf Extension Example::            How to define a `printf'
                                         handler function.
* Predefined Printf Handlers::          Predefined `printf' handlers.

Formatted Input

* Formatted Input Basics::      Some basics to get you started.
* Input Conversion Syntax::     Syntax of conversion specifications.
* Table of Input Conversions::  Summary of input conversions and what they do.
* Numeric Input Conversions::   Details of conversions for reading numbers.
* String Input Conversions::    Details of conversions for reading strings.
* Dynamic String Input::	String conversions that `malloc' the buffer.
* Other Input Conversions::     Details of miscellaneous other conversions.
* Formatted Input Functions::   Descriptions of the actual functions.
* Variable Arguments Input::    `vscanf' and friends.

Stream Buffering

* Buffering Concepts::          Terminology is defined here.
* Flushing Buffers::            How to ensure that output buffers are flushed.
* Controlling Buffering::       How to specify what kind of buffering to use.

Other Kinds of Streams

* String Streams::              Streams that get data from or put data in
                                 a string or memory buffer.
* Obstack Streams::		Streams that store data in an obstack.
* Custom Streams::              Defining your own streams with an arbitrary
                                 input data source and/or output data sink.

Custom Streams

* Streams and Cookies::         The "cookie" records where to fetch or
                                 store data that is read or written.
* Hook Functions::              How you should define the four "hook
                                 functions" that a custom stream needs.

Formatted Messages

* Printing Formatted Messages::   The `fmtmsg' function.
* Adding Severity Classes::       Add more severity classes.
* Example::                       How to use `fmtmsg' and `addseverity'.

Low-Level I/O

* Opening and Closing Files::           How to open and close file
                                         descriptors.
* I/O Primitives::                      Reading and writing data.
* File Position Primitive::             Setting a descriptor's file
                                         position.
* Descriptors and Streams::             Converting descriptor to stream
                                         or vice-versa.
* Stream/Descriptor Precautions::       Precautions needed if you use both
                                         descriptors and streams.
* Scatter-Gather::                      Fast I/O to discontinuous buffers.
* Memory-mapped I/O::                   Using files like memory.
* Waiting for I/O::                     How to check for input or output
					 on multiple file descriptors.
* Synchronizing I/O::                   Making sure all I/O actions completed.
* Asynchronous I/O::                    Perform I/O in parallel.
* Control Operations::                  Various other operations on file
					 descriptors.
* Duplicating Descriptors::             Fcntl commands for duplicating
                                         file descriptors.
* Descriptor Flags::                    Fcntl commands for manipulating
                                         flags associated with file
                                         descriptors.
* File Status Flags::                   Fcntl commands for manipulating
                                         flags associated with open files.
* File Locks::                          Fcntl commands for implementing
                                         file locking.
* Interrupt Input::                     Getting an asynchronous signal when
                                         input arrives.
* IOCTLs::                              Generic I/O Control operations.

Stream/Descriptor Precautions

* Linked Channels::	   Dealing with channels sharing a file position.
* Independent Channels::   Dealing with separately opened, unlinked channels.
* Cleaning Streams::	   Cleaning a stream makes it safe to use
                            another channel.

Asynchronous I/O

* Asynchronous Reads/Writes::    Asynchronous Read and Write Operations.
* Status of AIO Operations::     Getting the Status of AIO Operations.
* Synchronizing AIO Operations:: Getting into a consistent state.
* Cancel AIO Operations::        Cancellation of AIO Operations.
* Configuration of AIO::         How to optimize the AIO implementation.

File Status Flags

* Access Modes::                Whether the descriptor can read or write.
* Open-time Flags::             Details of `open'.
* Operating Modes::             Special modes to control I/O operations.
* Getting File Status Flags::   Fetching and changing these flags.

File System Interface

* Working Directory::           This is used to resolve relative
				 file names.
* Accessing Directories::       Finding out what files a directory
				 contains.
* Working with Directory Trees:: Apply actions to all files or a selectable
                                 subset of a directory hierarchy.
* Hard Links::                  Adding alternate names to a file.
* Symbolic Links::              A file that ``points to'' a file name.
* Deleting Files::              How to delete a file, and what that means.
* Renaming Files::              Changing a file's name.
* Creating Directories::        A system call just for creating a directory.
* File Attributes::             Attributes of individual files.
* Making Special Files::        How to create special files.
* Temporary Files::             Naming and creating temporary files.

Accessing Directories

* Directory Entries::           Format of one directory entry.
* Opening a Directory::         How to open a directory stream.
* Reading/Closing Directory::   How to read directory entries from the stream.
* Simple Directory Lister::     A very simple directory listing program.
* Random Access Directory::     Rereading part of the directory
                                 already read with the same stream.
* Scanning Directory Content::  Get entries for user selected subset of
                                 contents in given directory.
* Simple Directory Lister Mark II::  Revised version of the program.

File Attributes

* Attribute Meanings::          The names of the file attributes,
                                 and what their values mean.
* Reading Attributes::          How to read the attributes of a file.
* Testing File Type::           Distinguishing ordinary files,
                                 directories, links...
* File Owner::                  How ownership for new files is determined,
			         and how to change it.
* Permission Bits::             How information about a file's access
                                 mode is stored.
* Access Permission::           How the system decides who can access a file.
* Setting Permissions::         How permissions for new files are assigned,
			         and how to change them.
* Testing File Access::         How to find out if your process can
                                 access a file.
* File Times::                  About the time attributes of a file.
* File Size::			Manually changing the size of a file.

Pipes and FIFOs

* Creating a Pipe::             Making a pipe with the `pipe' function.
* Pipe to a Subprocess::        Using a pipe to communicate with a
				 child process.
* FIFO Special Files::          Making a FIFO special file.
* Pipe Atomicity::		When pipe (or FIFO) I/O is atomic.

Sockets

* Socket Concepts::	Basic concepts you need to know about.
* Communication Styles::Stream communication, datagrams and other styles.
* Socket Addresses::	How socket names (``addresses'') work.
* Interface Naming::	Identifying specific network interfaces.
* Local Namespace::	Details about the local namespace.
* Internet Namespace::	Details about the Internet namespace.
* Misc Namespaces::	Other namespaces not documented fully here.
* Open/Close Sockets::  Creating sockets and destroying them.
* Connections::		Operations on sockets with connection state.
* Datagrams::		Operations on datagram sockets.
* Inetd::		Inetd is a daemon that starts servers on request.
			   The most convenient way to write a server
			   is to make it work with Inetd.
* Socket Options::	Miscellaneous low-level socket options.
* Networks Database::   Accessing the database of network names.

Socket Addresses

* Address Formats::		About `struct sockaddr'.
* Setting Address::		Binding an address to a socket.
* Reading Address::		Reading the address of a socket.

Local Namespace

* Concepts: Local Namespace Concepts. What you need to understand.
* Details: Local Namespace Details.   Address format, symbolic names, etc.
* Example: Local Socket Example.      Example of creating a socket.

Internet Namespace

* Internet Address Formats::    How socket addresses are specified in the
                                 Internet namespace.
* Host Addresses::	        All about host addresses of Internet host.
* Protocols Database::		Referring to protocols by name.
* Ports::			Internet port numbers.
* Services Database::           Ports may have symbolic names.
* Byte Order::		        Different hosts may use different byte
                                 ordering conventions; you need to
                                 canonicalize host address and port number.
* Inet Example::	        Putting it all together.

Host Addresses

* Abstract Host Addresses::	What a host number consists of.
* Data type: Host Address Data Type.	Data type for a host number.
* Functions: Host Address Functions.	Functions to operate on them.
* Names: Host Names.		Translating host names to host numbers.

Open/Close Sockets

* Creating a Socket::           How to open a socket.
* Closing a Socket::            How to close a socket.
* Socket Pairs::                These are created like pipes.

Connections

* Connecting::    	     What the client program must do.
* Listening::		     How a server program waits for requests.
* Accepting Connections::    What the server does when it gets a request.
* Who is Connected::	     Getting the address of the
				other side of a connection.
* Transferring Data::        How to send and receive data.
* Byte Stream Example::	     An example program: a client for communicating
			      over a byte stream socket in the Internet namespace.
* Server Example::	     A corresponding server program.
* Out-of-Band Data::         This is an advanced feature.

Transferring Data

* Sending Data::		Sending data with `send'.
* Receiving Data::		Reading data with `recv'.
* Socket Data Options::		Using `send' and `recv'.

Datagrams

* Sending Datagrams::    Sending packets on a datagram socket.
* Receiving Datagrams::  Receiving packets on a datagram socket.
* Datagram Example::     An example program: packets sent over a
                           datagram socket in the local namespace.
* Example Receiver::	 Another program, that receives those packets.

Inetd

* Inetd Servers::
* Configuring Inetd::

Socket Options

* Socket Option Functions::     The basic functions for setting and getting
                                 socket options.
* Socket-Level Options::        Details of the options at the socket level.

Low-Level Terminal Interface

* Is It a Terminal::            How to determine if a file is a terminal
			         device, and what its name is.
* I/O Queues::                  About flow control and typeahead.
* Canonical or Not::            Two basic styles of input processing.
* Terminal Modes::              How to examine and modify flags controlling
			         details of terminal I/O: echoing,
                                 signals, editing.  Posix.
* BSD Terminal Modes::          BSD compatible terminal mode setting
* Line Control::                Sending break sequences, clearing
                                 terminal buffers ...
* Noncanon Example::            How to read single characters without echo.
* Pseudo-Terminals::            How to open a pseudo-terminal.

Terminal Modes

* Mode Data Types::             The data type `struct termios' and
                                 related types.
* Mode Functions::              Functions to read and set the terminal
                                 attributes.
* Setting Modes::               The right way to set terminal attributes
                                 reliably.
* Input Modes::                 Flags controlling low-level input handling.
* Output Modes::                Flags controlling low-level output handling.
* Control Modes::               Flags controlling serial port behavior.
* Local Modes::                 Flags controlling high-level input handling.
* Line Speed::                  How to read and set the terminal line speed.
* Special Characters::          Characters that have special effects,
			         and how to change them.
* Noncanonical Input::          Controlling how long to wait for input.

Special Characters

* Editing Characters::          Special characters that terminate lines and
                                  delete text, and other editing functions.
* Signal Characters::           Special characters that send or raise signals
                                  to or for certain classes of processes.
* Start/Stop Characters::       Special characters that suspend or resume
                                  suspended output.
* Other Special::		Other special characters for BSD systems:
				  they can discard output, and print status.

Pseudo-Terminals

* Allocation::             Allocating a pseudo terminal.
* Pseudo-Terminal Pairs::  How to open both sides of a
                            pseudo-terminal in a single operation.

Syslog

* Overview of Syslog::           Overview of a system's Syslog facility
* Submitting Syslog Messages::   Functions to submit messages to Syslog

Submitting Syslog Messages

* openlog::                      Open connection to Syslog
* syslog; vsyslog::              Submit message to Syslog
* closelog::                     Close connection to Syslog
* setlogmask::                   Cause certain messages to be ignored
* Syslog Example::               Example of all of the above

Mathematics

* Mathematical Constants::      Precise numeric values for often-used
                                 constants.
* Trig Functions::              Sine, cosine, tangent, and friends.
* Inverse Trig Functions::      Arcsine, arccosine, etc.
* Exponents and Logarithms::    Also pow and sqrt.
* Hyperbolic Functions::        sinh, cosh, tanh, etc.
* Special Functions::           Bessel, gamma, erf.
* Errors in Math Functions::    Known Maximum Errors in Math Functions.
* Pseudo-Random Numbers::       Functions for generating pseudo-random
				 numbers.
* FP Function Optimizations::   Fast code or small code.

Pseudo-Random Numbers

* ISO Random::                  `rand' and friends.
* BSD Random::                  `random' and friends.
* SVID Random::                 `drand48' and friends.

Arithmetic

* Integers::                    Basic integer types and concepts
* Integer Division::            Integer division with guaranteed rounding.
* Floating Point Numbers::      Basic concepts.  IEEE 754.
* Floating Point Classes::      The five kinds of floating-point number.
* Floating Point Errors::       When something goes wrong in a calculation.
* Rounding::                    Controlling how results are rounded.
* Control Functions::           Saving and restoring the FPU's state.
* Arithmetic Functions::        Fundamental operations provided by the library.
* Complex Numbers::             The types.  Writing complex constants.
* Operations on Complex::       Projection, conjugation, decomposition.
* Parsing of Numbers::          Converting strings to numbers.
* System V Number Conversion::  An archaic way to convert numbers to strings.

Floating Point Errors

* FP Exceptions::               IEEE 754 math exceptions and how to detect them.
* Infinity and NaN::            Special values returned by calculations.
* Status bit operations::       Checking for exceptions after the fact.
* Math Error Reporting::        How the math functions report errors.

Arithmetic Functions

* Absolute Value::              Absolute values of integers and floats.
* Normalization Functions::     Extracting exponents and putting them back.
* Rounding Functions::          Rounding floats to integers.
* Remainder Functions::         Remainders on division, precisely defined.
* FP Bit Twiddling::            Sign bit adjustment.  Adding epsilon.
* FP Comparison Functions::     Comparisons without risk of exceptions.
* Misc FP Arithmetic::          Max, min, positive difference, multiply-add.

Parsing of Numbers

* Parsing of Integers::         Functions for conversion of integer values.
* Parsing of Floats::           Functions for conversion of floating-point
				 values.

Date and Time

* Time Basics::                 Concepts and definitions.
* Elapsed Time::                Data types to represent elapsed times
* Processor And CPU Time::      Time a program has spent executing.
* Calendar Time::               Manipulation of ``real'' dates and times.
* Setting an Alarm::            Sending a signal after a specified time.
* Sleeping::                    Waiting for a period of time.

Processor And CPU Time

* CPU Time::                    The `clock' function.
* Processor Time::              The `times' function.

Calendar Time

* Simple Calendar Time::        Facilities for manipulating calendar time.
* High-Resolution Calendar::    A time representation with greater precision.
* Broken-down Time::            Facilities for manipulating local time.
* High Accuracy Clock::         Maintaining a high accuracy system clock.
* Formatting Calendar Time::    Converting times to strings.
* Parsing Date and Time::       Convert textual time and date information back
                                 into broken-down time values.
* TZ Variable::                 How users specify the time zone.
* Time Zone Functions::         Functions to examine or specify the time zone.
* Time Functions Example::      An example program showing use of some of
				 the time functions.

Parsing Date and Time

* Low-Level Time String Parsing::  Interpret string according to given format.
* General Time String Parsing::    User-friendly function to parse data and
                                    time strings.

Resource Usage And Limitation

* Resource Usage::		Measuring various resources used.
* Limits on Resources::		Specifying limits on resource usage.
* Priority::			Reading or setting process run priority.
* Memory Resources::            Querying memory available resources.
* Processor Resources::         Learn about the processors available.

Priority

* Absolute Priority::               The first tier of priority.  Posix
* Realtime Scheduling::             Scheduling among the process nobility
* Basic Scheduling Functions::      Get/set scheduling policy, priority
* Traditional Scheduling::          Scheduling among the vulgar masses
* CPU Affinity::                    Limiting execution to certain CPUs

Traditional Scheduling

* Traditional Scheduling Intro::
* Traditional Scheduling Functions::

Memory Resources

* Memory Subsystem::           Overview about traditional Unix memory handling.
* Query Memory Parameters::    How to get information about the memory
                                subsystem?

Non-Local Exits

* Intro: Non-Local Intro.        When and how to use these facilities.
* Details: Non-Local Details.    Functions for non-local exits.
* Non-Local Exits and Signals::  Portability issues.
* System V contexts::            Complete context control a la System V.

Signal Handling

* Concepts of Signals::         Introduction to the signal facilities.
* Standard Signals::            Particular kinds of signals with
                                 standard names and meanings.
* Signal Actions::              Specifying what happens when a
                                 particular signal is delivered.
* Defining Handlers::           How to write a signal handler function.
* Interrupted Primitives::	Signal handlers affect use of `open',
				 `read', `write' and other functions.
* Generating Signals::          How to send a signal to a process.
* Blocking Signals::            Making the system hold signals temporarily.
* Waiting for a Signal::        Suspending your program until a signal
                                 arrives.
* Signal Stack::                Using a Separate Signal Stack.
* BSD Signal Handling::         Additional functions for backward
			         compatibility with BSD.

Concepts of Signals

* Kinds of Signals::            Some examples of what can cause a signal.
* Signal Generation::           Concepts of why and how signals occur.
* Delivery of Signal::          Concepts of what a signal does to the
                                 process.

Standard Signals

* Program Error Signals::       Used to report serious program errors.
* Termination Signals::         Used to interrupt and/or terminate the
                                 program.
* Alarm Signals::               Used to indicate expiration of timers.
* Asynchronous I/O Signals::    Used to indicate input is available.
* Job Control Signals::         Signals used to support job control.
* Operation Error Signals::     Used to report operational system errors.
* Miscellaneous Signals::       Miscellaneous Signals.
* Signal Messages::             Printing a message describing a signal.

Signal Actions

* Basic Signal Handling::       The simple `signal' function.
* Advanced Signal Handling::    The more powerful `sigaction' function.
* Signal and Sigaction::        How those two functions interact.
* Sigaction Function Example::  An example of using the sigaction function.
* Flags for Sigaction::         Specifying options for signal handling.
* Initial Signal Actions::      How programs inherit signal actions.

Defining Handlers

* Handler Returns::             Handlers that return normally, and what
                                 this means.
* Termination in Handler::      How handler functions terminate a program.
* Longjmp in Handler::          Nonlocal transfer of control out of a
                                 signal handler.
* Signals in Handler::          What happens when signals arrive while
                                 the handler is already occupied.
* Merged Signals::		When a second signal arrives before the
				 first is handled.
* Nonreentrancy::               Do not call any functions unless you know they
                                 are reentrant with respect to signals.
* Atomic Data Access::          A single handler can run in the middle of
                                 reading or writing a single object.

Atomic Data Access

* Non-atomic Example::		A program illustrating interrupted access.
* Types: Atomic Types.		Data types that guarantee no interruption.
* Usage: Atomic Usage.		Proving that interruption is harmless.

Generating Signals

* Signaling Yourself::          A process can send a signal to itself.
* Signaling Another Process::   Send a signal to another process.
* Permission for kill::         Permission for using `kill'.
* Kill Example::                Using `kill' for Communication.

Blocking Signals

* Why Block::                           The purpose of blocking signals.
* Signal Sets::                         How to specify which signals to
                                         block.
* Process Signal Mask::                 Blocking delivery of signals to your
				         process during normal execution.
* Testing for Delivery::                Blocking to Test for Delivery of
                                         a Signal.
* Blocking for Handler::                Blocking additional signals while a
				         handler is being run.
* Checking for Pending Signals::        Checking for Pending Signals
* Remembering a Signal::                How you can get almost the same
                                         effect as blocking a signal, by
                                         handling it and setting a flag
                                         to be tested later.

Waiting for a Signal

* Using Pause::                 The simple way, using `pause'.
* Pause Problems::              Why the simple way is often not very good.
* Sigsuspend::                  Reliably waiting for a specific signal.

BSD Signal Handling

* BSD Handler::                 BSD Function to Establish a Handler.
* Blocking in BSD::             BSD Functions for Blocking Signals.

Program Basics

* Program Arguments::           Parsing your program's command-line arguments.
* Environment Variables::       Less direct parameters affecting your program
* System Calls::                Requesting service from the system
* Program Termination::         Telling the system you're done; return status

Program Arguments

* Argument Syntax::             By convention, options start with a hyphen.
* Parsing Program Arguments::   Ways to parse program options and arguments.

Parsing Program Arguments

* Getopt::                      Parsing program options using `getopt'.
* Argp::                        Parsing program options using `argp_parse'.
* Suboptions::                  Some programs need more detailed options.
* Suboptions Example::          This shows how it could be done for `mount'.

Environment Variables

* Environment Access::          How to get and set the values of
				 environment variables.
* Standard Environment::        These environment variables have
                		 standard interpretations.

Program Termination

* Normal Termination::          If a program calls `exit', a
                                 process terminates normally.
* Exit Status::                 The `exit status' provides information
                                 about why the process terminated.
* Cleanups on Exit::            A process can run its own cleanup
                                 functions upon normal termination.
* Aborting a Program::          The `abort' function causes
                                 abnormal program termination.
* Termination Internals::       What happens when a process terminates.

Processes

* Running a Command::           The easy way to run another program.
* Process Creation Concepts::   An overview of the hard way to do it.
* Process Identification::      How to get the process ID of a process.
* Creating a Process::          How to fork a child process.
* Executing a File::            How to make a process execute another program.
* Process Completion::          How to tell when a child process has completed.
* Process Completion Status::   How to interpret the status value
                                 returned from a child process.
* BSD Wait Functions::  	More functions, for backward compatibility.
* Process Creation Example::    A complete example program.

Job Control

* Concepts of Job Control::     Jobs can be controlled by a shell.
* Job Control is Optional::     Not all POSIX systems support job control.
* Controlling Terminal::        How a process gets its controlling terminal.
* Access to the Terminal::      How processes share the controlling terminal.
* Orphaned Process Groups::     Jobs left after the user logs out.
* Implementing a Shell::        What a shell must do to implement job control.
* Functions for Job Control::   Functions to control process groups.

Implementing a Shell

* Data Structures::             Introduction to the sample shell.
* Initializing the Shell::      What the shell must do to take
				 responsibility for job control.
* Launching Jobs::              Creating jobs to execute commands.
* Foreground and Background::   Putting a job in foreground of background.
* Stopped and Terminated Jobs::  Reporting job status.
* Continuing Stopped Jobs::     How to continue a stopped job in
				 the foreground or background.
* Missing Pieces::              Other parts of the shell.

Functions for Job Control

* Identifying the Terminal::    Determining the controlling terminal's name.
* Process Group Functions::     Functions for manipulating process groups.
* Terminal Access Functions::   Functions for controlling terminal access.

Name Service Switch

* NSS Basics::                  What is this NSS good for.
* NSS Configuration File::      Configuring NSS.
* NSS Module Internals::        How does it work internally.
* Extending NSS::               What to do to add services or databases.

NSS Configuration File

* Services in the NSS configuration::  Service names in the NSS configuration.
* Actions in the NSS configuration::  React appropriately to the lookup result.
* Notes on NSS Configuration File::  Things to take care about while
                                     configuring NSS.

NSS Module Internals

* NSS Module Names::            Construction of the interface function of
                                the NSS modules.
* NSS Modules Interface::       Programming interface in the NSS module
                                functions.

Extending NSS

* Adding another Service to NSS::  What is to do to add a new service.
* NSS Module Function Internals::  Guidelines for writing new NSS
                                        service functions.

Users and Groups

* User and Group IDs::          Each user has a unique numeric ID;
				 likewise for groups.
* Process Persona::             The user IDs and group IDs of a process.
* Why Change Persona::          Why a program might need to change
				 its user and/or group IDs.
* How Change Persona::          Changing the user and group IDs.
* Reading Persona::             How to examine the user and group IDs.

* Setting User ID::             Functions for setting the user ID.
* Setting Groups::              Functions for setting the group IDs.

* Enable/Disable Setuid::       Turning setuid access on and off.
* Setuid Program Example::      The pertinent parts of one sample program.
* Tips for Setuid::             How to avoid granting unlimited access.

* Who Logged In::               Getting the name of the user who logged in,
				 or of the real user ID of the current process.

* User Accounting Database::    Keeping information about users and various
                                 actions in databases.

* User Database::               Functions and data structures for
                        	 accessing the user database.
* Group Database::              Functions and data structures for
                        	 accessing the group database.
* Database Example::            Example program showing the use of database
				 inquiry functions.
* Netgroup Database::           Functions for accessing the netgroup database.

User Accounting Database

* Manipulating the Database::   Scanning and modifying the user
                                 accounting database.
* XPG Functions::               A standardized way for doing the same thing.
* Logging In and Out::          Functions from BSD that modify the user
                                 accounting database.

User Database

* User Data Structure::         What each user record contains.
* Lookup User::                 How to look for a particular user.
* Scanning All Users::          Scanning the list of all users, one by one.
* Writing a User Entry::        How a program can rewrite a user's record.

Group Database

* Group Data Structure::        What each group record contains.
* Lookup Group::                How to look for a particular group.
* Scanning All Groups::         Scanning the list of all groups.

Netgroup Database

* Netgroup Data::                  Data in the Netgroup database and where
                                   it comes from.
* Lookup Netgroup::                How to look for a particular netgroup.
* Netgroup Membership::            How to test for netgroup membership.

System Management

* Host Identification::         Determining the name of the machine.
* Platform Type::               Determining operating system and basic
                                  machine type
* Filesystem Handling::         Controlling/querying mounts
* System Parameters::           Getting and setting various system parameters

Filesystem Handling

* Mount Information::           What is or could be mounted?
* Mount-Unmount-Remount::       Controlling what is mounted and how

Mount Information

* fstab::                       The `fstab' file
* mtab::                        The `mtab' file
* Other Mount Information::     Other (non-libc) sources of mount information

System Configuration

* General Limits::           Constants and functions that describe
				various process-related limits that have
				one uniform value for any given machine.
* System Options::           Optional POSIX features.
* Version Supported::        Version numbers of POSIX.1 and POSIX.2.
* Sysconf::                  Getting specific configuration values
                                of general limits and system options.
* Minimums::                 Minimum values for general limits.

* Limits for Files::         Size limitations that pertain to individual files.
                                These can vary between file systems
                                or even from file to file.
* Options for Files::        Optional features that some files may support.
* File Minimums::            Minimum values for file limits.
* Pathconf::                 Getting the limit values for a particular file.

* Utility Limits::           Capacity limits of some POSIX.2 utility programs.
* Utility Minimums::         Minimum allowable values of those limits.

* String Parameters::        Getting the default search path.

Sysconf

* Sysconf Definition::        Detailed specifications of `sysconf'.
* Constants for Sysconf::     The list of parameters `sysconf' can read.
* Examples of Sysconf::       How to use `sysconf' and the parameter
				 macros properly together.

Cryptographic Functions

* Legal Problems::              This software can get you locked up, or worse.
* getpass::                     Prompting the user for a password.
* crypt::                       A one-way function for passwords.
* DES Encryption::              Routines for DES encryption.

Debugging Support

* Backtraces::                Obtaining and printing a back trace of the
                               current stack.

Language Features

* Consistency Checking::        Using `assert' to abort if
				 something ``impossible'' happens.
* Variadic Functions::          Defining functions with varying numbers
                                 of args.
* Null Pointer Constant::       The macro `NULL'.
* Important Data Types::        Data types for object sizes.
* Data Type Measurements::      Parameters of data type representations.

Variadic Functions

* Why Variadic::                Reasons for making functions take
                                 variable arguments.
* How Variadic::                How to define and call variadic functions.
* Variadic Example::            A complete example.

How Variadic

* Variadic Prototypes::  How to make a prototype for a function
			  with variable arguments.
* Receiving Arguments::  Steps you must follow to access the
			  optional argument values.
* How Many Arguments::   How to decide whether there are more arguments.
* Calling Variadics::    Things you need to know about calling
			  variable arguments functions.
* Argument Macros::      Detailed specification of the macros
        		  for accessing variable arguments.
* Old Varargs::		 The pre-ISO way of defining variadic functions.

Data Type Measurements

* Width of Type::           How many bits does an integer type hold?
* Range of Type::           What are the largest and smallest values
			     that an integer type can hold?
* Floating Type Macros::    Parameters that measure the floating point types.
* Structure Measurement::   Getting measurements on structure types.

Floating Type Macros

* Floating Point Concepts::     Definitions of terminology.
* Floating Point Parameters::   Details of specific macros.
* IEEE Floating Point::         The measurements for one common
                                 representation.

Installation

* Configuring and compiling::   How to compile and test GNU libc.
* Running make install::        How to install it once you've got it
 compiled.
* Tools for Compilation::       You'll need these first.
* Linux::                       Specific advice for GNU/Linux systems.
* Reporting Bugs::              So they'll get fixed.

Maintenance

* Source Layout::         How to add new functions or header files
                             to the GNU C library.
* Porting::               How to port the GNU C library to
                             a new machine or operating system.

Porting

* Hierarchy Conventions::       The layout of the `sysdeps' hierarchy.
* Porting to Unix::             Porting the library to an average
                                   Unix-like system.

File: libc.info,  Node: Introduction,  Next: Error Reporting,  Prev: Top,  Up: Top

1 Introduction
**************

The C language provides no built-in facilities for performing such
common operations as input/output, memory management, string
manipulation, and the like.  Instead, these facilities are defined in a
standard "library", which you compile and link with your programs.

   The GNU C library, described in this document, defines all of the
library functions that are specified by the ISO C standard, as well as
additional features specific to POSIX and other derivatives of the Unix
operating system, and extensions specific to the GNU system.

   The purpose of this manual is to tell you how to use the facilities
of the GNU library.  We have mentioned which features belong to which
standards to help you identify things that are potentially non-portable
to other systems.  But the emphasis in this manual is not on strict
portability.

* Menu:

* Getting Started::             What this manual is for and how to use it.
* Standards and Portability::   Standards and sources upon which the GNU
                                 C library is based.
* Using the Library::           Some practical uses for the library.
* Roadmap to the Manual::       Overview of the remaining chapters in
                                 this manual.

File: libc.info,  Node: Getting Started,  Next: Standards and Portability,  Up: Introduction

1.1 Getting Started
===================

This manual is written with the assumption that you are at least
somewhat familiar with the C programming language and basic programming
concepts.  Specifically, familiarity with ISO standard C (*note ISO
C::), rather than "traditional" pre-ISO C dialects, is assumed.

   The GNU C library includes several "header files", each of which
provides definitions and declarations for a group of related facilities;
this information is used by the C compiler when processing your program.
For example, the header file `stdio.h' declares facilities for
performing input and output, and the header file `string.h' declares
string processing utilities.  The organization of this manual generally
follows the same division as the header files.

   If you are reading this manual for the first time, you should read
all of the introductory material and skim the remaining chapters.
There are a _lot_ of functions in the GNU C library and it's not
realistic to expect that you will be able to remember exactly _how_ to
use each and every one of them.  It's more important to become
generally familiar with the kinds of facilities that the library
provides, so that when you are writing your programs you can recognize
_when_ to make use of library functions, and _where_ in this manual you
can find more specific information about them.

File: libc.info,  Node: Standards and Portability,  Next: Using the Library,  Prev: Getting Started,  Up: Introduction

1.2 Standards and Portability
=============================

This section discusses the various standards and other sources that the
GNU C library is based upon.  These sources include the ISO C and POSIX
standards, and the System V and Berkeley Unix implementations.

   The primary focus of this manual is to tell you how to make effective
use of the GNU library facilities.  But if you are concerned about
making your programs compatible with these standards, or portable to
operating systems other than GNU, this can affect how you use the
library.  This section gives you an overview of these standards, so that
you will know what they are when they are mentioned in other parts of
the manual.

   *Note Library Summary::, for an alphabetical list of the functions
and other symbols provided by the library.  This list also states which
standards each function or symbol comes from.

* Menu:

* ISO C::                       The international standard for the C
                                 programming language.
* POSIX::                       The ISO/IEC 9945 (aka IEEE 1003) standards
                                 for operating systems.
* Berkeley Unix::               BSD and SunOS.
* SVID::                        The System V Interface Description.
* XPG::                         The X/Open Portability Guide.

File: libc.info,  Node: ISO C,  Next: POSIX,  Up: Standards and Portability

1.2.1 ISO C
-----------

The GNU C library is compatible with the C standard adopted by the
American National Standards Institute (ANSI): `American National
Standard X3.159-1989--"ANSI C"' and later by the International
Standardization Organization (ISO): `ISO/IEC 9899:1990, "Programming
languages--C"'.  We here refer to the standard as ISO C since this is
the more general standard in respect of ratification.  The header files
and library facilities that make up the GNU library are a superset of
those specified by the ISO C standard.

   If you are concerned about strict adherence to the ISO C standard,
you should use the `-ansi' option when you compile your programs with
the GNU C compiler.  This tells the compiler to define _only_ ISO
standard features from the library header files, unless you explicitly
ask for additional features.  *Note Feature Test Macros::, for
information on how to do this.

   Being able to restrict the library to include only ISO C features is
important because ISO C puts limitations on what names can be defined
by the library implementation, and the GNU extensions don't fit these
limitations.  *Note Reserved Names::, for more information about these
restrictions.

   This manual does not attempt to give you complete details on the
differences between ISO C and older dialects.  It gives advice on how
to write programs to work portably under multiple C dialects, but does
not aim for completeness.

File: libc.info,  Node: POSIX,  Next: Berkeley Unix,  Prev: ISO C,  Up: Standards and Portability

1.2.2 POSIX (The Portable Operating System Interface)
-----------------------------------------------------

The GNU library is also compatible with the ISO "POSIX" family of
standards, known more formally as the "Portable Operating System
Interface for Computer Environments" (ISO/IEC 9945).  They were also
published as ANSI/IEEE Std 1003.  POSIX is derived mostly from various
versions of the Unix operating system.

   The library facilities specified by the POSIX standards are a
superset of those required by ISO C; POSIX specifies additional
features for ISO C functions, as well as specifying new additional
functions.  In general, the additional requirements and functionality
defined by the POSIX standards are aimed at providing lower-level
support for a particular kind of operating system environment, rather
than general programming language support which can run in many diverse
operating system environments.

   The GNU C library implements all of the functions specified in
`ISO/IEC 9945-1:1996, the POSIX System Application Program Interface',
commonly referred to as POSIX.1.  The primary extensions to the ISO C
facilities specified by this standard include file system interface
primitives (*note File System Interface::), device-specific terminal
control functions (*note Low-Level Terminal Interface::), and process
control functions (*note Processes::).

   Some facilities from `ISO/IEC 9945-2:1993, the POSIX Shell and
Utilities standard' (POSIX.2) are also implemented in the GNU library.
These include utilities for dealing with regular expressions and other
pattern matching facilities (*note Pattern Matching::).

File: libc.info,  Node: Berkeley Unix,  Next: SVID,  Prev: POSIX,  Up: Standards and Portability

1.2.3 Berkeley Unix
-------------------

The GNU C library defines facilities from some versions of Unix which
are not formally standardized, specifically from the 4.2 BSD, 4.3 BSD,
and 4.4 BSD Unix systems (also known as "Berkeley Unix") and from
"SunOS" (a popular 4.2 BSD derivative that includes some Unix System V
functionality).  These systems support most of the ISO C and POSIX
facilities, and 4.4 BSD and newer releases of SunOS in fact support
them all.

   The BSD facilities include symbolic links (*note Symbolic Links::),
the `select' function (*note Waiting for I/O::), the BSD signal
functions (*note BSD Signal Handling::), and sockets (*note Sockets::).

File: libc.info,  Node: SVID,  Next: XPG,  Prev: Berkeley Unix,  Up: Standards and Portability

1.2.4 SVID (The System V Interface Description)
-----------------------------------------------

The "System V Interface Description" (SVID) is a document describing
the AT&T Unix System V operating system.  It is to some extent a
superset of the POSIX standard (*note POSIX::).

   The GNU C library defines most of the facilities required by the SVID
that are not also required by the ISO C or POSIX standards, for
compatibility with  System V Unix and other Unix systems (such as
SunOS) which include these facilities.  However, many of the more
obscure and less generally useful facilities required by the SVID are
not included.  (In fact, Unix System V itself does not provide them
all.)

   The supported facilities from System V include the methods for
inter-process communication and shared memory, the `hsearch' and
`drand48' families of functions, `fmtmsg' and several of the
mathematical functions.

File: libc.info,  Node: XPG,  Prev: SVID,  Up: Standards and Portability

1.2.5 XPG (The X/Open Portability Guide)
----------------------------------------

The X/Open Portability Guide, published by the X/Open Company, Ltd., is
a more general standard than POSIX.  X/Open owns the Unix copyright and
the XPG specifies the requirements for systems which are intended to be
a Unix system.

   The GNU C library complies to the X/Open Portability Guide, Issue
4.2, with all extensions common to XSI (X/Open System Interface)
compliant systems and also all X/Open UNIX extensions.

   The additions on top of POSIX are mainly derived from functionality
available in System V and BSD systems.  Some of the really bad mistakes
in System V systems were corrected, though.  Since fulfilling the XPG
standard with the Unix extensions is a precondition for getting the
Unix brand chances are good that the functionality is available on
commercial systems.

File: libc.info,  Node: Using the Library,  Next: Roadmap to the Manual,  Prev: Standards and Portability,  Up: Introduction

1.3 Using the Library
=====================

This section describes some of the practical issues involved in using
the GNU C library.

* Menu:

* Header Files::                How to include the header files in your
                                 programs.
* Macro Definitions::           Some functions in the library may really
                                 be implemented as macros.
* Reserved Names::              The C standard reserves some names for
                                 the library, and some for users.
* Feature Test Macros::         How to control what names are defined.

File: libc.info,  Node: Header Files,  Next: Macro Definitions,  Up: Using the Library

1.3.1 Header Files
------------------

Libraries for use by C programs really consist of two parts: "header
files" that define types and macros and declare variables and
functions; and the actual library or "archive" that contains the
definitions of the variables and functions.

   (Recall that in C, a "declaration" merely provides information that
a function or variable exists and gives its type.  For a function
declaration, information about the types of its arguments might be
provided as well.  The purpose of declarations is to allow the compiler
to correctly process references to the declared variables and functions.
A "definition", on the other hand, actually allocates storage for a
variable or says what a function does.)

   In order to use the facilities in the GNU C library, you should be
sure that your program source files include the appropriate header
files.  This is so that the compiler has declarations of these
facilities available and can correctly process references to them.
Once your program has been compiled, the linker resolves these
references to the actual definitions provided in the archive file.

   Header files are included into a program source file by the
`#include' preprocessor directive.  The C language supports two forms
of this directive; the first,

     #include "HEADER"

is typically used to include a header file HEADER that you write
yourself; this would contain definitions and declarations describing the
interfaces between the different parts of your particular application.
By contrast,

     #include <file.h>

is typically used to include a header file `file.h' that contains
definitions and declarations for a standard library.  This file would
normally be installed in a standard place by your system administrator.
You should use this second form for the C library header files.

   Typically, `#include' directives are placed at the top of the C
source file, before any other code.  If you begin your source files with
some comments explaining what the code in the file does (a good idea),
put the `#include' directives immediately afterwards, following the
feature test macro definition (*note Feature Test Macros::).

   For more information about the use of header files and `#include'
directives, *note Header Files: (cpp.info)Header Files.

   The GNU C library provides several header files, each of which
contains the type and macro definitions and variable and function
declarations for a group of related facilities.  This means that your
programs may need to include several header files, depending on exactly
which facilities you are using.

   Some library header files include other library header files
automatically.  However, as a matter of programming style, you should
not rely on this; it is better to explicitly include all the header
files required for the library facilities you are using.  The GNU C
library header files have been written in such a way that it doesn't
matter if a header file is accidentally included more than once;
including a header file a second time has no effect.  Likewise, if your
program needs to include multiple header files, the order in which they
are included doesn't matter.

   *Compatibility Note:* Inclusion of standard header files in any
order and any number of times works in any ISO C implementation.
However, this has traditionally not been the case in many older C
implementations.

   Strictly speaking, you don't _have to_ include a header file to use
a function it declares; you could declare the function explicitly
yourself, according to the specifications in this manual.  But it is
usually better to include the header file because it may define types
and macros that are not otherwise available and because it may define
more efficient macro replacements for some functions.  It is also a sure
way to have the correct declaration.

File: libc.info,  Node: Macro Definitions,  Next: Reserved Names,  Prev: Header Files,  Up: Using the Library

1.3.2 Macro Definitions of Functions
------------------------------------

If we describe something as a function in this manual, it may have a
macro definition as well.  This normally has no effect on how your
program runs--the macro definition does the same thing as the function
would.  In particular, macro equivalents for library functions evaluate
arguments exactly once, in the same way that a function call would.  The
main reason for these macro definitions is that sometimes they can
produce an inline expansion that is considerably faster than an actual
function call.

   Taking the address of a library function works even if it is also
defined as a macro.  This is because, in this context, the name of the
function isn't followed by the left parenthesis that is syntactically
necessary to recognize a macro call.

   You might occasionally want to avoid using the macro definition of a
function--perhaps to make your program easier to debug.  There are two
ways you can do this:

   * You can avoid a macro definition in a specific use by enclosing
     the name of the function in parentheses.  This works because the
     name of the function doesn't appear in a syntactic context where
     it is recognizable as a macro call.

   * You can suppress any macro definition for a whole source file by
     using the `#undef' preprocessor directive, unless otherwise stated
     explicitly in the description of that facility.

   For example, suppose the header file `stdlib.h' declares a function
named `abs' with

     extern int abs (int);

and also provides a macro definition for `abs'.  Then, in:

     #include <stdlib.h>
     int f (int *i) { return abs (++*i); }

the reference to `abs' might refer to either a macro or a function.  On
the other hand, in each of the following examples the reference is to a
function and not a macro.

     #include <stdlib.h>
     int g (int *i) { return (abs) (++*i); }

     #undef abs
     int h (int *i) { return abs (++*i); }

   Since macro definitions that double for a function behave in exactly
the same way as the actual function version, there is usually no need
for any of these methods.  In fact, removing macro definitions usually
just makes your program slower.

File: libc.info,  Node: Reserved Names,  Next: Feature Test Macros,  Prev: Macro Definitions,  Up: Using the Library

1.3.3 Reserved Names
--------------------

The names of all library types, macros, variables and functions that
come from the ISO C standard are reserved unconditionally; your program
*may not* redefine these names.  All other library names are reserved
if your program explicitly includes the header file that defines or
declares them.  There are several reasons for these restrictions:

   * Other people reading your code could get very confused if you were
     using a function named `exit' to do something completely different
     from what the standard `exit' function does, for example.
     Preventing this situation helps to make your programs easier to
     understand and contributes to modularity and maintainability.

   * It avoids the possibility of a user accidentally redefining a
     library function that is called by other library functions.  If
     redefinition were allowed, those other functions would not work
     properly.

   * It allows the compiler to do whatever special optimizations it
     pleases on calls to these functions, without the possibility that
     they may have been redefined by the user.  Some library
     facilities, such as those for dealing with variadic arguments
     (*note Variadic Functions::) and non-local exits (*note Non-Local
     Exits::), actually require a considerable amount of cooperation on
     the part of the C compiler, and with respect to the
     implementation, it might be easier for the compiler to treat these
     as built-in parts of the language.

   In addition to the names documented in this manual, reserved names
include all external identifiers (global functions and variables) that
begin with an underscore (`_') and all identifiers regardless of use
that begin with either two underscores or an underscore followed by a
capital letter are reserved names.  This is so that the library and
header files can define functions, variables, and macros for internal
purposes without risk of conflict with names in user programs.

   Some additional classes of identifier names are reserved for future
extensions to the C language or the POSIX.1 environment.  While using
these names for your own purposes right now might not cause a problem,
they do raise the possibility of conflict with future versions of the C
or POSIX standards, so you should avoid these names.

   * Names beginning with a capital `E' followed a digit or uppercase
     letter may be used for additional error code names.  *Note Error
     Reporting::.

   * Names that begin with either `is' or `to' followed by a lowercase
     letter may be used for additional character testing and conversion
     functions.  *Note Character Handling::.

   * Names that begin with `LC_' followed by an uppercase letter may be
     used for additional macros specifying locale attributes.  *Note
     Locales::.

   * Names of all existing mathematics functions (*note Mathematics::)
     suffixed with `f' or `l' are reserved for corresponding functions
     that operate on `float' and `long double' arguments, respectively.

   * Names that begin with `SIG' followed by an uppercase letter are
     reserved for additional signal names.  *Note Standard Signals::.

   * Names that begin with `SIG_' followed by an uppercase letter are
     reserved for additional signal actions.  *Note Basic Signal
     Handling::.

   * Names beginning with `str', `mem', or `wcs' followed by a
     lowercase letter are reserved for additional string and array
     functions.  *Note String and Array Utilities::.

   * Names that end with `_t' are reserved for additional type names.

   In addition, some individual header files reserve names beyond those
that they actually define.  You only need to worry about these
restrictions if your program includes that particular header file.

   * The header file `dirent.h' reserves names prefixed with `d_'.

   * The header file `fcntl.h' reserves names prefixed with `l_', `F_',
     `O_', and `S_'.

   * The header file `grp.h' reserves names prefixed with `gr_'.

   * The header file `limits.h' reserves names suffixed with `_MAX'.

   * The header file `pwd.h' reserves names prefixed with `pw_'.

   * The header file `signal.h' reserves names prefixed with `sa_' and
     `SA_'.

   * The header file `sys/stat.h' reserves names prefixed with `st_'
     and `S_'.

   * The header file `sys/times.h' reserves names prefixed with `tms_'.

   * The header file `termios.h' reserves names prefixed with `c_',
     `V', `I', `O', and `TC'; and names prefixed with `B' followed by a
     digit.

File: libc.info,  Node: Feature Test Macros,  Prev: Reserved Names,  Up: Using the Library

1.3.4 Feature Test Macros
-------------------------

The exact set of features available when you compile a source file is
controlled by which "feature test macros" you define.

   If you compile your programs using `gcc -ansi', you get only the
ISO C library features, unless you explicitly request additional
features by defining one or more of the feature macros.  *Note GNU CC
Command Options: (gcc.info)Invoking GCC, for more information about GCC
options.

   You should define these macros by using `#define' preprocessor
directives at the top of your source code files.  These directives
_must_ come before any `#include' of a system header file.  It is best
to make them the very first thing in the file, preceded only by
comments.  You could also use the `-D' option to GCC, but it's better
if you make the source files indicate their own meaning in a
self-contained way.

   This system exists to allow the library to conform to multiple
standards.  Although the different standards are often described as
supersets of each other, they are usually incompatible because larger
standards require functions with names that smaller ones reserve to the
user program.  This is not mere pedantry -- it has been a problem in
practice.  For instance, some non-GNU programs define functions named
`getline' that have nothing to do with this library's `getline'.  They
would not be compilable if all features were enabled indiscriminately.

   This should not be used to verify that a program conforms to a
limited standard.  It is insufficient for this purpose, as it will not
protect you from including header files outside the standard, or
relying on semantics undefined within the standard.

 -- Macro: _POSIX_SOURCE
     If you define this macro, then the functionality from the POSIX.1
     standard (IEEE Standard 1003.1) is available, as well as all of the
     ISO C facilities.

     The state of `_POSIX_SOURCE' is irrelevant if you define the macro
     `_POSIX_C_SOURCE' to a positive integer.

 -- Macro: _POSIX_C_SOURCE
     Define this macro to a positive integer to control which POSIX
     functionality is made available.  The greater the value of this
     macro, the more functionality is made available.

     If you define this macro to a value greater than or equal to `1',
     then the functionality from the 1990 edition of the POSIX.1
     standard (IEEE Standard 1003.1-1990) is made available.

     If you define this macro to a value greater than or equal to `2',
     then the functionality from the 1992 edition of the POSIX.2
     standard (IEEE Standard 1003.2-1992) is made available.

     If you define this macro to a value greater than or equal to
     `199309L', then the functionality from the 1993 edition of the
     POSIX.1b standard (IEEE Standard 1003.1b-1993) is made available.

     Greater values for `_POSIX_C_SOURCE' will enable future extensions.
     The POSIX standards process will define these values as necessary,
     and the GNU C Library should support them some time after they
     become standardized.  The 1996 edition of POSIX.1 (ISO/IEC 9945-1:
     1996) states that if you define `_POSIX_C_SOURCE' to a value
     greater than or equal to `199506L', then the functionality from
     the 1996 edition is made available.

 -- Macro: _BSD_SOURCE
     If you define this macro, functionality derived from 4.3 BSD Unix
     is included as well as the ISO C, POSIX.1, and POSIX.2 material.

     Some of the features derived from 4.3 BSD Unix conflict with the
     corresponding features specified by the POSIX.1 standard.  If this
     macro is defined, the 4.3 BSD definitions take precedence over the
     POSIX definitions.

     Due to the nature of some of the conflicts between 4.3 BSD and
     POSIX.1, you need to use a special "BSD compatibility library"
     when linking programs compiled for BSD compatibility.  This is
     because some functions must be defined in two different ways, one
     of them in the normal C library, and one of them in the
     compatibility library.  If your program defines `_BSD_SOURCE', you
     must give the option `-lbsd-compat' to the compiler or linker when
     linking the program, to tell it to find functions in this special
     compatibility library before looking for them in the normal C
     library.

 -- Macro: _SVID_SOURCE
     If you define this macro, functionality derived from SVID is
     included as well as the ISO C, POSIX.1, POSIX.2, and X/Open
     material.

 -- Macro: _XOPEN_SOURCE
 -- Macro: _XOPEN_SOURCE_EXTENDED
     If you define this macro, functionality described in the X/Open
     Portability Guide is included.  This is a superset of the POSIX.1
     and POSIX.2 functionality and in fact `_POSIX_SOURCE' and
     `_POSIX_C_SOURCE' are automatically defined.

     As the unification of all Unices, functionality only available in
     BSD and SVID is also included.

     If the macro `_XOPEN_SOURCE_EXTENDED' is also defined, even more
     functionality is available.  The extra functions will make all
     functions available which are necessary for the X/Open Unix brand.

     If the macro `_XOPEN_SOURCE' has the value 500 this includes all
     functionality described so far plus some new definitions from the
     Single Unix Specification, version 2.

 -- Macro: _LARGEFILE_SOURCE
     If this macro is defined some extra functions are available which
     rectify a few shortcomings in all previous standards.
     Specifically, the functions `fseeko' and `ftello' are available.
     Without these functions the difference between the ISO C interface
     (`fseek', `ftell') and the low-level POSIX interface (`lseek')
     would lead to problems.

     This macro was introduced as part of the Large File Support
     extension (LFS).

 -- Macro: _LARGEFILE64_SOURCE
     If you define this macro an additional set of functions is made
     available which enables 32 bit systems to use files of sizes beyond
     the usual limit of 2GB.  This interface is not available if the
     system does not support files that large.  On systems where the
     natural file size limit is greater than 2GB (i.e., on 64 bit
     systems) the new functions are identical to the replaced functions.

     The new functionality is made available by a new set of types and
     functions which replace the existing ones.  The names of these new
     objects contain `64' to indicate the intention, e.g., `off_t' vs.
     `off64_t' and `fseeko' vs. `fseeko64'.

     This macro was introduced as part of the Large File Support
     extension (LFS).  It is a transition interface for the period when
     64 bit offsets are not generally used (see `_FILE_OFFSET_BITS').

 -- Macro: _FILE_OFFSET_BITS
     This macro determines which file system interface shall be used,
     one replacing the other.  Whereas `_LARGEFILE64_SOURCE' makes the
     64 bit interface available as an additional interface,
     `_FILE_OFFSET_BITS' allows the 64 bit interface to replace the old
     interface.

     If `_FILE_OFFSET_BITS' is undefined, or if it is defined to the
     value `32', nothing changes.  The 32 bit interface is used and
     types like `off_t' have a size of 32 bits on 32 bit systems.

     If the macro is defined to the value `64', the large file interface
     replaces the old interface.  I.e., the functions are not made
     available under different names (as they are with
     `_LARGEFILE64_SOURCE').  Instead the old function names now
     reference the new functions, e.g., a call to `fseeko' now indeed
     calls `fseeko64'.

     This macro should only be selected if the system provides
     mechanisms for handling large files.  On 64 bit systems this macro
     has no effect since the `*64' functions are identical to the
     normal functions.

     This macro was introduced as part of the Large File Support
     extension (LFS).

 -- Macro: _ISOC99_SOURCE
     Until the revised ISO C standard is widely adopted the new features
     are not automatically enabled.  The GNU libc nevertheless has a
     complete implementation of the new standard and to enable the new
     features the macro `_ISOC99_SOURCE' should be defined.

 -- Macro: _GNU_SOURCE
     If you define this macro, everything is included: ISO C89,
     ISO C99, POSIX.1, POSIX.2, BSD, SVID, X/Open, LFS, and GNU
     extensions.  In the cases where POSIX.1 conflicts with BSD, the
     POSIX definitions take precedence.

     If you want to get the full effect of `_GNU_SOURCE' but make the
     BSD definitions take precedence over the POSIX definitions, use
     this sequence of definitions:

          #define _GNU_SOURCE
          #define _BSD_SOURCE
          #define _SVID_SOURCE

     Note that if you do this, you must link your program with the BSD
     compatibility library by passing the `-lbsd-compat' option to the
     compiler or linker.  *NB:* If you forget to do this, you may get
     very strange errors at run time.

 -- Macro: _REENTRANT
 -- Macro: _THREAD_SAFE
     If you define one of these macros, reentrant versions of several
     functions get declared.  Some of the functions are specified in
     POSIX.1c but many others are only available on a few other systems
     or are unique to GNU libc.  The problem is the delay in the
     standardization of the thread safe C library interface.

     Unlike on some other systems, no special version of the C library
     must be used for linking.  There is only one version but while
     compiling this it must have been specified to compile as thread
     safe.

   We recommend you use `_GNU_SOURCE' in new programs.  If you don't
specify the `-ansi' option to GCC and don't define any of these macros
explicitly, the effect is the same as defining `_POSIX_C_SOURCE' to 2
and `_POSIX_SOURCE', `_SVID_SOURCE', and `_BSD_SOURCE' to 1.

   When you define a feature test macro to request a larger class of
features, it is harmless to define in addition a feature test macro for
a subset of those features.  For example, if you define
`_POSIX_C_SOURCE', then defining `_POSIX_SOURCE' as well has no effect.
Likewise, if you define `_GNU_SOURCE', then defining either
`_POSIX_SOURCE' or `_POSIX_C_SOURCE' or `_SVID_SOURCE' as well has no
effect.

   Note, however, that the features of `_BSD_SOURCE' are not a subset of
any of the other feature test macros supported.  This is because it
defines BSD features that take precedence over the POSIX features that
are requested by the other macros.  For this reason, defining
`_BSD_SOURCE' in addition to the other feature test macros does have an
effect: it causes the BSD features to take priority over the conflicting
POSIX features.

File: libc.info,  Node: Roadmap to the Manual,  Prev: Using the Library,  Up: Introduction

1.4 Roadmap to the Manual
=========================

Here is an overview of the contents of the remaining chapters of this
manual.

   * *note Error Reporting::, describes how errors detected by the
     library are reported.

   * *note Language Features::, contains information about library
     support for standard parts of the C language, including things
     like the `sizeof' operator and the symbolic constant `NULL', how
     to write functions accepting variable numbers of arguments, and
     constants describing the ranges and other properties of the
     numerical types.  There is also a simple debugging mechanism which
     allows you to put assertions in your code, and have diagnostic
     messages printed if the tests fail.

   * *note Memory::, describes the GNU library's facilities for
     managing and using virtual and real memory, including dynamic
     allocation of virtual memory.  If you do not know in advance how
     much memory your program needs, you can allocate it dynamically
     instead, and manipulate it via pointers.

   * *note Character Handling::, contains information about character
     classification functions (such as `isspace') and functions for
     performing case conversion.

   * *note String and Array Utilities::, has descriptions of functions
     for manipulating strings (null-terminated character arrays) and
     general byte arrays, including operations such as copying and
     comparison.

   * *note I/O Overview::, gives an overall look at the input and output
     facilities in the library, and contains information about basic
     concepts such as file names.

   * *note I/O on Streams::, describes I/O operations involving streams
     (or `FILE *' objects).  These are the normal C library functions
     from `stdio.h'.

   * *note Low-Level I/O::, contains information about I/O operations
     on file descriptors.  File descriptors are a lower-level mechanism
     specific to the Unix family of operating systems.

   * *note File System Interface::, has descriptions of operations on
     entire files, such as functions for deleting and renaming them and
     for creating new directories.  This chapter also contains
     information about how you can access the attributes of a file,
     such as its owner and file protection modes.

   * *note Pipes and FIFOs::, contains information about simple
     interprocess communication mechanisms.  Pipes allow communication
     between two related processes (such as between a parent and
     child), while FIFOs allow communication between processes sharing
     a common file system on the same machine.

   * *note Sockets::, describes a more complicated interprocess
     communication mechanism that allows processes running on different
     machines to communicate over a network.  This chapter also
     contains information about Internet host addressing and how to use
     the system network databases.

   * *note Low-Level Terminal Interface::, describes how you can change
     the attributes of a terminal device.  If you want to disable echo
     of characters typed by the user, for example, read this chapter.

   * *note Mathematics::, contains information about the math library
     functions.  These include things like random-number generators and
     remainder functions on integers as well as the usual trigonometric
     and exponential functions on floating-point numbers.

   * *note Low-Level Arithmetic Functions: Arithmetic, describes
     functions for simple arithmetic, analysis of floating-point
     values, and reading numbers from strings.

   * *note Searching and Sorting::, contains information about functions
     for searching and sorting arrays.  You can use these functions on
     any kind of array by providing an appropriate comparison function.

   * *note Pattern Matching::, presents functions for matching regular
     expressions and shell file name patterns, and for expanding words
     as the shell does.

   * *note Date and Time::, describes functions for measuring both
     calendar time and CPU time, as well as functions for setting
     alarms and timers.

   * *note Character Set Handling::, contains information about
     manipulating characters and strings using character sets larger
     than will fit in the usual `char' data type.

   * *note Locales::, describes how selecting a particular country or
     language affects the behavior of the library.  For example, the
     locale affects collation sequences for strings and how monetary
     values are formatted.

   * *note Non-Local Exits::, contains descriptions of the `setjmp' and
     `longjmp' functions.  These functions provide a facility for
     `goto'-like jumps which can jump from one function to another.

   * *note Signal Handling::, tells you all about signals--what they
     are, how to establish a handler that is called when a particular
     kind of signal is delivered, and how to prevent signals from
     arriving during critical sections of your program.

   * *note Program Basics::, tells how your programs can access their
     command-line arguments and environment variables.

   * *note Processes::, contains information about how to start new
     processes and run programs.

   * *note Job Control::, describes functions for manipulating process
     groups and the controlling terminal.  This material is probably
     only of interest if you are writing a shell or other program which
     handles job control specially.

   * *note Name Service Switch::, describes the services which are
     available for looking up names in the system databases, how to
     determine which service is used for which database, and how these
     services are implemented so that contributors can design their own
     services.

   * *note User Database::, and *note Group Database::, tell you how to
     access the system user and group databases.

   * *note System Management::, describes functions for controlling and
     getting information about the hardware and software configuration
     your program is executing under.

   * *note System Configuration::, tells you how you can get
     information about various operating system limits.  Most of these
     parameters are provided for compatibility with POSIX.

   * *note Library Summary::, gives a summary of all the functions,
     variables, and macros in the library, with complete data types and
     function prototypes, and says what standard or system each is
     derived from.

   * *note Maintenance::, explains how to build and install the GNU C
     library on your system, how to report any bugs you might find, and
     how to add new functions or port the library to a new system.

   If you already know the name of the facility you are interested in,
you can look it up in *note Library Summary::.  This gives you a
summary of its syntax and a pointer to where you can find a more
detailed description.  This appendix is particularly useful if you just
want to verify the order and type of arguments to a function, for
example.  It also tells you what standard or system each function,
variable, or macro is derived from.

File: libc.info,  Node: Error Reporting,  Next: Memory,  Prev: Introduction,  Up: Top

2 Error Reporting
*****************

Many functions in the GNU C library detect and report error conditions,
and sometimes your programs need to check for these error conditions.
For example, when you open an input file, you should verify that the
file was actually opened correctly, and print an error message or take
other appropriate action if the call to the library function failed.

   This chapter describes how the error reporting facility works.  Your
program should include the header file `errno.h' to use this facility.

* Menu:

* Checking for Errors::         How errors are reported by library functions.
* Error Codes::                 Error code macros; all of these expand
                                 into integer constant values.
* Error Messages::              Mapping error codes onto error messages.

File: libc.info,  Node: Checking for Errors,  Next: Error Codes,  Up: Error Reporting

2.1 Checking for Errors
=======================

Most library functions return a special value to indicate that they have
failed.  The special value is typically `-1', a null pointer, or a
constant such as `EOF' that is defined for that purpose.  But this
return value tells you only that an error has occurred.  To find out
what kind of error it was, you need to look at the error code stored in
the variable `errno'.  This variable is declared in the header file
`errno.h'.

 -- Variable: volatile int errno
     The variable `errno' contains the system error number.  You can
     change the value of `errno'.

     Since `errno' is declared `volatile', it might be changed
     asynchronously by a signal handler; see *note Defining Handlers::.
     However, a properly written signal handler saves and restores the
     value of `errno', so you generally do not need to worry about this
     possibility except when writing signal handlers.

     The initial value of `errno' at program startup is zero.  Many
     library functions are guaranteed to set it to certain nonzero
     values when they encounter certain kinds of errors.  These error
     conditions are listed for each function.  These functions do not
     change `errno' when they succeed; thus, the value of `errno' after
     a successful call is not necessarily zero, and you should not use
     `errno' to determine _whether_ a call failed.  The proper way to
     do that is documented for each function.  _If_ the call failed,
     you can examine `errno'.

     Many library functions can set `errno' to a nonzero value as a
     result of calling other library functions which might fail.  You
     should assume that any library function might alter `errno' when
     the function returns an error.

     *Portability Note:* ISO C specifies `errno' as a "modifiable
     lvalue" rather than as a variable, permitting it to be implemented
     as a macro.  For example, its expansion might involve a function
     call, like `*_errno ()'.  In fact, that is what it is on the GNU
     system itself.  The GNU library, on non-GNU systems, does whatever
     is right for the particular system.

     There are a few library functions, like `sqrt' and `atan', that
     return a perfectly legitimate value in case of an error, but also
     set `errno'.  For these functions, if you want to check to see
     whether an error occurred, the recommended method is to set `errno'
     to zero before calling the function, and then check its value
     afterward.

   All the error codes have symbolic names; they are macros defined in
`errno.h'.  The names start with `E' and an upper-case letter or digit;
you should consider names of this form to be reserved names.  *Note
Reserved Names::.

   The error code values are all positive integers and are all distinct,
with one exception: `EWOULDBLOCK' and `EAGAIN' are the same.  Since the
values are distinct, you can use them as labels in a `switch'
statement; just don't use both `EWOULDBLOCK' and `EAGAIN'.  Your
program should not make any other assumptions about the specific values
of these symbolic constants.

   The value of `errno' doesn't necessarily have to correspond to any
of these macros, since some library functions might return other error
codes of their own for other situations.  The only values that are
guaranteed to be meaningful for a particular library function are the
ones that this manual lists for that function.

   On non-GNU systems, almost any system call can return `EFAULT' if it
is given an invalid pointer as an argument.  Since this could only
happen as a result of a bug in your program, and since it will not
happen on the GNU system, we have saved space by not mentioning
`EFAULT' in the descriptions of individual functions.

   In some Unix systems, many system calls can also return `EFAULT' if
given as an argument a pointer into the stack, and the kernel for some
obscure reason fails in its attempt to extend the stack.  If this ever
happens, you should probably try using statically or dynamically
allocated memory instead of stack memory on that system.

File: libc.info,  Node: Error Codes,  Next: Error Messages,  Prev: Checking for Errors,  Up: Error Reporting

2.2 Error Codes
===============

The error code macros are defined in the header file `errno.h'.  All of
them expand into integer constant values.  Some of these error codes
can't occur on the GNU system, but they can occur using the GNU library
on other systems.

 -- Macro: int EPERM
     Operation not permitted; only the owner of the file (or other
     resource) or processes with special privileges can perform the
     operation.

 -- Macro: int ENOENT
     No such file or directory.  This is a "file doesn't exist" error
     for ordinary files that are referenced in contexts where they are
     expected to already exist.

 -- Macro: int ESRCH
     No process matches the specified process ID.

 -- Macro: int EINTR
     Interrupted function call; an asynchronous signal occurred and
     prevented completion of the call.  When this happens, you should
     try the call again.

     You can choose to have functions resume after a signal that is
     handled, rather than failing with `EINTR'; see *note Interrupted
     Primitives::.

 -- Macro: int EIO
     Input/output error; usually used for physical read or write errors.

 -- Macro: int ENXIO
     No such device or address.  The system tried to use the device
     represented by a file you specified, and it couldn't find the
     device.  This can mean that the device file was installed
     incorrectly, or that the physical device is missing or not
     correctly attached to the computer.

 -- Macro: int E2BIG
     Argument list too long; used when the arguments passed to a new
     program being executed with one of the `exec' functions (*note
     Executing a File::) occupy too much memory space.  This condition
     never arises in the GNU system.

 -- Macro: int ENOEXEC
     Invalid executable file format.  This condition is detected by the
     `exec' functions; see *note Executing a File::.

 -- Macro: int EBADF
     Bad file descriptor; for example, I/O on a descriptor that has been
     closed or reading from a descriptor open only for writing (or vice
     versa).

 -- Macro: int ECHILD
     There are no child processes.  This error happens on operations
     that are supposed to manipulate child processes, when there aren't
     any processes to manipulate.

 -- Macro: int EDEADLK
     Deadlock avoided; allocating a system resource would have resulted
     in a deadlock situation.  The system does not guarantee that it
     will notice all such situations.  This error means you got lucky
     and the system noticed; it might just hang.  *Note File Locks::,
     for an example.

 -- Macro: int ENOMEM
     No memory available.  The system cannot allocate more virtual
     memory because its capacity is full.

 -- Macro: int EACCES
     Permission denied; the file permissions do not allow the attempted
     operation.

 -- Macro: int EFAULT
     Bad address; an invalid pointer was detected.  In the GNU system,
     this error never happens; you get a signal instead.

 -- Macro: int ENOTBLK
     A file that isn't a block special file was given in a situation
     that requires one.  For example, trying to mount an ordinary file
     as a file system in Unix gives this error.

 -- Macro: int EBUSY
     Resource busy; a system resource that can't be shared is already
     in use.  For example, if you try to delete a file that is the root
     of a currently mounted filesystem, you get this error.

 -- Macro: int EEXIST
     File exists; an existing file was specified in a context where it
     only makes sense to specify a new file.

 -- Macro: int EXDEV
     An attempt to make an improper link across file systems was
     detected.  This happens not only when you use `link' (*note Hard
     Links::) but also when you rename a file with `rename' (*note
     Renaming Files::).

 -- Macro: int ENODEV
     The wrong type of device was given to a function that expects a
     particular sort of device.

 -- Macro: int ENOTDIR
     A file that isn't a directory was specified when a directory is
     required.

 -- Macro: int EISDIR
     File is a directory; you cannot open a directory for writing, or
     create or remove hard links to it.

 -- Macro: int EINVAL
     Invalid argument.  This is used to indicate various kinds of
     problems with passing the wrong argument to a library function.

 -- Macro: int EMFILE
     The current process has too many files open and can't open any
     more.  Duplicate descriptors do count toward this limit.

     In BSD and GNU, the number of open files is controlled by a
     resource limit that can usually be increased.  If you get this
     error, you might want to increase the `RLIMIT_NOFILE' limit or
     make it unlimited; *note Limits on Resources::.

 -- Macro: int ENFILE
     There are too many distinct file openings in the entire system.
     Note that any number of linked channels count as just one file
     opening; see *note Linked Channels::.  This error never occurs in
     the GNU system.

 -- Macro: int ENOTTY
     Inappropriate I/O control operation, such as trying to set terminal
     modes on an ordinary file.

 -- Macro: int ETXTBSY
     An attempt to execute a file that is currently open for writing, or
     write to a file that is currently being executed.  Often using a
     debugger to run a program is considered having it open for writing
     and will cause this error.  (The name stands for "text file
     busy".)  This is not an error in the GNU system; the text is
     copied as necessary.

 -- Macro: int EFBIG
     File too big; the size of a file would be larger than allowed by
     the system.

 -- Macro: int ENOSPC
     No space left on device; write operation on a file failed because
     the disk is full.

 -- Macro: int ESPIPE
     Invalid seek operation (such as on a pipe).

 -- Macro: int EROFS
     An attempt was made to modify something on a read-only file system.

 -- Macro: int EMLINK
     Too many links; the link count of a single file would become too
     large.  `rename' can cause this error if the file being renamed
     already has as many links as it can take (*note Renaming Files::).

 -- Macro: int EPIPE
     Broken pipe; there is no process reading from the other end of a
     pipe.  Every library function that returns this error code also
     generates a `SIGPIPE' signal; this signal terminates the program
     if not handled or blocked.  Thus, your program will never actually
     see `EPIPE' unless it has handled or blocked `SIGPIPE'.

 -- Macro: int EDOM
     Domain error; used by mathematical functions when an argument
     value does not fall into the domain over which the function is
     defined.

 -- Macro: int ERANGE
     Range error; used by mathematical functions when the result value
     is not representable because of overflow or underflow.

 -- Macro: int EAGAIN
     Resource temporarily unavailable; the call might work if you try
     again later.  The macro `EWOULDBLOCK' is another name for `EAGAIN';
     they are always the same in the GNU C library.

     This error can happen in a few different situations:

        * An operation that would block was attempted on an object that
          has non-blocking mode selected.  Trying the same operation
          again will block until some external condition makes it
          possible to read, write, or connect (whatever the operation).
          You can use `select' to find out when the operation will be
          possible; *note Waiting for I/O::.

          *Portability Note:* In many older Unix systems, this condition
          was indicated by `EWOULDBLOCK', which was a distinct error
          code different from `EAGAIN'.  To make your program portable,
          you should check for both codes and treat them the same.

        * A temporary resource shortage made an operation impossible.
          `fork' can return this error.  It indicates that the shortage
          is expected to pass, so your program can try the call again
          later and it may succeed.  It is probably a good idea to
          delay for a few seconds before trying it again, to allow time
          for other processes to release scarce resources.  Such
          shortages are usually fairly serious and affect the whole
          system, so usually an interactive program should report the
          error to the user and return to its command loop.

 -- Macro: int EWOULDBLOCK
     In the GNU C library, this is another name for `EAGAIN' (above).
     The values are always the same, on every operating system.

     C libraries in many older Unix systems have `EWOULDBLOCK' as a
     separate error code.

 -- Macro: int EINPROGRESS
     An operation that cannot complete immediately was initiated on an
     object that has non-blocking mode selected.  Some functions that
     must always block (such as `connect'; *note Connecting::) never
     return `EAGAIN'.  Instead, they return `EINPROGRESS' to indicate
     that the operation has begun and will take some time.  Attempts to
     manipulate the object before the call completes return `EALREADY'.
     You can use the `select' function to find out when the pending
     operation has completed; *note Waiting for I/O::.

 -- Macro: int EALREADY
     An operation is already in progress on an object that has
     non-blocking mode selected.

 -- Macro: int ENOTSOCK
     A file that isn't a socket was specified when a socket is required.

 -- Macro: int EMSGSIZE
     The size of a message sent on a socket was larger than the
     supported maximum size.

 -- Macro: int EPROTOTYPE
     The socket type does not support the requested communications
     protocol.

 -- Macro: int ENOPROTOOPT
     You specified a socket option that doesn't make sense for the
     particular protocol being used by the socket.  *Note Socket
     Options::.

 -- Macro: int EPROTONOSUPPORT
     The socket domain does not support the requested communications
     protocol (perhaps because the requested protocol is completely
     invalid).  *Note Creating a Socket::.

 -- Macro: int ESOCKTNOSUPPORT
     The socket type is not supported.

 -- Macro: int EOPNOTSUPP
     The operation you requested is not supported.  Some socket
     functions don't make sense for all types of sockets, and others
     may not be implemented for all communications protocols.  In the
     GNU system, this error can happen for many calls when the object
     does not support the particular operation; it is a generic
     indication that the server knows nothing to do for that call.

 -- Macro: int EPFNOSUPPORT
     The socket communications protocol family you requested is not
     supported.

 -- Macro: int EAFNOSUPPORT
     The address family specified for a socket is not supported; it is
     inconsistent with the protocol being used on the socket.  *Note
     Sockets::.

 -- Macro: int EADDRINUSE
     The requested socket address is already in use.  *Note Socket
     Addresses::.

 -- Macro: int EADDRNOTAVAIL
     The requested socket address is not available; for example, you
     tried to give a socket a name that doesn't match the local host
     name.  *Note Socket Addresses::.

 -- Macro: int ENETDOWN
     A socket operation failed because the network was down.

 -- Macro: int ENETUNREACH
     A socket operation failed because the subnet containing the remote
     host was unreachable.

 -- Macro: int ENETRESET
     A network connection was reset because the remote host crashed.

 -- Macro: int ECONNABORTED
     A network connection was aborted locally.

 -- Macro: int ECONNRESET
     A network connection was closed for reasons outside the control of
     the local host, such as by the remote machine rebooting or an
     unrecoverable protocol violation.

 -- Macro: int ENOBUFS
     The kernel's buffers for I/O operations are all in use.  In GNU,
     this error is always synonymous with `ENOMEM'; you may get one or
     the other from network operations.

 -- Macro: int EISCONN
     You tried to connect a socket that is already connected.  *Note
     Connecting::.

 -- Macro: int ENOTCONN
     The socket is not connected to anything.  You get this error when
     you try to transmit data over a socket, without first specifying a
     destination for the data.  For a connectionless socket (for
     datagram protocols, such as UDP), you get `EDESTADDRREQ' instead.

 -- Macro: int EDESTADDRREQ
     No default destination address was set for the socket.  You get
     this error when you try to transmit data over a connectionless
     socket, without first specifying a destination for the data with
     `connect'.

 -- Macro: int ESHUTDOWN
     The socket has already been shut down.

 -- Macro: int ETOOMANYREFS
     ???

 -- Macro: int ETIMEDOUT
     A socket operation with a specified timeout received no response
     during the timeout period.

 -- Macro: int ECONNREFUSED
     A remote host refused to allow the network connection (typically
     because it is not running the requested service).

 -- Macro: int ELOOP
     Too many levels of symbolic links were encountered in looking up a
     file name.  This often indicates a cycle of symbolic links.

 -- Macro: int ENAMETOOLONG
     Filename too long (longer than `PATH_MAX'; *note Limits for
     Files::) or host name too long (in `gethostname' or `sethostname';
     *note Host Identification::).

 -- Macro: int EHOSTDOWN
     The remote host for a requested network connection is down.

 -- Macro: int EHOSTUNREACH
     The remote host for a requested network connection is not
     reachable.

 -- Macro: int ENOTEMPTY
     Directory not empty, where an empty directory was expected.
     Typically, this error occurs when you are trying to delete a
     directory.

 -- Macro: int EPROCLIM
     This means that the per-user limit on new process would be
     exceeded by an attempted `fork'.  *Note Limits on Resources::, for
     details on the `RLIMIT_NPROC' limit.

 -- Macro: int EUSERS
     The file quota system is confused because there are too many users.

 -- Macro: int EDQUOT
     The user's disk quota was exceeded.

 -- Macro: int ESTALE
     Stale NFS file handle.  This indicates an internal confusion in
     the NFS system which is due to file system rearrangements on the
     server host.  Repairing this condition usually requires unmounting
     and remounting the NFS file system on the local host.

 -- Macro: int EREMOTE
     An attempt was made to NFS-mount a remote file system with a file
     name that already specifies an NFS-mounted file.  (This is an
     error on some operating systems, but we expect it to work properly
     on the GNU system, making this error code impossible.)

 -- Macro: int EBADRPC
     ???

 -- Macro: int ERPCMISMATCH
     ???

 -- Macro: int EPROGUNAVAIL
     ???

 -- Macro: int EPROGMISMATCH
     ???

 -- Macro: int EPROCUNAVAIL
     ???

 -- Macro: int ENOLCK
     No locks available.  This is used by the file locking facilities;
     see *note File Locks::.  This error is never generated by the GNU
     system, but it can result from an operation to an NFS server
     running another operating system.

 -- Macro: int EFTYPE
     Inappropriate file type or format.  The file was the wrong type
     for the operation, or a data file had the wrong format.

     On some systems `chmod' returns this error if you try to set the
     sticky bit on a non-directory file; *note Setting Permissions::.

 -- Macro: int EAUTH
     ???

 -- Macro: int ENEEDAUTH
     ???

 -- Macro: int ENOSYS
     Function not implemented.  This indicates that the function called
     is not implemented at all, either in the C library itself or in the
     operating system.  When you get this error, you can be sure that
     this particular function will always fail with `ENOSYS' unless you
     install a new version of the C library or the operating system.

 -- Macro: int ENOTSUP
     Not supported.  A function returns this error when certain
     parameter values are valid, but the functionality they request is
     not available.  This can mean that the function does not implement
     a particular command or option value or flag bit at all.  For
     functions that operate on some object given in a parameter, such
     as a file descriptor or a port, it might instead mean that only
     _that specific object_ (file descriptor, port, etc.) is unable to
     support the other parameters given; different file descriptors
     might support different ranges of parameter values.

     If the entire function is not available at all in the
     implementation, it returns `ENOSYS' instead.

 -- Macro: int EILSEQ
     While decoding a multibyte character the function came along an
     invalid or an incomplete sequence of bytes or the given wide
     character is invalid.

 -- Macro: int EBACKGROUND
     In the GNU system, servers supporting the `term' protocol return
     this error for certain operations when the caller is not in the
     foreground process group of the terminal.  Users do not usually
     see this error because functions such as `read' and `write'
     translate it into a `SIGTTIN' or `SIGTTOU' signal.  *Note Job
     Control::, for information on process groups and these signals.

 -- Macro: int EDIED
     In the GNU system, opening a file returns this error when the file
     is translated by a program and the translator program dies while
     starting up, before it has connected to the file.

 -- Macro: int ED
     The experienced user will know what is wrong.

 -- Macro: int EGREGIOUS
     You did *what*?

 -- Macro: int EIEIO
     Go home and have a glass of warm, dairy-fresh milk.

 -- Macro: int EGRATUITOUS
     This error code has no purpose.

 -- Macro: int EBADMSG

 -- Macro: int EIDRM

 -- Macro: int EMULTIHOP

 -- Macro: int ENODATA

 -- Macro: int ENOLINK

 -- Macro: int ENOMSG

 -- Macro: int ENOSR

 -- Macro: int ENOSTR

 -- Macro: int EOVERFLOW

 -- Macro: int EPROTO

 -- Macro: int ETIME

 -- Macro: int ECANCELED
     Operation canceled; an asynchronous operation was canceled before
     it completed.  *Note Asynchronous I/O::.  When you call
     `aio_cancel', the normal result is for the operations affected to
     complete with this error; *note Cancel AIO Operations::.

   _The following error codes are defined by the Linux/i386 kernel.
They are not yet documented._

 -- Macro: int ERESTART

 -- Macro: int ECHRNG

 -- Macro: int EL2NSYNC

 -- Macro: int EL3HLT

 -- Macro: int EL3RST

 -- Macro: int ELNRNG

 -- Macro: int EUNATCH

 -- Macro: int ENOCSI

 -- Macro: int EL2HLT

 -- Macro: int EBADE

 -- Macro: int EBADR

 -- Macro: int EXFULL

 -- Macro: int ENOANO

 -- Macro: int EBADRQC

 -- Macro: int EBADSLT

 -- Macro: int EDEADLOCK

 -- Macro: int EBFONT

 -- Macro: int ENONET

 -- Macro: int ENOPKG

 -- Macro: int EADV

 -- Macro: int ESRMNT

 -- Macro: int ECOMM

 -- Macro: int EDOTDOT

 -- Macro: int ENOTUNIQ

 -- Macro: int EBADFD

 -- Macro: int EREMCHG

 -- Macro: int ELIBACC

 -- Macro: int ELIBBAD

 -- Macro: int ELIBSCN

 -- Macro: int ELIBMAX

 -- Macro: int ELIBEXEC

 -- Macro: int ESTRPIPE

 -- Macro: int EUCLEAN

 -- Macro: int ENOTNAM

 -- Macro: int ENAVAIL

 -- Macro: int EISNAM

 -- Macro: int EREMOTEIO

 -- Macro: int ENOMEDIUM

 -- Macro: int EMEDIUMTYPE

 -- Macro: int ENOKEY

 -- Macro: int EKEYEXPIRED

 -- Macro: int EKEYREVOKED

 -- Macro: int EKEYREJECTED

 -- Macro: int EOWNERDEAD

 -- Macro: int ENOTRECOVERABLE

 -- Macro: int ERFKILL

File: libc.info,  Node: Error Messages,  Prev: Error Codes,  Up: Error Reporting

2.3 Error Messages
==================

The library has functions and variables designed to make it easy for
your program to report informative error messages in the customary
format about the failure of a library call.  The functions `strerror'
and `perror' give you the standard error message for a given error
code; the variable `program_invocation_short_name' gives you convenient
access to the name of the program that encountered the error.

 -- Function: char * strerror (int ERRNUM)
     The `strerror' function maps the error code (*note Checking for
     Errors::) specified by the ERRNUM argument to a descriptive error
     message string.  The return value is a pointer to this string.

     The value ERRNUM normally comes from the variable `errno'.

     You should not modify the string returned by `strerror'.  Also, if
     you make subsequent calls to `strerror', the string might be
     overwritten.  (But it's guaranteed that no library function ever
     calls `strerror' behind your back.)

     The function `strerror' is declared in `string.h'.

 -- Function: char * strerror_r (int ERRNUM, char *BUF, size_t N)
     The `strerror_r' function works like `strerror' but instead of
     returning the error message in a statically allocated buffer
     shared by all threads in the process, it returns a private copy
     for the thread. This might be either some permanent global data or
     a message string in the user supplied buffer starting at BUF with
     the length of N bytes.

     At most N characters are written (including the NUL byte) so it is
     up to the user to select the buffer large enough.

     This function should always be used in multi-threaded programs
     since there is no way to guarantee the string returned by
     `strerror' really belongs to the last call of the current thread.

     This function `strerror_r' is a GNU extension and it is declared in
     `string.h'.

 -- Function: void perror (const char *MESSAGE)
     This function prints an error message to the stream `stderr'; see
     *note Standard Streams::.  The orientation of `stderr' is not
     changed.

     If you call `perror' with a MESSAGE that is either a null pointer
     or an empty string, `perror' just prints the error message
     corresponding to `errno', adding a trailing newline.

     If you supply a non-null MESSAGE argument, then `perror' prefixes
     its output with this string.  It adds a colon and a space
     character to separate the MESSAGE from the error string
     corresponding to `errno'.

     The function `perror' is declared in `stdio.h'.

   `strerror' and `perror' produce the exact same message for any given
error code; the precise text varies from system to system.  On the GNU
system, the messages are fairly short; there are no multi-line messages
or embedded newlines.  Each error message begins with a capital letter
and does not include any terminating punctuation.

   *Compatibility Note:* The `strerror' function was introduced in
ISO C89.  Many older C systems do not support this function yet.

   Many programs that don't read input from the terminal are designed to
exit if any system call fails.  By convention, the error message from
such a program should start with the program's name, sans directories.
You can find that name in the variable `program_invocation_short_name';
the full file name is stored the variable `program_invocation_name'.

 -- Variable: char * program_invocation_name
     This variable's value is the name that was used to invoke the
     program running in the current process.  It is the same as
     `argv[0]'.  Note that this is not necessarily a useful file name;
     often it contains no directory names.  *Note Program Arguments::.

 -- Variable: char * program_invocation_short_name
     This variable's value is the name that was used to invoke the
     program running in the current process, with directory names
     removed.  (That is to say, it is the same as
     `program_invocation_name' minus everything up to the last slash,
     if any.)

   The library initialization code sets up both of these variables
before calling `main'.

   *Portability Note:* These two variables are GNU extensions.  If you
want your program to work with non-GNU libraries, you must save the
value of `argv[0]' in `main', and then strip off the directory names
yourself.  We added these extensions to make it possible to write
self-contained error-reporting subroutines that require no explicit
cooperation from `main'.

   Here is an example showing how to handle failure to open a file
correctly.  The function `open_sesame' tries to open the named file for
reading and returns a stream if successful.  The `fopen' library
function returns a null pointer if it couldn't open the file for some
reason.  In that situation, `open_sesame' constructs an appropriate
error message using the `strerror' function, and terminates the
program.  If we were going to make some other library calls before
passing the error code to `strerror', we'd have to save it in a local
variable instead, because those other library functions might overwrite
`errno' in the meantime.

     #include <errno.h>
     #include <stdio.h>
     #include <stdlib.h>
     #include <string.h>

     FILE *
     open_sesame (char *name)
     {
       FILE *stream;

       errno = 0;
       stream = fopen (name, "r");
       if (stream == NULL)
         {
           fprintf (stderr, "%s: Couldn't open file %s; %s\n",
                    program_invocation_short_name, name, strerror (errno));
           exit (EXIT_FAILURE);
         }
       else
         return stream;
     }

   Using `perror' has the advantage that the function is portable and
available on all systems implementing ISO C.  But often the text
`perror' generates is not what is wanted and there is no way to extend
or change what `perror' does.  The GNU coding standard, for instance,
requires error messages to be preceded by the program name and programs
which read some input files should provide information about the input
file name and the line number in case an error is encountered while
reading the file.  For these occasions there are two functions
available which are widely used throughout the GNU project.  These
functions are declared in `error.h'.

 -- Function: void error (int STATUS, int ERRNUM, const char *FORMAT,
          ...)
     The `error' function can be used to report general problems during
     program execution.  The FORMAT argument is a format string just
     like those given to the `printf' family of functions.  The
     arguments required for the format can follow the FORMAT parameter.
     Just like `perror', `error' also can report an error code in
     textual form.  But unlike `perror' the error value is explicitly
     passed to the function in the ERRNUM parameter.  This eliminates
     the problem mentioned above that the error reporting function must
     be called immediately after the function causing the error since
     otherwise `errno' might have a different value.

     The `error' prints first the program name.  If the application
     defined a global variable `error_print_progname' and points it to a
     function this function will be called to print the program name.
     Otherwise the string from the global variable `program_name' is
     used.  The program name is followed by a colon and a space which
     in turn is followed by the output produced by the format string.
     If the ERRNUM parameter is non-zero the format string output is
     followed by a colon and a space, followed by the error message for
     the error code ERRNUM.  In any case is the output terminated with
     a newline.

     The output is directed to the `stderr' stream.  If the `stderr'
     wasn't oriented before the call it will be narrow-oriented
     afterwards.

     The function will return unless the STATUS parameter has a
     non-zero value.  In this case the function will call `exit' with
     the STATUS value for its parameter and therefore never return.  If
     `error' returns the global variable `error_message_count' is
     incremented by one to keep track of the number of errors reported.

 -- Function: void error_at_line (int STATUS, int ERRNUM, const char
          *FNAME, unsigned int LINENO, const char *FORMAT, ...)
     The `error_at_line' function is very similar to the `error'
     function.  The only difference are the additional parameters FNAME
     and LINENO.  The handling of the other parameters is identical to
     that of `error' except that between the program name and the string
     generated by the format string additional text is inserted.

     Directly following the program name a colon, followed by the file
     name pointer to by FNAME, another colon, and a value of LINENO is
     printed.

     This additional output of course is meant to be used to locate an
     error in an input file (like a programming language source code
     file etc).

     If the global variable `error_one_per_line' is set to a non-zero
     value `error_at_line' will avoid printing consecutive messages for
     the same file and line.  Repetition which are not directly
     following each other are not caught.

     Just like `error' this function only returned if STATUS is zero.
     Otherwise `exit' is called with the non-zero value.  If `error'
     returns the global variable `error_message_count' is incremented
     by one to keep track of the number of errors reported.

   As mentioned above the `error' and `error_at_line' functions can be
customized by defining a variable named `error_print_progname'.

 -- Variable: void (*) error_print_progname  (void)
     If the `error_print_progname' variable is defined to a non-zero
     value the function pointed to is called by `error' or
     `error_at_line'.  It is expected to print the program name or do
     something similarly useful.

     The function is expected to be print to the `stderr' stream and
     must be able to handle whatever orientation the stream has.

     The variable is global and shared by all threads.

 -- Variable: unsigned int error_message_count
     The `error_message_count' variable is incremented whenever one of
     the functions `error' or `error_at_line' returns.  The variable is
     global and shared by all threads.

 -- Variable: int error_one_per_line
     The `error_one_per_line' variable influences only `error_at_line'.
     Normally the `error_at_line' function creates output for every
     invocation.  If `error_one_per_line' is set to a non-zero value
     `error_at_line' keeps track of the last file name and line number
     for which an error was reported and avoid directly following
     messages for the same file and line.  This variable is global and
     shared by all threads.

A program which read some input file and reports errors in it could look
like this:

     {
       char *line = NULL;
       size_t len = 0;
       unsigned int lineno = 0;

       error_message_count = 0;
       while (! feof_unlocked (fp))
         {
           ssize_t n = getline (&line, &len, fp);
           if (n <= 0)
             /* End of file or error.  */
             break;
           ++lineno;

           /* Process the line.  */
           ...

           if (Detect error in line)
             error_at_line (0, errval, filename, lineno,
                            "some error text %s", some_variable);
         }

       if (error_message_count != 0)
         error (EXIT_FAILURE, 0, "%u errors found", error_message_count);
     }

   `error' and `error_at_line' are clearly the functions of choice and
enable the programmer to write applications which follow the GNU coding
standard.  The GNU libc additionally contains functions which are used
in BSD for the same purpose.  These functions are declared in `err.h'.
It is generally advised to not use these functions.  They are included
only for compatibility.

 -- Function: void warn (const char *FORMAT, ...)
     The `warn' function is roughly equivalent to a call like
            error (0, errno, format, the parameters)
     except that the global variables `error' respects and modifies are
     not used.

 -- Function: void vwarn (const char *FORMAT, va_list)
     The `vwarn' function is just like `warn' except that the
     parameters for the handling of the format string FORMAT are passed
     in as an value of type `va_list'.

 -- Function: void warnx (const char *FORMAT, ...)
     The `warnx' function is roughly equivalent to a call like
            error (0, 0, format, the parameters)
     except that the global variables `error' respects and modifies are
     not used.  The difference to `warn' is that no error number string
     is printed.

 -- Function: void vwarnx (const char *FORMAT, va_list)
     The `vwarnx' function is just like `warnx' except that the
     parameters for the handling of the format string FORMAT are passed
     in as an value of type `va_list'.

 -- Function: void err (int STATUS, const char *FORMAT, ...)
     The `err' function is roughly equivalent to a call like
            error (status, errno, format, the parameters)
     except that the global variables `error' respects and modifies are
     not used and that the program is exited even if STATUS is zero.

 -- Function: void verr (int STATUS, const char *FORMAT, va_list)
     The `verr' function is just like `err' except that the parameters
     for the handling of the format string FORMAT are passed in as an
     value of type `va_list'.

 -- Function: void errx (int STATUS, const char *FORMAT, ...)
     The `errx' function is roughly equivalent to a call like
            error (status, 0, format, the parameters)
     except that the global variables `error' respects and modifies are
     not used and that the program is exited even if STATUS is zero.
     The difference to `err' is that no error number string is printed.

 -- Function: void verrx (int STATUS, const char *FORMAT, va_list)
     The `verrx' function is just like `errx' except that the
     parameters for the handling of the format string FORMAT are passed
     in as an value of type `va_list'.

File: libc.info,  Node: Memory,  Next: Character Handling,  Prev: Error Reporting,  Up: Top

3 Virtual Memory Allocation And Paging
**************************************

This chapter describes how processes manage and use memory in a system
that uses the GNU C library.

   The GNU C Library has several functions for dynamically allocating
virtual memory in various ways.  They vary in generality and in
efficiency.  The library also provides functions for controlling paging
and allocation of real memory.

* Menu:

* Memory Concepts::             An introduction to concepts and terminology.
* Memory Allocation::           Allocating storage for your program data
* Locking Pages::               Preventing page faults
* Resizing the Data Segment::   `brk', `sbrk'

   Memory mapped I/O is not discussed in this chapter.  *Note
Memory-mapped I/O::.

File: libc.info,  Node: Memory Concepts,  Next: Memory Allocation,  Up: Memory

3.1 Process Memory Concepts
===========================

One of the most basic resources a process has available to it is memory.
There are a lot of different ways systems organize memory, but in a
typical one, each process has one linear virtual address space, with
addresses running from zero to some huge maximum.  It need not be
contiguous; i.e., not all of these addresses actually can be used to
store data.

   The virtual memory is divided into pages (4 kilobytes is typical).
Backing each page of virtual memory is a page of real memory (called a
"frame") or some secondary storage, usually disk space.  The disk space
might be swap space or just some ordinary disk file.  Actually, a page
of all zeroes sometimes has nothing at all backing it - there's just a
flag saying it is all zeroes.

   The same frame of real memory or backing store can back multiple
virtual pages belonging to multiple processes.  This is normally the
case, for example, with virtual memory occupied by GNU C library code.
The same real memory frame containing the `printf' function backs a
virtual memory page in each of the existing processes that has a
`printf' call in its program.

   In order for a program to access any part of a virtual page, the page
must at that moment be backed by ("connected to") a real frame.  But
because there is usually a lot more virtual memory than real memory, the
pages must move back and forth between real memory and backing store
regularly, coming into real memory when a process needs to access them
and then retreating to backing store when not needed anymore.  This
movement is called "paging".

   When a program attempts to access a page which is not at that moment
backed by real memory, this is known as a "page fault".  When a page
fault occurs, the kernel suspends the process, places the page into a
real page frame (this is called "paging in" or "faulting in"), then
resumes the process so that from the process' point of view, the page
was in real memory all along.  In fact, to the process, all pages always
seem to be in real memory.  Except for one thing: the elapsed execution
time of an instruction that would normally be a few nanoseconds is
suddenly much, much, longer (because the kernel normally has to do I/O
to complete the page-in).  For programs sensitive to that, the functions
described in *note Locking Pages:: can control it.

   Within each virtual address space, a process has to keep track of
what is at which addresses, and that process is called memory
allocation.  Allocation usually brings to mind meting out scarce
resources, but in the case of virtual memory, that's not a major goal,
because there is generally much more of it than anyone needs.  Memory
allocation within a process is mainly just a matter of making sure that
the same byte of memory isn't used to store two different things.

   Processes allocate memory in two major ways: by exec and
programmatically.  Actually, forking is a third way, but it's not very
interesting.  *Note Creating a Process::.

   Exec is the operation of creating a virtual address space for a
process, loading its basic program into it, and executing the program.
It is done by the "exec" family of functions (e.g. `execl').  The
operation takes a program file (an executable), it allocates space to
load all the data in the executable, loads it, and transfers control to
it.  That data is most notably the instructions of the program (the
"text"), but also literals and constants in the program and even some
variables: C variables with the static storage class (*note Memory
Allocation and C::).

   Once that program begins to execute, it uses programmatic allocation
to gain additional memory.  In a C program with the GNU C library, there
are two kinds of programmatic allocation: automatic and dynamic.  *Note
Memory Allocation and C::.

   Memory-mapped I/O is another form of dynamic virtual memory
allocation.  Mapping memory to a file means declaring that the contents
of certain range of a process' addresses shall be identical to the
contents of a specified regular file.  The system makes the virtual
memory initially contain the contents of the file, and if you modify
the memory, the system writes the same modification to the file.  Note
that due to the magic of virtual memory and page faults, there is no
reason for the system to do I/O to read the file, or allocate real
memory for its contents, until the program accesses the virtual memory.
*Note Memory-mapped I/O::.

   Just as it programmatically allocates memory, the program can
programmatically deallocate ("free") it.  You can't free the memory
that was allocated by exec.  When the program exits or execs, you might
say that all its memory gets freed, but since in both cases the address
space ceases to exist, the point is really moot.  *Note Program
Termination::.

   A process' virtual address space is divided into segments.  A
segment is a contiguous range of virtual addresses.  Three important
segments are:

   *  The "text segment" contains a program's instructions and literals
     and static constants.  It is allocated by exec and stays the same
     size for the life of the virtual address space.

   * The "data segment" is working storage for the program.  It can be
     preallocated and preloaded by exec and the process can extend or
     shrink it by calling functions as described in *Note Resizing the
     Data Segment::.  Its lower end is fixed.

   * The "stack segment" contains a program stack.  It grows as the
     stack grows, but doesn't shrink when the stack shrinks.


File: libc.info,  Node: Memory Allocation,  Next: Locking Pages,  Prev: Memory Concepts,  Up: Memory

3.2 Allocating Storage For Program Data
=======================================

This section covers how ordinary programs manage storage for their data,
including the famous `malloc' function and some fancier facilities
special the GNU C library and GNU Compiler.

* Menu:

* Memory Allocation and C::     How to get different kinds of allocation in C.
* Unconstrained Allocation::    The `malloc' facility allows fully general
		 		 dynamic allocation.
* Allocation Debugging::        Finding memory leaks and not freed memory.
* Obstacks::                    Obstacks are less general than malloc
				 but more efficient and convenient.
* Variable Size Automatic::     Allocation of variable-sized blocks
				 of automatic storage that are freed when the
				 calling function returns.

File: libc.info,  Node: Memory Allocation and C,  Next: Unconstrained Allocation,  Up: Memory Allocation

3.2.1 Memory Allocation in C Programs
-------------------------------------

The C language supports two kinds of memory allocation through the
variables in C programs:

   * "Static allocation" is what happens when you declare a static or
     global variable.  Each static or global variable defines one block
     of space, of a fixed size.  The space is allocated once, when your
     program is started (part of the exec operation), and is never
     freed.

   * "Automatic allocation" happens when you declare an automatic
     variable, such as a function argument or a local variable.  The
     space for an automatic variable is allocated when the compound
     statement containing the declaration is entered, and is freed when
     that compound statement is exited.

     In GNU C, the size of the automatic storage can be an expression
     that varies.  In other C implementations, it must be a constant.

   A third important kind of memory allocation, "dynamic allocation",
is not supported by C variables but is available via GNU C library
functions.

3.2.1.1 Dynamic Memory Allocation
.................................

"Dynamic memory allocation" is a technique in which programs determine
as they are running where to store some information.  You need dynamic
allocation when the amount of memory you need, or how long you continue
to need it, depends on factors that are not known before the program
runs.

   For example, you may need a block to store a line read from an input
file; since there is no limit to how long a line can be, you must
allocate the memory dynamically and make it dynamically larger as you
read more of the line.

   Or, you may need a block for each record or each definition in the
input data; since you can't know in advance how many there will be, you
must allocate a new block for each record or definition as you read it.

   When you use dynamic allocation, the allocation of a block of memory
is an action that the program requests explicitly.  You call a function
or macro when you want to allocate space, and specify the size with an
argument.  If you want to free the space, you do so by calling another
function or macro.  You can do these things whenever you want, as often
as you want.

   Dynamic allocation is not supported by C variables; there is no
storage class "dynamic", and there can never be a C variable whose
value is stored in dynamically allocated space.  The only way to get
dynamically allocated memory is via a system call (which is generally
via a GNU C library function call), and the only way to refer to
dynamically allocated space is through a pointer.  Because it is less
convenient, and because the actual process of dynamic allocation
requires more computation time, programmers generally use dynamic
allocation only when neither static nor automatic allocation will serve.

   For example, if you want to allocate dynamically some space to hold a
`struct foobar', you cannot declare a variable of type `struct foobar'
whose contents are the dynamically allocated space.  But you can
declare a variable of pointer type `struct foobar *' and assign it the
address of the space.  Then you can use the operators `*' and `->' on
this pointer variable to refer to the contents of the space:

     {
       struct foobar *ptr
          = (struct foobar *) malloc (sizeof (struct foobar));
       ptr->name = x;
       ptr->next = current_foobar;
       current_foobar = ptr;
     }

File: libc.info,  Node: Unconstrained Allocation,  Next: Allocation Debugging,  Prev: Memory Allocation and C,  Up: Memory Allocation

3.2.2 Unconstrained Allocation
------------------------------

The most general dynamic allocation facility is `malloc'.  It allows
you to allocate blocks of memory of any size at any time, make them
bigger or smaller at any time, and free the blocks individually at any
time (or never).

* Menu:

* Basic Allocation::            Simple use of `malloc'.
* Malloc Examples::             Examples of `malloc'.  `xmalloc'.
* Freeing after Malloc::        Use `free' to free a block you
				 got with `malloc'.
* Changing Block Size::         Use `realloc' to make a block
				 bigger or smaller.
* Allocating Cleared Space::    Use `calloc' to allocate a
				 block and clear it.
* Efficiency and Malloc::       Efficiency considerations in use of
				 these functions.
* Aligned Memory Blocks::       Allocating specially aligned memory.
* Malloc Tunable Parameters::   Use `mallopt' to adjust allocation
                                 parameters.
* Heap Consistency Checking::   Automatic checking for errors.
* Hooks for Malloc::            You can use these hooks for debugging
				 programs that use `malloc'.
* Statistics of Malloc::        Getting information about how much
				 memory your program is using.
* Summary of Malloc::           Summary of `malloc' and related functions.

File: libc.info,  Node: Basic Allocation,  Next: Malloc Examples,  Up: Unconstrained Allocation

3.2.2.1 Basic Memory Allocation
...............................

To allocate a block of memory, call `malloc'.  The prototype for this
function is in `stdlib.h'.

 -- Function: void * malloc (size_t SIZE)
     This function returns a pointer to a newly allocated block SIZE
     bytes long, or a null pointer if the block could not be allocated.

   The contents of the block are undefined; you must initialize it
yourself (or use `calloc' instead; *note Allocating Cleared Space::).
Normally you would cast the value as a pointer to the kind of object
that you want to store in the block.  Here we show an example of doing
so, and of initializing the space with zeros using the library function
`memset' (*note Copying and Concatenation::):

     struct foo *ptr;
     ...
     ptr = (struct foo *) malloc (sizeof (struct foo));
     if (ptr == 0) abort ();
     memset (ptr, 0, sizeof (struct foo));

   You can store the result of `malloc' into any pointer variable
without a cast, because ISO C automatically converts the type `void *'
to another type of pointer when necessary.  But the cast is necessary
in contexts other than assignment operators or if you might want your
code to run in traditional C.

   Remember that when allocating space for a string, the argument to
`malloc' must be one plus the length of the string.  This is because a
string is terminated with a null character that doesn't count in the
"length" of the string but does need space.  For example:

     char *ptr;
     ...
     ptr = (char *) malloc (length + 1);

*Note Representation of Strings::, for more information about this.

File: libc.info,  Node: Malloc Examples,  Next: Freeing after Malloc,  Prev: Basic Allocation,  Up: Unconstrained Allocation

3.2.2.2 Examples of `malloc'
............................

If no more space is available, `malloc' returns a null pointer.  You
should check the value of _every_ call to `malloc'.  It is useful to
write a subroutine that calls `malloc' and reports an error if the
value is a null pointer, returning only if the value is nonzero.  This
function is conventionally called `xmalloc'.  Here it is:

     void *
     xmalloc (size_t size)
     {
       register void *value = malloc (size);
       if (value == 0)
         fatal ("virtual memory exhausted");
       return value;
     }

   Here is a real example of using `malloc' (by way of `xmalloc').  The
function `savestring' will copy a sequence of characters into a newly
allocated null-terminated string:

     char *
     savestring (const char *ptr, size_t len)
     {
       register char *value = (char *) xmalloc (len + 1);
       value[len] = '\0';
       return (char *) memcpy (value, ptr, len);
     }

   The block that `malloc' gives you is guaranteed to be aligned so
that it can hold any type of data.  In the GNU system, the address is
always a multiple of eight on most systems, and a multiple of 16 on
64-bit systems.  Only rarely is any higher boundary (such as a page
boundary) necessary; for those cases, use `memalign', `posix_memalign'
or `valloc' (*note Aligned Memory Blocks::).

   Note that the memory located after the end of the block is likely to
be in use for something else; perhaps a block already allocated by
another call to `malloc'.  If you attempt to treat the block as longer
than you asked for it to be, you are liable to destroy the data that
`malloc' uses to keep track of its blocks, or you may destroy the
contents of another block.  If you have already allocated a block and
discover you want it to be bigger, use `realloc' (*note Changing Block
Size::).

File: libc.info,  Node: Freeing after Malloc,  Next: Changing Block Size,  Prev: Malloc Examples,  Up: Unconstrained Allocation

3.2.2.3 Freeing Memory Allocated with `malloc'
..............................................

When you no longer need a block that you got with `malloc', use the
function `free' to make the block available to be allocated again.  The
prototype for this function is in `stdlib.h'.

 -- Function: void free (void *PTR)
     The `free' function deallocates the block of memory pointed at by
     PTR.

 -- Function: void cfree (void *PTR)
     This function does the same thing as `free'.  It's provided for
     backward compatibility with SunOS; you should use `free' instead.

   Freeing a block alters the contents of the block.  *Do not expect to
find any data (such as a pointer to the next block in a chain of
blocks) in the block after freeing it.*  Copy whatever you need out of
the block before freeing it!  Here is an example of the proper way to
free all the blocks in a chain, and the strings that they point to:

     struct chain
       {
         struct chain *next;
         char *name;
       }

     void
     free_chain (struct chain *chain)
     {
       while (chain != 0)
         {
           struct chain *next = chain->next;
           free (chain->name);
           free (chain);
           chain = next;
         }
     }

   Occasionally, `free' can actually return memory to the operating
system and make the process smaller.  Usually, all it can do is allow a
later call to `malloc' to reuse the space.  In the meantime, the space
remains in your program as part of a free-list used internally by
`malloc'.

   There is no point in freeing blocks at the end of a program, because
all of the program's space is given back to the system when the process
terminates.

File: libc.info,  Node: Changing Block Size,  Next: Allocating Cleared Space,  Prev: Freeing after Malloc,  Up: Unconstrained Allocation

3.2.2.4 Changing the Size of a Block
....................................

Often you do not know for certain how big a block you will ultimately
need at the time you must begin to use the block.  For example, the
block might be a buffer that you use to hold a line being read from a
file; no matter how long you make the buffer initially, you may
encounter a line that is longer.

   You can make the block longer by calling `realloc'.  This function
is declared in `stdlib.h'.

 -- Function: void * realloc (void *PTR, size_t NEWSIZE)
     The `realloc' function changes the size of the block whose address
     is PTR to be NEWSIZE.

     Since the space after the end of the block may be in use, `realloc'
     may find it necessary to copy the block to a new address where
     more free space is available.  The value of `realloc' is the new
     address of the block.  If the block needs to be moved, `realloc'
     copies the old contents.

     If you pass a null pointer for PTR, `realloc' behaves just like
     `malloc (NEWSIZE)'.  This can be convenient, but beware that older
     implementations (before ISO C) may not support this behavior, and
     will probably crash when `realloc' is passed a null pointer.

   Like `malloc', `realloc' may return a null pointer if no memory
space is available to make the block bigger.  When this happens, the
original block is untouched; it has not been modified or relocated.

   In most cases it makes no difference what happens to the original
block when `realloc' fails, because the application program cannot
continue when it is out of memory, and the only thing to do is to give
a fatal error message.  Often it is convenient to write and use a
subroutine, conventionally called `xrealloc', that takes care of the
error message as `xmalloc' does for `malloc':

     void *
     xrealloc (void *ptr, size_t size)
     {
       register void *value = realloc (ptr, size);
       if (value == 0)
         fatal ("Virtual memory exhausted");
       return value;
     }

   You can also use `realloc' to make a block smaller.  The reason you
would do this is to avoid tying up a lot of memory space when only a
little is needed.  In several allocation implementations, making a
block smaller sometimes necessitates copying it, so it can fail if no
other space is available.

   If the new size you specify is the same as the old size, `realloc'
is guaranteed to change nothing and return the same address that you
gave.

File: libc.info,  Node: Allocating Cleared Space,  Next: Efficiency and Malloc,  Prev: Changing Block Size,  Up: Unconstrained Allocation

3.2.2.5 Allocating Cleared Space
................................

The function `calloc' allocates memory and clears it to zero.  It is
declared in `stdlib.h'.

 -- Function: void * calloc (size_t COUNT, size_t ELTSIZE)
     This function allocates a block long enough to contain a vector of
     COUNT elements, each of size ELTSIZE.  Its contents are cleared to
     zero before `calloc' returns.

   You could define `calloc' as follows:

     void *
     calloc (size_t count, size_t eltsize)
     {
       size_t size = count * eltsize;
       void *value = malloc (size);
       if (value != 0)
         memset (value, 0, size);
       return value;
     }

   But in general, it is not guaranteed that `calloc' calls `malloc'
internally.  Therefore, if an application provides its own
`malloc'/`realloc'/`free' outside the C library, it should always
define `calloc', too.

File: libc.info,  Node: Efficiency and Malloc,  Next: Aligned Memory Blocks,  Prev: Allocating Cleared Space,  Up: Unconstrained Allocation

3.2.2.6 Efficiency Considerations for `malloc'
..............................................

As opposed to other versions, the `malloc' in the GNU C Library does
not round up block sizes to powers of two, neither for large nor for
small sizes.  Neighboring chunks can be coalesced on a `free' no matter
what their size is.  This makes the implementation suitable for all
kinds of allocation patterns without generally incurring high memory
waste through fragmentation.

   Very large blocks (much larger than a page) are allocated with
`mmap' (anonymous or via `/dev/zero') by this implementation.  This has
the great advantage that these chunks are returned to the system
immediately when they are freed.  Therefore, it cannot happen that a
large chunk becomes "locked" in between smaller ones and even after
calling `free' wastes memory.  The size threshold for `mmap' to be used
can be adjusted with `mallopt'.  The use of `mmap' can also be disabled
completely.

File: libc.info,  Node: Aligned Memory Blocks,  Next: Malloc Tunable Parameters,  Prev: Efficiency and Malloc,  Up: Unconstrained Allocation

3.2.2.7 Allocating Aligned Memory Blocks
........................................

The address of a block returned by `malloc' or `realloc' in the GNU
system is always a multiple of eight (or sixteen on 64-bit systems).
If you need a block whose address is a multiple of a higher power of
two than that, use `memalign', `posix_memalign', or `valloc'.
`memalign' is declared in `malloc.h' and `posix_memalign' is declared
in `stdlib.h'.

   With the GNU library, you can use `free' to free the blocks that
`memalign', `posix_memalign', and `valloc' return.  That does not work
in BSD, however--BSD does not provide any way to free such blocks.

 -- Function: void * memalign (size_t BOUNDARY, size_t SIZE)
     The `memalign' function allocates a block of SIZE bytes whose
     address is a multiple of BOUNDARY.  The BOUNDARY must be a power
     of two!  The function `memalign' works by allocating a somewhat
     larger block, and then returning an address within the block that
     is on the specified boundary.

 -- Function: int posix_memalign (void **MEMPTR, size_t ALIGNMENT,
          size_t SIZE)
     The `posix_memalign' function is similar to the `memalign'
     function in that it returns a buffer of SIZE bytes aligned to a
     multiple of ALIGNMENT.  But it adds one requirement to the
     parameter ALIGNMENT: the value must be a power of two multiple of
     `sizeof (void *)'.

     If the function succeeds in allocation memory a pointer to the
     allocated memory is returned in `*MEMPTR' and the return value is
     zero.  Otherwise the function returns an error value indicating
     the problem.

     This function was introduced in POSIX 1003.1d.

 -- Function: void * valloc (size_t SIZE)
     Using `valloc' is like using `memalign' and passing the page size
     as the value of the second argument.  It is implemented like this:

          void *
          valloc (size_t size)
          {
            return memalign (getpagesize (), size);
          }

     *note Query Memory Parameters:: for more information about the
     memory subsystem.

File: libc.info,  Node: Malloc Tunable Parameters,  Next: Heap Consistency Checking,  Prev: Aligned Memory Blocks,  Up: Unconstrained Allocation

3.2.2.8 Malloc Tunable Parameters
.................................

You can adjust some parameters for dynamic memory allocation with the
`mallopt' function.  This function is the general SVID/XPG interface,
defined in `malloc.h'.

 -- Function: int mallopt (int PARAM, int VALUE)
     When calling `mallopt', the PARAM argument specifies the parameter
     to be set, and VALUE the new value to be set.  Possible choices
     for PARAM, as defined in `malloc.h', are:

    `M_TRIM_THRESHOLD'
          This is the minimum size (in bytes) of the top-most,
          releasable chunk that will cause `sbrk' to be called with a
          negative argument in order to return memory to the system.

    `M_TOP_PAD'
          This parameter determines the amount of extra memory to
          obtain from the system when a call to `sbrk' is required.  It
          also specifies the number of bytes to retain when shrinking
          the heap by calling `sbrk' with a negative argument.  This
          provides the necessary hysteresis in heap size such that
          excessive amounts of system calls can be avoided.

    `M_MMAP_THRESHOLD'
          All chunks larger than this value are allocated outside the
          normal heap, using the `mmap' system call.  This way it is
          guaranteed that the memory for these chunks can be returned
          to the system on `free'.  Note that requests smaller than
          this threshold might still be allocated via `mmap'.

    `M_MMAP_MAX'
          The maximum number of chunks to allocate with `mmap'.
          Setting this to zero disables all use of `mmap'.

    `M_PERTURB'
          If non-zero, memory blocks are filled with values depending
          on some low order bits of this parameter when they are
          allocated (except when allocated by `calloc') and freed.
          This can be used to debug the use of uninitialized or freed
          heap memory.


File: libc.info,  Node: Heap Consistency Checking,  Next: Hooks for Malloc,  Prev: Malloc Tunable Parameters,  Up: Unconstrained Allocation

3.2.2.9 Heap Consistency Checking
.................................

You can ask `malloc' to check the consistency of dynamic memory by
using the `mcheck' function.  This function is a GNU extension,
declared in `mcheck.h'.

 -- Function: int mcheck (void (*ABORTFN) (enum mcheck_status STATUS))
     Calling `mcheck' tells `malloc' to perform occasional consistency
     checks.  These will catch things such as writing past the end of a
     block that was allocated with `malloc'.

     The ABORTFN argument is the function to call when an inconsistency
     is found.  If you supply a null pointer, then `mcheck' uses a
     default function which prints a message and calls `abort' (*note
     Aborting a Program::).  The function you supply is called with one
     argument, which says what sort of inconsistency was detected; its
     type is described below.

     It is too late to begin allocation checking once you have allocated
     anything with `malloc'.  So `mcheck' does nothing in that case.
     The function returns `-1' if you call it too late, and `0'
     otherwise (when it is successful).

     The easiest way to arrange to call `mcheck' early enough is to use
     the option `-lmcheck' when you link your program; then you don't
     need to modify your program source at all.  Alternatively you
     might use a debugger to insert a call to `mcheck' whenever the
     program is started, for example these gdb commands will
     automatically call `mcheck' whenever the program starts:

          (gdb) break main
          Breakpoint 1, main (argc=2, argv=0xbffff964) at whatever.c:10
          (gdb) command 1
          Type commands for when breakpoint 1 is hit, one per line.
          End with a line saying just "end".
          >call mcheck(0)
          >continue
          >end
          (gdb) ...

     This will however only work if no initialization function of any
     object involved calls any of the `malloc' functions since `mcheck'
     must be called before the first such function.


 -- Function: enum mcheck_status mprobe (void *POINTER)
     The `mprobe' function lets you explicitly check for inconsistencies
     in a particular allocated block.  You must have already called
     `mcheck' at the beginning of the program, to do its occasional
     checks; calling `mprobe' requests an additional consistency check
     to be done at the time of the call.

     The argument POINTER must be a pointer returned by `malloc' or
     `realloc'.  `mprobe' returns a value that says what inconsistency,
     if any, was found.  The values are described below.

 -- Data Type: enum mcheck_status
     This enumerated type describes what kind of inconsistency was
     detected in an allocated block, if any.  Here are the possible
     values:

    `MCHECK_DISABLED'
          `mcheck' was not called before the first allocation.  No
          consistency checking can be done.

    `MCHECK_OK'
          No inconsistency detected.

    `MCHECK_HEAD'
          The data immediately before the block was modified.  This
          commonly happens when an array index or pointer is
          decremented too far.

    `MCHECK_TAIL'
          The data immediately after the block was modified.  This
          commonly happens when an array index or pointer is
          incremented too far.

    `MCHECK_FREE'
          The block was already freed.

   Another possibility to check for and guard against bugs in the use of
`malloc', `realloc' and `free' is to set the environment variable
`MALLOC_CHECK_'.  When `MALLOC_CHECK_' is set, a special (less
efficient) implementation is used which is designed to be tolerant
against simple errors, such as double calls of `free' with the same
argument, or overruns of a single byte (off-by-one bugs).  Not all such
errors can be protected against, however, and memory leaks can result.
If `MALLOC_CHECK_' is set to `0', any detected heap corruption is
silently ignored; if set to `1', a diagnostic is printed on `stderr';
if set to `2', `abort' is called immediately.  This can be useful
because otherwise a crash may happen much later, and the true cause for
the problem is then very hard to track down.

   There is one problem with `MALLOC_CHECK_': in SUID or SGID binaries
it could possibly be exploited since diverging from the normal programs
behavior it now writes something to the standard error descriptor.
Therefore the use of `MALLOC_CHECK_' is disabled by default for SUID
and SGID binaries.  It can be enabled again by the system administrator
by adding a file `/etc/suid-debug' (the content is not important it
could be empty).

   So, what's the difference between using `MALLOC_CHECK_' and linking
with `-lmcheck'?  `MALLOC_CHECK_' is orthogonal with respect to
`-lmcheck'.  `-lmcheck' has been added for backward compatibility.
Both `MALLOC_CHECK_' and `-lmcheck' should uncover the same bugs - but
using `MALLOC_CHECK_' you don't need to recompile your application.

File: libc.info,  Node: Hooks for Malloc,  Next: Statistics of Malloc,  Prev: Heap Consistency Checking,  Up: Unconstrained Allocation

3.2.2.10 Memory Allocation Hooks
................................

The GNU C library lets you modify the behavior of `malloc', `realloc',
and `free' by specifying appropriate hook functions.  You can use these
hooks to help you debug programs that use dynamic memory allocation,
for example.

   The hook variables are declared in `malloc.h'.

 -- Variable: __malloc_hook
     The value of this variable is a pointer to the function that
     `malloc' uses whenever it is called.  You should define this
     function to look like `malloc'; that is, like:

          void *FUNCTION (size_t SIZE, const void *CALLER)

     The value of CALLER is the return address found on the stack when
     the `malloc' function was called.  This value allows you to trace
     the memory consumption of the program.

 -- Variable: __realloc_hook
     The value of this variable is a pointer to function that `realloc'
     uses whenever it is called.  You should define this function to
     look like `realloc'; that is, like:

          void *FUNCTION (void *PTR, size_t SIZE, const void *CALLER)

     The value of CALLER is the return address found on the stack when
     the `realloc' function was called.  This value allows you to trace
     the memory consumption of the program.

 -- Variable: __free_hook
     The value of this variable is a pointer to function that `free'
     uses whenever it is called.  You should define this function to
     look like `free'; that is, like:

          void FUNCTION (void *PTR, const void *CALLER)

     The value of CALLER is the return address found on the stack when
     the `free' function was called.  This value allows you to trace the
     memory consumption of the program.

 -- Variable: __memalign_hook
     The value of this variable is a pointer to function that `memalign'
     uses whenever it is called.  You should define this function to
     look like `memalign'; that is, like:

          void *FUNCTION (size_t ALIGNMENT, size_t SIZE, const void *CALLER)

     The value of CALLER is the return address found on the stack when
     the `memalign' function was called.  This value allows you to
     trace the memory consumption of the program.

   You must make sure that the function you install as a hook for one of
these functions does not call that function recursively without
restoring the old value of the hook first!  Otherwise, your program
will get stuck in an infinite recursion.  Before calling the function
recursively, one should make sure to restore all the hooks to their
previous value.  When coming back from the recursive call, all the
hooks should be resaved since a hook might modify itself.

 -- Variable: __malloc_initialize_hook
     The value of this variable is a pointer to a function that is
     called once when the malloc implementation is initialized.  This
     is a weak variable, so it can be overridden in the application
     with a definition like the following:

          void (*__MALLOC_INITIALIZE_HOOK) (void) = my_init_hook;

   An issue to look out for is the time at which the malloc hook
functions can be safely installed.  If the hook functions call the
malloc-related functions recursively, it is necessary that malloc has
already properly initialized itself at the time when `__malloc_hook'
etc. is assigned to.  On the other hand, if the hook functions provide a
complete malloc implementation of their own, it is vital that the hooks
are assigned to _before_ the very first `malloc' call has completed,
because otherwise a chunk obtained from the ordinary, un-hooked malloc
may later be handed to `__free_hook', for example.

   In both cases, the problem can be solved by setting up the hooks from
within a user-defined function pointed to by
`__malloc_initialize_hook'--then the hooks will be set up safely at the
right time.

   Here is an example showing how to use `__malloc_hook' and
`__free_hook' properly.  It installs a function that prints out
information every time `malloc' or `free' is called.  We just assume
here that `realloc' and `memalign' are not used in our program.

     /* Prototypes for __malloc_hook, __free_hook */
     #include <malloc.h>

     /* Prototypes for our hooks.  */
     static void my_init_hook (void);
     static void *my_malloc_hook (size_t, const void *);
     static void my_free_hook (void*, const void *);

     /* Override initializing hook from the C library. */
     void (*__malloc_initialize_hook) (void) = my_init_hook;

     static void
     my_init_hook (void)
     {
       old_malloc_hook = __malloc_hook;
       old_free_hook = __free_hook;
       __malloc_hook = my_malloc_hook;
       __free_hook = my_free_hook;
     }

     static void *
     my_malloc_hook (size_t size, const void *caller)
     {
       void *result;
       /* Restore all old hooks */
       __malloc_hook = old_malloc_hook;
       __free_hook = old_free_hook;
       /* Call recursively */
       result = malloc (size);
       /* Save underlying hooks */
       old_malloc_hook = __malloc_hook;
       old_free_hook = __free_hook;
       /* `printf' might call `malloc', so protect it too. */
       printf ("malloc (%u) returns %p\n", (unsigned int) size, result);
       /* Restore our own hooks */
       __malloc_hook = my_malloc_hook;
       __free_hook = my_free_hook;
       return result;
     }

     static void
     my_free_hook (void *ptr, const void *caller)
     {
       /* Restore all old hooks */
       __malloc_hook = old_malloc_hook;
       __free_hook = old_free_hook;
       /* Call recursively */
       free (ptr);
       /* Save underlying hooks */
       old_malloc_hook = __malloc_hook;
       old_free_hook = __free_hook;
       /* `printf' might call `free', so protect it too. */
       printf ("freed pointer %p\n", ptr);
       /* Restore our own hooks */
       __malloc_hook = my_malloc_hook;
       __free_hook = my_free_hook;
     }

     main ()
     {
       ...
     }

   The `mcheck' function (*note Heap Consistency Checking::) works by
installing such hooks.

File: libc.info,  Node: Statistics of Malloc,  Next: Summary of Malloc,  Prev: Hooks for Malloc,  Up: Unconstrained Allocation

3.2.2.11 Statistics for Memory Allocation with `malloc'
.......................................................

You can get information about dynamic memory allocation by calling the
`mallinfo' function.  This function and its associated data type are
declared in `malloc.h'; they are an extension of the standard SVID/XPG
version.

 -- Data Type: struct mallinfo
     This structure type is used to return information about the dynamic
     memory allocator.  It contains the following members:

    `int arena'
          This is the total size of memory allocated with `sbrk' by
          `malloc', in bytes.

    `int ordblks'
          This is the number of chunks not in use.  (The memory
          allocator internally gets chunks of memory from the operating
          system, and then carves them up to satisfy individual
          `malloc' requests; see *note Efficiency and Malloc::.)

    `int smblks'
          This field is unused.

    `int hblks'
          This is the total number of chunks allocated with `mmap'.

    `int hblkhd'
          This is the total size of memory allocated with `mmap', in
          bytes.

    `int usmblks'
          This field is unused.

    `int fsmblks'
          This field is unused.

    `int uordblks'
          This is the total size of memory occupied by chunks handed
          out by `malloc'.

    `int fordblks'
          This is the total size of memory occupied by free (not in
          use) chunks.

    `int keepcost'
          This is the size of the top-most releasable chunk that
          normally borders the end of the heap (i.e., the high end of
          the virtual address space's data segment).


 -- Function: struct mallinfo mallinfo (void)
     This function returns information about the current dynamic memory
     usage in a structure of type `struct mallinfo'.

File: libc.info,  Node: Summary of Malloc,  Prev: Statistics of Malloc,  Up: Unconstrained Allocation

3.2.2.12 Summary of `malloc'-Related Functions
..............................................

Here is a summary of the functions that work with `malloc':

`void *malloc (size_t SIZE)'
     Allocate a block of SIZE bytes.  *Note Basic Allocation::.

`void free (void *ADDR)'
     Free a block previously allocated by `malloc'.  *Note Freeing
     after Malloc::.

`void *realloc (void *ADDR, size_t SIZE)'
     Make a block previously allocated by `malloc' larger or smaller,
     possibly by copying it to a new location.  *Note Changing Block
     Size::.

`void *calloc (size_t COUNT, size_t ELTSIZE)'
     Allocate a block of COUNT * ELTSIZE bytes using `malloc', and set
     its contents to zero.  *Note Allocating Cleared Space::.

`void *valloc (size_t SIZE)'
     Allocate a block of SIZE bytes, starting on a page boundary.
     *Note Aligned Memory Blocks::.

`void *memalign (size_t SIZE, size_t BOUNDARY)'
     Allocate a block of SIZE bytes, starting on an address that is a
     multiple of BOUNDARY.  *Note Aligned Memory Blocks::.

`int mallopt (int PARAM, int VALUE)'
     Adjust a tunable parameter.  *Note Malloc Tunable Parameters::.

`int mcheck (void (*ABORTFN) (void))'
     Tell `malloc' to perform occasional consistency checks on
     dynamically allocated memory, and to call ABORTFN when an
     inconsistency is found.  *Note Heap Consistency Checking::.

`void *(*__malloc_hook) (size_t SIZE, const void *CALLER)'
     A pointer to a function that `malloc' uses whenever it is called.

`void *(*__realloc_hook) (void *PTR, size_t SIZE, const void *CALLER)'
     A pointer to a function that `realloc' uses whenever it is called.

`void (*__free_hook) (void *PTR, const void *CALLER)'
     A pointer to a function that `free' uses whenever it is called.

`void (*__memalign_hook) (size_t SIZE, size_t ALIGNMENT, const void *CALLER)'
     A pointer to a function that `memalign' uses whenever it is called.

`struct mallinfo mallinfo (void)'
     Return information about the current dynamic memory usage.  *Note
     Statistics of Malloc::.

File: libc.info,  Node: Allocation Debugging,  Next: Obstacks,  Prev: Unconstrained Allocation,  Up: Memory Allocation

3.2.3 Allocation Debugging
--------------------------

A complicated task when programming with languages which do not use
garbage collected dynamic memory allocation is to find memory leaks.
Long running programs must assure that dynamically allocated objects are
freed at the end of their lifetime.  If this does not happen the system
runs out of memory, sooner or later.

   The `malloc' implementation in the GNU C library provides some
simple means to detect such leaks and obtain some information to find
the location.  To do this the application must be started in a special
mode which is enabled by an environment variable.  There are no speed
penalties for the program if the debugging mode is not enabled.

* Menu:

* Tracing malloc::               How to install the tracing functionality.
* Using the Memory Debugger::    Example programs excerpts.
* Tips for the Memory Debugger:: Some more or less clever ideas.
* Interpreting the traces::      What do all these lines mean?

File: libc.info,  Node: Tracing malloc,  Next: Using the Memory Debugger,  Up: Allocation Debugging

3.2.3.1 How to install the tracing functionality
................................................

 -- Function: void mtrace (void)
     When the `mtrace' function is called it looks for an environment
     variable named `MALLOC_TRACE'.  This variable is supposed to
     contain a valid file name.  The user must have write access.  If
     the file already exists it is truncated.  If the environment
     variable is not set or it does not name a valid file which can be
     opened for writing nothing is done.  The behavior of `malloc' etc.
     is not changed.  For obvious reasons this also happens if the
     application is installed with the SUID or SGID bit set.

     If the named file is successfully opened, `mtrace' installs special
     handlers for the functions `malloc', `realloc', and `free' (*note
     Hooks for Malloc::).  From then on, all uses of these functions
     are traced and protocolled into the file.  There is now of course
     a speed penalty for all calls to the traced functions so tracing
     should not be enabled during normal use.

     This function is a GNU extension and generally not available on
     other systems.  The prototype can be found in `mcheck.h'.

 -- Function: void muntrace (void)
     The `muntrace' function can be called after `mtrace' was used to
     enable tracing the `malloc' calls.  If no (successful) call of
     `mtrace' was made `muntrace' does nothing.

     Otherwise it deinstalls the handlers for `malloc', `realloc', and
     `free' and then closes the protocol file.  No calls are
     protocolled anymore and the program runs again at full speed.

     This function is a GNU extension and generally not available on
     other systems.  The prototype can be found in `mcheck.h'.

File: libc.info,  Node: Using the Memory Debugger,  Next: Tips for the Memory Debugger,  Prev: Tracing malloc,  Up: Allocation Debugging

3.2.3.2 Example program excerpts
................................

Even though the tracing functionality does not influence the runtime
behavior of the program it is not a good idea to call `mtrace' in all
programs.  Just imagine that you debug a program using `mtrace' and all
other programs used in the debugging session also trace their `malloc'
calls.  The output file would be the same for all programs and thus is
unusable.  Therefore one should call `mtrace' only if compiled for
debugging.  A program could therefore start like this:

     #include <mcheck.h>

     int
     main (int argc, char *argv[])
     {
     #ifdef DEBUGGING
       mtrace ();
     #endif
       ...
     }

   This is all what is needed if you want to trace the calls during the
whole runtime of the program.  Alternatively you can stop the tracing at
any time with a call to `muntrace'.  It is even possible to restart the
tracing again with a new call to `mtrace'.  But this can cause
unreliable results since there may be calls of the functions which are
not called.  Please note that not only the application uses the traced
functions, also libraries (including the C library itself) use these
functions.

   This last point is also why it is no good idea to call `muntrace'
before the program terminated.  The libraries are informed about the
termination of the program only after the program returns from `main'
or calls `exit' and so cannot free the memory they use before this time.

   So the best thing one can do is to call `mtrace' as the very first
function in the program and never call `muntrace'.  So the program
traces almost all uses of the `malloc' functions (except those calls
which are executed by constructors of the program or used libraries).

File: libc.info,  Node: Tips for the Memory Debugger,  Next: Interpreting the traces,  Prev: Using the Memory Debugger,  Up: Allocation Debugging

3.2.3.3 Some more or less clever ideas
......................................

You know the situation.  The program is prepared for debugging and in
all debugging sessions it runs well.  But once it is started without
debugging the error shows up.  A typical example is a memory leak that
becomes visible only when we turn off the debugging.  If you foresee
such situations you can still win.  Simply use something equivalent to
the following little program:

     #include <mcheck.h>
     #include <signal.h>

     static void
     enable (int sig)
     {
       mtrace ();
       signal (SIGUSR1, enable);
     }

     static void
     disable (int sig)
     {
       muntrace ();
       signal (SIGUSR2, disable);
     }

     int
     main (int argc, char *argv[])
     {
       ...

       signal (SIGUSR1, enable);
       signal (SIGUSR2, disable);

       ...
     }

   I.e., the user can start the memory debugger any time s/he wants if
the program was started with `MALLOC_TRACE' set in the environment.
The output will of course not show the allocations which happened before
the first signal but if there is a memory leak this will show up
nevertheless.

File: libc.info,  Node: Interpreting the traces,  Prev: Tips for the Memory Debugger,  Up: Allocation Debugging

3.2.3.4 Interpreting the traces
...............................

If you take a look at the output it will look similar to this:

     = Start
      [0x8048209] - 0x8064cc8
      [0x8048209] - 0x8064ce0
      [0x8048209] - 0x8064cf8
      [0x80481eb] + 0x8064c48 0x14
      [0x80481eb] + 0x8064c60 0x14
      [0x80481eb] + 0x8064c78 0x14
      [0x80481eb] + 0x8064c90 0x14
     = End

   What this all means is not really important since the trace file is
not meant to be read by a human.  Therefore no attention is given to
readability.  Instead there is a program which comes with the GNU C
library which interprets the traces and outputs a summary in an
user-friendly way.  The program is called `mtrace' (it is in fact a
Perl script) and it takes one or two arguments.  In any case the name of
the file with the trace output must be specified.  If an optional
argument precedes the name of the trace file this must be the name of
the program which generated the trace.

     drepper$ mtrace tst-mtrace log
     No memory leaks.

   In this case the program `tst-mtrace' was run and it produced a
trace file `log'.  The message printed by `mtrace' shows there are no
problems with the code, all allocated memory was freed afterwards.

   If we call `mtrace' on the example trace given above we would get a
different outout:

     drepper$ mtrace errlog
     - 0x08064cc8 Free 2 was never alloc'd 0x8048209
     - 0x08064ce0 Free 3 was never alloc'd 0x8048209
     - 0x08064cf8 Free 4 was never alloc'd 0x8048209

     Memory not freed:
     -----------------
        Address     Size     Caller
     0x08064c48     0x14  at 0x80481eb
     0x08064c60     0x14  at 0x80481eb
     0x08064c78     0x14  at 0x80481eb
     0x08064c90     0x14  at 0x80481eb

   We have called `mtrace' with only one argument and so the script has
no chance to find out what is meant with the addresses given in the
trace.  We can do better:

     drepper$ mtrace tst errlog
     - 0x08064cc8 Free 2 was never alloc'd /home/drepper/tst.c:39
     - 0x08064ce0 Free 3 was never alloc'd /home/drepper/tst.c:39
     - 0x08064cf8 Free 4 was never alloc'd /home/drepper/tst.c:39

     Memory not freed:
     -----------------
        Address     Size     Caller
     0x08064c48     0x14  at /home/drepper/tst.c:33
     0x08064c60     0x14  at /home/drepper/tst.c:33
     0x08064c78     0x14  at /home/drepper/tst.c:33
     0x08064c90     0x14  at /home/drepper/tst.c:33

   Suddenly the output makes much more sense and the user can see
immediately where the function calls causing the trouble can be found.

   Interpreting this output is not complicated.  There are at most two
different situations being detected.  First, `free' was called for
pointers which were never returned by one of the allocation functions.
This is usually a very bad problem and what this looks like is shown in
the first three lines of the output.  Situations like this are quite
rare and if they appear they show up very drastically: the program
normally crashes.

   The other situation which is much harder to detect are memory leaks.
As you can see in the output the `mtrace' function collects all this
information and so can say that the program calls an allocation function
from line 33 in the source file `/home/drepper/tst-mtrace.c' four times
without freeing this memory before the program terminates.  Whether
this is a real problem remains to be investigated.

File: libc.info,  Node: Obstacks,  Next: Variable Size Automatic,  Prev: Allocation Debugging,  Up: Memory Allocation

3.2.4 Obstacks
--------------

An "obstack" is a pool of memory containing a stack of objects.  You
can create any number of separate obstacks, and then allocate objects in
specified obstacks.  Within each obstack, the last object allocated must
always be the first one freed, but distinct obstacks are independent of
each other.

   Aside from this one constraint of order of freeing, obstacks are
totally general: an obstack can contain any number of objects of any
size.  They are implemented with macros, so allocation is usually very
fast as long as the objects are usually small.  And the only space
overhead per object is the padding needed to start each object on a
suitable boundary.

* Menu:

* Creating Obstacks::		How to declare an obstack in your program.
* Preparing for Obstacks::	Preparations needed before you can
				 use obstacks.
* Allocation in an Obstack::    Allocating objects in an obstack.
* Freeing Obstack Objects::     Freeing objects in an obstack.
* Obstack Functions::		The obstack functions are both
				 functions and macros.
* Growing Objects::             Making an object bigger by stages.
* Extra Fast Growing::		Extra-high-efficiency (though more
				 complicated) growing objects.
* Status of an Obstack::        Inquiries about the status of an obstack.
* Obstacks Data Alignment::     Controlling alignment of objects in obstacks.
* Obstack Chunks::              How obstacks obtain and release chunks;
				 efficiency considerations.
* Summary of Obstacks::

File: libc.info,  Node: Creating Obstacks,  Next: Preparing for Obstacks,  Up: Obstacks

3.2.4.1 Creating Obstacks
.........................

The utilities for manipulating obstacks are declared in the header file
`obstack.h'.

 -- Data Type: struct obstack
     An obstack is represented by a data structure of type `struct
     obstack'.  This structure has a small fixed size; it records the
     status of the obstack and how to find the space in which objects
     are allocated.  It does not contain any of the objects themselves.
     You should not try to access the contents of the structure
     directly; use only the functions described in this chapter.

   You can declare variables of type `struct obstack' and use them as
obstacks, or you can allocate obstacks dynamically like any other kind
of object.  Dynamic allocation of obstacks allows your program to have a
variable number of different stacks.  (You can even allocate an obstack
structure in another obstack, but this is rarely useful.)

   All the functions that work with obstacks require you to specify
which obstack to use.  You do this with a pointer of type `struct
obstack *'.  In the following, we often say "an obstack" when strictly
speaking the object at hand is such a pointer.

   The objects in the obstack are packed into large blocks called
"chunks".  The `struct obstack' structure points to a chain of the
chunks currently in use.

   The obstack library obtains a new chunk whenever you allocate an
object that won't fit in the previous chunk.  Since the obstack library
manages chunks automatically, you don't need to pay much attention to
them, but you do need to supply a function which the obstack library
should use to get a chunk.  Usually you supply a function which uses
`malloc' directly or indirectly.  You must also supply a function to
free a chunk.  These matters are described in the following section.

File: libc.info,  Node: Preparing for Obstacks,  Next: Allocation in an Obstack,  Prev: Creating Obstacks,  Up: Obstacks

3.2.4.2 Preparing for Using Obstacks
....................................

Each source file in which you plan to use the obstack functions must
include the header file `obstack.h', like this:

     #include <obstack.h>

   Also, if the source file uses the macro `obstack_init', it must
declare or define two functions or macros that will be called by the
obstack library.  One, `obstack_chunk_alloc', is used to allocate the
chunks of memory into which objects are packed.  The other,
`obstack_chunk_free', is used to return chunks when the objects in them
are freed.  These macros should appear before any use of obstacks in
the source file.

   Usually these are defined to use `malloc' via the intermediary
`xmalloc' (*note Unconstrained Allocation::).  This is done with the
following pair of macro definitions:

     #define obstack_chunk_alloc xmalloc
     #define obstack_chunk_free free

Though the memory you get using obstacks really comes from `malloc',
using obstacks is faster because `malloc' is called less often, for
larger blocks of memory.  *Note Obstack Chunks::, for full details.

   At run time, before the program can use a `struct obstack' object as
an obstack, it must initialize the obstack by calling `obstack_init'.

 -- Function: int obstack_init (struct obstack *OBSTACK-PTR)
     Initialize obstack OBSTACK-PTR for allocation of objects.  This
     function calls the obstack's `obstack_chunk_alloc' function.  If
     allocation of memory fails, the function pointed to by
     `obstack_alloc_failed_handler' is called.  The `obstack_init'
     function always returns 1 (Compatibility notice: Former versions of
     obstack returned 0 if allocation failed).

   Here are two examples of how to allocate the space for an obstack and
initialize it.  First, an obstack that is a static variable:

     static struct obstack myobstack;
     ...
     obstack_init (&myobstack);

Second, an obstack that is itself dynamically allocated:

     struct obstack *myobstack_ptr
       = (struct obstack *) xmalloc (sizeof (struct obstack));

     obstack_init (myobstack_ptr);

 -- Variable: obstack_alloc_failed_handler
     The value of this variable is a pointer to a function that
     `obstack' uses when `obstack_chunk_alloc' fails to allocate
     memory.  The default action is to print a message and abort.  You
     should supply a function that either calls `exit' (*note Program
     Termination::) or `longjmp' (*note Non-Local Exits::) and doesn't
     return.

          void my_obstack_alloc_failed (void)
          ...
          obstack_alloc_failed_handler = &my_obstack_alloc_failed;


File: libc.info,  Node: Allocation in an Obstack,  Next: Freeing Obstack Objects,  Prev: Preparing for Obstacks,  Up: Obstacks

3.2.4.3 Allocation in an Obstack
................................

The most direct way to allocate an object in an obstack is with
`obstack_alloc', which is invoked almost like `malloc'.

 -- Function: void * obstack_alloc (struct obstack *OBSTACK-PTR, int
          SIZE)
     This allocates an uninitialized block of SIZE bytes in an obstack
     and returns its address.  Here OBSTACK-PTR specifies which obstack
     to allocate the block in; it is the address of the `struct obstack'
     object which represents the obstack.  Each obstack function or
     macro requires you to specify an OBSTACK-PTR as the first argument.

     This function calls the obstack's `obstack_chunk_alloc' function if
     it needs to allocate a new chunk of memory; it calls
     `obstack_alloc_failed_handler' if allocation of memory by
     `obstack_chunk_alloc' failed.

   For example, here is a function that allocates a copy of a string STR
in a specific obstack, which is in the variable `string_obstack':

     struct obstack string_obstack;

     char *
     copystring (char *string)
     {
       size_t len = strlen (string) + 1;
       char *s = (char *) obstack_alloc (&string_obstack, len);
       memcpy (s, string, len);
       return s;
     }

   To allocate a block with specified contents, use the function
`obstack_copy', declared like this:

 -- Function: void * obstack_copy (struct obstack *OBSTACK-PTR, void
          *ADDRESS, int SIZE)
     This allocates a block and initializes it by copying SIZE bytes of
     data starting at ADDRESS.  It calls `obstack_alloc_failed_handler'
     if allocation of memory by `obstack_chunk_alloc' failed.

 -- Function: void * obstack_copy0 (struct obstack *OBSTACK-PTR, void
          *ADDRESS, int SIZE)
     Like `obstack_copy', but appends an extra byte containing a null
     character.  This extra byte is not counted in the argument SIZE.

   The `obstack_copy0' function is convenient for copying a sequence of
characters into an obstack as a null-terminated string.  Here is an
example of its use:

     char *
     obstack_savestring (char *addr, int size)
     {
       return obstack_copy0 (&myobstack, addr, size);
     }

Contrast this with the previous example of `savestring' using `malloc'
(*note Basic Allocation::).

File: libc.info,  Node: Freeing Obstack Objects,  Next: Obstack Functions,  Prev: Allocation in an Obstack,  Up: Obstacks

3.2.4.4 Freeing Objects in an Obstack
.....................................

To free an object allocated in an obstack, use the function
`obstack_free'.  Since the obstack is a stack of objects, freeing one
object automatically frees all other objects allocated more recently in
the same obstack.

 -- Function: void obstack_free (struct obstack *OBSTACK-PTR, void
          *OBJECT)
     If OBJECT is a null pointer, everything allocated in the obstack
     is freed.  Otherwise, OBJECT must be the address of an object
     allocated in the obstack.  Then OBJECT is freed, along with
     everything allocated in OBSTACK since OBJECT.

   Note that if OBJECT is a null pointer, the result is an
uninitialized obstack.  To free all memory in an obstack but leave it
valid for further allocation, call `obstack_free' with the address of
the first object allocated on the obstack:

     obstack_free (obstack_ptr, first_object_allocated_ptr);

   Recall that the objects in an obstack are grouped into chunks.  When
all the objects in a chunk become free, the obstack library
automatically frees the chunk (*note Preparing for Obstacks::).  Then
other obstacks, or non-obstack allocation, can reuse the space of the
chunk.

File: libc.info,  Node: Obstack Functions,  Next: Growing Objects,  Prev: Freeing Obstack Objects,  Up: Obstacks

3.2.4.5 Obstack Functions and Macros
....................................

The interfaces for using obstacks may be defined either as functions or
as macros, depending on the compiler.  The obstack facility works with
all C compilers, including both ISO C and traditional C, but there are
precautions you must take if you plan to use compilers other than GNU C.

   If you are using an old-fashioned non-ISO C compiler, all the obstack
"functions" are actually defined only as macros.  You can call these
macros like functions, but you cannot use them in any other way (for
example, you cannot take their address).

   Calling the macros requires a special precaution: namely, the first
operand (the obstack pointer) may not contain any side effects, because
it may be computed more than once.  For example, if you write this:

     obstack_alloc (get_obstack (), 4);

you will find that `get_obstack' may be called several times.  If you
use `*obstack_list_ptr++' as the obstack pointer argument, you will get
very strange results since the incrementation may occur several times.

   In ISO C, each function has both a macro definition and a function
definition.  The function definition is used if you take the address of
the function without calling it.  An ordinary call uses the macro
definition by default, but you can request the function definition
instead by writing the function name in parentheses, as shown here:

     char *x;
     void *(*funcp) ();
     /* Use the macro.  */
     x = (char *) obstack_alloc (obptr, size);
     /* Call the function.  */
     x = (char *) (obstack_alloc) (obptr, size);
     /* Take the address of the function.  */
     funcp = obstack_alloc;

This is the same situation that exists in ISO C for the standard library
functions.  *Note Macro Definitions::.

   *Warning:* When you do use the macros, you must observe the
precaution of avoiding side effects in the first operand, even in ISO C.

   If you use the GNU C compiler, this precaution is not necessary,
because various language extensions in GNU C permit defining the macros
so as to compute each argument only once.

File: libc.info,  Node: Growing Objects,  Next: Extra Fast Growing,  Prev: Obstack Functions,  Up: Obstacks

3.2.4.6 Growing Objects
.......................

Because memory in obstack chunks is used sequentially, it is possible to
build up an object step by step, adding one or more bytes at a time to
the end of the object.  With this technique, you do not need to know
how much data you will put in the object until you come to the end of
it.  We call this the technique of "growing objects".  The special
functions for adding data to the growing object are described in this
section.

   You don't need to do anything special when you start to grow an
object.  Using one of the functions to add data to the object
automatically starts it.  However, it is necessary to say explicitly
when the object is finished.  This is done with the function
`obstack_finish'.

   The actual address of the object thus built up is not known until the
object is finished.  Until then, it always remains possible that you
will add so much data that the object must be copied into a new chunk.

   While the obstack is in use for a growing object, you cannot use it
for ordinary allocation of another object.  If you try to do so, the
space already added to the growing object will become part of the other
object.

 -- Function: void obstack_blank (struct obstack *OBSTACK-PTR, int SIZE)
     The most basic function for adding to a growing object is
     `obstack_blank', which adds space without initializing it.

 -- Function: void obstack_grow (struct obstack *OBSTACK-PTR, void
          *DATA, int SIZE)
     To add a block of initialized space, use `obstack_grow', which is
     the growing-object analogue of `obstack_copy'.  It adds SIZE bytes
     of data to the growing object, copying the contents from DATA.

 -- Function: void obstack_grow0 (struct obstack *OBSTACK-PTR, void
          *DATA, int SIZE)
     This is the growing-object analogue of `obstack_copy0'.  It adds
     SIZE bytes copied from DATA, followed by an additional null
     character.

 -- Function: void obstack_1grow (struct obstack *OBSTACK-PTR, char C)
     To add one character at a time, use the function `obstack_1grow'.
     It adds a single byte containing C to the growing object.

 -- Function: void obstack_ptr_grow (struct obstack *OBSTACK-PTR, void
          *DATA)
     Adding the value of a pointer one can use the function
     `obstack_ptr_grow'.  It adds `sizeof (void *)' bytes containing
     the value of DATA.

 -- Function: void obstack_int_grow (struct obstack *OBSTACK-PTR, int
          DATA)
     A single value of type `int' can be added by using the
     `obstack_int_grow' function.  It adds `sizeof (int)' bytes to the
     growing object and initializes them with the value of DATA.

 -- Function: void * obstack_finish (struct obstack *OBSTACK-PTR)
     When you are finished growing the object, use the function
     `obstack_finish' to close it off and return its final address.

     Once you have finished the object, the obstack is available for
     ordinary allocation or for growing another object.

     This function can return a null pointer under the same conditions
     as `obstack_alloc' (*note Allocation in an Obstack::).

   When you build an object by growing it, you will probably need to
know afterward how long it became.  You need not keep track of this as
you grow the object, because you can find out the length from the
obstack just before finishing the object with the function
`obstack_object_size', declared as follows:

 -- Function: int obstack_object_size (struct obstack *OBSTACK-PTR)
     This function returns the current size of the growing object, in
     bytes.  Remember to call this function _before_ finishing the
     object.  After it is finished, `obstack_object_size' will return
     zero.

   If you have started growing an object and wish to cancel it, you
should finish it and then free it, like this:

     obstack_free (obstack_ptr, obstack_finish (obstack_ptr));

This has no effect if no object was growing.

   You can use `obstack_blank' with a negative size argument to make
the current object smaller.  Just don't try to shrink it beyond zero
length--there's no telling what will happen if you do that.

File: libc.info,  Node: Extra Fast Growing,  Next: Status of an Obstack,  Prev: Growing Objects,  Up: Obstacks

3.2.4.7 Extra Fast Growing Objects
..................................

The usual functions for growing objects incur overhead for checking
whether there is room for the new growth in the current chunk.  If you
are frequently constructing objects in small steps of growth, this
overhead can be significant.

   You can reduce the overhead by using special "fast growth" functions
that grow the object without checking.  In order to have a robust
program, you must do the checking yourself.  If you do this checking in
the simplest way each time you are about to add data to the object, you
have not saved anything, because that is what the ordinary growth
functions do.  But if you can arrange to check less often, or check
more efficiently, then you make the program faster.

   The function `obstack_room' returns the amount of room available in
the current chunk.  It is declared as follows:

 -- Function: int obstack_room (struct obstack *OBSTACK-PTR)
     This returns the number of bytes that can be added safely to the
     current growing object (or to an object about to be started) in
     obstack OBSTACK using the fast growth functions.

   While you know there is room, you can use these fast growth functions
for adding data to a growing object:

 -- Function: void obstack_1grow_fast (struct obstack *OBSTACK-PTR,
          char C)
     The function `obstack_1grow_fast' adds one byte containing the
     character C to the growing object in obstack OBSTACK-PTR.

 -- Function: void obstack_ptr_grow_fast (struct obstack *OBSTACK-PTR,
          void *DATA)
     The function `obstack_ptr_grow_fast' adds `sizeof (void *)' bytes
     containing the value of DATA to the growing object in obstack
     OBSTACK-PTR.

 -- Function: void obstack_int_grow_fast (struct obstack *OBSTACK-PTR,
          int DATA)
     The function `obstack_int_grow_fast' adds `sizeof (int)' bytes
     containing the value of DATA to the growing object in obstack
     OBSTACK-PTR.

 -- Function: void obstack_blank_fast (struct obstack *OBSTACK-PTR, int
          SIZE)
     The function `obstack_blank_fast' adds SIZE bytes to the growing
     object in obstack OBSTACK-PTR without initializing them.

   When you check for space using `obstack_room' and there is not
enough room for what you want to add, the fast growth functions are not
safe.  In this case, simply use the corresponding ordinary growth
function instead.  Very soon this will copy the object to a new chunk;
then there will be lots of room available again.

   So, each time you use an ordinary growth function, check afterward
for sufficient space using `obstack_room'.  Once the object is copied
to a new chunk, there will be plenty of space again, so the program will
start using the fast growth functions again.

   Here is an example:

     void
     add_string (struct obstack *obstack, const char *ptr, int len)
     {
       while (len > 0)
         {
           int room = obstack_room (obstack);
           if (room == 0)
             {
               /* Not enough room. Add one character slowly,
                  which may copy to a new chunk and make room.  */
               obstack_1grow (obstack, *ptr++);
               len--;
             }
           else
             {
               if (room > len)
                 room = len;
               /* Add fast as much as we have room for. */
               len -= room;
               while (room-- > 0)
                 obstack_1grow_fast (obstack, *ptr++);
             }
         }
     }

File: libc.info,  Node: Status of an Obstack,  Next: Obstacks Data Alignment,  Prev: Extra Fast Growing,  Up: Obstacks

3.2.4.8 Status of an Obstack
............................

Here are functions that provide information on the current status of
allocation in an obstack.  You can use them to learn about an object
while still growing it.

 -- Function: void * obstack_base (struct obstack *OBSTACK-PTR)
     This function returns the tentative address of the beginning of the
     currently growing object in OBSTACK-PTR.  If you finish the object
     immediately, it will have that address.  If you make it larger
     first, it may outgrow the current chunk--then its address will
     change!

     If no object is growing, this value says where the next object you
     allocate will start (once again assuming it fits in the current
     chunk).

 -- Function: void * obstack_next_free (struct obstack *OBSTACK-PTR)
     This function returns the address of the first free byte in the
     current chunk of obstack OBSTACK-PTR.  This is the end of the
     currently growing object.  If no object is growing,
     `obstack_next_free' returns the same value as `obstack_base'.

 -- Function: int obstack_object_size (struct obstack *OBSTACK-PTR)
     This function returns the size in bytes of the currently growing
     object.  This is equivalent to

          obstack_next_free (OBSTACK-PTR) - obstack_base (OBSTACK-PTR)

File: libc.info,  Node: Obstacks Data Alignment,  Next: Obstack Chunks,  Prev: Status of an Obstack,  Up: Obstacks

3.2.4.9 Alignment of Data in Obstacks
.....................................

Each obstack has an "alignment boundary"; each object allocated in the
obstack automatically starts on an address that is a multiple of the
specified boundary.  By default, this boundary is aligned so that the
object can hold any type of data.

   To access an obstack's alignment boundary, use the macro
`obstack_alignment_mask', whose function prototype looks like this:

 -- Macro: int obstack_alignment_mask (struct obstack *OBSTACK-PTR)
     The value is a bit mask; a bit that is 1 indicates that the
     corresponding bit in the address of an object should be 0.  The
     mask value should be one less than a power of 2; the effect is
     that all object addresses are multiples of that power of 2.  The
     default value of the mask is a value that allows aligned objects
     to hold any type of data: for example, if its value is 3, any type
     of data can be stored at locations whose addresses are multiples
     of 4.  A mask value of 0 means an object can start on any multiple
     of 1 (that is, no alignment is required).

     The expansion of the macro `obstack_alignment_mask' is an lvalue,
     so you can alter the mask by assignment.  For example, this
     statement:

          obstack_alignment_mask (obstack_ptr) = 0;

     has the effect of turning off alignment processing in the
     specified obstack.

   Note that a change in alignment mask does not take effect until
_after_ the next time an object is allocated or finished in the
obstack.  If you are not growing an object, you can make the new
alignment mask take effect immediately by calling `obstack_finish'.
This will finish a zero-length object and then do proper alignment for
the next object.

File: libc.info,  Node: Obstack Chunks,  Next: Summary of Obstacks,  Prev: Obstacks Data Alignment,  Up: Obstacks

3.2.4.10 Obstack Chunks
.......................

Obstacks work by allocating space for themselves in large chunks, and
then parceling out space in the chunks to satisfy your requests.  Chunks
are normally 4096 bytes long unless you specify a different chunk size.
The chunk size includes 8 bytes of overhead that are not actually used
for storing objects.  Regardless of the specified size, longer chunks
will be allocated when necessary for long objects.

   The obstack library allocates chunks by calling the function
`obstack_chunk_alloc', which you must define.  When a chunk is no
longer needed because you have freed all the objects in it, the obstack
library frees the chunk by calling `obstack_chunk_free', which you must
also define.

   These two must be defined (as macros) or declared (as functions) in
each source file that uses `obstack_init' (*note Creating Obstacks::).
Most often they are defined as macros like this:

     #define obstack_chunk_alloc malloc
     #define obstack_chunk_free free

   Note that these are simple macros (no arguments).  Macro definitions
with arguments will not work!  It is necessary that
`obstack_chunk_alloc' or `obstack_chunk_free', alone, expand into a
function name if it is not itself a function name.

   If you allocate chunks with `malloc', the chunk size should be a
power of 2.  The default chunk size, 4096, was chosen because it is long
enough to satisfy many typical requests on the obstack yet short enough
not to waste too much memory in the portion of the last chunk not yet
used.

 -- Macro: int obstack_chunk_size (struct obstack *OBSTACK-PTR)
     This returns the chunk size of the given obstack.

   Since this macro expands to an lvalue, you can specify a new chunk
size by assigning it a new value.  Doing so does not affect the chunks
already allocated, but will change the size of chunks allocated for
that particular obstack in the future.  It is unlikely to be useful to
make the chunk size smaller, but making it larger might improve
efficiency if you are allocating many objects whose size is comparable
to the chunk size.  Here is how to do so cleanly:

     if (obstack_chunk_size (obstack_ptr) < NEW-CHUNK-SIZE)
       obstack_chunk_size (obstack_ptr) = NEW-CHUNK-SIZE;

File: libc.info,  Node: Summary of Obstacks,  Prev: Obstack Chunks,  Up: Obstacks

3.2.4.11 Summary of Obstack Functions
.....................................

Here is a summary of all the functions associated with obstacks.  Each
takes the address of an obstack (`struct obstack *') as its first
argument.

`void obstack_init (struct obstack *OBSTACK-PTR)'
     Initialize use of an obstack.  *Note Creating Obstacks::.

`void *obstack_alloc (struct obstack *OBSTACK-PTR, int SIZE)'
     Allocate an object of SIZE uninitialized bytes.  *Note Allocation
     in an Obstack::.

`void *obstack_copy (struct obstack *OBSTACK-PTR, void *ADDRESS, int SIZE)'
     Allocate an object of SIZE bytes, with contents copied from
     ADDRESS.  *Note Allocation in an Obstack::.

`void *obstack_copy0 (struct obstack *OBSTACK-PTR, void *ADDRESS, int SIZE)'
     Allocate an object of SIZE+1 bytes, with SIZE of them copied from
     ADDRESS, followed by a null character at the end.  *Note
     Allocation in an Obstack::.

`void obstack_free (struct obstack *OBSTACK-PTR, void *OBJECT)'
     Free OBJECT (and everything allocated in the specified obstack
     more recently than OBJECT).  *Note Freeing Obstack Objects::.

`void obstack_blank (struct obstack *OBSTACK-PTR, int SIZE)'
     Add SIZE uninitialized bytes to a growing object.  *Note Growing
     Objects::.

`void obstack_grow (struct obstack *OBSTACK-PTR, void *ADDRESS, int SIZE)'
     Add SIZE bytes, copied from ADDRESS, to a growing object.  *Note
     Growing Objects::.

`void obstack_grow0 (struct obstack *OBSTACK-PTR, void *ADDRESS, int SIZE)'
     Add SIZE bytes, copied from ADDRESS, to a growing object, and then
     add another byte containing a null character.  *Note Growing
     Objects::.

`void obstack_1grow (struct obstack *OBSTACK-PTR, char DATA-CHAR)'
     Add one byte containing DATA-CHAR to a growing object.  *Note
     Growing Objects::.

`void *obstack_finish (struct obstack *OBSTACK-PTR)'
     Finalize the object that is growing and return its permanent
     address.  *Note Growing Objects::.

`int obstack_object_size (struct obstack *OBSTACK-PTR)'
     Get the current size of the currently growing object.  *Note
     Growing Objects::.

`void obstack_blank_fast (struct obstack *OBSTACK-PTR, int SIZE)'
     Add SIZE uninitialized bytes to a growing object without checking
     that there is enough room.  *Note Extra Fast Growing::.

`void obstack_1grow_fast (struct obstack *OBSTACK-PTR, char DATA-CHAR)'
     Add one byte containing DATA-CHAR to a growing object without
     checking that there is enough room.  *Note Extra Fast Growing::.

`int obstack_room (struct obstack *OBSTACK-PTR)'
     Get the amount of room now available for growing the current
     object.  *Note Extra Fast Growing::.

`int obstack_alignment_mask (struct obstack *OBSTACK-PTR)'
     The mask used for aligning the beginning of an object.  This is an
     lvalue.  *Note Obstacks Data Alignment::.

`int obstack_chunk_size (struct obstack *OBSTACK-PTR)'
     The size for allocating chunks.  This is an lvalue.  *Note Obstack
     Chunks::.

`void *obstack_base (struct obstack *OBSTACK-PTR)'
     Tentative starting address of the currently growing object.  *Note
     Status of an Obstack::.

`void *obstack_next_free (struct obstack *OBSTACK-PTR)'
     Address just after the end of the currently growing object.  *Note
     Status of an Obstack::.

File: libc.info,  Node: Variable Size Automatic,  Prev: Obstacks,  Up: Memory Allocation

3.2.5 Automatic Storage with Variable Size
------------------------------------------

The function `alloca' supports a kind of half-dynamic allocation in
which blocks are allocated dynamically but freed automatically.

   Allocating a block with `alloca' is an explicit action; you can
allocate as many blocks as you wish, and compute the size at run time.
But all the blocks are freed when you exit the function that `alloca'
was called from, just as if they were automatic variables declared in
that function.  There is no way to free the space explicitly.

   The prototype for `alloca' is in `stdlib.h'.  This function is a BSD
extension.

 -- Function: void * alloca (size_t SIZE);
     The return value of `alloca' is the address of a block of SIZE
     bytes of memory, allocated in the stack frame of the calling
     function.

   Do not use `alloca' inside the arguments of a function call--you
will get unpredictable results, because the stack space for the
`alloca' would appear on the stack in the middle of the space for the
function arguments.  An example of what to avoid is `foo (x, alloca
(4), y)'.

* Menu:

* Alloca Example::              Example of using `alloca'.
* Advantages of Alloca::        Reasons to use `alloca'.
* Disadvantages of Alloca::     Reasons to avoid `alloca'.
* GNU C Variable-Size Arrays::  Only in GNU C, here is an alternative
				 method of allocating dynamically and
				 freeing automatically.

File: libc.info,  Node: Alloca Example,  Next: Advantages of Alloca,  Up: Variable Size Automatic

3.2.5.1 `alloca' Example
........................

As an example of the use of `alloca', here is a function that opens a
file name made from concatenating two argument strings, and returns a
file descriptor or minus one signifying failure:

     int
     open2 (char *str1, char *str2, int flags, int mode)
     {
       char *name = (char *) alloca (strlen (str1) + strlen (str2) + 1);
       stpcpy (stpcpy (name, str1), str2);
       return open (name, flags, mode);
     }

Here is how you would get the same results with `malloc' and `free':

     int
     open2 (char *str1, char *str2, int flags, int mode)
     {
       char *name = (char *) malloc (strlen (str1) + strlen (str2) + 1);
       int desc;
       if (name == 0)
         fatal ("virtual memory exceeded");
       stpcpy (stpcpy (name, str1), str2);
       desc = open (name, flags, mode);
       free (name);
       return desc;
     }

   As you can see, it is simpler with `alloca'.  But `alloca' has
other, more important advantages, and some disadvantages.

File: libc.info,  Node: Advantages of Alloca,  Next: Disadvantages of Alloca,  Prev: Alloca Example,  Up: Variable Size Automatic

3.2.5.2 Advantages of `alloca'
..............................

Here are the reasons why `alloca' may be preferable to `malloc':

   * Using `alloca' wastes very little space and is very fast.  (It is
     open-coded by the GNU C compiler.)

   * Since `alloca' does not have separate pools for different sizes of
     block, space used for any size block can be reused for any other
     size.  `alloca' does not cause memory fragmentation.

   * Nonlocal exits done with `longjmp' (*note Non-Local Exits::)
     automatically free the space allocated with `alloca' when they exit
     through the function that called `alloca'.  This is the most
     important reason to use `alloca'.

     To illustrate this, suppose you have a function
     `open_or_report_error' which returns a descriptor, like `open', if
     it succeeds, but does not return to its caller if it fails.  If
     the file cannot be opened, it prints an error message and jumps
     out to the command level of your program using `longjmp'.  Let's
     change `open2' (*note Alloca Example::) to use this subroutine:

          int
          open2 (char *str1, char *str2, int flags, int mode)
          {
            char *name = (char *) alloca (strlen (str1) + strlen (str2) + 1);
            stpcpy (stpcpy (name, str1), str2);
            return open_or_report_error (name, flags, mode);
          }

     Because of the way `alloca' works, the memory it allocates is
     freed even when an error occurs, with no special effort required.

     By contrast, the previous definition of `open2' (which uses
     `malloc' and `free') would develop a memory leak if it were
     changed in this way.  Even if you are willing to make more changes
     to fix it, there is no easy way to do so.

File: libc.info,  Node: Disadvantages of Alloca,  Next: GNU C Variable-Size Arrays,  Prev: Advantages of Alloca,  Up: Variable Size Automatic

3.2.5.3 Disadvantages of `alloca'
.................................

These are the disadvantages of `alloca' in comparison with `malloc':

   * If you try to allocate more memory than the machine can provide,
     you don't get a clean error message.  Instead you get a fatal
     signal like the one you would get from an infinite recursion;
     probably a segmentation violation (*note Program Error Signals::).

   * Some non-GNU systems fail to support `alloca', so it is less
     portable.  However, a slower emulation of `alloca' written in C is
     available for use on systems with this deficiency.

File: libc.info,  Node: GNU C Variable-Size Arrays,  Prev: Disadvantages of Alloca,  Up: Variable Size Automatic

3.2.5.4 GNU C Variable-Size Arrays
..................................

In GNU C, you can replace most uses of `alloca' with an array of
variable size.  Here is how `open2' would look then:

     int open2 (char *str1, char *str2, int flags, int mode)
     {
       char name[strlen (str1) + strlen (str2) + 1];
       stpcpy (stpcpy (name, str1), str2);
       return open (name, flags, mode);
     }

   But `alloca' is not always equivalent to a variable-sized array, for
several reasons:

   * A variable size array's space is freed at the end of the scope of
     the name of the array.  The space allocated with `alloca' remains
     until the end of the function.

   * It is possible to use `alloca' within a loop, allocating an
     additional block on each iteration.  This is impossible with
     variable-sized arrays.

   *NB:* If you mix use of `alloca' and variable-sized arrays within
one function, exiting a scope in which a variable-sized array was
declared frees all blocks allocated with `alloca' during the execution
of that scope.

File: libc.info,  Node: Locking Pages,  Next: Resizing the Data Segment,  Prev: Memory Allocation,  Up: Memory

3.4 Locking Pages
=================

You can tell the system to associate a particular virtual memory page
with a real page frame and keep it that way -- i.e., cause the page to
be paged in if it isn't already and mark it so it will never be paged
out and consequently will never cause a page fault.  This is called
"locking" a page.

   The functions in this chapter lock and unlock the calling process'
pages.

* Menu:

* Why Lock Pages::                Reasons to read this section.
* Locked Memory Details::         Everything you need to know locked
                                    memory
* Page Lock Functions::           Here's how to do it.

File: libc.info,  Node: Why Lock Pages,  Next: Locked Memory Details,  Up: Locking Pages

3.4.1 Why Lock Pages
--------------------

Because page faults cause paged out pages to be paged in transparently,
a process rarely needs to be concerned about locking pages.  However,
there are two reasons people sometimes are:

   * Speed.  A page fault is transparent only insofar as the process is
     not sensitive to how long it takes to do a simple memory access.
     Time-critical processes, especially realtime processes, may not be
     able to wait or may not be able to tolerate variance in execution
     speed.

     A process that needs to lock pages for this reason probably also
     needs priority among other processes for use of the CPU.  *Note
     Priority::.

     In some cases, the programmer knows better than the system's demand
     paging allocator which pages should remain in real memory to
     optimize system performance.  In this case, locking pages can help.

   * Privacy.  If you keep secrets in virtual memory and that virtual
     memory gets paged out, that increases the chance that the secrets
     will get out.  If a password gets written out to disk swap space,
     for example, it might still be there long after virtual and real
     memory have been wiped clean.


   Be aware that when you lock a page, that's one fewer page frame that
can be used to back other virtual memory (by the same or other
processes), which can mean more page faults, which means the system
runs more slowly.  In fact, if you lock enough memory, some programs
may not be able to run at all for lack of real memory.

File: libc.info,  Node: Locked Memory Details,  Next: Page Lock Functions,  Prev: Why Lock Pages,  Up: Locking Pages

3.4.2 Locked Memory Details
---------------------------

A memory lock is associated with a virtual page, not a real frame.  The
paging rule is: If a frame backs at least one locked page, don't page it
out.

   Memory locks do not stack.  I.e., you can't lock a particular page
twice so that it has to be unlocked twice before it is truly unlocked.
It is either locked or it isn't.

   A memory lock persists until the process that owns the memory
explicitly unlocks it.  (But process termination and exec cause the
virtual memory to cease to exist, which you might say means it isn't
locked any more).

   Memory locks are not inherited by child processes.  (But note that
on a modern Unix system, immediately after a fork, the parent's and the
child's virtual address space are backed by the same real page frames,
so the child enjoys the parent's locks).  *Note Creating a Process::.

   Because of its ability to impact other processes, only the superuser
can lock a page.  Any process can unlock its own page.

   The system sets limits on the amount of memory a process can have
locked and the amount of real memory it can have dedicated to it.
*Note Limits on Resources::.

   In Linux, locked pages aren't as locked as you might think.  Two
virtual pages that are not shared memory can nonetheless be backed by
the same real frame.  The kernel does this in the name of efficiency
when it knows both virtual pages contain identical data, and does it
even if one or both of the virtual pages are locked.

   But when a process modifies one of those pages, the kernel must get
it a separate frame and fill it with the page's data.  This is known as
a "copy-on-write page fault".  It takes a small amount of time and in a
pathological case, getting that frame may require I/O.

   To make sure this doesn't happen to your program, don't just lock the
pages.  Write to them as well, unless you know you won't write to them
ever.  And to make sure you have pre-allocated frames for your stack,
enter a scope that declares a C automatic variable larger than the
maximum stack size you will need, set it to something, then return from
its scope.

File: libc.info,  Node: Page Lock Functions,  Prev: Locked Memory Details,  Up: Locking Pages

3.4.3 Functions To Lock And Unlock Pages
----------------------------------------

The symbols in this section are declared in `sys/mman.h'.  These
functions are defined by POSIX.1b, but their availability depends on
your kernel.  If your kernel doesn't allow these functions, they exist
but always fail.  They _are_ available with a Linux kernel.

   *Portability Note:* POSIX.1b requires that when the `mlock' and
`munlock' functions are available, the file `unistd.h' define the macro
`_POSIX_MEMLOCK_RANGE' and the file `limits.h' define the macro
`PAGESIZE' to be the size of a memory page in bytes.  It requires that
when the `mlockall' and `munlockall' functions are available, the
`unistd.h' file define the macro `_POSIX_MEMLOCK'.  The GNU C library
conforms to this requirement.

 -- Function: int mlock (const void *ADDR, size_t LEN)
     `mlock' locks a range of the calling process' virtual pages.

     The range of memory starts at address ADDR and is LEN bytes long.
     Actually, since you must lock whole pages, it is the range of
     pages that include any part of the specified range.

     When the function returns successfully, each of those pages is
     backed by (connected to) a real frame (is resident) and is marked
     to stay that way.  This means the function may cause page-ins and
     have to wait for them.

     When the function fails, it does not affect the lock status of any
     pages.

     The return value is zero if the function succeeds.  Otherwise, it
     is `-1' and `errno' is set accordingly.  `errno' values specific
     to this function are:

    `ENOMEM'
             * At least some of the specified address range does not
               exist in the calling process' virtual address space.

             * The locking would cause the process to exceed its locked
               page limit.

    `EPERM'
          The calling process is not superuser.

    `EINVAL'
          LEN is not positive.

    `ENOSYS'
          The kernel does not provide `mlock' capability.


     You can lock _all_ a process' memory with `mlockall'.  You unlock
     memory with `munlock' or `munlockall'.

     To avoid all page faults in a C program, you have to use
     `mlockall', because some of the memory a program uses is hidden
     from the C code, e.g. the stack and automatic variables, and you
     wouldn't know what address to tell `mlock'.


 -- Function: int munlock (const void *ADDR, size_t LEN)
     `munlock' unlocks a range of the calling process' virtual pages.

     `munlock' is the inverse of `mlock' and functions completely
     analogously to `mlock', except that there is no `EPERM' failure.


 -- Function: int mlockall (int FLAGS)
     `mlockall' locks all the pages in a process' virtual memory address
     space, and/or any that are added to it in the future.  This
     includes the pages of the code, data and stack segment, as well as
     shared libraries, user space kernel data, shared memory, and
     memory mapped files.

     FLAGS is a string of single bit flags represented by the following
     macros.  They tell `mlockall' which of its functions you want.  All
     other bits must be zero.

    `MCL_CURRENT'
          Lock all pages which currently exist in the calling process'
          virtual address space.

    `MCL_FUTURE'
          Set a mode such that any pages added to the process' virtual
          address space in the future will be locked from birth.  This
          mode does not affect future address spaces owned by the same
          process so exec, which replaces a process' address space,
          wipes out `MCL_FUTURE'.  *Note Executing a File::.


     When the function returns successfully, and you specified
     `MCL_CURRENT', all of the process' pages are backed by (connected
     to) real frames (they are resident) and are marked to stay that
     way.  This means the function may cause page-ins and have to wait
     for them.

     When the process is in `MCL_FUTURE' mode because it successfully
     executed this function and specified `MCL_CURRENT', any system call
     by the process that requires space be added to its virtual address
     space fails with `errno' = `ENOMEM' if locking the additional space
     would cause the process to exceed its locked page limit.  In the
     case that the address space addition that can't be accommodated is
     stack expansion, the stack expansion fails and the kernel sends a
     `SIGSEGV' signal to the process.

     When the function fails, it does not affect the lock status of any
     pages or the future locking mode.

     The return value is zero if the function succeeds.  Otherwise, it
     is `-1' and `errno' is set accordingly.  `errno' values specific
     to this function are:

    `ENOMEM'
             * At least some of the specified address range does not
               exist in the calling process' virtual address space.

             * The locking would cause the process to exceed its locked
               page limit.

    `EPERM'
          The calling process is not superuser.

    `EINVAL'
          Undefined bits in FLAGS are not zero.

    `ENOSYS'
          The kernel does not provide `mlockall' capability.


     You can lock just specific pages with `mlock'.  You unlock pages
     with `munlockall' and `munlock'.


 -- Function: int munlockall (void)
     `munlockall' unlocks every page in the calling process' virtual
     address space and turn off `MCL_FUTURE' future locking mode.

     The return value is zero if the function succeeds.  Otherwise, it
     is `-1' and `errno' is set accordingly.  The only way this
     function can fail is for generic reasons that all functions and
     system calls can fail, so there are no specific `errno' values.


File: libc.info,  Node: Resizing the Data Segment,  Prev: Locking Pages,  Up: Memory

3.3 Resizing the Data Segment
=============================

The symbols in this section are declared in `unistd.h'.

   You will not normally use the functions in this section, because the
functions described in *note Memory Allocation:: are easier to use.
Those are interfaces to a GNU C Library memory allocator that uses the
functions below itself.  The functions below are simple interfaces to
system calls.

 -- Function: int brk (void *ADDR)
     `brk' sets the high end of the calling process' data segment to
     ADDR.

     The address of the end of a segment is defined to be the address
     of the last byte in the segment plus 1.

     The function has no effect if ADDR is lower than the low end of
     the data segment.  (This is considered success, by the way).

     The function fails if it would cause the data segment to overlap
     another segment or exceed the process' data storage limit (*note
     Limits on Resources::).

     The function is named for a common historical case where data
     storage and the stack are in the same segment.  Data storage
     allocation grows upward from the bottom of the segment while the
     stack grows downward toward it from the top of the segment and the
     curtain between them is called the "break".

     The return value is zero on success.  On failure, the return value
     is `-1' and `errno' is set accordingly.  The following `errno'
     values are specific to this function:

    `ENOMEM'
          The request would cause the data segment to overlap another
          segment or exceed the process' data storage limit.


 -- Function: void *sbrk (ptrdiff_t DELTA)
     This function is the same as `brk' except that you specify the new
     end of the data segment as an offset DELTA from the current end
     and on success the return value is the address of the resulting
     end of the data segment instead of zero.

     This means you can use `sbrk(0)' to find out what the current end
     of the data segment is.


File: libc.info,  Node: Character Handling,  Next: String and Array Utilities,  Prev: Memory,  Up: Top

4 Character Handling
********************

Programs that work with characters and strings often need to classify a
character--is it alphabetic, is it a digit, is it whitespace, and so
on--and perform case conversion operations on characters.  The
functions in the header file `ctype.h' are provided for this purpose.

   Since the choice of locale and character set can alter the
classifications of particular character codes, all of these functions
are affected by the current locale.  (More precisely, they are affected
by the locale currently selected for character classification--the
`LC_CTYPE' category; see *note Locale Categories::.)

   The ISO C standard specifies two different sets of functions.  The
one set works on `char' type characters, the other one on `wchar_t'
wide characters (*note Extended Char Intro::).

* Menu:

* Classification of Characters::       Testing whether characters are
			                letters, digits, punctuation, etc.

* Case Conversion::                    Case mapping, and the like.
* Classification of Wide Characters::  Character class determination for
                                        wide characters.
* Using Wide Char Classes::            Notes on using the wide character
                                        classes.
* Wide Character Case Conversion::     Mapping of wide characters.

File: libc.info,  Node: Classification of Characters,  Next: Case Conversion,  Up: Character Handling

4.1 Classification of Characters
================================

This section explains the library functions for classifying characters.
For example, `isalpha' is the function to test for an alphabetic
character.  It takes one argument, the character to test, and returns a
nonzero integer if the character is alphabetic, and zero otherwise.  You
would use it like this:

     if (isalpha (c))
       printf ("The character `%c' is alphabetic.\n", c);

   Each of the functions in this section tests for membership in a
particular class of characters; each has a name starting with `is'.
Each of them takes one argument, which is a character to test, and
returns an `int' which is treated as a boolean value.  The character
argument is passed as an `int', and it may be the constant value `EOF'
instead of a real character.

   The attributes of any given character can vary between locales.
*Note Locales::, for more information on locales.

   These functions are declared in the header file `ctype.h'.

 -- Function: int islower (int C)
     Returns true if C is a lower-case letter.  The letter need not be
     from the Latin alphabet, any alphabet representable is valid.

 -- Function: int isupper (int C)
     Returns true if C is an upper-case letter.  The letter need not be
     from the Latin alphabet, any alphabet representable is valid.

 -- Function: int isalpha (int C)
     Returns true if C is an alphabetic character (a letter).  If
     `islower' or `isupper' is true of a character, then `isalpha' is
     also true.

     In some locales, there may be additional characters for which
     `isalpha' is true--letters which are neither upper case nor lower
     case.  But in the standard `"C"' locale, there are no such
     additional characters.

 -- Function: int isdigit (int C)
     Returns true if C is a decimal digit (`0' through `9').

 -- Function: int isalnum (int C)
     Returns true if C is an alphanumeric character (a letter or
     number); in other words, if either `isalpha' or `isdigit' is true
     of a character, then `isalnum' is also true.

 -- Function: int isxdigit (int C)
     Returns true if C is a hexadecimal digit.  Hexadecimal digits
     include the normal decimal digits `0' through `9' and the letters
     `A' through `F' and `a' through `f'.

 -- Function: int ispunct (int C)
     Returns true if C is a punctuation character.  This means any
     printing character that is not alphanumeric or a space character.

 -- Function: int isspace (int C)
     Returns true if C is a "whitespace" character.  In the standard
     `"C"' locale, `isspace' returns true for only the standard
     whitespace characters:

    `' ''
          space

    `'\f''
          formfeed

    `'\n''
          newline

    `'\r''
          carriage return

    `'\t''
          horizontal tab

    `'\v''
          vertical tab

 -- Function: int isblank (int C)
     Returns true if C is a blank character; that is, a space or a tab.
     This function was originally a GNU extension, but was added in
     ISO C99.

 -- Function: int isgraph (int C)
     Returns true if C is a graphic character; that is, a character
     that has a glyph associated with it.  The whitespace characters
     are not considered graphic.

 -- Function: int isprint (int C)
     Returns true if C is a printing character.  Printing characters
     include all the graphic characters, plus the space (` ') character.

 -- Function: int iscntrl (int C)
     Returns true if C is a control character (that is, a character that
     is not a printing character).

 -- Function: int isascii (int C)
     Returns true if C is a 7-bit `unsigned char' value that fits into
     the US/UK ASCII character set.  This function is a BSD extension
     and is also an SVID extension.

File: libc.info,  Node: Case Conversion,  Next: Classification of Wide Characters,  Prev: Classification of Characters,  Up: Character Handling

4.2 Case Conversion
===================

This section explains the library functions for performing conversions
such as case mappings on characters.  For example, `toupper' converts
any character to upper case if possible.  If the character can't be
converted, `toupper' returns it unchanged.

   These functions take one argument of type `int', which is the
character to convert, and return the converted character as an `int'.
If the conversion is not applicable to the argument given, the argument
is returned unchanged.

   *Compatibility Note:* In pre-ISO C dialects, instead of returning
the argument unchanged, these functions may fail when the argument is
not suitable for the conversion.  Thus for portability, you may need to
write `islower(c) ? toupper(c) : c' rather than just `toupper(c)'.

   These functions are declared in the header file `ctype.h'.

 -- Function: int tolower (int C)
     If C is an upper-case letter, `tolower' returns the corresponding
     lower-case letter.  If C is not an upper-case letter, C is
     returned unchanged.

 -- Function: int toupper (int C)
     If C is a lower-case letter, `toupper' returns the corresponding
     upper-case letter.  Otherwise C is returned unchanged.

 -- Function: int toascii (int C)
     This function converts C to a 7-bit `unsigned char' value that
     fits into the US/UK ASCII character set, by clearing the high-order
     bits.  This function is a BSD extension and is also an SVID
     extension.

 -- Function: int _tolower (int C)
     This is identical to `tolower', and is provided for compatibility
     with the SVID.  *Note SVID::.

 -- Function: int _toupper (int C)
     This is identical to `toupper', and is provided for compatibility
     with the SVID.

File: libc.info,  Node: Classification of Wide Characters,  Next: Using Wide Char Classes,  Prev: Case Conversion,  Up: Character Handling

4.3 Character class determination for wide characters
=====================================================

Amendment 1 to ISO C90 defines functions to classify wide characters.
Although the original ISO C90 standard already defined the type
`wchar_t', no functions operating on them were defined.

   The general design of the classification functions for wide
characters is more general.  It allows extensions to the set of
available classifications, beyond those which are always available.
The POSIX standard specifies how extensions can be made, and this is
already implemented in the GNU C library implementation of the
`localedef' program.

   The character class functions are normally implemented with bitsets,
with a bitset per character.  For a given character, the appropriate
bitset is read from a table and a test is performed as to whether a
certain bit is set.  Which bit is tested for is determined by the class.

   For the wide character classification functions this is made visible.
There is a type classification type defined, a function to retrieve this
value for a given class, and a function to test whether a given
character is in this class, using the classification value.  On top of
this the normal character classification functions as used for `char'
objects can be defined.

 -- Data type: wctype_t
     The `wctype_t' can hold a value which represents a character class.
     The only defined way to generate such a value is by using the
     `wctype' function.

     This type is defined in `wctype.h'.

 -- Function: wctype_t wctype (const char *PROPERTY)
     The `wctype' returns a value representing a class of wide
     characters which is identified by the string PROPERTY.  Beside
     some standard properties each locale can define its own ones.  In
     case no property with the given name is known for the current
     locale selected for the `LC_CTYPE' category, the function returns
     zero.

     The properties known in every locale are:

     `"alnum"'         `"alpha"'         `"cntrl"'         `"digit"'
     `"graph"'         `"lower"'         `"print"'         `"punct"'
     `"space"'         `"upper"'         `"xdigit"'

     This function is declared in `wctype.h'.

   To test the membership of a character to one of the non-standard
classes the ISO C standard defines a completely new function.

 -- Function: int iswctype (wint_t WC, wctype_t DESC)
     This function returns a nonzero value if WC is in the character
     class specified by DESC.  DESC must previously be returned by a
     successful call to `wctype'.

     This function is declared in `wctype.h'.

   To make it easier to use the commonly-used classification functions,
they are defined in the C library.  There is no need to use `wctype' if
the property string is one of the known character classes.  In some
situations it is desirable to construct the property strings, and then
it is important that `wctype' can also handle the standard classes.

 -- Function: int iswalnum (wint_t WC)
     This function returns a nonzero value if WC is an alphanumeric
     character (a letter or number); in other words, if either
     `iswalpha' or `iswdigit' is true of a character, then `iswalnum'
     is also true.

     This function can be implemented using

          iswctype (wc, wctype ("alnum"))

     It is declared in `wctype.h'.

 -- Function: int iswalpha (wint_t WC)
     Returns true if WC is an alphabetic character (a letter).  If
     `iswlower' or `iswupper' is true of a character, then `iswalpha'
     is also true.

     In some locales, there may be additional characters for which
     `iswalpha' is true--letters which are neither upper case nor lower
     case.  But in the standard `"C"' locale, there are no such
     additional characters.

     This function can be implemented using

          iswctype (wc, wctype ("alpha"))

     It is declared in `wctype.h'.

 -- Function: int iswcntrl (wint_t WC)
     Returns true if WC is a control character (that is, a character
     that is not a printing character).

     This function can be implemented using

          iswctype (wc, wctype ("cntrl"))

     It is declared in `wctype.h'.

 -- Function: int iswdigit (wint_t WC)
     Returns true if WC is a digit (e.g., `0' through `9').  Please
     note that this function does not only return a nonzero value for
     _decimal_ digits, but for all kinds of digits.  A consequence is
     that code like the following will *not* work unconditionally for
     wide characters:

          n = 0;
          while (iswdigit (*wc))
            {
              n *= 10;
              n += *wc++ - L'0';
            }

     This function can be implemented using

          iswctype (wc, wctype ("digit"))

     It is declared in `wctype.h'.

 -- Function: int iswgraph (wint_t WC)
     Returns true if WC is a graphic character; that is, a character
     that has a glyph associated with it.  The whitespace characters
     are not considered graphic.

     This function can be implemented using

          iswctype (wc, wctype ("graph"))

     It is declared in `wctype.h'.

 -- Function: int iswlower (wint_t WC)
     Returns true if WC is a lower-case letter.  The letter need not be
     from the Latin alphabet, any alphabet representable is valid.

     This function can be implemented using

          iswctype (wc, wctype ("lower"))

     It is declared in `wctype.h'.

 -- Function: int iswprint (wint_t WC)
     Returns true if WC is a printing character.  Printing characters
     include all the graphic characters, plus the space (` ') character.

     This function can be implemented using

          iswctype (wc, wctype ("print"))

     It is declared in `wctype.h'.

 -- Function: int iswpunct (wint_t WC)
     Returns true if WC is a punctuation character.  This means any
     printing character that is not alphanumeric or a space character.

     This function can be implemented using

          iswctype (wc, wctype ("punct"))

     It is declared in `wctype.h'.

 -- Function: int iswspace (wint_t WC)
     Returns true if WC is a "whitespace" character.  In the standard
     `"C"' locale, `iswspace' returns true for only the standard
     whitespace characters:

    `L' ''
          space

    `L'\f''
          formfeed

    `L'\n''
          newline

    `L'\r''
          carriage return

    `L'\t''
          horizontal tab

    `L'\v''
          vertical tab

     This function can be implemented using

          iswctype (wc, wctype ("space"))

     It is declared in `wctype.h'.

 -- Function: int iswupper (wint_t WC)
     Returns true if WC is an upper-case letter.  The letter need not be
     from the Latin alphabet, any alphabet representable is valid.

     This function can be implemented using

          iswctype (wc, wctype ("upper"))

     It is declared in `wctype.h'.

 -- Function: int iswxdigit (wint_t WC)
     Returns true if WC is a hexadecimal digit.  Hexadecimal digits
     include the normal decimal digits `0' through `9' and the letters
     `A' through `F' and `a' through `f'.

     This function can be implemented using

          iswctype (wc, wctype ("xdigit"))

     It is declared in `wctype.h'.

   The GNU C library also provides a function which is not defined in
the ISO C standard but which is available as a version for single byte
characters as well.

 -- Function: int iswblank (wint_t WC)
     Returns true if WC is a blank character; that is, a space or a tab.
     This function was originally a GNU extension, but was added in
     ISO C99.  It is declared in `wchar.h'.

File: libc.info,  Node: Using Wide Char Classes,  Next: Wide Character Case Conversion,  Prev: Classification of Wide Characters,  Up: Character Handling

4.4 Notes on using the wide character classes
=============================================

The first note is probably not astonishing but still occasionally a
cause of problems.  The `iswXXX' functions can be implemented using
macros and in fact, the GNU C library does this.  They are still
available as real functions but when the `wctype.h' header is included
the macros will be used.  This is the same as the `char' type versions
of these functions.

   The second note covers something new.  It can be best illustrated by
a (real-world) example.  The first piece of code is an excerpt from the
original code.  It is truncated a bit but the intention should be clear.

     int
     is_in_class (int c, const char *class)
     {
       if (strcmp (class, "alnum") == 0)
         return isalnum (c);
       if (strcmp (class, "alpha") == 0)
         return isalpha (c);
       if (strcmp (class, "cntrl") == 0)
         return iscntrl (c);
       ...
       return 0;
     }

   Now, with the `wctype' and `iswctype' you can avoid the `if'
cascades, but rewriting the code as follows is wrong:

     int
     is_in_class (int c, const char *class)
     {
       wctype_t desc = wctype (class);
       return desc ? iswctype ((wint_t) c, desc) : 0;
     }

   The problem is that it is not guaranteed that the wide character
representation of a single-byte character can be found using casting.
In fact, usually this fails miserably.  The correct solution to this
problem is to write the code as follows:

     int
     is_in_class (int c, const char *class)
     {
       wctype_t desc = wctype (class);
       return desc ? iswctype (btowc (c), desc) : 0;
     }

   *Note Converting a Character::, for more information on `btowc'.
Note that this change probably does not improve the performance of the
program a lot since the `wctype' function still has to make the string
comparisons.  It gets really interesting if the `is_in_class' function
is called more than once for the same class name.  In this case the
variable DESC could be computed once and reused for all the calls.
Therefore the above form of the function is probably not the final one.

File: libc.info,  Node: Wide Character Case Conversion,  Prev: Using Wide Char Classes,  Up: Character Handling

4.5 Mapping of wide characters.
===============================

The classification functions are also generalized by the ISO C
standard.  Instead of just allowing the two standard mappings, a locale
can contain others.  Again, the `localedef' program already supports
generating such locale data files.

 -- Data Type: wctrans_t
     This data type is defined as a scalar type which can hold a value
     representing the locale-dependent character mapping.  There is no
     way to construct such a value apart from using the return value of
     the `wctrans' function.

     This type is defined in `wctype.h'.

 -- Function: wctrans_t wctrans (const char *PROPERTY)
     The `wctrans' function has to be used to find out whether a named
     mapping is defined in the current locale selected for the
     `LC_CTYPE' category.  If the returned value is non-zero, you can
     use it afterwards in calls to `towctrans'.  If the return value is
     zero no such mapping is known in the current locale.

     Beside locale-specific mappings there are two mappings which are
     guaranteed to be available in every locale:

     `"tolower"'                        `"toupper"'

     These functions are declared in `wctype.h'.

 -- Function: wint_t towctrans (wint_t WC, wctrans_t DESC)
     `towctrans' maps the input character WC according to the rules of
     the mapping for which DESC is a descriptor, and returns the value
     it finds.  DESC must be obtained by a successful call to `wctrans'.

     This function is declared in `wctype.h'.

   For the generally available mappings, the ISO C standard defines
convenient shortcuts so that it is not necessary to call `wctrans' for
them.

 -- Function: wint_t towlower (wint_t WC)
     If WC is an upper-case letter, `towlower' returns the corresponding
     lower-case letter.  If WC is not an upper-case letter, WC is
     returned unchanged.

     `towlower' can be implemented using

          towctrans (wc, wctrans ("tolower"))

     This function is declared in `wctype.h'.

 -- Function: wint_t towupper (wint_t WC)
     If WC is a lower-case letter, `towupper' returns the corresponding
     upper-case letter.  Otherwise WC is returned unchanged.

     `towupper' can be implemented using

          towctrans (wc, wctrans ("toupper"))

     This function is declared in `wctype.h'.

   The same warnings given in the last section for the use of the wide
character classification functions apply here.  It is not possible to
simply cast a `char' type value to a `wint_t' and use it as an argument
to `towctrans' calls.

File: libc.info,  Node: String and Array Utilities,  Next: Character Set Handling,  Prev: Character Handling,  Up: Top

5 String and Array Utilities
****************************

Operations on strings (or arrays of characters) are an important part of
many programs.  The GNU C library provides an extensive set of string
utility functions, including functions for copying, concatenating,
comparing, and searching strings.  Many of these functions can also
operate on arbitrary regions of storage; for example, the `memcpy'
function can be used to copy the contents of any kind of array.

   It's fairly common for beginning C programmers to "reinvent the
wheel" by duplicating this functionality in their own code, but it pays
to become familiar with the library functions and to make use of them,
since this offers benefits in maintenance, efficiency, and portability.

   For instance, you could easily compare one string to another in two
lines of C code, but if you use the built-in `strcmp' function, you're
less likely to make a mistake.  And, since these library functions are
typically highly optimized, your program may run faster too.

* Menu:

* Representation of Strings::   Introduction to basic concepts.
* String/Array Conventions::    Whether to use a string function or an
				 arbitrary array function.
* String Length::               Determining the length of a string.
* Copying and Concatenation::   Functions to copy the contents of strings
				 and arrays.
* String/Array Comparison::     Functions for byte-wise and character-wise
				 comparison.
* Collation Functions::         Functions for collating strings.
* Search Functions::            Searching for a specific element or substring.
* Finding Tokens in a String::  Splitting a string into tokens by looking
				 for delimiters.
* strfry::                      Function for flash-cooking a string.
* Trivial Encryption::          Obscuring data.
* Encode Binary Data::          Encoding and Decoding of Binary Data.
* Argz and Envz Vectors::       Null-separated string vectors.

File: libc.info,  Node: Representation of Strings,  Next: String/Array Conventions,  Up: String and Array Utilities

5.1 Representation of Strings
=============================

This section is a quick summary of string concepts for beginning C
programmers.  It describes how character strings are represented in C
and some common pitfalls.  If you are already familiar with this
material, you can skip this section.

   A "string" is an array of `char' objects.  But string-valued
variables are usually declared to be pointers of type `char *'.  Such
variables do not include space for the text of a string; that has to be
stored somewhere else--in an array variable, a string constant, or
dynamically allocated memory (*note Memory Allocation::).  It's up to
you to store the address of the chosen memory space into the pointer
variable.  Alternatively you can store a "null pointer" in the pointer
variable.  The null pointer does not point anywhere, so attempting to
reference the string it points to gets an error.

   "string" normally refers to multibyte character strings as opposed to
wide character strings.  Wide character strings are arrays of type
`wchar_t' and as for multibyte character strings usually pointers of
type `wchar_t *' are used.

   By convention, a "null character", `'\0'', marks the end of a
multibyte character string and the "null wide character", `L'\0'',
marks the end of a wide character string.  For example, in testing to
see whether the `char *' variable P points to a null character marking
the end of a string, you can write `!*P' or `*P == '\0''.

   A null character is quite different conceptually from a null pointer,
although both are represented by the integer `0'.

   "String literals" appear in C program source as strings of
characters between double-quote characters (`"') where the initial
double-quote character is immediately preceded by a capital `L' (ell)
character (as in `L"foo"').  In ISO C, string literals can also be
formed by "string concatenation": `"a" "b"' is the same as `"ab"'.  For
wide character strings one can either use `L"a" L"b"' or `L"a" "b"'.
Modification of string literals is not allowed by the GNU C compiler,
because literals are placed in read-only storage.

   Character arrays that are declared `const' cannot be modified
either.  It's generally good style to declare non-modifiable string
pointers to be of type `const char *', since this often allows the C
compiler to detect accidental modifications as well as providing some
amount of documentation about what your program intends to do with the
string.

   The amount of memory allocated for the character array may extend
past the null character that normally marks the end of the string.  In
this document, the term "allocated size" is always used to refer to the
total amount of memory allocated for the string, while the term
"length" refers to the number of characters up to (but not including)
the terminating null character.

   A notorious source of program bugs is trying to put more characters
in a string than fit in its allocated size.  When writing code that
extends strings or moves characters into a pre-allocated array, you
should be very careful to keep track of the length of the text and make
explicit checks for overflowing the array.  Many of the library
functions _do not_ do this for you!  Remember also that you need to
allocate an extra byte to hold the null character that marks the end of
the string.

   Originally strings were sequences of bytes where each byte
represents a single character.  This is still true today if the strings
are encoded using a single-byte character encoding.  Things are
different if the strings are encoded using a multibyte encoding (for
more information on encodings see *note Extended Char Intro::).  There
is no difference in the programming interface for these two kind of
strings; the programmer has to be aware of this and interpret the byte
sequences accordingly.

   But since there is no separate interface taking care of these
differences the byte-based string functions are sometimes hard to use.
Since the count parameters of these functions specify bytes a call to
`strncpy' could cut a multibyte character in the middle and put an
incomplete (and therefore unusable) byte sequence in the target buffer.

   To avoid these problems later versions of the ISO C standard
introduce a second set of functions which are operating on "wide
characters" (*note Extended Char Intro::).  These functions don't have
the problems the single-byte versions have since every wide character is
a legal, interpretable value.  This does not mean that cutting wide
character strings at arbitrary points is without problems.  It normally
is for alphabet-based languages (except for non-normalized text) but
languages based on syllables still have the problem that more than one
wide character is necessary to complete a logical unit.  This is a
higher level problem which the C library functions are not designed to
solve.  But it is at least good that no invalid byte sequences can be
created.  Also, the higher level functions can also much easier operate
on wide character than on multibyte characters so that a general advise
is to use wide characters internally whenever text is more than simply
copied.

   The remaining of this chapter will discuss the functions for handling
wide character strings in parallel with the discussion of the multibyte
character strings since there is almost always an exact equivalent
available.

File: libc.info,  Node: String/Array Conventions,  Next: String Length,  Prev: Representation of Strings,  Up: String and Array Utilities

5.2 String and Array Conventions
================================

This chapter describes both functions that work on arbitrary arrays or
blocks of memory, and functions that are specific to null-terminated
arrays of characters and wide characters.

   Functions that operate on arbitrary blocks of memory have names
beginning with `mem' and `wmem' (such as `memcpy' and `wmemcpy') and
invariably take an argument which specifies the size (in bytes and wide
characters respectively) of the block of memory to operate on.  The
array arguments and return values for these functions have type `void
*' or `wchar_t'.  As a matter of style, the elements of the arrays used
with the `mem' functions are referred to as "bytes".  You can pass any
kind of pointer to these functions, and the `sizeof' operator is useful
in computing the value for the size argument.  Parameters to the `wmem'
functions must be of type `wchar_t *'.  These functions are not really
usable with anything but arrays of this type.

   In contrast, functions that operate specifically on strings and wide
character strings have names beginning with `str' and `wcs'
respectively (such as `strcpy' and `wcscpy') and look for a null
character to terminate the string instead of requiring an explicit size
argument to be passed.  (Some of these functions accept a specified
maximum length, but they also check for premature termination with a
null character.)  The array arguments and return values for these
functions have type `char *' and `wchar_t *' respectively, and the
array elements are referred to as "characters" and "wide characters".

   In many cases, there are both `mem' and `str'/`wcs' versions of a
function.  The one that is more appropriate to use depends on the exact
situation.  When your program is manipulating arbitrary arrays or
blocks of storage, then you should always use the `mem' functions.  On
the other hand, when you are manipulating null-terminated strings it is
usually more convenient to use the `str'/`wcs' functions, unless you
already know the length of the string in advance.  The `wmem' functions
should be used for wide character arrays with known size.

   Some of the memory and string functions take single characters as
arguments.  Since a value of type `char' is automatically promoted into
an value of type `int' when used as a parameter, the functions are
declared with `int' as the type of the parameter in question.  In case
of the wide character function the situation is similarly: the
parameter type for a single wide character is `wint_t' and not
`wchar_t'.  This would for many implementations not be necessary since
the `wchar_t' is large enough to not be automatically promoted, but
since the ISO C standard does not require such a choice of types the
`wint_t' type is used.

File: libc.info,  Node: String Length,  Next: Copying and Concatenation,  Prev: String/Array Conventions,  Up: String and Array Utilities

5.3 String Length
=================

You can get the length of a string using the `strlen' function.  This
function is declared in the header file `string.h'.

 -- Function: size_t strlen (const char *S)
     The `strlen' function returns the length of the null-terminated
     string S in bytes.  (In other words, it returns the offset of the
     terminating null character within the array.)

     For example,
          strlen ("hello, world")
              => 12

     When applied to a character array, the `strlen' function returns
     the length of the string stored there, not its allocated size.
     You can get the allocated size of the character array that holds a
     string using the `sizeof' operator:

          char string[32] = "hello, world";
          sizeof (string)
              => 32
          strlen (string)
              => 12

     But beware, this will not work unless STRING is the character
     array itself, not a pointer to it.  For example:

          char string[32] = "hello, world";
          char *ptr = string;
          sizeof (string)
              => 32
          sizeof (ptr)
              => 4  /* (on a machine with 4 byte pointers) */

     This is an easy mistake to make when you are working with
     functions that take string arguments; those arguments are always
     pointers, not arrays.

     It must also be noted that for multibyte encoded strings the return
     value does not have to correspond to the number of characters in
     the string.  To get this value the string can be converted to wide
     characters and `wcslen' can be used or something like the following
     code can be used:

          /* The input is in `string'.
             The length is expected in `n'.  */
          {
            mbstate_t t;
            char *scopy = string;
            /* In initial state.  */
            memset (&t, '\0', sizeof (t));
            /* Determine number of characters.  */
            n = mbsrtowcs (NULL, &scopy, strlen (scopy), &t);
          }

     This is cumbersome to do so if the number of characters (as
     opposed to bytes) is needed often it is better to work with wide
     characters.

   The wide character equivalent is declared in `wchar.h'.

 -- Function: size_t wcslen (const wchar_t *WS)
     The `wcslen' function is the wide character equivalent to
     `strlen'.  The return value is the number of wide characters in the
     wide character string pointed to by WS (this is also the offset of
     the terminating null wide character of WS).

     Since there are no multi wide character sequences making up one
     character the return value is not only the offset in the array, it
     is also the number of wide characters.

     This function was introduced in Amendment 1 to ISO C90.

 -- Function: size_t strnlen (const char *S, size_t MAXLEN)
     The `strnlen' function returns the length of the string S in bytes
     if this length is smaller than MAXLEN bytes.  Otherwise it returns
     MAXLEN.  Therefore this function is equivalent to `(strlen (S) < n
     ? strlen (S) : MAXLEN)' but it is more efficient and works even if
     the string S is not null-terminated.

          char string[32] = "hello, world";
          strnlen (string, 32)
              => 12
          strnlen (string, 5)
              => 5

     This function is a GNU extension and is declared in `string.h'.

 -- Function: size_t wcsnlen (const wchar_t *WS, size_t MAXLEN)
     `wcsnlen' is the wide character equivalent to `strnlen'.  The
     MAXLEN parameter specifies the maximum number of wide characters.

     This function is a GNU extension and is declared in `wchar.h'.

File: libc.info,  Node: Copying and Concatenation,  Next: String/Array Comparison,  Prev: String Length,  Up: String and Array Utilities

5.4 Copying and Concatenation
=============================

You can use the functions described in this section to copy the contents
of strings and arrays, or to append the contents of one string to
another.  The `str' and `mem' functions are declared in the header file
`string.h' while the `wstr' and `wmem' functions are declared in the
file `wchar.h'.

   A helpful way to remember the ordering of the arguments to the
functions in this section is that it corresponds to an assignment
expression, with the destination array specified to the left of the
source array.  All of these functions return the address of the
destination array.

   Most of these functions do not work properly if the source and
destination arrays overlap.  For example, if the beginning of the
destination array overlaps the end of the source array, the original
contents of that part of the source array may get overwritten before it
is copied.  Even worse, in the case of the string functions, the null
character marking the end of the string may be lost, and the copy
function might get stuck in a loop trashing all the memory allocated to
your program.

   All functions that have problems copying between overlapping arrays
are explicitly identified in this manual.  In addition to functions in
this section, there are a few others like `sprintf' (*note Formatted
Output Functions::) and `scanf' (*note Formatted Input Functions::).

 -- Function: void * memcpy (void *restrict TO, const void *restrict
          FROM, size_t SIZE)
     The `memcpy' function copies SIZE bytes from the object beginning
     at FROM into the object beginning at TO.  The behavior of this
     function is undefined if the two arrays TO and FROM overlap; use
     `memmove' instead if overlapping is possible.

     The value returned by `memcpy' is the value of TO.

     Here is an example of how you might use `memcpy' to copy the
     contents of an array:

          struct foo *oldarray, *newarray;
          int arraysize;
          ...
          memcpy (new, old, arraysize * sizeof (struct foo));

 -- Function: wchar_t * wmemcpy (wchar_t *restrict WTO, const wchar_t
          *restrict WFROM, size_t SIZE)
     The `wmemcpy' function copies SIZE wide characters from the object
     beginning at WFROM into the object beginning at WTO.  The behavior
     of this function is undefined if the two arrays WTO and WFROM
     overlap; use `wmemmove' instead if overlapping is possible.

     The following is a possible implementation of `wmemcpy' but there
     are more optimizations possible.

          wchar_t *
          wmemcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom,
                   size_t size)
          {
            return (wchar_t *) memcpy (wto, wfrom, size * sizeof (wchar_t));
          }

     The value returned by `wmemcpy' is the value of WTO.

     This function was introduced in Amendment 1 to ISO C90.

 -- Function: void * mempcpy (void *restrict TO, const void *restrict
          FROM, size_t SIZE)
     The `mempcpy' function is nearly identical to the `memcpy'
     function.  It copies SIZE bytes from the object beginning at
     `from' into the object pointed to by TO.  But instead of returning
     the value of TO it returns a pointer to the byte following the
     last written byte in the object beginning at TO.  I.e., the value
     is `((void *) ((char *) TO + SIZE))'.

     This function is useful in situations where a number of objects
     shall be copied to consecutive memory positions.

          void *
          combine (void *o1, size_t s1, void *o2, size_t s2)
          {
            void *result = malloc (s1 + s2);
            if (result != NULL)
              mempcpy (mempcpy (result, o1, s1), o2, s2);
            return result;
          }

     This function is a GNU extension.

 -- Function: wchar_t * wmempcpy (wchar_t *restrict WTO, const wchar_t
          *restrict WFROM, size_t SIZE)
     The `wmempcpy' function is nearly identical to the `wmemcpy'
     function.  It copies SIZE wide characters from the object
     beginning at `wfrom' into the object pointed to by WTO.  But
     instead of returning the value of WTO it returns a pointer to the
     wide character following the last written wide character in the
     object beginning at WTO.  I.e., the value is `WTO + SIZE'.

     This function is useful in situations where a number of objects
     shall be copied to consecutive memory positions.

     The following is a possible implementation of `wmemcpy' but there
     are more optimizations possible.

          wchar_t *
          wmempcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom,
                    size_t size)
          {
            return (wchar_t *) mempcpy (wto, wfrom, size * sizeof (wchar_t));
          }

     This function is a GNU extension.

 -- Function: void * memmove (void *TO, const void *FROM, size_t SIZE)
     `memmove' copies the SIZE bytes at FROM into the SIZE bytes at TO,
     even if those two blocks of space overlap.  In the case of
     overlap, `memmove' is careful to copy the original values of the
     bytes in the block at FROM, including those bytes which also
     belong to the block at TO.

     The value returned by `memmove' is the value of TO.

 -- Function: wchar_t * wmemmove (wchar *WTO, const wchar_t *WFROM,
          size_t SIZE)
     `wmemmove' copies the SIZE wide characters at WFROM into the SIZE
     wide characters at WTO, even if those two blocks of space overlap.
     In the case of overlap, `memmove' is careful to copy the original
     values of the wide characters in the block at WFROM, including
     those wide characters which also belong to the block at WTO.

     The following is a possible implementation of `wmemcpy' but there
     are more optimizations possible.

          wchar_t *
          wmempcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom,
                    size_t size)
          {
            return (wchar_t *) mempcpy (wto, wfrom, size * sizeof (wchar_t));
          }

     The value returned by `wmemmove' is the value of WTO.

     This function is a GNU extension.

 -- Function: void * memccpy (void *restrict TO, const void *restrict
          FROM, int C, size_t SIZE)
     This function copies no more than SIZE bytes from FROM to TO,
     stopping if a byte matching C is found.  The return value is a
     pointer into TO one byte past where C was copied, or a null
     pointer if no byte matching C appeared in the first SIZE bytes of
     FROM.

 -- Function: void * memset (void *BLOCK, int C, size_t SIZE)
     This function copies the value of C (converted to an `unsigned
     char') into each of the first SIZE bytes of the object beginning
     at BLOCK.  It returns the value of BLOCK.

 -- Function: wchar_t * wmemset (wchar_t *BLOCK, wchar_t WC, size_t
          SIZE)
     This function copies the value of WC into each of the first SIZE
     wide characters of the object beginning at BLOCK.  It returns the
     value of BLOCK.

 -- Function: char * strcpy (char *restrict TO, const char *restrict
          FROM)
     This copies characters from the string FROM (up to and including
     the terminating null character) into the string TO.  Like
     `memcpy', this function has undefined results if the strings
     overlap.  The return value is the value of TO.

 -- Function: wchar_t * wcscpy (wchar_t *restrict WTO, const wchar_t
          *restrict WFROM)
     This copies wide characters from the string WFROM (up to and
     including the terminating null wide character) into the string
     WTO.  Like `wmemcpy', this function has undefined results if the
     strings overlap.  The return value is the value of WTO.

 -- Function: char * strncpy (char *restrict TO, const char *restrict
          FROM, size_t SIZE)
     This function is similar to `strcpy' but always copies exactly
     SIZE characters into TO.

     If the length of FROM is more than SIZE, then `strncpy' copies
     just the first SIZE characters.  Note that in this case there is
     no null terminator written into TO.

     If the length of FROM is less than SIZE, then `strncpy' copies all
     of FROM, followed by enough null characters to add up to SIZE
     characters in all.  This behavior is rarely useful, but it is
     specified by the ISO C standard.

     The behavior of `strncpy' is undefined if the strings overlap.

     Using `strncpy' as opposed to `strcpy' is a way to avoid bugs
     relating to writing past the end of the allocated space for TO.
     However, it can also make your program much slower in one common
     case: copying a string which is probably small into a potentially
     large buffer.  In this case, SIZE may be large, and when it is,
     `strncpy' will waste a considerable amount of time copying null
     characters.

 -- Function: wchar_t * wcsncpy (wchar_t *restrict WTO, const wchar_t
          *restrict WFROM, size_t SIZE)
     This function is similar to `wcscpy' but always copies exactly
     SIZE wide characters into WTO.

     If the length of WFROM is more than SIZE, then `wcsncpy' copies
     just the first SIZE wide characters.  Note that in this case there
     is no null terminator written into WTO.

     If the length of WFROM is less than SIZE, then `wcsncpy' copies
     all of WFROM, followed by enough null wide characters to add up to
     SIZE wide characters in all.  This behavior is rarely useful, but
     it is specified by the ISO C standard.

     The behavior of `wcsncpy' is undefined if the strings overlap.

     Using `wcsncpy' as opposed to `wcscpy' is a way to avoid bugs
     relating to writing past the end of the allocated space for WTO.
     However, it can also make your program much slower in one common
     case: copying a string which is probably small into a potentially
     large buffer.  In this case, SIZE may be large, and when it is,
     `wcsncpy' will waste a considerable amount of time copying null
     wide characters.

 -- Function: char * strdup (const char *S)
     This function copies the null-terminated string S into a newly
     allocated string.  The string is allocated using `malloc'; see
     *note Unconstrained Allocation::.  If `malloc' cannot allocate
     space for the new string, `strdup' returns a null pointer.
     Otherwise it returns a pointer to the new string.

 -- Function: wchar_t * wcsdup (const wchar_t *WS)
     This function copies the null-terminated wide character string WS
     into a newly allocated string.  The string is allocated using
     `malloc'; see *note Unconstrained Allocation::.  If `malloc'
     cannot allocate space for the new string, `wcsdup' returns a null
     pointer.  Otherwise it returns a pointer to the new wide character
     string.

     This function is a GNU extension.

 -- Function: char * strndup (const char *S, size_t SIZE)
     This function is similar to `strdup' but always copies at most
     SIZE characters into the newly allocated string.

     If the length of S is more than SIZE, then `strndup' copies just
     the first SIZE characters and adds a closing null terminator.
     Otherwise all characters are copied and the string is terminated.

     This function is different to `strncpy' in that it always
     terminates the destination string.

     `strndup' is a GNU extension.

 -- Function: char * stpcpy (char *restrict TO, const char *restrict
          FROM)
     This function is like `strcpy', except that it returns a pointer to
     the end of the string TO (that is, the address of the terminating
     null character `to + strlen (from)') rather than the beginning.

     For example, this program uses `stpcpy' to concatenate `foo' and
     `bar' to produce `foobar', which it then prints.

          #include <string.h>
          #include <stdio.h>

          int
          main (void)
          {
            char buffer[10];
            char *to = buffer;
            to = stpcpy (to, "foo");
            to = stpcpy (to, "bar");
            puts (buffer);
            return 0;
          }

     This function is not part of the ISO or POSIX standards, and is not
     customary on Unix systems, but we did not invent it either.
     Perhaps it comes from MS-DOG.

     Its behavior is undefined if the strings overlap.  The function is
     declared in `string.h'.

 -- Function: wchar_t * wcpcpy (wchar_t *restrict WTO, const wchar_t
          *restrict WFROM)
     This function is like `wcscpy', except that it returns a pointer to
     the end of the string WTO (that is, the address of the terminating
     null character `wto + strlen (wfrom)') rather than the beginning.

     This function is not part of ISO or POSIX but was found useful
     while developing the GNU C Library itself.

     The behavior of `wcpcpy' is undefined if the strings overlap.

     `wcpcpy' is a GNU extension and is declared in `wchar.h'.

 -- Function: char * stpncpy (char *restrict TO, const char *restrict
          FROM, size_t SIZE)
     This function is similar to `stpcpy' but copies always exactly
     SIZE characters into TO.

     If the length of FROM is more then SIZE, then `stpncpy' copies
     just the first SIZE characters and returns a pointer to the
     character directly following the one which was copied last.  Note
     that in this case there is no null terminator written into TO.

     If the length of FROM is less than SIZE, then `stpncpy' copies all
     of FROM, followed by enough null characters to add up to SIZE
     characters in all.  This behavior is rarely useful, but it is
     implemented to be useful in contexts where this behavior of the
     `strncpy' is used.  `stpncpy' returns a pointer to the _first_
     written null character.

     This function is not part of ISO or POSIX but was found useful
     while developing the GNU C Library itself.

     Its behavior is undefined if the strings overlap.  The function is
     declared in `string.h'.

 -- Function: wchar_t * wcpncpy (wchar_t *restrict WTO, const wchar_t
          *restrict WFROM, size_t SIZE)
     This function is similar to `wcpcpy' but copies always exactly
     WSIZE characters into WTO.

     If the length of WFROM is more then SIZE, then `wcpncpy' copies
     just the first SIZE wide characters and returns a pointer to the
     wide character directly following the last non-null wide character
     which was copied last.  Note that in this case there is no null
     terminator written into WTO.

     If the length of WFROM is less than SIZE, then `wcpncpy' copies
     all of WFROM, followed by enough null characters to add up to SIZE
     characters in all.  This behavior is rarely useful, but it is
     implemented to be useful in contexts where this behavior of the
     `wcsncpy' is used.  `wcpncpy' returns a pointer to the _first_
     written null character.

     This function is not part of ISO or POSIX but was found useful
     while developing the GNU C Library itself.

     Its behavior is undefined if the strings overlap.

     `wcpncpy' is a GNU extension and is declared in `wchar.h'.

 -- Macro: char * strdupa (const char *S)
     This macro is similar to `strdup' but allocates the new string
     using `alloca' instead of `malloc' (*note Variable Size
     Automatic::).  This means of course the returned string has the
     same limitations as any block of memory allocated using `alloca'.

     For obvious reasons `strdupa' is implemented only as a macro; you
     cannot get the address of this function.  Despite this limitation
     it is a useful function.  The following code shows a situation
     where using `malloc' would be a lot more expensive.

          #include <paths.h>
          #include <string.h>
          #include <stdio.h>

          const char path[] = _PATH_STDPATH;

          int
          main (void)
          {
            char *wr_path = strdupa (path);
            char *cp = strtok (wr_path, ":");

            while (cp != NULL)
              {
                puts (cp);
                cp = strtok (NULL, ":");
              }
            return 0;
          }

     Please note that calling `strtok' using PATH directly is invalid.
     It is also not allowed to call `strdupa' in the argument list of
     `strtok' since `strdupa' uses `alloca' (*note Variable Size
     Automatic::) can interfere with the parameter passing.

     This function is only available if GNU CC is used.

 -- Macro: char * strndupa (const char *S, size_t SIZE)
     This function is similar to `strndup' but like `strdupa' it
     allocates the new string using `alloca' *note Variable Size
     Automatic::.  The same advantages and limitations of `strdupa' are
     valid for `strndupa', too.

     This function is implemented only as a macro, just like `strdupa'.
     Just as `strdupa' this macro also must not be used inside the
     parameter list in a function call.

     `strndupa' is only available if GNU CC is used.

 -- Function: char * strcat (char *restrict TO, const char *restrict
          FROM)
     The `strcat' function is similar to `strcpy', except that the
     characters from FROM are concatenated or appended to the end of
     TO, instead of overwriting it.  That is, the first character from
     FROM overwrites the null character marking the end of TO.

     An equivalent definition for `strcat' would be:

          char *
          strcat (char *restrict to, const char *restrict from)
          {
            strcpy (to + strlen (to), from);
            return to;
          }

     This function has undefined results if the strings overlap.

 -- Function: wchar_t * wcscat (wchar_t *restrict WTO, const wchar_t
          *restrict WFROM)
     The `wcscat' function is similar to `wcscpy', except that the
     characters from WFROM are concatenated or appended to the end of
     WTO, instead of overwriting it.  That is, the first character from
     WFROM overwrites the null character marking the end of WTO.

     An equivalent definition for `wcscat' would be:

          wchar_t *
          wcscat (wchar_t *wto, const wchar_t *wfrom)
          {
            wcscpy (wto + wcslen (wto), wfrom);
            return wto;
          }

     This function has undefined results if the strings overlap.

   Programmers using the `strcat' or `wcscat' function (or the
following `strncat' or `wcsncar' functions for that matter) can easily
be recognized as lazy and reckless.  In almost all situations the
lengths of the participating strings are known (it better should be
since how can one otherwise ensure the allocated size of the buffer is
sufficient?)  Or at least, one could know them if one keeps track of the
results of the various function calls.  But then it is very inefficient
to use `strcat'/`wcscat'.  A lot of time is wasted finding the end of
the destination string so that the actual copying can start.  This is a
common example:

     /* This function concatenates arbitrarily many strings.  The last
        parameter must be `NULL'.  */
     char *
     concat (const char *str, ...)
     {
       va_list ap, ap2;
       size_t total = 1;
       const char *s;
       char *result;

       va_start (ap, str);
       /* Actually `va_copy', but this is the name more gcc versions
          understand.  */
       __va_copy (ap2, ap);

       /* Determine how much space we need.  */
       for (s = str; s != NULL; s = va_arg (ap, const char *))
         total += strlen (s);

       va_end (ap);

       result = (char *) malloc (total);
       if (result != NULL)
         {
           result[0] = '\0';

           /* Copy the strings.  */
           for (s = str; s != NULL; s = va_arg (ap2, const char *))
             strcat (result, s);
         }

       va_end (ap2);

       return result;
     }

   This looks quite simple, especially the second loop where the strings
are actually copied.  But these innocent lines hide a major performance
penalty.  Just imagine that ten strings of 100 bytes each have to be
concatenated.  For the second string we search the already stored 100
bytes for the end of the string so that we can append the next string.
For all strings in total the comparisons necessary to find the end of
the intermediate results sums up to 5500!  If we combine the copying
with the search for the allocation we can write this function more
efficient:

     char *
     concat (const char *str, ...)
     {
       va_list ap;
       size_t allocated = 100;
       char *result = (char *) malloc (allocated);

       if (result != NULL)
         {
           char *newp;
           char *wp;

           va_start (ap, str);

           wp = result;
           for (s = str; s != NULL; s = va_arg (ap, const char *))
             {
               size_t len = strlen (s);

               /* Resize the allocated memory if necessary.  */
               if (wp + len + 1 > result + allocated)
                 {
                   allocated = (allocated + len) * 2;
                   newp = (char *) realloc (result, allocated);
                   if (newp == NULL)
                     {
                       free (result);
                       return NULL;
                     }
                   wp = newp + (wp - result);
                   result = newp;
                 }

               wp = mempcpy (wp, s, len);
             }

           /* Terminate the result string.  */
           *wp++ = '\0';

           /* Resize memory to the optimal size.  */
           newp = realloc (result, wp - result);
           if (newp != NULL)
             result = newp;

           va_end (ap);
         }

       return result;
     }

   With a bit more knowledge about the input strings one could fine-tune
the memory allocation.  The difference we are pointing to here is that
we don't use `strcat' anymore.  We always keep track of the length of
the current intermediate result so we can safe us the search for the
end of the string and use `mempcpy'.  Please note that we also don't
use `stpcpy' which might seem more natural since we handle with
strings.  But this is not necessary since we already know the length of
the string and therefore can use the faster memory copying function.
The example would work for wide characters the same way.

   Whenever a programmer feels the need to use `strcat' she or he
should think twice and look through the program whether the code cannot
be rewritten to take advantage of already calculated results.  Again: it
is almost always unnecessary to use `strcat'.

 -- Function: char * strncat (char *restrict TO, const char *restrict
          FROM, size_t SIZE)
     This function is like `strcat' except that not more than SIZE
     characters from FROM are appended to the end of TO.  A single null
     character is also always appended to TO, so the total allocated
     size of TO must be at least `SIZE + 1' bytes longer than its
     initial length.

     The `strncat' function could be implemented like this:

          char *
          strncat (char *to, const char *from, size_t size)
          {
            to[strlen (to) + size] = '\0';
            strncpy (to + strlen (to), from, size);
            return to;
          }

     The behavior of `strncat' is undefined if the strings overlap.

 -- Function: wchar_t * wcsncat (wchar_t *restrict WTO, const wchar_t
          *restrict WFROM, size_t SIZE)
     This function is like `wcscat' except that not more than SIZE
     characters from FROM are appended to the end of TO.  A single null
     character is also always appended to TO, so the total allocated
     size of TO must be at least `SIZE + 1' bytes longer than its
     initial length.

     The `wcsncat' function could be implemented like this:

          wchar_t *
          wcsncat (wchar_t *restrict wto, const wchar_t *restrict wfrom,
                   size_t size)
          {
            wto[wcslen (to) + size] = L'\0';
            wcsncpy (wto + wcslen (wto), wfrom, size);
            return wto;
          }

     The behavior of `wcsncat' is undefined if the strings overlap.

   Here is an example showing the use of `strncpy' and `strncat' (the
wide character version is equivalent).  Notice how, in the call to
`strncat', the SIZE parameter is computed to avoid overflowing the
character array `buffer'.

     #include <string.h>
     #include <stdio.h>

     #define SIZE 10

     static char buffer[SIZE];

     main ()
     {
       strncpy (buffer, "hello", SIZE);
       puts (buffer);
       strncat (buffer, ", world", SIZE - strlen (buffer) - 1);
       puts (buffer);
     }

The output produced by this program looks like:

     hello
     hello, wo

 -- Function: void bcopy (const void *FROM, void *TO, size_t SIZE)
     This is a partially obsolete alternative for `memmove', derived
     from BSD.  Note that it is not quite equivalent to `memmove',
     because the arguments are not in the same order and there is no
     return value.

 -- Function: void bzero (void *BLOCK, size_t SIZE)
     This is a partially obsolete alternative for `memset', derived from
     BSD.  Note that it is not as general as `memset', because the only
     value it can store is zero.

File: libc.info,  Node: String/Array Comparison,  Next: Collation Functions,  Prev: Copying and Concatenation,  Up: String and Array Utilities

5.5 String/Array Comparison
===========================

You can use the functions in this section to perform comparisons on the
contents of strings and arrays.  As well as checking for equality, these
functions can also be used as the ordering functions for sorting
operations.  *Note Searching and Sorting::, for an example of this.

   Unlike most comparison operations in C, the string comparison
functions return a nonzero value if the strings are _not_ equivalent
rather than if they are.  The sign of the value indicates the relative
ordering of the first characters in the strings that are not
equivalent:  a negative value indicates that the first string is "less"
than the second, while a positive value indicates that the first string
is "greater".

   The most common use of these functions is to check only for equality.
This is canonically done with an expression like `! strcmp (s1, s2)'.

   All of these functions are declared in the header file `string.h'.

 -- Function: int memcmp (const void *A1, const void *A2, size_t SIZE)
     The function `memcmp' compares the SIZE bytes of memory beginning
     at A1 against the SIZE bytes of memory beginning at A2.  The value
     returned has the same sign as the difference between the first
     differing pair of bytes (interpreted as `unsigned char' objects,
     then promoted to `int').

     If the contents of the two blocks are equal, `memcmp' returns `0'.

 -- Function: int wmemcmp (const wchar_t *A1, const wchar_t *A2, size_t
          SIZE)
     The function `wmemcmp' compares the SIZE wide characters beginning
     at A1 against the SIZE wide characters beginning at A2.  The value
     returned is smaller than or larger than zero depending on whether
     the first differing wide character is A1 is smaller or larger than
     the corresponding character in A2.

     If the contents of the two blocks are equal, `wmemcmp' returns `0'.

   On arbitrary arrays, the `memcmp' function is mostly useful for
testing equality.  It usually isn't meaningful to do byte-wise ordering
comparisons on arrays of things other than bytes.  For example, a
byte-wise comparison on the bytes that make up floating-point numbers
isn't likely to tell you anything about the relationship between the
values of the floating-point numbers.

   `wmemcmp' is really only useful to compare arrays of type `wchar_t'
since the function looks at `sizeof (wchar_t)' bytes at a time and this
number of bytes is system dependent.

   You should also be careful about using `memcmp' to compare objects
that can contain "holes", such as the padding inserted into structure
objects to enforce alignment requirements, extra space at the end of
unions, and extra characters at the ends of strings whose length is less
than their allocated size.  The contents of these "holes" are
indeterminate and may cause strange behavior when performing byte-wise
comparisons.  For more predictable results, perform an explicit
component-wise comparison.

   For example, given a structure type definition like:

     struct foo
       {
         unsigned char tag;
         union
           {
             double f;
             long i;
             char *p;
           } value;
       };

you are better off writing a specialized comparison function to compare
`struct foo' objects instead of comparing them with `memcmp'.

 -- Function: int strcmp (const char *S1, const char *S2)
     The `strcmp' function compares the string S1 against S2, returning
     a value that has the same sign as the difference between the first
     differing pair of characters (interpreted as `unsigned char'
     objects, then promoted to `int').

     If the two strings are equal, `strcmp' returns `0'.

     A consequence of the ordering used by `strcmp' is that if S1 is an
     initial substring of S2, then S1 is considered to be "less than"
     S2.

     `strcmp' does not take sorting conventions of the language the
     strings are written in into account.  To get that one has to use
     `strcoll'.

 -- Function: int wcscmp (const wchar_t *WS1, const wchar_t *WS2)
     The `wcscmp' function compares the wide character string WS1
     against WS2.  The value returned is smaller than or larger than
     zero depending on whether the first differing wide character is
     WS1 is smaller or larger than the corresponding character in WS2.

     If the two strings are equal, `wcscmp' returns `0'.

     A consequence of the ordering used by `wcscmp' is that if WS1 is
     an initial substring of WS2, then WS1 is considered to be "less
     than" WS2.

     `wcscmp' does not take sorting conventions of the language the
     strings are written in into account.  To get that one has to use
     `wcscoll'.

 -- Function: int strcasecmp (const char *S1, const char *S2)
     This function is like `strcmp', except that differences in case are
     ignored.  How uppercase and lowercase characters are related is
     determined by the currently selected locale.  In the standard `"C"'
     locale the characters A" and a" do not match but in a locale which
     regards these characters as parts of the alphabet they do match.

     `strcasecmp' is derived from BSD.

 -- Function: int wcscasecmp (const wchar_t *WS1, const wchar_T *WS2)
     This function is like `wcscmp', except that differences in case are
     ignored.  How uppercase and lowercase characters are related is
     determined by the currently selected locale.  In the standard `"C"'
     locale the characters A" and a" do not match but in a locale which
     regards these characters as parts of the alphabet they do match.

     `wcscasecmp' is a GNU extension.

 -- Function: int strncmp (const char *S1, const char *S2, size_t SIZE)
     This function is the similar to `strcmp', except that no more than
     SIZE characters are compared.  In other words, if the two strings
     are the same in their first SIZE characters, the return value is
     zero.

 -- Function: int wcsncmp (const wchar_t *WS1, const wchar_t *WS2,
          size_t SIZE)
     This function is the similar to `wcscmp', except that no more than
     SIZE wide characters are compared.  In other words, if the two
     strings are the same in their first SIZE wide characters, the
     return value is zero.

 -- Function: int strncasecmp (const char *S1, const char *S2, size_t N)
     This function is like `strncmp', except that differences in case
     are ignored.  Like `strcasecmp', it is locale dependent how
     uppercase and lowercase characters are related.

     `strncasecmp' is a GNU extension.

 -- Function: int wcsncasecmp (const wchar_t *WS1, const wchar_t *S2,
          size_t N)
     This function is like `wcsncmp', except that differences in case
     are ignored.  Like `wcscasecmp', it is locale dependent how
     uppercase and lowercase characters are related.

     `wcsncasecmp' is a GNU extension.

   Here are some examples showing the use of `strcmp' and `strncmp'
(equivalent examples can be constructed for the wide character
functions).  These examples assume the use of the ASCII character set.
(If some other character set--say, EBCDIC--is used instead, then the
glyphs are associated with different numeric codes, and the return
values and ordering may differ.)

     strcmp ("hello", "hello")
         => 0    /* These two strings are the same. */
     strcmp ("hello", "Hello")
         => 32   /* Comparisons are case-sensitive. */
     strcmp ("hello", "world")
         => -15  /* The character `'h'' comes before `'w''. */
     strcmp ("hello", "hello, world")
         => -44  /* Comparing a null character against a comma. */
     strncmp ("hello", "hello, world", 5)
         => 0    /* The initial 5 characters are the same. */
     strncmp ("hello, world", "hello, stupid world!!!", 5)
         => 0    /* The initial 5 characters are the same. */

 -- Function: int strverscmp (const char *S1, const char *S2)
     The `strverscmp' function compares the string S1 against S2,
     considering them as holding indices/version numbers.  Return value
     follows the same conventions as found in the `strverscmp'
     function.  In fact, if S1 and S2 contain no digits, `strverscmp'
     behaves like `strcmp'.

     Basically, we compare strings normally (character by character),
     until we find a digit in each string - then we enter a special
     comparison mode, where each sequence of digits is taken as a
     whole.  If we reach the end of these two parts without noticing a
     difference, we return to the standard comparison mode.  There are
     two types of numeric parts: "integral" and "fractional" (those
     begin with a '0'). The types of the numeric parts affect the way
     we sort them:

        * integral/integral: we compare values as you would expect.

        * fractional/integral: the fractional part is less than the
          integral one.  Again, no surprise.

        * fractional/fractional: the things become a bit more complex.
          If the common prefix contains only leading zeroes, the
          longest part is less than the other one; else the comparison
          behaves normally.

          strverscmp ("no digit", "no digit")
              => 0    /* same behavior as strcmp. */
          strverscmp ("item#99", "item#100")
              => <0   /* same prefix, but 99 < 100. */
          strverscmp ("alpha1", "alpha001")
              => >0   /* fractional part inferior to integral one. */
          strverscmp ("part1_f012", "part1_f01")
              => >0   /* two fractional parts. */
          strverscmp ("foo.009", "foo.0")
              => <0   /* idem, but with leading zeroes only. */

     This function is especially useful when dealing with filename
     sorting, because filenames frequently hold indices/version numbers.

     `strverscmp' is a GNU extension.

 -- Function: int bcmp (const void *A1, const void *A2, size_t SIZE)
     This is an obsolete alias for `memcmp', derived from BSD.

File: libc.info,  Node: Collation Functions,  Next: Search Functions,  Prev: String/Array Comparison,  Up: String and Array Utilities

5.6 Collation Functions
=======================

In some locales, the conventions for lexicographic ordering differ from
the strict numeric ordering of character codes.  For example, in Spanish
most glyphs with diacritical marks such as accents are not considered
distinct letters for the purposes of collation.  On the other hand, the
two-character sequence `ll' is treated as a single letter that is
collated immediately after `l'.

   You can use the functions `strcoll' and `strxfrm' (declared in the
headers file `string.h') and `wcscoll' and `wcsxfrm' (declared in the
headers file `wchar') to compare strings using a collation ordering
appropriate for the current locale.  The locale used by these functions
in particular can be specified by setting the locale for the
`LC_COLLATE' category; see *note Locales::.

   In the standard C locale, the collation sequence for `strcoll' is
the same as that for `strcmp'.  Similarly, `wcscoll' and `wcscmp' are
the same in this situation.

   Effectively, the way these functions work is by applying a mapping to
transform the characters in a string to a byte sequence that represents
the string's position in the collating sequence of the current locale.
Comparing two such byte sequences in a simple fashion is equivalent to
comparing the strings with the locale's collating sequence.

   The functions `strcoll' and `wcscoll' perform this translation
implicitly, in order to do one comparison.  By contrast, `strxfrm' and
`wcsxfrm' perform the mapping explicitly.  If you are making multiple
comparisons using the same string or set of strings, it is likely to be
more efficient to use `strxfrm' or `wcsxfrm' to transform all the
strings just once, and subsequently compare the transformed strings
with `strcmp' or `wcscmp'.

 -- Function: int strcoll (const char *S1, const char *S2)
     The `strcoll' function is similar to `strcmp' but uses the
     collating sequence of the current locale for collation (the
     `LC_COLLATE' locale).

 -- Function: int wcscoll (const wchar_t *WS1, const wchar_t *WS2)
     The `wcscoll' function is similar to `wcscmp' but uses the
     collating sequence of the current locale for collation (the
     `LC_COLLATE' locale).

   Here is an example of sorting an array of strings, using `strcoll'
to compare them.  The actual sort algorithm is not written here; it
comes from `qsort' (*note Array Sort Function::).  The job of the code
shown here is to say how to compare the strings while sorting them.
(Later on in this section, we will show a way to do this more
efficiently using `strxfrm'.)

     /* This is the comparison function used with `qsort'. */

     int
     compare_elements (char **p1, char **p2)
     {
       return strcoll (*p1, *p2);
     }

     /* This is the entry point--the function to sort
        strings using the locale's collating sequence. */

     void
     sort_strings (char **array, int nstrings)
     {
       /* Sort `temp_array' by comparing the strings. */
       qsort (array, nstrings,
              sizeof (char *), compare_elements);
     }

 -- Function: size_t strxfrm (char *restrict TO, const char *restrict
          FROM, size_t SIZE)
     The function `strxfrm' transforms the string FROM using the
     collation transformation determined by the locale currently
     selected for collation, and stores the transformed string in the
     array TO.  Up to SIZE characters (including a terminating null
     character) are stored.

     The behavior is undefined if the strings TO and FROM overlap; see
     *note Copying and Concatenation::.

     The return value is the length of the entire transformed string.
     This value is not affected by the value of SIZE, but if it is
     greater or equal than SIZE, it means that the transformed string
     did not entirely fit in the array TO.  In this case, only as much
     of the string as actually fits was stored.  To get the whole
     transformed string, call `strxfrm' again with a bigger output
     array.

     The transformed string may be longer than the original string, and
     it may also be shorter.

     If SIZE is zero, no characters are stored in TO.  In this case,
     `strxfrm' simply returns the number of characters that would be
     the length of the transformed string.  This is useful for
     determining what size the allocated array should be.  It does not
     matter what TO is if SIZE is zero; TO may even be a null pointer.

 -- Function: size_t wcsxfrm (wchar_t *restrict WTO, const wchar_t
          *WFROM, size_t SIZE)
     The function `wcsxfrm' transforms wide character string WFROM
     using the collation transformation determined by the locale
     currently selected for collation, and stores the transformed
     string in the array WTO.  Up to SIZE wide characters (including a
     terminating null character) are stored.

     The behavior is undefined if the strings WTO and WFROM overlap;
     see *note Copying and Concatenation::.

     The return value is the length of the entire transformed wide
     character string.  This value is not affected by the value of
     SIZE, but if it is greater or equal than SIZE, it means that the
     transformed wide character string did not entirely fit in the
     array WTO.  In this case, only as much of the wide character
     string as actually fits was stored.  To get the whole transformed
     wide character string, call `wcsxfrm' again with a bigger output
     array.

     The transformed wide character string may be longer than the
     original wide character string, and it may also be shorter.

     If SIZE is zero, no characters are stored in TO.  In this case,
     `wcsxfrm' simply returns the number of wide characters that would
     be the length of the transformed wide character string.  This is
     useful for determining what size the allocated array should be
     (remember to multiply with `sizeof (wchar_t)').  It does not
     matter what WTO is if SIZE is zero; WTO may even be a null pointer.

   Here is an example of how you can use `strxfrm' when you plan to do
many comparisons.  It does the same thing as the previous example, but
much faster, because it has to transform each string only once, no
matter how many times it is compared with other strings.  Even the time
needed to allocate and free storage is much less than the time we save,
when there are many strings.

     struct sorter { char *input; char *transformed; };

     /* This is the comparison function used with `qsort'
        to sort an array of `struct sorter'. */

     int
     compare_elements (struct sorter *p1, struct sorter *p2)
     {
       return strcmp (p1->transformed, p2->transformed);
     }

     /* This is the entry point--the function to sort
        strings using the locale's collating sequence. */

     void
     sort_strings_fast (char **array, int nstrings)
     {
       struct sorter temp_array[nstrings];
       int i;

       /* Set up `temp_array'.  Each element contains
          one input string and its transformed string. */
       for (i = 0; i < nstrings; i++)
         {
           size_t length = strlen (array[i]) * 2;
           char *transformed;
           size_t transformed_length;

           temp_array[i].input = array[i];

           /* First try a buffer perhaps big enough.  */
           transformed = (char *) xmalloc (length);

           /* Transform `array[i]'.  */
           transformed_length = strxfrm (transformed, array[i], length);

           /* If the buffer was not large enough, resize it
              and try again.  */
           if (transformed_length >= length)
             {
               /* Allocate the needed space. +1 for terminating
                  `NUL' character.  */
               transformed = (char *) xrealloc (transformed,
                                                transformed_length + 1);

               /* The return value is not interesting because we know
                  how long the transformed string is.  */
               (void) strxfrm (transformed, array[i],
                               transformed_length + 1);
             }

           temp_array[i].transformed = transformed;
         }

       /* Sort `temp_array' by comparing transformed strings. */
       qsort (temp_array, sizeof (struct sorter),
              nstrings, compare_elements);

       /* Put the elements back in the permanent array
          in their sorted order. */
       for (i = 0; i < nstrings; i++)
         array[i] = temp_array[i].input;

       /* Free the strings we allocated. */
       for (i = 0; i < nstrings; i++)
         free (temp_array[i].transformed);
     }

   The interesting part of this code for the wide character version
would look like this:

     void
     sort_strings_fast (wchar_t **array, int nstrings)
     {
       ...
           /* Transform `array[i]'.  */
           transformed_length = wcsxfrm (transformed, array[i], length);

           /* If the buffer was not large enough, resize it
              and try again.  */
           if (transformed_length >= length)
             {
               /* Allocate the needed space. +1 for terminating
                  `NUL' character.  */
               transformed = (wchar_t *) xrealloc (transformed,
                                                   (transformed_length + 1)
                                                   * sizeof (wchar_t));

               /* The return value is not interesting because we know
                  how long the transformed string is.  */
               (void) wcsxfrm (transformed, array[i],
                               transformed_length + 1);
             }
       ...

Note the additional multiplication with `sizeof (wchar_t)' in the
`realloc' call.

   *Compatibility Note:* The string collation functions are a new
feature of ISO C90.  Older C dialects have no equivalent feature.  The
wide character versions were introduced in Amendment 1 to ISO C90.

File: libc.info,  Node: Search Functions,  Next: Finding Tokens in a String,  Prev: Collation Functions,  Up: String and Array Utilities

5.7 Search Functions
====================

This section describes library functions which perform various kinds of
searching operations on strings and arrays.  These functions are
declared in the header file `string.h'.

 -- Function: void * memchr (const void *BLOCK, int C, size_t SIZE)
     This function finds the first occurrence of the byte C (converted
     to an `unsigned char') in the initial SIZE bytes of the object
     beginning at BLOCK.  The return value is a pointer to the located
     byte, or a null pointer if no match was found.

 -- Function: wchar_t * wmemchr (const wchar_t *BLOCK, wchar_t WC,
          size_t SIZE)
     This function finds the first occurrence of the wide character WC
     in the initial SIZE wide characters of the object beginning at
     BLOCK.  The return value is a pointer to the located wide
     character, or a null pointer if no match was found.

 -- Function: void * rawmemchr (const void *BLOCK, int C)
     Often the `memchr' function is used with the knowledge that the
     byte C is available in the memory block specified by the
     parameters.  But this means that the SIZE parameter is not really
     needed and that the tests performed with it at runtime (to check
     whether the end of the block is reached) are not needed.

     The `rawmemchr' function exists for just this situation which is
     surprisingly frequent.  The interface is similar to `memchr' except
     that the SIZE parameter is missing.  The function will look beyond
     the end of the block pointed to by BLOCK in case the programmer
     made an error in assuming that the byte C is present in the block.
     In this case the result is unspecified.  Otherwise the return
     value is a pointer to the located byte.

     This function is of special interest when looking for the end of a
     string.  Since all strings are terminated by a null byte a call
     like

             rawmemchr (str, '\0')

     will never go beyond the end of the string.

     This function is a GNU extension.

 -- Function: void * memrchr (const void *BLOCK, int C, size_t SIZE)
     The function `memrchr' is like `memchr', except that it searches
     backwards from the end of the block defined by BLOCK and SIZE
     (instead of forwards from the front).

     This function is a GNU extension.

 -- Function: char * strchr (const char *STRING, int C)
     The `strchr' function finds the first occurrence of the character
     C (converted to a `char') in the null-terminated string beginning
     at STRING.  The return value is a pointer to the located
     character, or a null pointer if no match was found.

     For example,
          strchr ("hello, world", 'l')
              => "llo, world"
          strchr ("hello, world", '?')
              => NULL

     The terminating null character is considered to be part of the
     string, so you can use this function get a pointer to the end of a
     string by specifying a null character as the value of the C
     argument.  It would be better (but less portable) to use
     `strchrnul' in this case, though.

 -- Function: wchar_t * wcschr (const wchar_t *WSTRING, int WC)
     The `wcschr' function finds the first occurrence of the wide
     character WC in the null-terminated wide character string
     beginning at WSTRING.  The return value is a pointer to the
     located wide character, or a null pointer if no match was found.

     The terminating null character is considered to be part of the wide
     character string, so you can use this function get a pointer to
     the end of a wide character string by specifying a null wude
     character as the value of the WC argument.  It would be better
     (but less portable) to use `wcschrnul' in this case, though.

 -- Function: char * strchrnul (const char *STRING, int C)
     `strchrnul' is the same as `strchr' except that if it does not
     find the character, it returns a pointer to string's terminating
     null character rather than a null pointer.

     This function is a GNU extension.

 -- Function: wchar_t * wcschrnul (const wchar_t *WSTRING, wchar_t WC)
     `wcschrnul' is the same as `wcschr' except that if it does not
     find the wide character, it returns a pointer to wide character
     string's terminating null wide character rather than a null
     pointer.

     This function is a GNU extension.

   One useful, but unusual, use of the `strchr' function is when one
wants to have a pointer pointing to the NUL byte terminating a string.
This is often written in this way:

       s += strlen (s);

This is almost optimal but the addition operation duplicated a bit of
the work already done in the `strlen' function.  A better solution is
this:

       s = strchr (s, '\0');

   There is no restriction on the second parameter of `strchr' so it
could very well also be the NUL character.  Those readers thinking very
hard about this might now point out that the `strchr' function is more
expensive than the `strlen' function since we have two abort criteria.
This is right.  But in the GNU C library the implementation of `strchr'
is optimized in a special way so that `strchr' actually is faster.

 -- Function: char * strrchr (const char *STRING, int C)
     The function `strrchr' is like `strchr', except that it searches
     backwards from the end of the string STRING (instead of forwards
     from the front).

     For example,
          strrchr ("hello, world", 'l')
              => "ld"

 -- Function: wchar_t * wcsrchr (const wchar_t *WSTRING, wchar_t C)
     The function `wcsrchr' is like `wcschr', except that it searches
     backwards from the end of the string WSTRING (instead of forwards
     from the front).

 -- Function: char * strstr (const char *HAYSTACK, const char *NEEDLE)
     This is like `strchr', except that it searches HAYSTACK for a
     substring NEEDLE rather than just a single character.  It returns
     a pointer into the string HAYSTACK that is the first character of
     the substring, or a null pointer if no match was found.  If NEEDLE
     is an empty string, the function returns HAYSTACK.

     For example,
          strstr ("hello, world", "l")
              => "llo, world"
          strstr ("hello, world", "wo")
              => "world"

 -- Function: wchar_t * wcsstr (const wchar_t *HAYSTACK, const wchar_t
          *NEEDLE)
     This is like `wcschr', except that it searches HAYSTACK for a
     substring NEEDLE rather than just a single wide character.  It
     returns a pointer into the string HAYSTACK that is the first wide
     character of the substring, or a null pointer if no match was
     found.  If NEEDLE is an empty string, the function returns
     HAYSTACK.

 -- Function: wchar_t * wcswcs (const wchar_t *HAYSTACK, const wchar_t
          *NEEDLE)
     `wcswcs' is an deprecated alias for `wcsstr'.  This is the name
     originally used in the X/Open Portability Guide before the
     Amendment 1 to ISO C90 was published.

 -- Function: char * strcasestr (const char *HAYSTACK, const char
          *NEEDLE)
     This is like `strstr', except that it ignores case in searching for
     the substring.   Like `strcasecmp', it is locale dependent how
     uppercase and lowercase characters are related.

     For example,
          strcasestr ("hello, world", "L")
              => "llo, world"
          strcasestr ("hello, World", "wo")
              => "World"

 -- Function: void * memmem (const void *HAYSTACK, size_t HAYSTACK-LEN,
          const void *NEEDLE, size_t NEEDLE-LEN)
     This is like `strstr', but NEEDLE and HAYSTACK are byte arrays
     rather than null-terminated strings.  NEEDLE-LEN is the length of
     NEEDLE and HAYSTACK-LEN is the length of HAYSTACK.

     This function is a GNU extension.

 -- Function: size_t strspn (const char *STRING, const char *SKIPSET)
     The `strspn' ("string span") function returns the length of the
     initial substring of STRING that consists entirely of characters
     that are members of the set specified by the string SKIPSET.  The
     order of the characters in SKIPSET is not important.

     For example,
          strspn ("hello, world", "abcdefghijklmnopqrstuvwxyz")
              => 5

     Note that "character" is here used in the sense of byte.  In a
     string using a multibyte character encoding (abstract) character
     consisting of more than one byte are not treated as an entity.
     Each byte is treated separately.  The function is not
     locale-dependent.

 -- Function: size_t wcsspn (const wchar_t *WSTRING, const wchar_t
          *SKIPSET)
     The `wcsspn' ("wide character string span") function returns the
     length of the initial substring of WSTRING that consists entirely
     of wide characters that are members of the set specified by the
     string SKIPSET.  The order of the wide characters in SKIPSET is not
     important.

 -- Function: size_t strcspn (const char *STRING, const char *STOPSET)
     The `strcspn' ("string complement span") function returns the
     length of the initial substring of STRING that consists entirely
     of characters that are _not_ members of the set specified by the
     string STOPSET.  (In other words, it returns the offset of the
     first character in STRING that is a member of the set STOPSET.)

     For example,
          strcspn ("hello, world", " \t\n,.;!?")
              => 5

     Note that "character" is here used in the sense of byte.  In a
     string using a multibyte character encoding (abstract) character
     consisting of more than one byte are not treated as an entity.
     Each byte is treated separately.  The function is not
     locale-dependent.

 -- Function: size_t wcscspn (const wchar_t *WSTRING, const wchar_t
          *STOPSET)
     The `wcscspn' ("wide character string complement span") function
     returns the length of the initial substring of WSTRING that
     consists entirely of wide characters that are _not_ members of the
     set specified by the string STOPSET.  (In other words, it returns
     the offset of the first character in STRING that is a member of
     the set STOPSET.)

 -- Function: char * strpbrk (const char *STRING, const char *STOPSET)
     The `strpbrk' ("string pointer break") function is related to
     `strcspn', except that it returns a pointer to the first character
     in STRING that is a member of the set STOPSET instead of the
     length of the initial substring.  It returns a null pointer if no
     such character from STOPSET is found.

     For example,

          strpbrk ("hello, world", " \t\n,.;!?")
              => ", world"

     Note that "character" is here used in the sense of byte.  In a
     string using a multibyte character encoding (abstract) character
     consisting of more than one byte are not treated as an entity.
     Each byte is treated separately.  The function is not
     locale-dependent.

 -- Function: wchar_t * wcspbrk (const wchar_t *WSTRING, const wchar_t
          *STOPSET)
     The `wcspbrk' ("wide character string pointer break") function is
     related to `wcscspn', except that it returns a pointer to the first
     wide character in WSTRING that is a member of the set STOPSET
     instead of the length of the initial substring.  It returns a null
     pointer if no such character from STOPSET is found.

5.7.1 Compatibility String Search Functions
-------------------------------------------

 -- Function: char * index (const char *STRING, int C)
     `index' is another name for `strchr'; they are exactly the same.
     New code should always use `strchr' since this name is defined in
     ISO C while `index' is a BSD invention which never was available
     on System V derived systems.

 -- Function: char * rindex (const char *STRING, int C)
     `rindex' is another name for `strrchr'; they are exactly the same.
     New code should always use `strrchr' since this name is defined in
     ISO C while `rindex' is a BSD invention which never was available
     on System V derived systems.

File: libc.info,  Node: Finding Tokens in a String,  Next: strfry,  Prev: Search Functions,  Up: String and Array Utilities

5.8 Finding Tokens in a String
==============================

It's fairly common for programs to have a need to do some simple kinds
of lexical analysis and parsing, such as splitting a command string up
into tokens.  You can do this with the `strtok' function, declared in
the header file `string.h'.

 -- Function: char * strtok (char *restrict NEWSTRING, const char
          *restrict DELIMITERS)
     A string can be split into tokens by making a series of calls to
     the function `strtok'.

     The string to be split up is passed as the NEWSTRING argument on
     the first call only.  The `strtok' function uses this to set up
     some internal state information.  Subsequent calls to get
     additional tokens from the same string are indicated by passing a
     null pointer as the NEWSTRING argument.  Calling `strtok' with
     another non-null NEWSTRING argument reinitializes the state
     information.  It is guaranteed that no other library function ever
     calls `strtok' behind your back (which would mess up this internal
     state information).

     The DELIMITERS argument is a string that specifies a set of
     delimiters that may surround the token being extracted.  All the
     initial characters that are members of this set are discarded.
     The first character that is _not_ a member of this set of
     delimiters marks the beginning of the next token.  The end of the
     token is found by looking for the next character that is a member
     of the delimiter set.  This character in the original string
     NEWSTRING is overwritten by a null character, and the pointer to
     the beginning of the token in NEWSTRING is returned.

     On the next call to `strtok', the searching begins at the next
     character beyond the one that marked the end of the previous token.
     Note that the set of delimiters DELIMITERS do not have to be the
     same on every call in a series of calls to `strtok'.

     If the end of the string NEWSTRING is reached, or if the remainder
     of string consists only of delimiter characters, `strtok' returns
     a null pointer.

     Note that "character" is here used in the sense of byte.  In a
     string using a multibyte character encoding (abstract) character
     consisting of more than one byte are not treated as an entity.
     Each byte is treated separately.  The function is not
     locale-dependent.

 -- Function: wchar_t * wcstok (wchar_t *NEWSTRING, const char
          *DELIMITERS)
     A string can be split into tokens by making a series of calls to
     the function `wcstok'.

     The string to be split up is passed as the NEWSTRING argument on
     the first call only.  The `wcstok' function uses this to set up
     some internal state information.  Subsequent calls to get
     additional tokens from the same wide character string are
     indicated by passing a null pointer as the NEWSTRING argument.
     Calling `wcstok' with another non-null NEWSTRING argument
     reinitializes the state information.  It is guaranteed that no
     other library function ever calls `wcstok' behind your back (which
     would mess up this internal state information).

     The DELIMITERS argument is a wide character string that specifies
     a set of delimiters that may surround the token being extracted.
     All the initial wide characters that are members of this set are
     discarded.  The first wide character that is _not_ a member of
     this set of delimiters marks the beginning of the next token.  The
     end of the token is found by looking for the next wide character
     that is a member of the delimiter set.  This wide character in the
     original wide character string NEWSTRING is overwritten by a null
     wide character, and the pointer to the beginning of the token in
     NEWSTRING is returned.

     On the next call to `wcstok', the searching begins at the next
     wide character beyond the one that marked the end of the previous
     token.  Note that the set of delimiters DELIMITERS do not have to
     be the same on every call in a series of calls to `wcstok'.

     If the end of the wide character string NEWSTRING is reached, or
     if the remainder of string consists only of delimiter wide
     characters, `wcstok' returns a null pointer.

     Note that "character" is here used in the sense of byte.  In a
     string using a multibyte character encoding (abstract) character
     consisting of more than one byte are not treated as an entity.
     Each byte is treated separately.  The function is not
     locale-dependent.

   *Warning:* Since `strtok' and `wcstok' alter the string they is
parsing, you should always copy the string to a temporary buffer before
parsing it with `strtok'/`wcstok' (*note Copying and Concatenation::).
If you allow `strtok' or `wcstok' to modify a string that came from
another part of your program, you are asking for trouble; that string
might be used for other purposes after `strtok' or `wcstok' has
modified it, and it would not have the expected value.

   The string that you are operating on might even be a constant.  Then
when `strtok' or `wcstok' tries to modify it, your program will get a
fatal signal for writing in read-only memory.  *Note Program Error
Signals::.  Even if the operation of `strtok' or `wcstok' would not
require a modification of the string (e.g., if there is exactly one
token) the string can (and in the GNU libc case will) be modified.

   This is a special case of a general principle: if a part of a program
does not have as its purpose the modification of a certain data
structure, then it is error-prone to modify the data structure
temporarily.

   The functions `strtok' and `wcstok' are not reentrant.  *Note
Nonreentrancy::, for a discussion of where and why reentrancy is
important.

   Here is a simple example showing the use of `strtok'.

     #include <string.h>
     #include <stddef.h>

     ...

     const char string[] = "words separated by spaces -- and, punctuation!";
     const char delimiters[] = " .,;:!-";
     char *token, *cp;

     ...

     cp = strdupa (string);                /* Make writable copy.  */
     token = strtok (cp, delimiters);      /* token => "words" */
     token = strtok (NULL, delimiters);    /* token => "separated" */
     token = strtok (NULL, delimiters);    /* token => "by" */
     token = strtok (NULL, delimiters);    /* token => "spaces" */
     token = strtok (NULL, delimiters);    /* token => "and" */
     token = strtok (NULL, delimiters);    /* token => "punctuation" */
     token = strtok (NULL, delimiters);    /* token => NULL */

   The GNU C library contains two more functions for tokenizing a string
which overcome the limitation of non-reentrancy.  They are only
available for multibyte character strings.

 -- Function: char * strtok_r (char *NEWSTRING, const char *DELIMITERS,
          char **SAVE_PTR)
     Just like `strtok', this function splits the string into several
     tokens which can be accessed by successive calls to `strtok_r'.
     The difference is that the information about the next token is
     stored in the space pointed to by the third argument, SAVE_PTR,
     which is a pointer to a string pointer.  Calling `strtok_r' with a
     null pointer for NEWSTRING and leaving SAVE_PTR between the calls
     unchanged does the job without hindering reentrancy.

     This function is defined in POSIX.1 and can be found on many
     systems which support multi-threading.

 -- Function: char * strsep (char **STRING_PTR, const char *DELIMITER)
     This function has a similar functionality as `strtok_r' with the
     NEWSTRING argument replaced by the SAVE_PTR argument.  The
     initialization of the moving pointer has to be done by the user.
     Successive calls to `strsep' move the pointer along the tokens
     separated by DELIMITER, returning the address of the next token
     and updating STRING_PTR to point to the beginning of the next
     token.

     One difference between `strsep' and `strtok_r' is that if the
     input string contains more than one character from DELIMITER in a
     row `strsep' returns an empty string for each pair of characters
     from DELIMITER.  This means that a program normally should test
     for `strsep' returning an empty string before processing it.

     This function was introduced in 4.3BSD and therefore is widely
     available.

   Here is how the above example looks like when `strsep' is used.

     #include <string.h>
     #include <stddef.h>

     ...

     const char string[] = "words separated by spaces -- and, punctuation!";
     const char delimiters[] = " .,;:!-";
     char *running;
     char *token;

     ...

     running = strdupa (string);
     token = strsep (&running, delimiters);    /* token => "words" */
     token = strsep (&running, delimiters);    /* token => "separated" */
     token = strsep (&running, delimiters);    /* token => "by" */
     token = strsep (&running, delimiters);    /* token => "spaces" */
     token = strsep (&running, delimiters);    /* token => "" */
     token = strsep (&running, delimiters);    /* token => "" */
     token = strsep (&running, delimiters);    /* token => "" */
     token = strsep (&running, delimiters);    /* token => "and" */
     token = strsep (&running, delimiters);    /* token => "" */
     token = strsep (&running, delimiters);    /* token => "punctuation" */
     token = strsep (&running, delimiters);    /* token => "" */
     token = strsep (&running, delimiters);    /* token => NULL */

 -- Function: char * basename (const char *FILENAME)
     The GNU version of the `basename' function returns the last
     component of the path in FILENAME.  This function is the preferred
     usage, since it does not modify the argument, FILENAME, and
     respects trailing slashes.  The prototype for `basename' can be
     found in `string.h'.  Note, this function is overriden by the XPG
     version, if `libgen.h' is included.

     Example of using GNU `basename':

          #include <string.h>

          int
          main (int argc, char *argv[])
          {
            char *prog = basename (argv[0]);

            if (argc < 2)
              {
                fprintf (stderr, "Usage %s <arg>\n", prog);
                exit (1);
              }

            ...
          }

     *Portability Note:* This function may produce different results on
     different systems.


 -- Function: char * basename (char *PATH)
     This is the standard XPG defined `basename'. It is similar in
     spirit to the GNU version, but may modify the PATH by removing
     trailing '/' characters.  If the PATH is made up entirely of '/'
     characters, then "/" will be returned.  Also, if PATH is `NULL' or
     an empty string, then "." is returned.  The prototype for the XPG
     version can be found in `libgen.h'.

     Example of using XPG `basename':

          #include <libgen.h>

          int
          main (int argc, char *argv[])
          {
            char *prog;
            char *path = strdupa (argv[0]);

            prog = basename (path);

            if (argc < 2)
              {
                fprintf (stderr, "Usage %s <arg>\n", prog);
                exit (1);
              }

            ...

          }

 -- Function: char * dirname (char *PATH)
     The `dirname' function is the compliment to the XPG version of
     `basename'.  It returns the parent directory of the file specified
     by PATH.  If PATH is `NULL', an empty string, or contains no '/'
     characters, then "." is returned.  The prototype for this function
     can be found in `libgen.h'.

File: libc.info,  Node: strfry,  Next: Trivial Encryption,  Prev: Finding Tokens in a String,  Up: String and Array Utilities

5.9 strfry
==========

The function below addresses the perennial programming quandary: "How do
I take good data in string form and painlessly turn it into garbage?"
This is actually a fairly simple task for C programmers who do not use
the GNU C library string functions, but for programs based on the GNU C
library, the `strfry' function is the preferred method for destroying
string data.

   The prototype for this function is in `string.h'.

 -- Function: char * strfry (char *STRING)
     `strfry' creates a pseudorandom anagram of a string, replacing the
     input with the anagram in place.  For each position in the string,
     `strfry' swaps it with a position in the string selected at random
     (from a uniform distribution).  The two positions may be the same.

     The return value of `strfry' is always STRING.

     *Portability Note:*  This function is unique to the GNU C library.


File: libc.info,  Node: Trivial Encryption,  Next: Encode Binary Data,  Prev: strfry,  Up: String and Array Utilities

5.10 Trivial Encryption
=======================

The `memfrob' function converts an array of data to something
unrecognizable and back again.  It is not encryption in its usual sense
since it is easy for someone to convert the encrypted data back to clear
text.  The transformation is analogous to Usenet's "Rot13" encryption
method for obscuring offensive jokes from sensitive eyes and such.
Unlike Rot13, `memfrob' works on arbitrary binary data, not just text.

   For true encryption, *Note Cryptographic Functions::.

   This function is declared in `string.h'.

 -- Function: void * memfrob (void *MEM, size_t LENGTH)
     `memfrob' transforms (frobnicates) each byte of the data structure
     at MEM, which is LENGTH bytes long, by bitwise exclusive oring it
     with binary 00101010.  It does the transformation in place and its
     return value is always MEM.

     Note that `memfrob' a second time on the same data structure
     returns it to its original state.

     This is a good function for hiding information from someone who
     doesn't want to see it or doesn't want to see it very much.  To
     really prevent people from retrieving the information, use
     stronger encryption such as that described in *Note Cryptographic
     Functions::.

     *Portability Note:*  This function is unique to the GNU C library.


File: libc.info,  Node: Encode Binary Data,  Next: Argz and Envz Vectors,  Prev: Trivial Encryption,  Up: String and Array Utilities

5.11 Encode Binary Data
=======================

To store or transfer binary data in environments which only support text
one has to encode the binary data by mapping the input bytes to
characters in the range allowed for storing or transfering.  SVID
systems (and nowadays XPG compliant systems) provide minimal support for
this task.

 -- Function: char * l64a (long int N)
     This function encodes a 32-bit input value using characters from
     the basic character set.  It returns a pointer to a 7 character
     buffer which contains an encoded version of N.  To encode a series
     of bytes the user must copy the returned string to a destination
     buffer.  It returns the empty string if N is zero, which is
     somewhat bizarre but mandated by the standard.
     *Warning:* Since a static buffer is used this function should not
     be used in multi-threaded programs.  There is no thread-safe
     alternative to this function in the C library.
     *Compatibility Note:* The XPG standard states that the return
     value of `l64a' is undefined if N is negative.  In the GNU
     implementation, `l64a' treats its argument as unsigned, so it will
     return a sensible encoding for any nonzero N; however, portable
     programs should not rely on this.

     To encode a large buffer `l64a' must be called in a loop, once for
     each 32-bit word of the buffer.  For example, one could do
     something like this:

          char *
          encode (const void *buf, size_t len)
          {
            /* We know in advance how long the buffer has to be. */
            unsigned char *in = (unsigned char *) buf;
            char *out = malloc (6 + ((len + 3) / 4) * 6 + 1);
            char *cp = out, *p;

            /* Encode the length. */
            /* Using `htonl' is necessary so that the data can be
               decoded even on machines with different byte order.
               `l64a' can return a string shorter than 6 bytes, so
               we pad it with encoding of 0 ('.') at the end by
               hand. */

            p = stpcpy (cp, l64a (htonl (len)));
            cp = mempcpy (p, "......", 6 - (p - cp));

            while (len > 3)
              {
                unsigned long int n = *in++;
                n = (n << 8) | *in++;
                n = (n << 8) | *in++;
                n = (n << 8) | *in++;
                len -= 4;
                p = stpcpy (cp, l64a (htonl (n)));
                cp = mempcpy (p, "......", 6 - (p - cp));
              }
            if (len > 0)
              {
                unsigned long int n = *in++;
                if (--len > 0)
                  {
                    n = (n << 8) | *in++;
                    if (--len > 0)
                      n = (n << 8) | *in;
                  }
                cp = stpcpy (cp, l64a (htonl (n)));
              }
            *cp = '\0';
            return out;
          }

     It is strange that the library does not provide the complete
     functionality needed but so be it.


   To decode data produced with `l64a' the following function should be
used.

 -- Function: long int a64l (const char *STRING)
     The parameter STRING should contain a string which was produced by
     a call to `l64a'.  The function processes at least 6 characters of
     this string, and decodes the characters it finds according to the
     table below.  It stops decoding when it finds a character not in
     the table, rather like `atoi'; if you have a buffer which has been
     broken into lines, you must be careful to skip over the
     end-of-line characters.

     The decoded number is returned as a `long int' value.

   The `l64a' and `a64l' functions use a base 64 encoding, in which
each character of an encoded string represents six bits of an input
word.  These symbols are used for the base 64 digits:

        0     1     2     3     4     5     6     7
0       `.'   `/'   `0'   `1'   `2'   `3'   `4'   `5'
8       `6'   `7'   `8'   `9'   `A'   `B'   `C'   `D'
16      `E'   `F'   `G'   `H'   `I'   `J'   `K'   `L'
24      `M'   `N'   `O'   `P'   `Q'   `R'   `S'   `T'
32      `U'   `V'   `W'   `X'   `Y'   `Z'   `a'   `b'
40      `c'   `d'   `e'   `f'   `g'   `h'   `i'   `j'
48      `k'   `l'   `m'   `n'   `o'   `p'   `q'   `r'
56      `s'   `t'   `u'   `v'   `w'   `x'   `y'   `z'

   This encoding scheme is not standard.  There are some other encoding
methods which are much more widely used (UU encoding, MIME encoding).
Generally, it is better to use one of these encodings.

File: libc.info,  Node: Argz and Envz Vectors,  Prev: Encode Binary Data,  Up: String and Array Utilities

5.12 Argz and Envz Vectors
==========================

"argz vectors" are vectors of strings in a contiguous block of memory,
each element separated from its neighbors by null-characters (`'\0'').

   "Envz vectors" are an extension of argz vectors where each element
is a name-value pair, separated by a `'='' character (as in a Unix
environment).

* Menu:

* Argz Functions::              Operations on argz vectors.
* Envz Functions::              Additional operations on environment vectors.

File: libc.info,  Node: Argz Functions,  Next: Envz Functions,  Up: Argz and Envz Vectors

5.12.1 Argz Functions
---------------------

Each argz vector is represented by a pointer to the first element, of
type `char *', and a size, of type `size_t', both of which can be
initialized to `0' to represent an empty argz vector.  All argz
functions accept either a pointer and a size argument, or pointers to
them, if they will be modified.

   The argz functions use `malloc'/`realloc' to allocate/grow argz
vectors, and so any argz vector creating using these functions may be
freed by using `free'; conversely, any argz function that may grow a
string expects that string to have been allocated using `malloc' (those
argz functions that only examine their arguments or modify them in
place will work on any sort of memory).  *Note Unconstrained
Allocation::.

   All argz functions that do memory allocation have a return type of
`error_t', and return `0' for success, and `ENOMEM' if an allocation
error occurs.

   These functions are declared in the standard include file `argz.h'.

 -- Function: error_t argz_create (char *const ARGV[], char **ARGZ,
          size_t *ARGZ_LEN)
     The `argz_create' function converts the Unix-style argument vector
     ARGV (a vector of pointers to normal C strings, terminated by
     `(char *)0'; *note Program Arguments::) into an argz vector with
     the same elements, which is returned in ARGZ and ARGZ_LEN.

 -- Function: error_t argz_create_sep (const char *STRING, int SEP,
          char **ARGZ, size_t *ARGZ_LEN)
     The `argz_create_sep' function converts the null-terminated string
     STRING into an argz vector (returned in ARGZ and ARGZ_LEN) by
     splitting it into elements at every occurrence of the character
     SEP.

 -- Function: size_t argz_count (const char *ARGZ, size_t ARG_LEN)
     Returns the number of elements in the argz vector ARGZ and
     ARGZ_LEN.

 -- Function: void argz_extract (char *ARGZ, size_t ARGZ_LEN, char
          **ARGV)
     The `argz_extract' function converts the argz vector ARGZ and
     ARGZ_LEN into a Unix-style argument vector stored in ARGV, by
     putting pointers to every element in ARGZ into successive
     positions in ARGV, followed by a terminator of `0'.  ARGV must be
     pre-allocated with enough space to hold all the elements in ARGZ
     plus the terminating `(char *)0' (`(argz_count (ARGZ, ARGZ_LEN) +
     1) * sizeof (char *)' bytes should be enough).  Note that the
     string pointers stored into ARGV point into ARGZ--they are not
     copies--and so ARGZ must be copied if it will be changed while
     ARGV is still active.  This function is useful for passing the
     elements in ARGZ to an exec function (*note Executing a File::).

 -- Function: void argz_stringify (char *ARGZ, size_t LEN, int SEP)
     The `argz_stringify' converts ARGZ into a normal string with the
     elements separated by the character SEP, by replacing each `'\0''
     inside ARGZ (except the last one, which terminates the string)
     with SEP.  This is handy for printing ARGZ in a readable manner.

 -- Function: error_t argz_add (char **ARGZ, size_t *ARGZ_LEN, const
          char *STR)
     The `argz_add' function adds the string STR to the end of the argz
     vector `*ARGZ', and updates `*ARGZ' and `*ARGZ_LEN' accordingly.

 -- Function: error_t argz_add_sep (char **ARGZ, size_t *ARGZ_LEN,
          const char *STR, int DELIM)
     The `argz_add_sep' function is similar to `argz_add', but STR is
     split into separate elements in the result at occurrences of the
     character DELIM.  This is useful, for instance, for adding the
     components of a Unix search path to an argz vector, by using a
     value of `':'' for DELIM.

 -- Function: error_t argz_append (char **ARGZ, size_t *ARGZ_LEN, const
          char *BUF, size_t BUF_LEN)
     The `argz_append' function appends BUF_LEN bytes starting at BUF
     to the argz vector `*ARGZ', reallocating `*ARGZ' to accommodate
     it, and adding BUF_LEN to `*ARGZ_LEN'.

 -- Function: void argz_delete (char **ARGZ, size_t *ARGZ_LEN, char
          *ENTRY)
     If ENTRY points to the beginning of one of the elements in the
     argz vector `*ARGZ', the `argz_delete' function will remove this
     entry and reallocate `*ARGZ', modifying `*ARGZ' and `*ARGZ_LEN'
     accordingly.  Note that as destructive argz functions usually
     reallocate their argz argument, pointers into argz vectors such as
     ENTRY will then become invalid.

 -- Function: error_t argz_insert (char **ARGZ, size_t *ARGZ_LEN, char
          *BEFORE, const char *ENTRY)
     The `argz_insert' function inserts the string ENTRY into the argz
     vector `*ARGZ' at a point just before the existing element pointed
     to by BEFORE, reallocating `*ARGZ' and updating `*ARGZ' and
     `*ARGZ_LEN'.  If BEFORE is `0', ENTRY is added to the end instead
     (as if by `argz_add').  Since the first element is in fact the
     same as `*ARGZ', passing in `*ARGZ' as the value of BEFORE will
     result in ENTRY being inserted at the beginning.

 -- Function: char * argz_next (char *ARGZ, size_t ARGZ_LEN, const char
          *ENTRY)
     The `argz_next' function provides a convenient way of iterating
     over the elements in the argz vector ARGZ.  It returns a pointer
     to the next element in ARGZ after the element ENTRY, or `0' if
     there are no elements following ENTRY.  If ENTRY is `0', the first
     element of ARGZ is returned.

     This behavior suggests two styles of iteration:

              char *entry = 0;
              while ((entry = argz_next (ARGZ, ARGZ_LEN, entry)))
                ACTION;

     (the double parentheses are necessary to make some C compilers
     shut up about what they consider a questionable `while'-test) and:

              char *entry;
              for (entry = ARGZ;
                   entry;
                   entry = argz_next (ARGZ, ARGZ_LEN, entry))
                ACTION;

     Note that the latter depends on ARGZ having a value of `0' if it
     is empty (rather than a pointer to an empty block of memory); this
     invariant is maintained for argz vectors created by the functions
     here.

 -- Function: error_t argz_replace (char **ARGZ, size_t *ARGZ_LEN,
          const char *STR, const char *WITH, unsigned *REPLACE_COUNT)
     Replace any occurrences of the string STR in ARGZ with WITH,
     reallocating ARGZ as necessary.  If REPLACE_COUNT is non-zero,
     `*REPLACE_COUNT' will be incremented by number of replacements
     performed.

File: libc.info,  Node: Envz Functions,  Prev: Argz Functions,  Up: Argz and Envz Vectors

5.12.2 Envz Functions
---------------------

Envz vectors are just argz vectors with additional constraints on the
form of each element; as such, argz functions can also be used on them,
where it makes sense.

   Each element in an envz vector is a name-value pair, separated by a
`'='' character; if multiple `'='' characters are present in an
element, those after the first are considered part of the value, and
treated like all other non-`'\0'' characters.

   If _no_ `'='' characters are present in an element, that element is
considered the name of a "null" entry, as distinct from an entry with an
empty value: `envz_get' will return `0' if given the name of null
entry, whereas an entry with an empty value would result in a value of
`""'; `envz_entry' will still find such entries, however.  Null entries
can be removed with `envz_strip' function.

   As with argz functions, envz functions that may allocate memory (and
thus fail) have a return type of `error_t', and return either `0' or
`ENOMEM'.

   These functions are declared in the standard include file `envz.h'.

 -- Function: char * envz_entry (const char *ENVZ, size_t ENVZ_LEN,
          const char *NAME)
     The `envz_entry' function finds the entry in ENVZ with the name
     NAME, and returns a pointer to the whole entry--that is, the argz
     element which begins with NAME followed by a `'='' character.  If
     there is no entry with that name, `0' is returned.

 -- Function: char * envz_get (const char *ENVZ, size_t ENVZ_LEN, const
          char *NAME)
     The `envz_get' function finds the entry in ENVZ with the name NAME
     (like `envz_entry'), and returns a pointer to the value portion of
     that entry (following the `'='').  If there is no entry with that
     name (or only a null entry), `0' is returned.

 -- Function: error_t envz_add (char **ENVZ, size_t *ENVZ_LEN, const
          char *NAME, const char *VALUE)
     The `envz_add' function adds an entry to `*ENVZ' (updating `*ENVZ'
     and `*ENVZ_LEN') with the name NAME, and value VALUE.  If an entry
     with the same name already exists in ENVZ, it is removed first.
     If VALUE is `0', then the new entry will the special null type of
     entry (mentioned above).

 -- Function: error_t envz_merge (char **ENVZ, size_t *ENVZ_LEN, const
          char *ENVZ2, size_t ENVZ2_LEN, int OVERRIDE)
     The `envz_merge' function adds each entry in ENVZ2 to ENVZ, as if
     with `envz_add', updating `*ENVZ' and `*ENVZ_LEN'.  If OVERRIDE is
     true, then values in ENVZ2 will supersede those with the same name
     in ENVZ, otherwise not.

     Null entries are treated just like other entries in this respect,
     so a null entry in ENVZ can prevent an entry of the same name in
     ENVZ2 from being added to ENVZ, if OVERRIDE is false.

 -- Function: void envz_strip (char **ENVZ, size_t *ENVZ_LEN)
     The `envz_strip' function removes any null entries from ENVZ,
     updating `*ENVZ' and `*ENVZ_LEN'.

File: libc.info,  Node: Character Set Handling,  Next: Locales,  Prev: String and Array Utilities,  Up: Top

6 Character Set Handling
************************

Character sets used in the early days of computing had only six, seven,
or eight bits for each character: there was never a case where more than
eight bits (one byte) were used to represent a single character.  The
limitations of this approach became more apparent as more people
grappled with non-Roman character sets, where not all the characters
that make up a language's character set can be represented by 2^8
choices.  This chapter shows the functionality that was added to the C
library to support multiple character sets.

* Menu:

* Extended Char Intro::              Introduction to Extended Characters.
* Charset Function Overview::        Overview about Character Handling
                                      Functions.
* Restartable multibyte conversion:: Restartable multibyte conversion
                                      Functions.
* Non-reentrant Conversion::         Non-reentrant Conversion Function.
* Generic Charset Conversion::       Generic Charset Conversion.

File: libc.info,  Node: Extended Char Intro,  Next: Charset Function Overview,  Up: Character Set Handling

6.1 Introduction to Extended Characters
=======================================

A variety of solutions is available to overcome the differences between
character sets with a 1:1 relation between bytes and characters and
character sets with ratios of 2:1 or 4:1.  The remainder of this
section gives a few examples to help understand the design decisions
made while developing the functionality of the C library.

   A distinction we have to make right away is between internal and
external representation.  "Internal representation" means the
representation used by a program while keeping the text in memory.
External representations are used when text is stored or transmitted
through some communication channel.  Examples of external
representations include files waiting in a directory to be read and
parsed.

   Traditionally there has been no difference between the two
representations.  It was equally comfortable and useful to use the same
single-byte representation internally and externally.  This comfort
level decreases with more and larger character sets.

   One of the problems to overcome with the internal representation is
handling text that is externally encoded using different character
sets.  Assume a program that reads two texts and compares them using
some metric.  The comparison can be usefully done only if the texts are
internally kept in a common format.

   For such a common format (= character set) eight bits are certainly
no longer enough.  So the smallest entity will have to grow: "wide
characters" will now be used.  Instead of one byte per character, two or
four will be used instead.  (Three are not good to address in memory and
more than four bytes seem not to be necessary).

   As shown in some other part of this manual, a completely new family
has been created of functions that can handle wide character texts in
memory.  The most commonly used character sets for such internal wide
character representations are Unicode and ISO 10646 (also known as UCS
for Universal Character Set).  Unicode was originally planned as a
16-bit character set; whereas, ISO 10646 was designed to be a 31-bit
large code space.  The two standards are practically identical.  They
have the same character repertoire and code table, but Unicode specifies
added semantics.  At the moment, only characters in the first `0x10000'
code positions (the so-called Basic Multilingual Plane, BMP) have been
assigned, but the assignment of more specialized characters outside this
16-bit space is already in progress.  A number of encodings have been
defined for Unicode and ISO 10646 characters: UCS-2 is a 16-bit word
that can only represent characters from the BMP, UCS-4 is a 32-bit word
than can represent any Unicode and ISO 10646 character, UTF-8 is an
ASCII compatible encoding where ASCII characters are represented by
ASCII bytes and non-ASCII characters by sequences of 2-6 non-ASCII
bytes, and finally UTF-16 is an extension of UCS-2 in which pairs of
certain UCS-2 words can be used to encode non-BMP characters up to
`0x10ffff'.

   To represent wide characters the `char' type is not suitable.  For
this reason the ISO C standard introduces a new type that is designed
to keep one character of a wide character string.  To maintain the
similarity there is also a type corresponding to `int' for those
functions that take a single wide character.

 -- Data type: wchar_t
     This data type is used as the base type for wide character strings.
     In other words, arrays of objects of this type are the equivalent
     of `char[]' for multibyte character strings.  The type is defined
     in `stddef.h'.

     The ISO C90 standard, where `wchar_t' was introduced, does not say
     anything specific about the representation.  It only requires that
     this type is capable of storing all elements of the basic
     character set.  Therefore it would be legitimate to define
     `wchar_t' as `char', which might make sense for embedded systems.

     But for GNU systems `wchar_t' is always 32 bits wide and,
     therefore, capable of representing all UCS-4 values and,
     therefore, covering all of ISO 10646.  Some Unix systems define
     `wchar_t' as a 16-bit type and thereby follow Unicode very
     strictly.  This definition is perfectly fine with the standard,
     but it also means that to represent all characters from Unicode
     and ISO 10646 one has to use UTF-16 surrogate characters, which is
     in fact a multi-wide-character encoding.  But resorting to
     multi-wide-character encoding contradicts the purpose of the
     `wchar_t' type.

 -- Data type: wint_t
     `wint_t' is a data type used for parameters and variables that
     contain a single wide character.  As the name suggests this type
     is the equivalent of `int' when using the normal `char' strings.
     The types `wchar_t' and `wint_t' often have the same
     representation if their size is 32 bits wide but if `wchar_t' is
     defined as `char' the type `wint_t' must be defined as `int' due
     to the parameter promotion.

     This type is defined in `wchar.h' and was introduced in
     Amendment 1 to ISO C90.

   As there are for the `char' data type macros are available for
specifying the minimum and maximum value representable in an object of
type `wchar_t'.

 -- Macro: wint_t WCHAR_MIN
     The macro `WCHAR_MIN' evaluates to the minimum value representable
     by an object of type `wint_t'.

     This macro was introduced in Amendment 1 to ISO C90.

 -- Macro: wint_t WCHAR_MAX
     The macro `WCHAR_MAX' evaluates to the maximum value representable
     by an object of type `wint_t'.

     This macro was introduced in Amendment 1 to ISO C90.

   Another special wide character value is the equivalent to `EOF'.

 -- Macro: wint_t WEOF
     The macro `WEOF' evaluates to a constant expression of type
     `wint_t' whose value is different from any member of the extended
     character set.

     `WEOF' need not be the same value as `EOF' and unlike `EOF' it
     also need _not_ be negative.  In other words, sloppy code like

          {
            int c;
            ...
            while ((c = getc (fp)) < 0)
              ...
          }

     has to be rewritten to use `WEOF' explicitly when wide characters
     are used:

          {
            wint_t c;
            ...
            while ((c = wgetc (fp)) != WEOF)
              ...
          }

     This macro was introduced in Amendment 1 to ISO C90 and is defined
     in `wchar.h'.

   These internal representations present problems when it comes to
storing and transmittal.  Because each single wide character consists
of more than one byte, they are effected by byte-ordering.  Thus,
machines with different endianesses would see different values when
accessing the same data.  This byte ordering concern also applies for
communication protocols that are all byte-based and therefore require
that the sender has to decide about splitting the wide character in
bytes.  A last (but not least important) point is that wide characters
often require more storage space than a customized byte-oriented
character set.

   For all the above reasons, an external encoding that is different
from the internal encoding is often used if the latter is UCS-2 or
UCS-4.  The external encoding is byte-based and can be chosen
appropriately for the environment and for the texts to be handled.  A
variety of different character sets can be used for this external
encoding (information that will not be exhaustively presented
here-instead, a description of the major groups will suffice).  All of
the ASCII-based character sets fulfill one requirement: they are
"filesystem safe."  This means that the character `'/'' is used in the
encoding _only_ to represent itself.  Things are a bit different for
character sets like EBCDIC (Extended Binary Coded Decimal Interchange
Code, a character set family used by IBM), but if the operation system
does not understand EBCDIC directly the parameters-to-system calls have
to be converted first anyhow.

   * The simplest character sets are single-byte character sets.  There
     can be only up to 256 characters (for 8 bit character sets), which
     is not sufficient to cover all languages but might be sufficient
     to handle a specific text.  Handling of a 8 bit character sets is
     simple.  This is not true for other kinds presented later, and
     therefore, the application one uses might require the use of 8 bit
     character sets.

   * The ISO 2022 standard defines a mechanism for extended character
     sets where one character _can_ be represented by more than one
     byte.  This is achieved by associating a state with the text.
     Characters that can be used to change the state can be embedded in
     the text.  Each byte in the text might have a different
     interpretation in each state.  The state might even influence
     whether a given byte stands for a character on its own or whether
     it has to be combined with some more bytes.

     In most uses of ISO 2022 the defined character sets do not allow
     state changes that cover more than the next character.  This has
     the big advantage that whenever one can identify the beginning of
     the byte sequence of a character one can interpret a text
     correctly.  Examples of character sets using this policy are the
     various EUC character sets (used by Sun's operations systems,
     EUC-JP, EUC-KR, EUC-TW, and EUC-CN) or Shift_JIS (SJIS, a Japanese
     encoding).

     But there are also character sets using a state that is valid for
     more than one character and has to be changed by another byte
     sequence.  Examples for this are ISO-2022-JP, ISO-2022-KR, and
     ISO-2022-CN.

   * Early attempts to fix 8 bit character sets for other languages
     using the Roman alphabet lead to character sets like ISO 6937.
     Here bytes representing characters like the acute accent do not
     produce output themselves: one has to combine them with other
     characters to get the desired result.  For example, the byte
     sequence `0xc2 0x61' (non-spacing acute accent, followed by
     lower-case `a') to get the "small a with  acute" character.  To
     get the acute accent character on its own, one has to write `0xc2
     0x20' (the non-spacing acute followed by a space).

     Character sets like ISO 6937 are used in some embedded systems such
     as teletex.

   * Instead of converting the Unicode or ISO 10646 text used
     internally, it is often also sufficient to simply use an encoding
     different than UCS-2/UCS-4.  The Unicode and ISO 10646 standards
     even specify such an encoding: UTF-8.  This encoding is able to
     represent all of ISO 10646 31 bits in a byte string of length one
     to six.

     There were a few other attempts to encode ISO 10646 such as UTF-7,
     but UTF-8 is today the only encoding that should be used.  In
     fact, with any luck UTF-8 will soon be the only external encoding
     that has to be supported.  It proves to be universally usable and
     its only disadvantage is that it favors Roman languages by making
     the byte string representation of other scripts (Cyrillic, Greek,
     Asian scripts) longer than necessary if using a specific character
     set for these scripts.  Methods like the Unicode compression
     scheme can alleviate these problems.

   The question remaining is: how to select the character set or
encoding to use.  The answer: you cannot decide about it yourself, it
is decided by the developers of the system or the majority of the
users.  Since the goal is interoperability one has to use whatever the
other people one works with use.  If there are no constraints, the
selection is based on the requirements the expected circle of users
will have.  In other words, if a project is expected to be used in
only, say, Russia it is fine to use KOI8-R or a similar character set.
But if at the same time people from, say, Greece are participating one
should use a character set that allows all people to collaborate.

   The most widely useful solution seems to be: go with the most general
character set, namely ISO 10646.  Use UTF-8 as the external encoding
and problems about users not being able to use their own language
adequately are a thing of the past.

   One final comment about the choice of the wide character
representation is necessary at this point.  We have said above that the
natural choice is using Unicode or ISO 10646.  This is not required,
but at least encouraged, by the ISO C standard.  The standard defines
at least a macro `__STDC_ISO_10646__' that is only defined on systems
where the `wchar_t' type encodes ISO 10646 characters.  If this symbol
is not defined one should avoid making assumptions about the wide
character representation.  If the programmer uses only the functions
provided by the C library to handle wide character strings there should
be no compatibility problems with other systems.

File: libc.info,  Node: Charset Function Overview,  Next: Restartable multibyte conversion,  Prev: Extended Char Intro,  Up: Character Set Handling

6.2 Overview about Character Handling Functions
===============================================

A Unix C library contains three different sets of functions in two
families to handle character set conversion.  One of the function
families (the most commonly used) is specified in the ISO C90 standard
and, therefore, is portable even beyond the Unix world.  Unfortunately
this family is the least useful one.  These functions should be avoided
whenever possible, especially when developing libraries (as opposed to
applications).

   The second family of functions got introduced in the early Unix
standards (XPG2) and is still part of the latest and greatest Unix
standard: Unix 98.  It is also the most powerful and useful set of
functions.  But we will start with the functions defined in Amendment 1
to ISO C90.

File: libc.info,  Node: Restartable multibyte conversion,  Next: Non-reentrant Conversion,  Prev: Charset Function Overview,  Up: Character Set Handling

6.3 Restartable Multibyte Conversion Functions
==============================================

The ISO C standard defines functions to convert strings from a
multibyte representation to wide character strings.  There are a number
of peculiarities:

   * The character set assumed for the multibyte encoding is not
     specified as an argument to the functions.  Instead the character
     set specified by the `LC_CTYPE' category of the current locale is
     used; see *note Locale Categories::.

   * The functions handling more than one character at a time require
     NUL terminated strings as the argument (i.e., converting blocks of
     text does not work unless one can add a NUL byte at an appropriate
     place).  The GNU C library contains some extensions to the
     standard that allow specifying a size, but basically they also
     expect terminated strings.

   Despite these limitations the ISO C functions can be used in many
contexts.  In graphical user interfaces, for instance, it is not
uncommon to have functions that require text to be displayed in a wide
character string if the text is not simple ASCII.  The text itself might
come from a file with translations and the user should decide about the
current locale, which determines the translation and therefore also the
external encoding used.  In such a situation (and many others) the
functions described here are perfect.  If more freedom while performing
the conversion is necessary take a look at the `iconv' functions (*note
Generic Charset Conversion::).

* Menu:

* Selecting the Conversion::     Selecting the conversion and its properties.
* Keeping the state::            Representing the state of the conversion.
* Converting a Character::       Converting Single Characters.
* Converting Strings::           Converting Multibyte and Wide Character
                                  Strings.
* Multibyte Conversion Example:: A Complete Multibyte Conversion Example.

File: libc.info,  Node: Selecting the Conversion,  Next: Keeping the state,  Up: Restartable multibyte conversion

6.3.1 Selecting the conversion and its properties
-------------------------------------------------

We already said above that the currently selected locale for the
`LC_CTYPE' category decides about the conversion that is performed by
the functions we are about to describe.  Each locale uses its own
character set (given as an argument to `localedef') and this is the one
assumed as the external multibyte encoding.  The wide character set is
always UCS-4, at least on GNU systems.

   A characteristic of each multibyte character set is the maximum
number of bytes that can be necessary to represent one character.  This
information is quite important when writing code that uses the
conversion functions (as shown in the examples below).  The ISO C
standard defines two macros that provide this information.

 -- Macro: int MB_LEN_MAX
     `MB_LEN_MAX' specifies the maximum number of bytes in the multibyte
     sequence for a single character in any of the supported locales.
     It is a compile-time constant and is defined in `limits.h'.

 -- Macro: int MB_CUR_MAX
     `MB_CUR_MAX' expands into a positive integer expression that is the
     maximum number of bytes in a multibyte character in the current
     locale.  The value is never greater than `MB_LEN_MAX'.  Unlike
     `MB_LEN_MAX' this macro need not be a compile-time constant, and in
     the GNU C library it is not.

     `MB_CUR_MAX' is defined in `stdlib.h'.

   Two different macros are necessary since strictly ISO C90 compilers
do not allow variable length array definitions, but still it is
desirable to avoid dynamic allocation.  This incomplete piece of code
shows the problem:

     {
       char buf[MB_LEN_MAX];
       ssize_t len = 0;

       while (! feof (fp))
         {
           fread (&buf[len], 1, MB_CUR_MAX - len, fp);
           /* ... process buf */
           len -= used;
         }
     }

   The code in the inner loop is expected to have always enough bytes in
the array BUF to convert one multibyte character.  The array BUF has to
be sized statically since many compilers do not allow a variable size.
The `fread' call makes sure that `MB_CUR_MAX' bytes are always
available in BUF.  Note that it isn't a problem if `MB_CUR_MAX' is not
a compile-time constant.

File: libc.info,  Node: Keeping the state,  Next: Converting a Character,  Prev: Selecting the Conversion,  Up: Restartable multibyte conversion

6.3.2 Representing the state of the conversion
----------------------------------------------

In the introduction of this chapter it was said that certain character
sets use a "stateful" encoding.  That is, the encoded values depend in
some way on the previous bytes in the text.

   Since the conversion functions allow converting a text in more than
one step we must have a way to pass this information from one call of
the functions to another.

 -- Data type: mbstate_t
     A variable of type `mbstate_t' can contain all the information
     about the "shift state" needed from one call to a conversion
     function to another.

     `mbstate_t' is defined in `wchar.h'.  It was introduced in
     Amendment 1 to ISO C90.

   To use objects of type `mbstate_t' the programmer has to define such
objects (normally as local variables on the stack) and pass a pointer to
the object to the conversion functions.  This way the conversion
function can update the object if the current multibyte character set
is stateful.

   There is no specific function or initializer to put the state object
in any specific state.  The rules are that the object should always
represent the initial state before the first use, and this is achieved
by clearing the whole variable with code such as follows:

     {
       mbstate_t state;
       memset (&state, '\0', sizeof (state));
       /* from now on STATE can be used.  */
       ...
     }

   When using the conversion functions to generate output it is often
necessary to test whether the current state corresponds to the initial
state.  This is necessary, for example, to decide whether to emit
escape sequences to set the state to the initial state at certain
sequence points.  Communication protocols often require this.

 -- Function: int mbsinit (const mbstate_t *PS)
     The `mbsinit' function determines whether the state object pointed
     to by PS is in the initial state.  If PS is a null pointer or the
     object is in the initial state the return value is nonzero.
     Otherwise it is zero.

     `mbsinit' was introduced in Amendment 1 to ISO C90 and is declared
     in `wchar.h'.

   Code using `mbsinit' often looks similar to this:

     {
       mbstate_t state;
       memset (&state, '\0', sizeof (state));
       /* Use STATE.  */
       ...
       if (! mbsinit (&state))
         {
           /* Emit code to return to initial state.  */
           const wchar_t empty[] = L"";
           const wchar_t *srcp = empty;
           wcsrtombs (outbuf, &srcp, outbuflen, &state);
         }
       ...
     }

   The code to emit the escape sequence to get back to the initial
state is interesting.  The `wcsrtombs' function can be used to
determine the necessary output code (*note Converting Strings::).
Please note that on GNU systems it is not necessary to perform this
extra action for the conversion from multibyte text to wide character
text since the wide character encoding is not stateful.  But there is
nothing mentioned in any standard that prohibits making `wchar_t' using
a stateful encoding.

File: libc.info,  Node: Converting a Character,  Next: Converting Strings,  Prev: Keeping the state,  Up: Restartable multibyte conversion

6.3.3 Converting Single Characters
----------------------------------

The most fundamental of the conversion functions are those dealing with
single characters.  Please note that this does not always mean single
bytes.  But since there is very often a subset of the multibyte
character set that consists of single byte sequences, there are
functions to help with converting bytes.  Frequently, ASCII is a subpart
of the multibyte character set.  In such a scenario, each ASCII
character stands for itself, and all other characters have at least a
first byte that is beyond the range 0 to 127.

 -- Function: wint_t btowc (int C)
     The `btowc' function ("byte to wide character") converts a valid
     single byte character C in the initial shift state into the wide
     character equivalent using the conversion rules from the currently
     selected locale of the `LC_CTYPE' category.

     If `(unsigned char) C' is no valid single byte multibyte character
     or if C is `EOF', the function returns `WEOF'.

     Please note the restriction of C being tested for validity only in
     the initial shift state.  No `mbstate_t' object is used from which
     the state information is taken, and the function also does not use
     any static state.

     The `btowc' function was introduced in Amendment 1 to ISO C90 and
     is declared in `wchar.h'.

   Despite the limitation that the single byte value is always
interpreted in the initial state, this function is actually useful most
of the time.  Most characters are either entirely single-byte character
sets or they are extension to ASCII.  But then it is possible to write
code like this (not that this specific example is very useful):

     wchar_t *
     itow (unsigned long int val)
     {
       static wchar_t buf[30];
       wchar_t *wcp = &buf[29];
       *wcp = L'\0';
       while (val != 0)
         {
           *--wcp = btowc ('0' + val % 10);
           val /= 10;
         }
       if (wcp == &buf[29])
         *--wcp = L'0';
       return wcp;
     }

   Why is it necessary to use such a complicated implementation and not
simply cast `'0' + val % 10' to a wide character?  The answer is that
there is no guarantee that one can perform this kind of arithmetic on
the character of the character set used for `wchar_t' representation.
In other situations the bytes are not constant at compile time and so
the compiler cannot do the work.  In situations like this, using
`btowc' is required.

There is also a function for the conversion in the other direction.

 -- Function: int wctob (wint_t C)
     The `wctob' function ("wide character to byte") takes as the
     parameter a valid wide character.  If the multibyte representation
     for this character in the initial state is exactly one byte long,
     the return value of this function is this character.  Otherwise
     the return value is `EOF'.

     `wctob' was introduced in Amendment 1 to ISO C90 and is declared
     in `wchar.h'.

   There are more general functions to convert single character from
multibyte representation to wide characters and vice versa.  These
functions pose no limit on the length of the multibyte representation
and they also do not require it to be in the initial state.

 -- Function: size_t mbrtowc (wchar_t *restrict PWC, const char
          *restrict S, size_t N, mbstate_t *restrict PS)
     The `mbrtowc' function ("multibyte restartable to wide character")
     converts the next multibyte character in the string pointed to by
     S into a wide character and stores it in the wide character string
     pointed to by PWC.  The conversion is performed according to the
     locale currently selected for the `LC_CTYPE' category.  If the
     conversion for the character set used in the locale requires a
     state, the multibyte string is interpreted in the state
     represented by the object pointed to by PS.  If PS is a null
     pointer, a static, internal state variable used only by the
     `mbrtowc' function is used.

     If the next multibyte character corresponds to the NUL wide
     character, the return value of the function is 0 and the state
     object is afterwards in the initial state.  If the next N or fewer
     bytes form a correct multibyte character, the return value is the
     number of bytes starting from S that form the multibyte character.
     The conversion state is updated according to the bytes consumed in
     the conversion.  In both cases the wide character (either the
     `L'\0'' or the one found in the conversion) is stored in the
     string pointed to by PWC if PWC is not null.

     If the first N bytes of the multibyte string possibly form a valid
     multibyte character but there are more than N bytes needed to
     complete it, the return value of the function is `(size_t) -2' and
     no value is stored.  Please note that this can happen even if N
     has a value greater than or equal to `MB_CUR_MAX' since the input
     might contain redundant shift sequences.

     If the first `n' bytes of the multibyte string cannot possibly form
     a valid multibyte character, no value is stored, the global
     variable `errno' is set to the value `EILSEQ', and the function
     returns `(size_t) -1'.  The conversion state is afterwards
     undefined.

     `mbrtowc' was introduced in Amendment 1 to ISO C90 and is declared
     in `wchar.h'.

   Use of `mbrtowc' is straightforward.  A function that copies a
multibyte string into a wide character string while at the same time
converting all lowercase characters into uppercase could look like this
(this is not the final version, just an example; it has no error
checking, and sometimes leaks memory):

     wchar_t *
     mbstouwcs (const char *s)
     {
       size_t len = strlen (s);
       wchar_t *result = malloc ((len + 1) * sizeof (wchar_t));
       wchar_t *wcp = result;
       wchar_t tmp[1];
       mbstate_t state;
       size_t nbytes;

       memset (&state, '\0', sizeof (state));
       while ((nbytes = mbrtowc (tmp, s, len, &state)) > 0)
         {
           if (nbytes >= (size_t) -2)
             /* Invalid input string.  */
             return NULL;
           *wcp++ = towupper (tmp[0]);
           len -= nbytes;
           s += nbytes;
         }
       return result;
     }

   The use of `mbrtowc' should be clear.  A single wide character is
stored in `TMP[0]', and the number of consumed bytes is stored in the
variable NBYTES.  If the conversion is successful, the uppercase
variant of the wide character is stored in the RESULT array and the
pointer to the input string and the number of available bytes is
adjusted.

   The only non-obvious thing about `mbrtowc' might be the way memory
is allocated for the result.  The above code uses the fact that there
can never be more wide characters in the converted results than there
are bytes in the multibyte input string.  This method yields a
pessimistic guess about the size of the result, and if many wide
character strings have to be constructed this way or if the strings are
long, the extra memory required to be allocated because the input
string contains multibyte characters might be significant.  The
allocated memory block can be resized to the correct size before
returning it, but a better solution might be to allocate just the right
amount of space for the result right away.  Unfortunately there is no
function to compute the length of the wide character string directly
from the multibyte string.  There is, however, a function that does
part of the work.

 -- Function: size_t mbrlen (const char *restrict S, size_t N,
          mbstate_t *PS)
     The `mbrlen' function ("multibyte restartable length") computes
     the number of at most N bytes starting at S, which form the next
     valid and complete multibyte character.

     If the next multibyte character corresponds to the NUL wide
     character, the return value is 0.  If the next N bytes form a valid
     multibyte character, the number of bytes belonging to this
     multibyte character byte sequence is returned.

     If the first N bytes possibly form a valid multibyte character but
     the character is incomplete, the return value is `(size_t) -2'.
     Otherwise the multibyte character sequence is invalid and the
     return value is `(size_t) -1'.

     The multibyte sequence is interpreted in the state represented by
     the object pointed to by PS.  If PS is a null pointer, a state
     object local to `mbrlen' is used.

     `mbrlen' was introduced in Amendment 1 to ISO C90 and is declared
     in `wchar.h'.

   The attentive reader now will note that `mbrlen' can be implemented
as

     mbrtowc (NULL, s, n, ps != NULL ? ps : &internal)

   This is true and in fact is mentioned in the official specification.
How can this function be used to determine the length of the wide
character string created from a multibyte character string?  It is not
directly usable, but we can define a function `mbslen' using it:

     size_t
     mbslen (const char *s)
     {
       mbstate_t state;
       size_t result = 0;
       size_t nbytes;
       memset (&state, '\0', sizeof (state));
       while ((nbytes = mbrlen (s, MB_LEN_MAX, &state)) > 0)
         {
           if (nbytes >= (size_t) -2)
             /* Something is wrong.  */
             return (size_t) -1;
           s += nbytes;
           ++result;
         }
       return result;
     }

   This function simply calls `mbrlen' for each multibyte character in
the string and counts the number of function calls.  Please note that
we here use `MB_LEN_MAX' as the size argument in the `mbrlen' call.
This is acceptable since a) this value is larger then the length of the
longest multibyte character sequence and b) we know that the string S
ends with a NUL byte, which cannot be part of any other multibyte
character sequence but the one representing the NUL wide character.
Therefore, the `mbrlen' function will never read invalid memory.

   Now that this function is available (just to make this clear, this
function is _not_ part of the GNU C library) we can compute the number
of wide character required to store the converted multibyte character
string S using

     wcs_bytes = (mbslen (s) + 1) * sizeof (wchar_t);

   Please note that the `mbslen' function is quite inefficient.  The
implementation of `mbstouwcs' with `mbslen' would have to perform the
conversion of the multibyte character input string twice, and this
conversion might be quite expensive.  So it is necessary to think about
the consequences of using the easier but imprecise method before doing
the work twice.

 -- Function: size_t wcrtomb (char *restrict S, wchar_t WC, mbstate_t
          *restrict PS)
     The `wcrtomb' function ("wide character restartable to multibyte")
     converts a single wide character into a multibyte string
     corresponding to that wide character.

     If S is a null pointer, the function resets the state stored in
     the objects pointed to by PS (or the internal `mbstate_t' object)
     to the initial state.  This can also be achieved by a call like
     this:

          wcrtombs (temp_buf, L'\0', ps)

     since, if S is a null pointer, `wcrtomb' performs as if it writes
     into an internal buffer, which is guaranteed to be large enough.

     If WC is the NUL wide character, `wcrtomb' emits, if necessary, a
     shift sequence to get the state PS into the initial state followed
     by a single NUL byte, which is stored in the string S.

     Otherwise a byte sequence (possibly including shift sequences) is
     written into the string S.  This only happens if WC is a valid wide
     character (i.e., it has a multibyte representation in the
     character set selected by locale of the `LC_CTYPE' category).  If
     WC is no valid wide character, nothing is stored in the strings S,
     `errno' is set to `EILSEQ', the conversion state in PS is
     undefined and the return value is `(size_t) -1'.

     If no error occurred the function returns the number of bytes
     stored in the string S.  This includes all bytes representing shift
     sequences.

     One word about the interface of the function: there is no parameter
     specifying the length of the array S.  Instead the function
     assumes that there are at least `MB_CUR_MAX' bytes available since
     this is the maximum length of any byte sequence representing a
     single character.  So the caller has to make sure that there is
     enough space available, otherwise buffer overruns can occur.

     `wcrtomb' was introduced in Amendment 1 to ISO C90 and is declared
     in `wchar.h'.

   Using `wcrtomb' is as easy as using `mbrtowc'.  The following
example appends a wide character string to a multibyte character string.
Again, the code is not really useful (or correct), it is simply here to
demonstrate the use and some problems.

     char *
     mbscatwcs (char *s, size_t len, const wchar_t *ws)
     {
       mbstate_t state;
       /* Find the end of the existing string.  */
       char *wp = strchr (s, '\0');
       len -= wp - s;
       memset (&state, '\0', sizeof (state));
       do
         {
           size_t nbytes;
           if (len < MB_CUR_LEN)
             {
               /* We cannot guarantee that the next
                  character fits into the buffer, so
                  return an error.  */
               errno = E2BIG;
               return NULL;
             }
           nbytes = wcrtomb (wp, *ws, &state);
           if (nbytes == (size_t) -1)
             /* Error in the conversion.  */
             return NULL;
           len -= nbytes;
           wp += nbytes;
         }
       while (*ws++ != L'\0');
       return s;
     }

   First the function has to find the end of the string currently in the
array S.  The `strchr' call does this very efficiently since a
requirement for multibyte character representations is that the NUL byte
is never used except to represent itself (and in this context, the end
of the string).

   After initializing the state object the loop is entered where the
first task is to make sure there is enough room in the array S.  We
abort if there are not at least `MB_CUR_LEN' bytes available.  This is
not always optimal but we have no other choice.  We might have less
than `MB_CUR_LEN' bytes available but the next multibyte character
might also be only one byte long.  At the time the `wcrtomb' call
returns it is too late to decide whether the buffer was large enough.
If this solution is unsuitable, there is a very slow but more accurate
solution.

       ...
       if (len < MB_CUR_LEN)
         {
           mbstate_t temp_state;
           memcpy (&temp_state, &state, sizeof (state));
           if (wcrtomb (NULL, *ws, &temp_state) > len)
             {
               /* We cannot guarantee that the next
                  character fits into the buffer, so
                  return an error.  */
               errno = E2BIG;
               return NULL;
             }
         }
       ...

   Here we perform the conversion that might overflow the buffer so that
we are afterwards in the position to make an exact decision about the
buffer size.  Please note the `NULL' argument for the destination
buffer in the new `wcrtomb' call; since we are not interested in the
converted text at this point, this is a nice way to express this.  The
most unusual thing about this piece of code certainly is the duplication
of the conversion state object, but if a change of the state is
necessary to emit the next multibyte character, we want to have the
same shift state change performed in the real conversion.  Therefore,
we have to preserve the initial shift state information.

   There are certainly many more and even better solutions to this
problem.  This example is only provided for educational purposes.

File: libc.info,  Node: Converting Strings,  Next: Multibyte Conversion Example,  Prev: Converting a Character,  Up: Restartable multibyte conversion

6.3.4 Converting Multibyte and Wide Character Strings
-----------------------------------------------------

The functions described in the previous section only convert a single
character at a time.  Most operations to be performed in real-world
programs include strings and therefore the ISO C standard also defines
conversions on entire strings.  However, the defined set of functions
is quite limited; therefore, the GNU C library contains a few
extensions that can help in some important situations.

 -- Function: size_t mbsrtowcs (wchar_t *restrict DST, const char
          **restrict SRC, size_t LEN, mbstate_t *restrict PS)
     The `mbsrtowcs' function ("multibyte string restartable to wide
     character string") converts an NUL-terminated multibyte character
     string at `*SRC' into an equivalent wide character string,
     including the NUL wide character at the end.  The conversion is
     started using the state information from the object pointed to by
     PS or from an internal object of `mbsrtowcs' if PS is a null
     pointer.  Before returning, the state object is updated to match
     the state after the last converted character.  The state is the
     initial state if the terminating NUL byte is reached and converted.

     If DST is not a null pointer, the result is stored in the array
     pointed to by DST; otherwise, the conversion result is not
     available since it is stored in an internal buffer.

     If LEN wide characters are stored in the array DST before reaching
     the end of the input string, the conversion stops and LEN is
     returned.  If DST is a null pointer, LEN is never checked.

     Another reason for a premature return from the function call is if
     the input string contains an invalid multibyte sequence.  In this
     case the global variable `errno' is set to `EILSEQ' and the
     function returns `(size_t) -1'.

     In all other cases the function returns the number of wide
     characters converted during this call.  If DST is not null,
     `mbsrtowcs' stores in the pointer pointed to by SRC either a null
     pointer (if the NUL byte in the input string was reached) or the
     address of the byte following the last converted multibyte
     character.

     `mbsrtowcs' was introduced in Amendment 1 to ISO C90 and is
     declared in `wchar.h'.

   The definition of the `mbsrtowcs' function has one important
limitation.  The requirement that DST has to be a NUL-terminated string
provides problems if one wants to convert buffers with text.  A buffer
is normally no collection of NUL-terminated strings but instead a
continuous collection of lines, separated by newline characters.  Now
assume that a function to convert one line from a buffer is needed.
Since the line is not NUL-terminated, the source pointer cannot
directly point into the unmodified text buffer.  This means, either one
inserts the NUL byte at the appropriate place for the time of the
`mbsrtowcs' function call (which is not doable for a read-only buffer
or in a multi-threaded application) or one copies the line in an extra
buffer where it can be terminated by a NUL byte.  Note that it is not
in general possible to limit the number of characters to convert by
setting the parameter LEN to any specific value.  Since it is not known
how many bytes each multibyte character sequence is in length, one can
only guess.

   There is still a problem with the method of NUL-terminating a line
right after the newline character, which could lead to very strange
results.  As said in the description of the `mbsrtowcs' function above
the conversion state is guaranteed to be in the initial shift state
after processing the NUL byte at the end of the input string.  But this
NUL byte is not really part of the text (i.e., the conversion state
after the newline in the original text could be something different
than the initial shift state and therefore the first character of the
next line is encoded using this state).  But the state in question is
never accessible to the user since the conversion stops after the NUL
byte (which resets the state).  Most stateful character sets in use
today require that the shift state after a newline be the initial
state-but this is not a strict guarantee.  Therefore, simply
NUL-terminating a piece of a running text is not always an adequate
solution and, therefore, should never be used in generally used code.

   The generic conversion interface (*note Generic Charset Conversion::)
does not have this limitation (it simply works on buffers, not
strings), and the GNU C library contains a set of functions that take
additional parameters specifying the maximal number of bytes that are
consumed from the input string.  This way the problem of `mbsrtowcs''s
example above could be solved by determining the line length and
passing this length to the function.

 -- Function: size_t wcsrtombs (char *restrict DST, const wchar_t
          **restrict SRC, size_t LEN, mbstate_t *restrict PS)
     The `wcsrtombs' function ("wide character string restartable to
     multibyte string") converts the NUL-terminated wide character
     string at `*SRC' into an equivalent multibyte character string and
     stores the result in the array pointed to by DST.  The NUL wide
     character is also converted.  The conversion starts in the state
     described in the object pointed to by PS or by a state object
     locally to `wcsrtombs' in case PS is a null pointer.  If DST is a
     null pointer, the conversion is performed as usual but the result
     is not available.  If all characters of the input string were
     successfully converted and if DST is not a null pointer, the
     pointer pointed to by SRC gets assigned a null pointer.

     If one of the wide characters in the input string has no valid
     multibyte character equivalent, the conversion stops early, sets
     the global variable `errno' to `EILSEQ', and returns `(size_t) -1'.

     Another reason for a premature stop is if DST is not a null
     pointer and the next converted character would require more than
     LEN bytes in total to the array DST.  In this case (and if DEST is
     not a null pointer) the pointer pointed to by SRC is assigned a
     value pointing to the wide character right after the last one
     successfully converted.

     Except in the case of an encoding error the return value of the
     `wcsrtombs' function is the number of bytes in all the multibyte
     character sequences stored in DST.  Before returning the state in
     the object pointed to by PS (or the internal object in case PS is
     a null pointer) is updated to reflect the state after the last
     conversion.  The state is the initial shift state in case the
     terminating NUL wide character was converted.

     The `wcsrtombs' function was introduced in Amendment 1 to ISO C90
     and is declared in `wchar.h'.

   The restriction mentioned above for the `mbsrtowcs' function applies
here also.  There is no possibility of directly controlling the number
of input characters.  One has to place the NUL wide character at the
correct place or control the consumed input indirectly via the
available output array size (the LEN parameter).

 -- Function: size_t mbsnrtowcs (wchar_t *restrict DST, const char
          **restrict SRC, size_t NMC, size_t LEN, mbstate_t *restrict
          PS)
     The `mbsnrtowcs' function is very similar to the `mbsrtowcs'
     function.  All the parameters are the same except for NMC, which is
     new.  The return value is the same as for `mbsrtowcs'.

     This new parameter specifies how many bytes at most can be used
     from the multibyte character string.  In other words, the
     multibyte character string `*SRC' need not be NUL-terminated.  But
     if a NUL byte is found within the NMC first bytes of the string,
     the conversion stops here.

     This function is a GNU extension.  It is meant to work around the
     problems mentioned above.  Now it is possible to convert a buffer
     with multibyte character text piece for piece without having to
     care about inserting NUL bytes and the effect of NUL bytes on the
     conversion state.

   A function to convert a multibyte string into a wide character string
and display it could be written like this (this is not a really useful
example):

     void
     showmbs (const char *src, FILE *fp)
     {
       mbstate_t state;
       int cnt = 0;
       memset (&state, '\0', sizeof (state));
       while (1)
         {
           wchar_t linebuf[100];
           const char *endp = strchr (src, '\n');
           size_t n;

           /* Exit if there is no more line.  */
           if (endp == NULL)
             break;

           n = mbsnrtowcs (linebuf, &src, endp - src, 99, &state);
           linebuf[n] = L'\0';
           fprintf (fp, "line %d: \"%S\"\n", linebuf);
         }
     }

   There is no problem with the state after a call to `mbsnrtowcs'.
Since we don't insert characters in the strings that were not in there
right from the beginning and we use STATE only for the conversion of
the given buffer, there is no problem with altering the state.

 -- Function: size_t wcsnrtombs (char *restrict DST, const wchar_t
          **restrict SRC, size_t NWC, size_t LEN, mbstate_t *restrict
          PS)
     The `wcsnrtombs' function implements the conversion from wide
     character strings to multibyte character strings.  It is similar to
     `wcsrtombs' but, just like `mbsnrtowcs', it takes an extra
     parameter, which specifies the length of the input string.

     No more than NWC wide characters from the input string `*SRC' are
     converted.  If the input string contains a NUL wide character in
     the first NWC characters, the conversion stops at this place.

     The `wcsnrtombs' function is a GNU extension and just like
     `mbsnrtowcs' helps in situations where no NUL-terminated input
     strings are available.

File: libc.info,  Node: Multibyte Conversion Example,  Prev: Converting Strings,  Up: Restartable multibyte conversion

6.3.5 A Complete Multibyte Conversion Example
---------------------------------------------

The example programs given in the last sections are only brief and do
not contain all the error checking, etc.  Presented here is a complete
and documented example.  It features the `mbrtowc' function but it
should be easy to derive versions using the other functions.

     int
     file_mbsrtowcs (int input, int output)
     {
       /* Note the use of `MB_LEN_MAX'.
          `MB_CUR_MAX' cannot portably be used here.  */
       char buffer[BUFSIZ + MB_LEN_MAX];
       mbstate_t state;
       int filled = 0;
       int eof = 0;

       /* Initialize the state.  */
       memset (&state, '\0', sizeof (state));

       while (!eof)
         {
           ssize_t nread;
           ssize_t nwrite;
           char *inp = buffer;
           wchar_t outbuf[BUFSIZ];
           wchar_t *outp = outbuf;

           /* Fill up the buffer from the input file.  */
           nread = read (input, buffer + filled, BUFSIZ);
           if (nread < 0)
             {
               perror ("read");
               return 0;
             }
           /* If we reach end of file, make a note to read no more. */
           if (nread == 0)
             eof = 1;

           /* `filled' is now the number of bytes in `buffer'. */
           filled += nread;

           /* Convert those bytes to wide characters-as many as we can. */
           while (1)
             {
               size_t thislen = mbrtowc (outp, inp, filled, &state);
               /* Stop converting at invalid character;
                  this can mean we have read just the first part
                  of a valid character.  */
               if (thislen == (size_t) -1)
                 break;
               /* We want to handle embedded NUL bytes
                  but the return value is 0.  Correct this.  */
               if (thislen == 0)
                 thislen = 1;
               /* Advance past this character. */
               inp += thislen;
               filled -= thislen;
               ++outp;
             }

           /* Write the wide characters we just made.  */
           nwrite = write (output, outbuf,
                           (outp - outbuf) * sizeof (wchar_t));
           if (nwrite < 0)
             {
               perror ("write");
               return 0;
             }

           /* See if we have a _real_ invalid character. */
           if ((eof && filled > 0) || filled >= MB_CUR_MAX)
             {
               error (0, 0, "invalid multibyte character");
               return 0;
             }

           /* If any characters must be carried forward,
              put them at the beginning of `buffer'. */
           if (filled > 0)
             memmove (buffer, inp, filled);
         }

       return 1;
     }

File: libc.info,  Node: Non-reentrant Conversion,  Next: Generic Charset Conversion,  Prev: Restartable multibyte conversion,  Up: Character Set Handling

6.4 Non-reentrant Conversion Function
=====================================

The functions described in the previous chapter are defined in
Amendment 1 to ISO C90, but the original ISO C90 standard also
contained functions for character set conversion.  The reason that
these original functions are not described first is that they are almost
entirely useless.

   The problem is that all the conversion functions described in the
original ISO C90 use a local state.  Using a local state implies that
multiple conversions at the same time (not only when using threads)
cannot be done, and that you cannot first convert single characters and
then strings since you cannot tell the conversion functions which state
to use.

   These original functions are therefore usable only in a very limited
set of situations.  One must complete converting the entire string
before starting a new one, and each string/text must be converted with
the same function (there is no problem with the library itself; it is
guaranteed that no library function changes the state of any of these
functions).  *For the above reasons it is highly requested that the
functions described in the previous section be used in place of
non-reentrant conversion functions.*

* Menu:

* Non-reentrant Character Conversion::  Non-reentrant Conversion of Single
                                         Characters.
* Non-reentrant String Conversion::     Non-reentrant Conversion of Strings.
* Shift State::                         States in Non-reentrant Functions.

File: libc.info,  Node: Non-reentrant Character Conversion,  Next: Non-reentrant String Conversion,  Up: Non-reentrant Conversion

6.4.1 Non-reentrant Conversion of Single Characters
---------------------------------------------------

 -- Function: int mbtowc (wchar_t *restrict RESULT, const char
          *restrict STRING, size_t SIZE)
     The `mbtowc' ("multibyte to wide character") function when called
     with non-null STRING converts the first multibyte character
     beginning at STRING to its corresponding wide character code.  It
     stores the result in `*RESULT'.

     `mbtowc' never examines more than SIZE bytes.  (The idea is to
     supply for SIZE the number of bytes of data you have in hand.)

     `mbtowc' with non-null STRING distinguishes three possibilities:
     the first SIZE bytes at STRING start with valid multibyte
     characters, they start with an invalid byte sequence or just part
     of a character, or STRING points to an empty string (a null
     character).

     For a valid multibyte character, `mbtowc' converts it to a wide
     character and stores that in `*RESULT', and returns the number of
     bytes in that character (always at least 1 and never more than
     SIZE).

     For an invalid byte sequence, `mbtowc' returns -1.  For an empty
     string, it returns 0, also storing `'\0'' in `*RESULT'.

     If the multibyte character code uses shift characters, then
     `mbtowc' maintains and updates a shift state as it scans.  If you
     call `mbtowc' with a null pointer for STRING, that initializes the
     shift state to its standard initial value.  It also returns
     nonzero if the multibyte character code in use actually has a
     shift state.  *Note Shift State::.

 -- Function: int wctomb (char *STRING, wchar_t WCHAR)
     The `wctomb' ("wide character to multibyte") function converts the
     wide character code WCHAR to its corresponding multibyte character
     sequence, and stores the result in bytes starting at STRING.  At
     most `MB_CUR_MAX' characters are stored.

     `wctomb' with non-null STRING distinguishes three possibilities
     for WCHAR: a valid wide character code (one that can be translated
     to a multibyte character), an invalid code, and `L'\0''.

     Given a valid code, `wctomb' converts it to a multibyte character,
     storing the bytes starting at STRING.  Then it returns the number
     of bytes in that character (always at least 1 and never more than
     `MB_CUR_MAX').

     If WCHAR is an invalid wide character code, `wctomb' returns -1.
     If WCHAR is `L'\0'', it returns `0', also storing `'\0'' in
     `*STRING'.

     If the multibyte character code uses shift characters, then
     `wctomb' maintains and updates a shift state as it scans.  If you
     call `wctomb' with a null pointer for STRING, that initializes the
     shift state to its standard initial value.  It also returns
     nonzero if the multibyte character code in use actually has a
     shift state.  *Note Shift State::.

     Calling this function with a WCHAR argument of zero when STRING is
     not null has the side-effect of reinitializing the stored shift
     state _as well as_ storing the multibyte character `'\0'' and
     returning 0.

   Similar to `mbrlen' there is also a non-reentrant function that
computes the length of a multibyte character.  It can be defined in
terms of `mbtowc'.

 -- Function: int mblen (const char *STRING, size_t SIZE)
     The `mblen' function with a non-null STRING argument returns the
     number of bytes that make up the multibyte character beginning at
     STRING, never examining more than SIZE bytes.  (The idea is to
     supply for SIZE the number of bytes of data you have in hand.)

     The return value of `mblen' distinguishes three possibilities: the
     first SIZE bytes at STRING start with valid multibyte characters,
     they start with an invalid byte sequence or just part of a
     character, or STRING points to an empty string (a null character).

     For a valid multibyte character, `mblen' returns the number of
     bytes in that character (always at least `1' and never more than
     SIZE).  For an invalid byte sequence, `mblen' returns -1.  For an
     empty string, it returns 0.

     If the multibyte character code uses shift characters, then `mblen'
     maintains and updates a shift state as it scans.  If you call
     `mblen' with a null pointer for STRING, that initializes the shift
     state to its standard initial value.  It also returns a nonzero
     value if the multibyte character code in use actually has a shift
     state.  *Note Shift State::.

     The function `mblen' is declared in `stdlib.h'.

File: libc.info,  Node: Non-reentrant String Conversion,  Next: Shift State,  Prev: Non-reentrant Character Conversion,  Up: Non-reentrant Conversion

6.4.2 Non-reentrant Conversion of Strings
-----------------------------------------

For convenience the ISO C90 standard also defines functions to convert
entire strings instead of single characters.  These functions suffer
from the same problems as their reentrant counterparts from Amendment 1
to ISO C90; see *note Converting Strings::.

 -- Function: size_t mbstowcs (wchar_t *WSTRING, const char *STRING,
          size_t SIZE)
     The `mbstowcs' ("multibyte string to wide character string")
     function converts the null-terminated string of multibyte
     characters STRING to an array of wide character codes, storing not
     more than SIZE wide characters into the array beginning at WSTRING.
     The terminating null character counts towards the size, so if SIZE
     is less than the actual number of wide characters resulting from
     STRING, no terminating null character is stored.

     The conversion of characters from STRING begins in the initial
     shift state.

     If an invalid multibyte character sequence is found, the `mbstowcs'
     function returns a value of -1.  Otherwise, it returns the number
     of wide characters stored in the array WSTRING.  This number does
     not include the terminating null character, which is present if the
     number is less than SIZE.

     Here is an example showing how to convert a string of multibyte
     characters, allocating enough space for the result.

          wchar_t *
          mbstowcs_alloc (const char *string)
          {
            size_t size = strlen (string) + 1;
            wchar_t *buf = xmalloc (size * sizeof (wchar_t));

            size = mbstowcs (buf, string, size);
            if (size == (size_t) -1)
              return NULL;
            buf = xrealloc (buf, (size + 1) * sizeof (wchar_t));
            return buf;
          }


 -- Function: size_t wcstombs (char *STRING, const wchar_t *WSTRING,
          size_t SIZE)
     The `wcstombs' ("wide character string to multibyte string")
     function converts the null-terminated wide character array WSTRING
     into a string containing multibyte characters, storing not more
     than SIZE bytes starting at STRING, followed by a terminating null
     character if there is room.  The conversion of characters begins in
     the initial shift state.

     The terminating null character counts towards the size, so if SIZE
     is less than or equal to the number of bytes needed in WSTRING, no
     terminating null character is stored.

     If a code that does not correspond to a valid multibyte character
     is found, the `wcstombs' function returns a value of -1.
     Otherwise, the return value is the number of bytes stored in the
     array STRING.  This number does not include the terminating null
     character, which is present if the number is less than SIZE.

File: libc.info,  Node: Shift State,  Prev: Non-reentrant String Conversion,  Up: Non-reentrant Conversion

6.4.3 States in Non-reentrant Functions
---------------------------------------

In some multibyte character codes, the _meaning_ of any particular byte
sequence is not fixed; it depends on what other sequences have come
earlier in the same string.  Typically there are just a few sequences
that can change the meaning of other sequences; these few are called
"shift sequences" and we say that they set the "shift state" for other
sequences that follow.

   To illustrate shift state and shift sequences, suppose we decide that
the sequence `0200' (just one byte) enters Japanese mode, in which
pairs of bytes in the range from `0240' to `0377' are single
characters, while `0201' enters Latin-1 mode, in which single bytes in
the range from `0240' to `0377' are characters, and interpreted
according to the ISO Latin-1 character set.  This is a multibyte code
that has two alternative shift states ("Japanese mode" and "Latin-1
mode"), and two shift sequences that specify particular shift states.

   When the multibyte character code in use has shift states, then
`mblen', `mbtowc', and `wctomb' must maintain and update the current
shift state as they scan the string.  To make this work properly, you
must follow these rules:

   * Before starting to scan a string, call the function with a null
     pointer for the multibyte character address--for example, `mblen
     (NULL, 0)'.  This initializes the shift state to its standard
     initial value.

   * Scan the string one character at a time, in order.  Do not "back
     up" and rescan characters already scanned, and do not intersperse
     the processing of different strings.

   Here is an example of using `mblen' following these rules:

     void
     scan_string (char *s)
     {
       int length = strlen (s);

       /* Initialize shift state.  */
       mblen (NULL, 0);

       while (1)
         {
           int thischar = mblen (s, length);
           /* Deal with end of string and invalid characters.  */
           if (thischar == 0)
             break;
           if (thischar == -1)
             {
               error ("invalid multibyte character");
               break;
             }
           /* Advance past this character.  */
           s += thischar;
           length -= thischar;
         }
     }

   The functions `mblen', `mbtowc' and `wctomb' are not reentrant when
using a multibyte code that uses a shift state.  However, no other
library functions call these functions, so you don't have to worry that
the shift state will be changed mysteriously.

File: libc.info,  Node: Generic Charset Conversion,  Prev: Non-reentrant Conversion,  Up: Character Set Handling

6.5 Generic Charset Conversion
==============================

The conversion functions mentioned so far in this chapter all had in
common that they operate on character sets that are not directly
specified by the functions.  The multibyte encoding used is specified by
the currently selected locale for the `LC_CTYPE' category.  The wide
character set is fixed by the implementation (in the case of GNU C
library it is always UCS-4 encoded ISO 10646.

   This has of course several problems when it comes to general
character conversion:

   * For every conversion where neither the source nor the destination
     character set is the character set of the locale for the `LC_CTYPE'
     category, one has to change the `LC_CTYPE' locale using
     `setlocale'.

     Changing the `LC_TYPE' locale introduces major problems for the
     rest of the programs since several more functions (e.g., the
     character classification functions, *note Classification of
     Characters::) use the `LC_CTYPE' category.

   * Parallel conversions to and from different character sets are not
     possible since the `LC_CTYPE' selection is global and shared by all
     threads.

   * If neither the source nor the destination character set is the
     character set used for `wchar_t' representation, there is at least
     a two-step process necessary to convert a text using the functions
     above.  One would have to select the source character set as the
     multibyte encoding, convert the text into a `wchar_t' text, select
     the destination character set as the multibyte encoding, and
     convert the wide character text to the multibyte (= destination)
     character set.

     Even if this is possible (which is not guaranteed) it is a very
     tiring work.  Plus it suffers from the other two raised points
     even more due to the steady changing of the locale.

   The XPG2 standard defines a completely new set of functions, which
has none of these limitations.  They are not at all coupled to the
selected locales, and they have no constraints on the character sets
selected for source and destination.  Only the set of available
conversions limits them.  The standard does not specify that any
conversion at all must be available.  Such availability is a measure of
the quality of the implementation.

   In the following text first the interface to `iconv' and then the
conversion function, will be described.  Comparisons with other
implementations will show what obstacles stand in the way of portable
applications.  Finally, the implementation is described in so far as
might interest the advanced user who wants to extend conversion
capabilities.

* Menu:

* Generic Conversion Interface::    Generic Character Set Conversion Interface.
* iconv Examples::                  A complete `iconv' example.
* Other iconv Implementations::     Some Details about other `iconv'
                                     Implementations.
* glibc iconv Implementation::      The `iconv' Implementation in the GNU C
                                     library.

File: libc.info,  Node: Generic Conversion Interface,  Next: iconv Examples,  Up: Generic Charset Conversion

6.5.1 Generic Character Set Conversion Interface
------------------------------------------------

This set of functions follows the traditional cycle of using a resource:
open-use-close.  The interface consists of three functions, each of
which implements one step.

   Before the interfaces are described it is necessary to introduce a
data type.  Just like other open-use-close interfaces the functions
introduced here work using handles and the `iconv.h' header defines a
special type for the handles used.

 -- Data Type: iconv_t
     This data type is an abstract type defined in `iconv.h'.  The user
     must not assume anything about the definition of this type; it
     must be completely opaque.

     Objects of this type can get assigned handles for the conversions
     using the `iconv' functions.  The objects themselves need not be
     freed, but the conversions for which the handles stand for have to.

The first step is the function to create a handle.

 -- Function: iconv_t iconv_open (const char *TOCODE, const char
          *FROMCODE)
     The `iconv_open' function has to be used before starting a
     conversion.  The two parameters this function takes determine the
     source and destination character set for the conversion, and if the
     implementation has the possibility to perform such a conversion,
     the function returns a handle.

     If the wanted conversion is not available, the `iconv_open'
     function returns `(iconv_t) -1'. In this case the global variable
     `errno' can have the following values:

    `EMFILE'
          The process already has `OPEN_MAX' file descriptors open.

    `ENFILE'
          The system limit of open file is reached.

    `ENOMEM'
          Not enough memory to carry out the operation.

    `EINVAL'
          The conversion from FROMCODE to TOCODE is not supported.

     It is not possible to use the same descriptor in different threads
     to perform independent conversions.  The data structures associated
     with the descriptor include information about the conversion state.
     This must not be messed up by using it in different conversions.

     An `iconv' descriptor is like a file descriptor as for every use a
     new descriptor must be created.  The descriptor does not stand for
     all of the conversions from FROMSET to TOSET.

     The GNU C library implementation of `iconv_open' has one
     significant extension to other implementations.  To ease the
     extension of the set of available conversions, the implementation
     allows storing the necessary files with data and code in an
     arbitrary number of directories.  How this extension must be
     written will be explained below (*note glibc iconv
     Implementation::).  Here it is only important to say that all
     directories mentioned in the `GCONV_PATH' environment variable are
     considered only if they contain a file `gconv-modules'.  These
     directories need not necessarily be created by the system
     administrator.  In fact, this extension is introduced to help users
     writing and using their own, new conversions.  Of course, this
     does not work for security reasons in SUID binaries; in this case
     only the system directory is considered and this normally is
     `PREFIX/lib/gconv'.  The `GCONV_PATH' environment variable is
     examined exactly once at the first call of the `iconv_open'
     function.  Later modifications of the variable have no effect.

     The `iconv_open' function was introduced early in the X/Open
     Portability Guide, version 2.  It is supported by all commercial
     Unices as it is required for the Unix branding.  However, the
     quality and completeness of the implementation varies widely.  The
     `iconv_open' function is declared in `iconv.h'.

   The `iconv' implementation can associate large data structure with
the handle returned by `iconv_open'.  Therefore, it is crucial to free
all the resources once all conversions are carried out and the
conversion is not needed anymore.

 -- Function: int iconv_close (iconv_t CD)
     The `iconv_close' function frees all resources associated with the
     handle CD, which must have been returned by a successful call to
     the `iconv_open' function.

     If the function call was successful the return value is 0.
     Otherwise it is -1 and `errno' is set appropriately.  Defined
     error are:

    `EBADF'
          The conversion descriptor is invalid.

     The `iconv_close' function was introduced together with the rest
     of the `iconv' functions in XPG2 and is declared in `iconv.h'.

   The standard defines only one actual conversion function.  This has,
therefore, the most general interface: it allows conversion from one
buffer to another.  Conversion from a file to a buffer, vice versa, or
even file to file can be implemented on top of it.

 -- Function: size_t iconv (iconv_t CD, char **INBUF, size_t
          *INBYTESLEFT, char **OUTBUF, size_t *OUTBYTESLEFT)
     The `iconv' function converts the text in the input buffer
     according to the rules associated with the descriptor CD and
     stores the result in the output buffer.  It is possible to call the
     function for the same text several times in a row since for
     stateful character sets the necessary state information is kept in
     the data structures associated with the descriptor.

     The input buffer is specified by `*INBUF' and it contains
     `*INBYTESLEFT' bytes.  The extra indirection is necessary for
     communicating the used input back to the caller (see below).  It is
     important to note that the buffer pointer is of type `char' and the
     length is measured in bytes even if the input text is encoded in
     wide characters.

     The output buffer is specified in a similar way.  `*OUTBUF' points
     to the beginning of the buffer with at least `*OUTBYTESLEFT' bytes
     room for the result.  The buffer pointer again is of type `char'
     and the length is measured in bytes.  If OUTBUF or `*OUTBUF' is a
     null pointer, the conversion is performed but no output is
     available.

     If INBUF is a null pointer, the `iconv' function performs the
     necessary action to put the state of the conversion into the
     initial state.  This is obviously a no-op for non-stateful
     encodings, but if the encoding has a state, such a function call
     might put some byte sequences in the output buffer, which perform
     the necessary state changes.  The next call with INBUF not being a
     null pointer then simply goes on from the initial state.  It is
     important that the programmer never makes any assumption as to
     whether the conversion has to deal with states.  Even if the input
     and output character sets are not stateful, the implementation
     might still have to keep states.  This is due to the
     implementation chosen for the GNU C library as it is described
     below.  Therefore an `iconv' call to reset the state should always
     be performed if some protocol requires this for the output text.

     The conversion stops for one of three reasons. The first is that
     all characters from the input buffer are converted.  This actually
     can mean two things: either all bytes from the input buffer are
     consumed or there are some bytes at the end of the buffer that
     possibly can form a complete character but the input is
     incomplete.  The second reason for a stop is that the output
     buffer is full.  And the third reason is that the input contains
     invalid characters.

     In all of these cases the buffer pointers after the last successful
     conversion, for input and output buffer, are stored in INBUF and
     OUTBUF, and the available room in each buffer is stored in
     INBYTESLEFT and OUTBYTESLEFT.

     Since the character sets selected in the `iconv_open' call can be
     almost arbitrary, there can be situations where the input buffer
     contains valid characters, which have no identical representation
     in the output character set.  The behavior in this situation is
     undefined.  The _current_ behavior of the GNU C library in this
     situation is to return with an error immediately.  This certainly
     is not the most desirable solution; therefore, future versions
     will provide better ones, but they are not yet finished.

     If all input from the input buffer is successfully converted and
     stored in the output buffer, the function returns the number of
     non-reversible conversions performed.  In all other cases the
     return value is `(size_t) -1' and `errno' is set appropriately.
     In such cases the value pointed to by INBYTESLEFT is nonzero.

    `EILSEQ'
          The conversion stopped because of an invalid byte sequence in
          the input.  After the call, `*INBUF' points at the first byte
          of the invalid byte sequence.

    `E2BIG'
          The conversion stopped because it ran out of space in the
          output buffer.

    `EINVAL'
          The conversion stopped because of an incomplete byte sequence
          at the end of the input buffer.

    `EBADF'
          The CD argument is invalid.

     The `iconv' function was introduced in the XPG2 standard and is
     declared in the `iconv.h' header.

   The definition of the `iconv' function is quite good overall.  It
provides quite flexible functionality.  The only problems lie in the
boundary cases, which are incomplete byte sequences at the end of the
input buffer and invalid input.  A third problem, which is not really a
design problem, is the way conversions are selected.  The standard does
not say anything about the legitimate names, a minimal set of available
conversions.  We will see how this negatively impacts other
implementations, as demonstrated below.

File: libc.info,  Node: iconv Examples,  Next: Other iconv Implementations,  Prev: Generic Conversion Interface,  Up: Generic Charset Conversion

6.5.2 A complete `iconv' example
--------------------------------

The example below features a solution for a common problem.  Given that
one knows the internal encoding used by the system for `wchar_t'
strings, one often is in the position to read text from a file and store
it in wide character buffers.  One can do this using `mbsrtowcs', but
then we run into the problems discussed above.

     int
     file2wcs (int fd, const char *charset, wchar_t *outbuf, size_t avail)
     {
       char inbuf[BUFSIZ];
       size_t insize = 0;
       char *wrptr = (char *) outbuf;
       int result = 0;
       iconv_t cd;

       cd = iconv_open ("WCHAR_T", charset);
       if (cd == (iconv_t) -1)
         {
           /* Something went wrong.  */
           if (errno == EINVAL)
             error (0, 0, "conversion from '%s' to wchar_t not available",
                    charset);
           else
             perror ("iconv_open");

           /* Terminate the output string.  */
           *outbuf = L'\0';

           return -1;
         }

       while (avail > 0)
         {
           size_t nread;
           size_t nconv;
           char *inptr = inbuf;

           /* Read more input.  */
           nread = read (fd, inbuf + insize, sizeof (inbuf) - insize);
           if (nread == 0)
             {
               /* When we come here the file is completely read.
                  This still could mean there are some unused
                  characters in the `inbuf'.  Put them back.  */
               if (lseek (fd, -insize, SEEK_CUR) == -1)
                 result = -1;

               /* Now write out the byte sequence to get into the
                  initial state if this is necessary.  */
               iconv (cd, NULL, NULL, &wrptr, &avail);

               break;
             }
           insize += nread;

           /* Do the conversion.  */
           nconv = iconv (cd, &inptr, &insize, &wrptr, &avail);
           if (nconv == (size_t) -1)
             {
               /* Not everything went right.  It might only be
                  an unfinished byte sequence at the end of the
                  buffer.  Or it is a real problem.  */
               if (errno == EINVAL)
                 /* This is harmless.  Simply move the unused
                    bytes to the beginning of the buffer so that
                    they can be used in the next round.  */
                 memmove (inbuf, inptr, insize);
               else
                 {
                   /* It is a real problem.  Maybe we ran out of
                      space in the output buffer or we have invalid
                      input.  In any case back the file pointer to
                      the position of the last processed byte.  */
                   lseek (fd, -insize, SEEK_CUR);
                   result = -1;
                   break;
                 }
             }
         }

       /* Terminate the output string.  */
       if (avail >= sizeof (wchar_t))
         *((wchar_t *) wrptr) = L'\0';

       if (iconv_close (cd) != 0)
         perror ("iconv_close");

       return (wchar_t *) wrptr - outbuf;
     }

   This example shows the most important aspects of using the `iconv'
functions.  It shows how successive calls to `iconv' can be used to
convert large amounts of text.  The user does not have to care about
stateful encodings as the functions take care of everything.

   An interesting point is the case where `iconv' returns an error and
`errno' is set to `EINVAL'.  This is not really an error in the
transformation.  It can happen whenever the input character set contains
byte sequences of more than one byte for some character and texts are
not processed in one piece.  In this case there is a chance that a
multibyte sequence is cut.  The caller can then simply read the
remainder of the takes and feed the offending bytes together with new
character from the input to `iconv' and continue the work.  The
internal state kept in the descriptor is _not_ unspecified after such
an event as is the case with the conversion functions from the ISO C
standard.

   The example also shows the problem of using wide character strings
with `iconv'.  As explained in the description of the `iconv' function
above, the function always takes a pointer to a `char' array and the
available space is measured in bytes.  In the example, the output
buffer is a wide character buffer; therefore, we use a local variable
WRPTR of type `char *', which is used in the `iconv' calls.

   This looks rather innocent but can lead to problems on platforms that
have tight restriction on alignment.  Therefore the caller of `iconv'
has to make sure that the pointers passed are suitable for access of
characters from the appropriate character set.  Since, in the above
case, the input parameter to the function is a `wchar_t' pointer, this
is the case (unless the user violates alignment when computing the
parameter).  But in other situations, especially when writing generic
functions where one does not know what type of character set one uses
and, therefore, treats text as a sequence of bytes, it might become
tricky.

File: libc.info,  Node: Other iconv Implementations,  Next: glibc iconv Implementation,  Prev: iconv Examples,  Up: Generic Charset Conversion

6.5.3 Some Details about other `iconv' Implementations
------------------------------------------------------

This is not really the place to discuss the `iconv' implementation of
other systems but it is necessary to know a bit about them to write
portable programs.  The above mentioned problems with the specification
of the `iconv' functions can lead to portability issues.

   The first thing to notice is that, due to the large number of
character sets in use, it is certainly not practical to encode the
conversions directly in the C library.  Therefore, the conversion
information must come from files outside the C library.  This is
usually done in one or both of the following ways:

   * The C library contains a set of generic conversion functions that
     can read the needed conversion tables and other information from
     data files.  These files get loaded when necessary.

     This solution is problematic as it requires a great deal of effort
     to apply to all character sets (potentially an infinite set).  The
     differences in the structure of the different character sets is so
     large that many different variants of the table-processing
     functions must be developed.  In addition, the generic nature of
     these functions make them slower than specifically implemented
     functions.

   * The C library only contains a framework that can dynamically load
     object files and execute the conversion functions contained
     therein.

     This solution provides much more flexibility.  The C library itself
     contains only very little code and therefore reduces the general
     memory footprint.  Also, with a documented interface between the C
     library and the loadable modules it is possible for third parties
     to extend the set of available conversion modules.  A drawback of
     this solution is that dynamic loading must be available.

   Some implementations in commercial Unices implement a mixture of
these possibilities; the majority implement only the second solution.
Using loadable modules moves the code out of the library itself and
keeps the door open for extensions and improvements, but this design is
also limiting on some platforms since not many platforms support dynamic
loading in statically linked programs.  On platforms without this
capability it is therefore not possible to use this interface in
statically linked programs.  The GNU C library has, on ELF platforms, no
problems with dynamic loading in these situations; therefore, this
point is moot.  The danger is that one gets acquainted with this
situation and forgets about the restrictions on other systems.

   A second thing to know about other `iconv' implementations is that
the number of available conversions is often very limited.  Some
implementations provide, in the standard release (not special
international or developer releases), at most 100 to 200 conversion
possibilities.  This does not mean 200 different character sets are
supported; for example, conversions from one character set to a set of
10 others might count as 10 conversions.  Together with the other
direction this makes 20 conversion possibilities used up by one
character set.  One can imagine the thin coverage these platform
provide.  Some Unix vendors even provide only a handful of conversions,
which renders them useless for almost all uses.

   This directly leads to a third and probably the most problematic
point.  The way the `iconv' conversion functions are implemented on all
known Unix systems and the availability of the conversion functions from
character set A to B and the conversion from B to C does _not_ imply
that the conversion from A to C is available.

   This might not seem unreasonable and problematic at first, but it is
a quite big problem as one will notice shortly after hitting it.  To
show the problem we assume to write a program that has to convert from
A to C.  A call like

     cd = iconv_open ("C", "A");

fails according to the assumption above.  But what does the program do
now?  The conversion is necessary; therefore, simply giving up is not
an option.

   This is a nuisance.  The `iconv' function should take care of this.
But how should the program proceed from here on?  If it tries to convert
to character set B, first the two `iconv_open' calls

     cd1 = iconv_open ("B", "A");

and

     cd2 = iconv_open ("C", "B");

will succeed, but how to find B?

   Unfortunately, the answer is: there is no general solution.  On some
systems guessing might help.  On those systems most character sets can
convert to and from UTF-8 encoded ISO 10646 or Unicode text. Beside
this only some very system-specific methods can help.  Since the
conversion functions come from loadable modules and these modules must
be stored somewhere in the filesystem, one _could_ try to find them and
determine from the available file which conversions are available and
whether there is an indirect route from A to C.

   This example shows one of the design errors of `iconv' mentioned
above.  It should at least be possible to determine the list of
available conversion programmatically so that if `iconv_open' says
there is no such conversion, one could make sure this also is true for
indirect routes.

File: libc.info,  Node: glibc iconv Implementation,  Prev: Other iconv Implementations,  Up: Generic Charset Conversion

6.5.4 The `iconv' Implementation in the GNU C library
-----------------------------------------------------

After reading about the problems of `iconv' implementations in the last
section it is certainly good to note that the implementation in the GNU
C library has none of the problems mentioned above.  What follows is a
step-by-step analysis of the points raised above.  The evaluation is
based on the current state of the development (as of January 1999).
The development of the `iconv' functions is not complete, but basic
functionality has solidified.

   The GNU C library's `iconv' implementation uses shared loadable
modules to implement the conversions.  A very small number of
conversions are built into the library itself but these are only rather
trivial conversions.

   All the benefits of loadable modules are available in the GNU C
library implementation.  This is especially appealing since the
interface is well documented (see below), and it, therefore, is easy to
write new conversion modules.  The drawback of using loadable objects
is not a problem in the GNU C library, at least on ELF systems.  Since
the library is able to load shared objects even in statically linked
binaries, static linking need not be forbidden in case one wants to use
`iconv'.

   The second mentioned problem is the number of supported conversions.
Currently, the GNU C library supports more than 150 character sets.  The
way the implementation is designed the number of supported conversions
is greater than 22350 (150 times 149).  If any conversion from or to a
character set is missing, it can be added easily.

   Particularly impressive as it may be, this high number is due to the
fact that the GNU C library implementation of `iconv' does not have the
third problem mentioned above (i.e., whenever there is a conversion
from a character set A to B and from B to C it is always possible to
convert from A to C directly).  If the `iconv_open' returns an error
and sets `errno' to `EINVAL', there is no known way, directly or
indirectly, to perform the wanted conversion.

   Triangulation is achieved by providing for each character set a
conversion from and to UCS-4 encoded ISO 10646.  Using ISO 10646 as an
intermediate representation it is possible to "triangulate" (i.e.,
convert with an intermediate representation).

   There is no inherent requirement to provide a conversion to
ISO 10646 for a new character set, and it is also possible to provide
other conversions where neither source nor destination character set is
ISO 10646.  The existing set of conversions is simply meant to cover all
conversions that might be of interest.

   All currently available conversions use the triangulation method
above, making conversion run unnecessarily slow.  If, for example,
somebody often needs the conversion from ISO-2022-JP to EUC-JP, a
quicker solution would involve direct conversion between the two
character sets, skipping the input to ISO 10646 first.  The two
character sets of interest are much more similar to each other than to
ISO 10646.

   In such a situation one easily can write a new conversion and
provide it as a better alternative.  The GNU C library `iconv'
implementation would automatically use the module implementing the
conversion if it is specified to be more efficient.

6.5.4.1 Format of `gconv-modules' files
.......................................

All information about the available conversions comes from a file named
`gconv-modules', which can be found in any of the directories along the
`GCONV_PATH'.  The `gconv-modules' files are line-oriented text files,
where each of the lines has one of the following formats:

   * If the first non-whitespace character is a `#' the line contains
     only comments and is ignored.

   * Lines starting with `alias' define an alias name for a character
     set.  Two more words are expected on the line.  The first word
     defines the alias name, and the second defines the original name
     of the character set.  The effect is that it is possible to use
     the alias name in the FROMSET or TOSET parameters of `iconv_open'
     and achieve the same result as when using the real character set
     name.

     This is quite important as a character set has often many different
     names.  There is normally an official name but this need not
     correspond to the most popular name.  Beside this many character
     sets have special names that are somehow constructed.  For
     example, all character sets specified by the ISO have an alias of
     the form `ISO-IR-NNN' where NNN is the registration number.  This
     allows programs that know about the registration number to
     construct character set names and use them in `iconv_open' calls.
     More on the available names and aliases follows below.

   * Lines starting with `module' introduce an available conversion
     module.  These lines must contain three or four more words.

     The first word specifies the source character set, the second word
     the destination character set of conversion implemented in this
     module, and the third word is the name of the loadable module.
     The filename is constructed by appending the usual shared object
     suffix (normally `.so') and this file is then supposed to be found
     in the same directory the `gconv-modules' file is in.  The last
     word on the line, which is optional, is a numeric value
     representing the cost of the conversion.  If this word is missing,
     a cost of 1 is assumed.  The numeric value itself does not matter
     that much; what counts are the relative values of the sums of
     costs for all possible conversion paths.  Below is a more precise
     description of the use of the cost value.

   Returning to the example above where one has written a module to
directly convert from ISO-2022-JP to EUC-JP and back.  All that has to
be done is to put the new module, let its name be ISO2022JP-EUCJP.so,
in a directory and add a file `gconv-modules' with the following
content in the same directory:

     module  ISO-2022-JP//   EUC-JP//        ISO2022JP-EUCJP    1
     module  EUC-JP//        ISO-2022-JP//   ISO2022JP-EUCJP    1

   To see why this is sufficient, it is necessary to understand how the
conversion used by `iconv' (and described in the descriptor) is
selected.  The approach to this problem is quite simple.

   At the first call of the `iconv_open' function the program reads all
available `gconv-modules' files and builds up two tables: one
containing all the known aliases and another that contains the
information about the conversions and which shared object implements
them.

6.5.4.2 Finding the conversion path in `iconv'
..............................................

The set of available conversions form a directed graph with weighted
edges.  The weights on the edges are the costs specified in the
`gconv-modules' files.  The `iconv_open' function uses an algorithm
suitable for search for the best path in such a graph and so constructs
a list of conversions that must be performed in succession to get the
transformation from the source to the destination character set.

   Explaining why the above `gconv-modules' files allows the `iconv'
implementation to resolve the specific ISO-2022-JP to EUC-JP conversion
module instead of the conversion coming with the library itself is
straightforward.  Since the latter conversion takes two steps (from
ISO-2022-JP to ISO 10646 and then from ISO 10646 to EUC-JP), the cost
is 1+1 = 2.  The above `gconv-modules' file, however, specifies that
the new conversion modules can perform this conversion with only the
cost of 1.

   A mysterious item about the `gconv-modules' file above (and also the
file coming with the GNU C library) are the names of the character sets
specified in the `module' lines.  Why do almost all the names end in
`//'?  And this is not all: the names can actually be regular
expressions.  At this point in time this mystery should not be
revealed, unless you have the relevant spell-casting materials: ashes
from an original DOS 6.2 boot disk burnt in effigy, a crucifix blessed
by St. Emacs, assorted herbal roots from Central America, sand from
Cebu, etc.  Sorry!  *The part of the implementation where this is used
is not yet finished.  For now please simply follow the existing
examples.  It'll become clearer once it is. -drepper*

   A last remark about the `gconv-modules' is about the names not
ending with `//'.  A character set named `INTERNAL' is often mentioned.
From the discussion above and the chosen name it should have become
clear that this is the name for the representation used in the
intermediate step of the triangulation.  We have said that this is UCS-4
but actually that is not quite right.  The UCS-4 specification also
includes the specification of the byte ordering used.  Since a UCS-4
value consists of four bytes, a stored value is effected by byte
ordering.  The internal representation is _not_ the same as UCS-4 in
case the byte ordering of the processor (or at least the running
process) is not the same as the one required for UCS-4.  This is done
for performance reasons as one does not want to perform unnecessary
byte-swapping operations if one is not interested in actually seeing
the result in UCS-4.  To avoid trouble with endianness, the internal
representation consistently is named `INTERNAL' even on big-endian
systems where the representations are identical.

6.5.4.3 `iconv' module data structures
......................................

So far this section has described how modules are located and considered
to be used.  What remains to be described is the interface of the
modules so that one can write new ones. This section describes the
interface as it is in use in January 1999.  The interface will change a
bit in the future but, with luck, only in an upwardly compatible way.

   The definitions necessary to write new modules are publicly available
in the non-standard header `gconv.h'.  The following text, therefore,
describes the definitions from this header file.  First, however, it is
necessary to get an overview.

   From the perspective of the user of `iconv' the interface is quite
simple: the `iconv_open' function returns a handle that can be used in
calls to `iconv', and finally the handle is freed with a call to
`iconv_close'.  The problem is that the handle has to be able to
represent the possibly long sequences of conversion steps and also the
state of each conversion since the handle is all that is passed to the
`iconv' function.  Therefore, the data structures are really the
elements necessary to understanding the implementation.

   We need two different kinds of data structures.  The first describes
the conversion and the second describes the state etc.  There are
really two type definitions like this in `gconv.h'.

 -- Data type: struct __gconv_step
     This data structure describes one conversion a module can perform.
     For each function in a loaded module with conversion functions
     there is exactly one object of this type.  This object is shared
     by all users of the conversion (i.e., this object does not contain
     any information corresponding to an actual conversion; it only
     describes the conversion itself).

    `struct __gconv_loaded_object *__shlib_handle'
    `const char *__modname'
    `int __counter'
          All these elements of the structure are used internally in
          the C library to coordinate loading and unloading the shared.
          One must not expect any of the other elements to be available
          or initialized.

    `const char *__from_name'
    `const char *__to_name'
          `__from_name' and `__to_name' contain the names of the source
          and destination character sets.  They can be used to identify
          the actual conversion to be carried out since one module
          might implement conversions for more than one character set
          and/or direction.

    `gconv_fct __fct'
    `gconv_init_fct __init_fct'
    `gconv_end_fct __end_fct'
          These elements contain pointers to the functions in the
          loadable module.  The interface will be explained below.

    `int __min_needed_from'
    `int __max_needed_from'
    `int __min_needed_to'
    `int __max_needed_to;'
          These values have to be supplied in the init function of the
          module.  The `__min_needed_from' value specifies how many
          bytes a character of the source character set at least needs.
          The `__max_needed_from' specifies the maximum value that also
          includes possible shift sequences.

          The `__min_needed_to' and `__max_needed_to' values serve the
          same purpose as `__min_needed_from' and `__max_needed_from'
          but this time for the destination character set.

          It is crucial that these values be accurate since otherwise
          the conversion functions will have problems or not work at
          all.

    `int __stateful'
          This element must also be initialized by the init function.
          `int __stateful' is nonzero if the source character set is
          stateful.  Otherwise it is zero.

    `void *__data'
          This element can be used freely by the conversion functions
          in the module.  `void *__data' can be used to communicate
          extra information from one call to another.  `void *__data'
          need not be initialized if not needed at all.  If `void
          *__data' element is assigned a pointer to dynamically
          allocated memory (presumably in the init function) it has to
          be made sure that the end function deallocates the memory.
          Otherwise the application will leak memory.

          It is important to be aware that this data structure is
          shared by all users of this specification conversion and
          therefore the `__data' element must not contain data specific
          to one specific use of the conversion function.

 -- Data type: struct __gconv_step_data
     This is the data structure that contains the information specific
     to each use of the conversion functions.

    `char *__outbuf'
    `char *__outbufend'
          These elements specify the output buffer for the conversion
          step.  The `__outbuf' element points to the beginning of the
          buffer, and `__outbufend' points to the byte following the
          last byte in the buffer.  The conversion function must not
          assume anything about the size of the buffer but it can be
          safely assumed the there is room for at least one complete
          character in the output buffer.

          Once the conversion is finished, if the conversion is the
          last step, the `__outbuf' element must be modified to point
          after the last byte written into the buffer to signal how
          much output is available.  If this conversion step is not the
          last one, the element must not be modified.  The
          `__outbufend' element must not be modified.

    `int __is_last'
          This element is nonzero if this conversion step is the last
          one.  This information is necessary for the recursion.  See
          the description of the conversion function internals below.
          This element must never be modified.

    `int __invocation_counter'
          The conversion function can use this element to see how many
          calls of the conversion function already happened.  Some
          character sets require a certain prolog when generating
          output, and by comparing this value with zero, one can find
          out whether it is the first call and whether, therefore, the
          prolog should be emitted.  This element must never be
          modified.

    `int __internal_use'
          This element is another one rarely used but needed in certain
          situations.  It is assigned a nonzero value in case the
          conversion functions are used to implement `mbsrtowcs' et.al.
          (i.e., the function is not used directly through the `iconv'
          interface).

          This sometimes makes a difference as it is expected that the
          `iconv' functions are used to translate entire texts while the
          `mbsrtowcs' functions are normally used only to convert single
          strings and might be used multiple times to convert entire
          texts.

          But in this situation we would have problem complying with
          some rules of the character set specification.  Some
          character sets require a prolog, which must appear exactly
          once for an entire text.  If a number of `mbsrtowcs' calls
          are used to convert the text, only the first call must add
          the prolog.  However, because there is no communication
          between the different calls of `mbsrtowcs', the conversion
          functions have no possibility to find this out.  The
          situation is different for sequences of `iconv' calls since
          the handle allows access to the needed information.

          The `int __internal_use' element is mostly used together with
          `__invocation_counter' as follows:

               if (!data->__internal_use
                    && data->__invocation_counter == 0)
                 /* Emit prolog.  */
                 ...

          This element must never be modified.

    `mbstate_t *__statep'
          The `__statep' element points to an object of type `mbstate_t'
          (*note Keeping the state::).  The conversion of a stateful
          character set must use the object pointed to by `__statep' to
          store information about the conversion state.  The `__statep'
          element itself must never be modified.

    `mbstate_t __state'
          This element must _never_ be used directly.  It is only part
          of this structure to have the needed space allocated.

6.5.4.4 `iconv' module interfaces
.................................

With the knowledge about the data structures we now can describe the
conversion function itself.  To understand the interface a bit of
knowledge is necessary about the functionality in the C library that
loads the objects with the conversions.

   It is often the case that one conversion is used more than once
(i.e., there are several `iconv_open' calls for the same set of
character sets during one program run).  The `mbsrtowcs' et.al.
functions in the GNU C library also use the `iconv' functionality, which
increases the number of uses of the same functions even more.

   Because of this multiple use of conversions, the modules do not get
loaded exclusively for one conversion.  Instead a module once loaded can
be used by an arbitrary number of `iconv' or `mbsrtowcs' calls at the
same time.  The splitting of the information between conversion-
function-specific information and conversion data makes this possible.
The last section showed the two data structures used to do this.

   This is of course also reflected in the interface and semantics of
the functions that the modules must provide.  There are three functions
that must have the following names:

`gconv_init'
     The `gconv_init' function initializes the conversion function
     specific data structure.  This very same object is shared by all
     conversions that use this conversion and, therefore, no state
     information about the conversion itself must be stored in here.
     If a module implements more than one conversion, the `gconv_init'
     function will be called multiple times.

`gconv_end'
     The `gconv_end' function is responsible for freeing all resources
     allocated by the `gconv_init' function.  If there is nothing to do,
     this function can be missing.  Special care must be taken if the
     module implements more than one conversion and the `gconv_init'
     function does not allocate the same resources for all conversions.

`gconv'
     This is the actual conversion function.  It is called to convert
     one block of text.  It gets passed the conversion step information
     initialized by `gconv_init' and the conversion data, specific to
     this use of the conversion functions.

   There are three data types defined for the three module interface
functions and these define the interface.

 -- Data type: int (*__gconv_init_fct) (struct __gconv_step *)
     This specifies the interface of the initialization function of the
     module.  It is called exactly once for each conversion the module
     implements.

     As explained in the description of the `struct __gconv_step' data
     structure above the initialization function has to initialize
     parts of it.

    `__min_needed_from'
    `__max_needed_from'
    `__min_needed_to'
    `__max_needed_to'
          These elements must be initialized to the exact numbers of
          the minimum and maximum number of bytes used by one character
          in the source and destination character sets, respectively.
          If the characters all have the same size, the minimum and
          maximum values are the same.

    `__stateful'
          This element must be initialized to an nonzero value if the
          source character set is stateful.  Otherwise it must be zero.

     If the initialization function needs to communicate some
     information to the conversion function, this communication can
     happen using the `__data' element of the `__gconv_step' structure.
     But since this data is shared by all the conversions, it must not
     be modified by the conversion function.  The example below shows
     how this can be used.

          #define MIN_NEEDED_FROM         1
          #define MAX_NEEDED_FROM         4
          #define MIN_NEEDED_TO           4
          #define MAX_NEEDED_TO           4

          int
          gconv_init (struct __gconv_step *step)
          {
            /* Determine which direction.  */
            struct iso2022jp_data *new_data;
            enum direction dir = illegal_dir;
            enum variant var = illegal_var;
            int result;

            if (__strcasecmp (step->__from_name, "ISO-2022-JP//") == 0)
              {
                dir = from_iso2022jp;
                var = iso2022jp;
              }
            else if (__strcasecmp (step->__to_name, "ISO-2022-JP//") == 0)
              {
                dir = to_iso2022jp;
                var = iso2022jp;
              }
            else if (__strcasecmp (step->__from_name, "ISO-2022-JP-2//") == 0)
              {
                dir = from_iso2022jp;
                var = iso2022jp2;
              }
            else if (__strcasecmp (step->__to_name, "ISO-2022-JP-2//") == 0)
              {
                dir = to_iso2022jp;
                var = iso2022jp2;
              }

            result = __GCONV_NOCONV;
            if (dir != illegal_dir)
              {
                new_data = (struct iso2022jp_data *)
                  malloc (sizeof (struct iso2022jp_data));

                result = __GCONV_NOMEM;
                if (new_data != NULL)
                  {
                    new_data->dir = dir;
                    new_data->var = var;
                    step->__data = new_data;

                    if (dir == from_iso2022jp)
                      {
                        step->__min_needed_from = MIN_NEEDED_FROM;
                        step->__max_needed_from = MAX_NEEDED_FROM;
                        step->__min_needed_to = MIN_NEEDED_TO;
                        step->__max_needed_to = MAX_NEEDED_TO;
                      }
                    else
                      {
                        step->__min_needed_from = MIN_NEEDED_TO;
                        step->__max_needed_from = MAX_NEEDED_TO;
                        step->__min_needed_to = MIN_NEEDED_FROM;
                        step->__max_needed_to = MAX_NEEDED_FROM + 2;
                      }

                    /* Yes, this is a stateful encoding.  */
                    step->__stateful = 1;

                    result = __GCONV_OK;
                  }
              }

            return result;
          }

     The function first checks which conversion is wanted.  The module
     from which this function is taken implements four different
     conversions; which one is selected can be determined by comparing
     the names.  The comparison should always be done without paying
     attention to the case.

     Next, a data structure, which contains the necessary information
     about which conversion is selected, is allocated.  The data
     structure `struct iso2022jp_data' is locally defined since,
     outside the module, this data is not used at all.  Please note
     that if all four conversions this modules supports are requested
     there are four data blocks.

     One interesting thing is the initialization of the `__min_' and
     `__max_' elements of the step data object.  A single ISO-2022-JP
     character can consist of one to four bytes.  Therefore the
     `MIN_NEEDED_FROM' and `MAX_NEEDED_FROM' macros are defined this
     way.  The output is always the `INTERNAL' character set (aka
     UCS-4) and therefore each character consists of exactly four
     bytes.  For the conversion from `INTERNAL' to ISO-2022-JP we have
     to take into account that escape sequences might be necessary to
     switch the character sets.  Therefore the `__max_needed_to'
     element for this direction gets assigned `MAX_NEEDED_FROM + 2'.
     This takes into account the two bytes needed for the escape
     sequences to single the switching.  The asymmetry in the maximum
     values for the two directions can be explained easily: when
     reading ISO-2022-JP text, escape sequences can be handled alone
     (i.e., it is not necessary to process a real character since the
     effect of the escape sequence can be recorded in the state
     information).  The situation is different for the other direction.
     Since it is in general not known which character comes next, one
     cannot emit escape sequences to change the state in advance.  This
     means the escape sequences that have to be emitted together with
     the next character.  Therefore one needs more room than only for
     the character itself.

     The possible return values of the initialization function are:

    `__GCONV_OK'
          The initialization succeeded

    `__GCONV_NOCONV'
          The requested conversion is not supported in the module.
          This can happen if the `gconv-modules' file has errors.

    `__GCONV_NOMEM'
          Memory required to store additional information could not be
          allocated.

   The function called before the module is unloaded is significantly
easier.  It often has nothing at all to do; in which case it can be left
out completely.

 -- Data type: void (*__gconv_end_fct) (struct gconv_step *)
     The task of this function is to free all resources allocated in the
     initialization function.  Therefore only the `__data' element of
     the object pointed to by the argument is of interest.  Continuing
     the example from the initialization function, the finalization
     function looks like this:

          void
          gconv_end (struct __gconv_step *data)
          {
            free (data->__data);
          }

   The most important function is the conversion function itself, which
can get quite complicated for complex character sets.  But since this
is not of interest here, we will only describe a possible skeleton for
the conversion function.

 -- Data type: int (*__gconv_fct) (struct __gconv_step *, struct
          __gconv_step_data *, const char **, const char *, size_t *,
          int)
     The conversion function can be called for two basic reason: to
     convert text or to reset the state.  From the description of the
     `iconv' function it can be seen why the flushing mode is
     necessary.  What mode is selected is determined by the sixth
     argument, an integer.  This argument being nonzero means that
     flushing is selected.

     Common to both modes is where the output buffer can be found.  The
     information about this buffer is stored in the conversion step
     data.  A pointer to this information is passed as the second
     argument to this function.  The description of the `struct
     __gconv_step_data' structure has more information on the
     conversion step data.

     What has to be done for flushing depends on the source character
     set.  If the source character set is not stateful, nothing has to
     be done.  Otherwise the function has to emit a byte sequence to
     bring the state object into the initial state.  Once this all
     happened the other conversion modules in the chain of conversions
     have to get the same chance.  Whether another step follows can be
     determined from the `__is_last' element of the step data structure
     to which the first parameter points.

     The more interesting mode is when actual text has to be converted.
     The first step in this case is to convert as much text as possible
     from the input buffer and store the result in the output buffer.
     The start of the input buffer is determined by the third argument,
     which is a pointer to a pointer variable referencing the beginning
     of the buffer.  The fourth argument is a pointer to the byte right
     after the last byte in the buffer.

     The conversion has to be performed according to the current state
     if the character set is stateful.  The state is stored in an
     object pointed to by the `__statep' element of the step data
     (second argument).  Once either the input buffer is empty or the
     output buffer is full the conversion stops.  At this point, the
     pointer variable referenced by the third parameter must point to
     the byte following the last processed byte (i.e., if all of the
     input is consumed, this pointer and the fourth parameter have the
     same value).

     What now happens depends on whether this step is the last one.  If
     it is the last step, the only thing that has to be done is to
     update the `__outbuf' element of the step data structure to point
     after the last written byte.  This update gives the caller the
     information on how much text is available in the output buffer.
     In addition, the variable pointed to by the fifth parameter, which
     is of type `size_t', must be incremented by the number of
     characters (_not bytes_) that were converted in a non-reversible
     way.  Then, the function can return.

     In case the step is not the last one, the later conversion
     functions have to get a chance to do their work.  Therefore, the
     appropriate conversion function has to be called.  The information
     about the functions is stored in the conversion data structures,
     passed as the first parameter.  This information and the step data
     are stored in arrays, so the next element in both cases can be
     found by simple pointer arithmetic:

          int
          gconv (struct __gconv_step *step, struct __gconv_step_data *data,
                 const char **inbuf, const char *inbufend, size_t *written,
                 int do_flush)
          {
            struct __gconv_step *next_step = step + 1;
            struct __gconv_step_data *next_data = data + 1;
            ...

     The `next_step' pointer references the next step information and
     `next_data' the next data record.  The call of the next function
     therefore will look similar to this:

            next_step->__fct (next_step, next_data, &outerr, outbuf,
                              written, 0)

     But this is not yet all.  Once the function call returns the
     conversion function might have some more to do.  If the return
     value of the function is `__GCONV_EMPTY_INPUT', more room is
     available in the output buffer.  Unless the input buffer is empty
     the conversion, functions start all over again and process the
     rest of the input buffer.  If the return value is not
     `__GCONV_EMPTY_INPUT', something went wrong and we have to recover
     from this.

     A requirement for the conversion function is that the input buffer
     pointer (the third argument) always point to the last character
     that was put in converted form into the output buffer.  This is
     trivially true after the conversion performed in the current step,
     but if the conversion functions deeper downstream stop
     prematurely, not all characters from the output buffer are
     consumed and, therefore, the input buffer pointers must be backed
     off to the right position.

     Correcting the input buffers is easy to do if the input and output
     character sets have a fixed width for all characters.  In this
     situation we can compute how many characters are left in the
     output buffer and, therefore, can correct the input buffer pointer
     appropriately with a similar computation.  Things are getting
     tricky if either character set has characters represented with
     variable length byte sequences, and it gets even more complicated
     if the conversion has to take care of the state.  In these cases
     the conversion has to be performed once again, from the known
     state before the initial conversion (i.e., if necessary the state
     of the conversion has to be reset and the conversion loop has to be
     executed again).  The difference now is that it is known how much
     input must be created, and the conversion can stop before
     converting the first unused character.  Once this is done the
     input buffer pointers must be updated again and the function can
     return.

     One final thing should be mentioned.  If it is necessary for the
     conversion to know whether it is the first invocation (in case a
     prolog has to be emitted), the conversion function should
     increment the `__invocation_counter' element of the step data
     structure just before returning to the caller.  See the
     description of the `struct __gconv_step_data' structure above for
     more information on how this can be used.

     The return value must be one of the following values:

    `__GCONV_EMPTY_INPUT'
          All input was consumed and there is room left in the output
          buffer.

    `__GCONV_FULL_OUTPUT'
          No more room in the output buffer.  In case this is not the
          last step this value is propagated down from the call of the
          next conversion function in the chain.

    `__GCONV_INCOMPLETE_INPUT'
          The input buffer is not entirely empty since it contains an
          incomplete character sequence.

     The following example provides a framework for a conversion
     function.  In case a new conversion has to be written the holes in
     this implementation have to be filled and that is it.

          int
          gconv (struct __gconv_step *step, struct __gconv_step_data *data,
                 const char **inbuf, const char *inbufend, size_t *written,
                 int do_flush)
          {
            struct __gconv_step *next_step = step + 1;
            struct __gconv_step_data *next_data = data + 1;
            gconv_fct fct = next_step->__fct;
            int status;

            /* If the function is called with no input this means we have
               to reset to the initial state.  The possibly partly
               converted input is dropped.  */
            if (do_flush)
              {
                status = __GCONV_OK;

                /* Possible emit a byte sequence which put the state object
                   into the initial state.  */

                /* Call the steps down the chain if there are any but only
                   if we successfully emitted the escape sequence.  */
                if (status == __GCONV_OK && ! data->__is_last)
                  status = fct (next_step, next_data, NULL, NULL,
                                written, 1);
              }
            else
              {
                /* We preserve the initial values of the pointer variables.  */
                const char *inptr = *inbuf;
                char *outbuf = data->__outbuf;
                char *outend = data->__outbufend;
                char *outptr;

                do
                  {
                    /* Remember the start value for this round.  */
                    inptr = *inbuf;
                    /* The outbuf buffer is empty.  */
                    outptr = outbuf;

                    /* For stateful encodings the state must be safe here.  */

                    /* Run the conversion loop.  `status' is set
                       appropriately afterwards.  */

                    /* If this is the last step, leave the loop. There is
                       nothing we can do.  */
                    if (data->__is_last)
                      {
                        /* Store information about how many bytes are
                           available.  */
                        data->__outbuf = outbuf;

                       /* If any non-reversible conversions were performed,
                          add the number to `*written'.  */

                       break;
                     }

                    /* Write out all output that was produced.  */
                    if (outbuf > outptr)
                      {
                        const char *outerr = data->__outbuf;
                        int result;

                        result = fct (next_step, next_data, &outerr,
                                      outbuf, written, 0);

                        if (result != __GCONV_EMPTY_INPUT)
                          {
                            if (outerr != outbuf)
                              {
                                /* Reset the input buffer pointer.  We
                                   document here the complex case.  */
                                size_t nstatus;

                                /* Reload the pointers.  */
                                *inbuf = inptr;
                                outbuf = outptr;

                                /* Possibly reset the state.  */

                                /* Redo the conversion, but this time
                                   the end of the output buffer is at
                                   `outerr'.  */
                              }

                            /* Change the status.  */
                            status = result;
                          }
                        else
                          /* All the output is consumed, we can make
                              another run if everything was ok.  */
                          if (status == __GCONV_FULL_OUTPUT)
                            status = __GCONV_OK;
                     }
                  }
                while (status == __GCONV_OK);

                /* We finished one use of this step.  */
                ++data->__invocation_counter;
              }

            return status;
          }

   This information should be sufficient to write new modules.  Anybody
doing so should also take a look at the available source code in the GNU
C library sources.  It contains many examples of working and optimized
modules.

File: libc.info,  Node: Locales,  Next: Message Translation,  Prev: Character Set Handling,  Up: Top

7 Locales and Internationalization
**********************************

Different countries and cultures have varying conventions for how to
communicate.  These conventions range from very simple ones, such as the
format for representing dates and times, to very complex ones, such as
the language spoken.

   "Internationalization" of software means programming it to be able
to adapt to the user's favorite conventions.  In ISO C,
internationalization works by means of "locales".  Each locale
specifies a collection of conventions, one convention for each purpose.
The user chooses a set of conventions by specifying a locale (via
environment variables).

   All programs inherit the chosen locale as part of their environment.
Provided the programs are written to obey the choice of locale, they
will follow the conventions preferred by the user.

* Menu:

* Effects of Locale::           Actions affected by the choice of
                                 locale.
* Choosing Locale::             How the user specifies a locale.
* Locale Categories::           Different purposes for which you can
                                 select a locale.
* Setting the Locale::          How a program specifies the locale
                                 with library functions.
* Standard Locales::            Locale names available on all systems.
* Locale Names::                Format of system-specific locale names.
* Locale Information::          How to access the information for the locale.
* Formatting Numbers::          A dedicated function to format numbers.
* Yes-or-No Questions::         Check a Response against the locale.

File: libc.info,  Node: Effects of Locale,  Next: Choosing Locale,  Up: Locales

7.1 What Effects a Locale Has
=============================

Each locale specifies conventions for several purposes, including the
following:

   * What multibyte character sequences are valid, and how they are
     interpreted (*note Character Set Handling::).

   * Classification of which characters in the local character set are
     considered alphabetic, and upper- and lower-case conversion
     conventions (*note Character Handling::).

   * The collating sequence for the local language and character set
     (*note Collation Functions::).

   * Formatting of numbers and currency amounts (*note General
     Numeric::).

   * Formatting of dates and times (*note Formatting Calendar Time::).

   * What language to use for output, including error messages (*note
     Message Translation::).

   * What language to use for user answers to yes-or-no questions
     (*note Yes-or-No Questions::).

   * What language to use for more complex user input.  (The C library
     doesn't yet help you implement this.)

   Some aspects of adapting to the specified locale are handled
automatically by the library subroutines.  For example, all your program
needs to do in order to use the collating sequence of the chosen locale
is to use `strcoll' or `strxfrm' to compare strings.

   Other aspects of locales are beyond the comprehension of the library.
For example, the library can't automatically translate your program's
output messages into other languages.  The only way you can support
output in the user's favorite language is to program this more or less
by hand.  The C library provides functions to handle translations for
multiple languages easily.

   This chapter discusses the mechanism by which you can modify the
current locale.  The effects of the current locale on specific library
functions are discussed in more detail in the descriptions of those
functions.

File: libc.info,  Node: Choosing Locale,  Next: Locale Categories,  Prev: Effects of Locale,  Up: Locales

7.2 Choosing a Locale
=====================

The simplest way for the user to choose a locale is to set the
environment variable `LANG'.  This specifies a single locale to use for
all purposes.  For example, a user could specify a hypothetical locale
named `espana-castellano' to use the standard conventions of most of
Spain.

   The set of locales supported depends on the operating system you are
using, and so do their names, except that the standard locale called
`C' or `POSIX' always exist.  *Note Locale Names::.

   In order to force the system to always use the default locale, the
user can set the `LC_ALL' environment variable to `C'.

   A user also has the option of specifying different locales for
different purposes--in effect, choosing a mixture of multiple locales.
*Note Locale Categories::.

   For example, the user might specify the locale `espana-castellano'
for most purposes, but specify the locale `usa-english' for currency
formatting.  This might make sense if the user is a Spanish-speaking
American, working in Spanish, but representing monetary amounts in US
dollars.

   Note that both locales `espana-castellano' and `usa-english', like
all locales, would include conventions for all of the purposes to which
locales apply.  However, the user can choose to use each locale for a
particular subset of those purposes.

File: libc.info,  Node: Locale Categories,  Next: Setting the Locale,  Prev: Choosing Locale,  Up: Locales

7.3 Locale Categories
=====================

The purposes that locales serve are grouped into "categories", so that
a user or a program can choose the locale for each category
independently.  Here is a table of categories; each name is both an
environment variable that a user can set, and a macro name that you can
use as the first argument to `setlocale'.

   The contents of the environment variable (or the string in the second
argument to `setlocale') has to be a valid locale name.  *Note Locale
Names::.

`LC_COLLATE'
     This category applies to collation of strings (functions `strcoll'
     and `strxfrm'); see *note Collation Functions::.

`LC_CTYPE'
     This category applies to classification and conversion of
     characters, and to multibyte and wide characters; see *note
     Character Handling::, and *note Character Set Handling::.

`LC_MONETARY'
     This category applies to formatting monetary values; see *note
     General Numeric::.

`LC_NUMERIC'
     This category applies to formatting numeric values that are not
     monetary; see *note General Numeric::.

`LC_TIME'
     This category applies to formatting date and time values; see
     *note Formatting Calendar Time::.

`LC_MESSAGES'
     This category applies to selecting the language used in the user
     interface for message translation (*note The Uniforum approach::;
     *note Message catalogs a la X/Open::)  and contains regular
     expressions for affirmative and negative responses.

`LC_ALL'
     This is not a category; it is only a macro that you can use with
     `setlocale' to set a single locale for all purposes.  Setting this
     environment variable overwrites all selections by the other `LC_*'
     variables or `LANG'.

`LANG'
     If this environment variable is defined, its value specifies the
     locale to use for all purposes except as overridden by the
     variables above.

   When developing the message translation functions it was felt that
the functionality provided by the variables above is not sufficient.
For example, it should be possible to specify more than one locale name.
Take a Swedish user who better speaks German than English, and a program
whose messages are output in English by default.  It should be possible
to specify that the first choice of language is Swedish, the second
German, and if this also fails to use English.  This is possible with
the variable `LANGUAGE'.  For further description of this GNU extension
see *note Using gettextized software::.

File: libc.info,  Node: Setting the Locale,  Next: Standard Locales,  Prev: Locale Categories,  Up: Locales

7.4 How Programs Set the Locale
===============================

A C program inherits its locale environment variables when it starts up.
This happens automatically.  However, these variables do not
automatically control the locale used by the library functions, because
ISO C says that all programs start by default in the standard `C'
locale.  To use the locales specified by the environment, you must call
`setlocale'.  Call it as follows:

     setlocale (LC_ALL, "");

to select a locale based on the user choice of the appropriate
environment variables.

   You can also use `setlocale' to specify a particular locale, for
general use or for a specific category.

   The symbols in this section are defined in the header file
`locale.h'.

 -- Function: char * setlocale (int CATEGORY, const char *LOCALE)
     The function `setlocale' sets the current locale for category
     CATEGORY to LOCALE.

     If CATEGORY is `LC_ALL', this specifies the locale for all
     purposes.  The other possible values of CATEGORY specify an single
     purpose (*note Locale Categories::).

     You can also use this function to find out the current locale by
     passing a null pointer as the LOCALE argument.  In this case,
     `setlocale' returns a string that is the name of the locale
     currently selected for category CATEGORY.

     The string returned by `setlocale' can be overwritten by subsequent
     calls, so you should make a copy of the string (*note Copying and
     Concatenation::) if you want to save it past any further calls to
     `setlocale'.  (The standard library is guaranteed never to call
     `setlocale' itself.)

     You should not modify the string returned by `setlocale'.  It might
     be the same string that was passed as an argument in a previous
     call to `setlocale'.  One requirement is that the CATEGORY must be
     the same in the call the string was returned and the one when the
     string is passed in as LOCALE parameter.

     When you read the current locale for category `LC_ALL', the value
     encodes the entire combination of selected locales for all
     categories.  If you specify the same "locale name" with `LC_ALL'
     in a subsequent call to `setlocale', it restores the same
     combination of locale selections.

     To be sure you can use the returned string encoding the currently
     selected locale at a later time, you must make a copy of the
     string.  It is not guaranteed that the returned pointer remains
     valid over time.

     When the LOCALE argument is not a null pointer, the string returned
     by `setlocale' reflects the newly-modified locale.

     If you specify an empty string for LOCALE, this means to read the
     appropriate environment variable and use its value to select the
     locale for CATEGORY.

     If a nonempty string is given for LOCALE, then the locale of that
     name is used if possible.

     The effective locale name (either the second argument to
     `setlocale', or if the argument is an empty string, the name
     obtained from the process environment) must be valid locale name.
     *Note Locale Names::.

     If you specify an invalid locale name, `setlocale' returns a null
     pointer and leaves the current locale unchanged.

   Here is an example showing how you might use `setlocale' to
temporarily switch to a new locale.

     #include <stddef.h>
     #include <locale.h>
     #include <stdlib.h>
     #include <string.h>

     void
     with_other_locale (char *new_locale,
                        void (*subroutine) (int),
                        int argument)
     {
       char *old_locale, *saved_locale;

       /* Get the name of the current locale.  */
       old_locale = setlocale (LC_ALL, NULL);

       /* Copy the name so it won't be clobbered by `setlocale'. */
       saved_locale = strdup (old_locale);
       if (saved_locale == NULL)
         fatal ("Out of memory");

       /* Now change the locale and do some stuff with it. */
       setlocale (LC_ALL, new_locale);
       (*subroutine) (argument);

       /* Restore the original locale. */
       setlocale (LC_ALL, saved_locale);
       free (saved_locale);
     }

   *Portability Note:* Some ISO C systems may define additional locale
categories, and future versions of the library will do so.  For
portability, assume that any symbol beginning with `LC_' might be
defined in `locale.h'.

File: libc.info,  Node: Standard Locales,  Next: Locale Names,  Prev: Setting the Locale,  Up: Locales

7.5 Standard Locales
====================

The only locale names you can count on finding on all operating systems
are these three standard ones:

`"C"'
     This is the standard C locale.  The attributes and behavior it
     provides are specified in the ISO C standard.  When your program
     starts up, it initially uses this locale by default.

`"POSIX"'
     This is the standard POSIX locale.  Currently, it is an alias for
     the standard C locale.

`""'
     The empty name says to select a locale based on environment
     variables.  *Note Locale Categories::.

   Defining and installing named locales is normally a responsibility of
the system administrator at your site (or the person who installed the
GNU C library).  It is also possible for the user to create private
locales.  All this will be discussed later when describing the tool to
do so.

   If your program needs to use something other than the `C' locale, it
will be more portable if you use whatever locale the user specifies
with the environment, rather than trying to specify some non-standard
locale explicitly by name.  Remember, different machines might have
different sets of locales installed.

File: libc.info,  Node: Locale Names,  Next: Locale Information,  Prev: Standard Locales,  Up: Locales

7.6 Locale Names
================

The following command prints a list of locales supported by the system:

       locale -a

   *Portability Note:* With the notable exception of the standard
locale names `C' and `POSIX', locale names are system-specific.

   Most locale names follow XPG syntax and consist of up to four parts:

     LANGUAGE[_TERRITORY[.CODESET]][@MODIFIER]

   Beside the first part, all of them are allowed to be missing.  If the
full specified locale is not found, less specific ones are looked for.
The various parts will be stripped off, in the following order:

  1. codeset

  2. normalized codeset

  3. territory

  4. modifier

   For example, the locale name `de_AT.iso885915@euro' denotes a
German-language locale for use in Austria, using the ISO-8859-15
(Latin-9) character set, and with the Euro as the currency symbol.

   In addition to locale names which follow XPG syntax, systems may
provide aliases such as `german'.  Both categories of names must not
contain the slash character `/'.

   If the locale name starts with a slash `/', it is treated as a path
relative to the configured locale directories; see `LOCPATH' below.
The specified path must not contain a component `..', or the name is
invalid, and `setlocale' will fail.

   *Portability Note:* POSIX suggests that if a locale name starts with
a slash `/', it is resolved as an absolute path.  However, the GNU C
Library treats it as a relative path under the directories listed in
`LOCPATH' (or the default locale directory if `LOCPATH' is unset).

   Locale names which are longer than an implementation-defined limit
are invalid and cause `setlocale' to fail.

   As a special case, locale names used with `LC_ALL' can combine
several locales, reflecting different locale settings for different
categories.  For example, you might want to use a U.S. locale with ISO
A4 paper format, so you set `LANG' to `en_US.UTF-8', and `LC_PAPER' to
`de_DE.UTF-8'.  In this case, the `LC_ALL'-style combined locale name is

     LC_CTYPE=en_US.UTF-8;LC_TIME=en_US.UTF-8;LC_PAPER=de_DE.UTF-8;...

   followed by other category settings not shown here.

   The path used for finding locale data can be set using the `LOCPATH'
environment variable.  This variable lists the directories in which to
search for locale definitions, separated by a colon `:'.

   The default path for finding locale data is system specific.  A
typical value for the `LOCPATH' default is:

     /usr/share/locale

   The value of `LOCPATH' is ignored by privileged programs for
security reasons, and only the default directory is used.

File: libc.info,  Node: Locale Information,  Next: Formatting Numbers,  Prev: Locale Names,  Up: Locales

7.7 Accessing Locale Information
================================

There are several ways to access locale information.  The simplest way
is to let the C library itself do the work.  Several of the functions
in this library implicitly access the locale data, and use what
information is provided by the currently selected locale.  This is how
the locale model is meant to work normally.

   As an example take the `strftime' function, which is meant to nicely
format date and time information (*note Formatting Calendar Time::).
Part of the standard information contained in the `LC_TIME' category is
the names of the months.  Instead of requiring the programmer to take
care of providing the translations the `strftime' function does this
all by itself.  `%A' in the format string is replaced by the
appropriate weekday name of the locale currently selected by `LC_TIME'.
This is an easy example, and wherever possible functions do things
automatically in this way.

   But there are quite often situations when there is simply no function
to perform the task, or it is simply not possible to do the work
automatically.  For these cases it is necessary to access the
information in the locale directly.  To do this the C library provides
two functions: `localeconv' and `nl_langinfo'.  The former is part of
ISO C and therefore portable, but has a brain-damaged interface.  The
second is part of the Unix interface and is portable in as far as the
system follows the Unix standards.

* Menu:

* The Lame Way to Locale Data::   ISO C's `localeconv'.
* The Elegant and Fast Way::      X/Open's `nl_langinfo'.

File: libc.info,  Node: The Lame Way to Locale Data,  Next: The Elegant and Fast Way,  Up: Locale Information

7.7.1 `localeconv': It is portable but ...
------------------------------------------

Together with the `setlocale' function the ISO C people invented the
`localeconv' function.  It is a masterpiece of poor design.  It is
expensive to use, not extendable, and not generally usable as it
provides access to only `LC_MONETARY' and `LC_NUMERIC' related
information.  Nevertheless, if it is applicable to a given situation it
should be used since it is very portable.  The function `strfmon'
formats monetary amounts according to the selected locale using this
information.

 -- Function: struct lconv * localeconv (void)
     The `localeconv' function returns a pointer to a structure whose
     components contain information about how numeric and monetary
     values should be formatted in the current locale.

     You should not modify the structure or its contents.  The
     structure might be overwritten by subsequent calls to
     `localeconv', or by calls to `setlocale', but no other function in
     the library overwrites this value.

 -- Data Type: struct lconv
     `localeconv''s return value is of this data type.  Its elements are
     described in the following subsections.

   If a member of the structure `struct lconv' has type `char', and the
value is `CHAR_MAX', it means that the current locale has no value for
that parameter.

* Menu:

* General Numeric::             Parameters for formatting numbers and
                                 currency amounts.
* Currency Symbol::             How to print the symbol that identifies an
                                 amount of money (e.g. `$').
* Sign of Money Amount::        How to print the (positive or negative) sign
                                 for a monetary amount, if one exists.

File: libc.info,  Node: General Numeric,  Next: Currency Symbol,  Up: The Lame Way to Locale Data

7.7.1.1 Generic Numeric Formatting Parameters
.............................................

These are the standard members of `struct lconv'; there may be others.

`char *decimal_point'
`char *mon_decimal_point'
     These are the decimal-point separators used in formatting
     non-monetary and monetary quantities, respectively.  In the `C'
     locale, the value of `decimal_point' is `"."', and the value of
     `mon_decimal_point' is `""'.

`char *thousands_sep'
`char *mon_thousands_sep'
     These are the separators used to delimit groups of digits to the
     left of the decimal point in formatting non-monetary and monetary
     quantities, respectively.  In the `C' locale, both members have a
     value of `""' (the empty string).

`char *grouping'
`char *mon_grouping'
     These are strings that specify how to group the digits to the left
     of the decimal point.  `grouping' applies to non-monetary
     quantities and `mon_grouping' applies to monetary quantities.  Use
     either `thousands_sep' or `mon_thousands_sep' to separate the digit
     groups.

     Each member of these strings is to be interpreted as an integer
     value of type `char'.  Successive numbers (from left to right)
     give the sizes of successive groups (from right to left, starting
     at the decimal point.)  The last member is either `0', in which
     case the previous member is used over and over again for all the
     remaining groups, or `CHAR_MAX', in which case there is no more
     grouping--or, put another way, any remaining digits form one large
     group without separators.

     For example, if `grouping' is `"\04\03\02"', the correct grouping
     for the number `123456787654321' is `12', `34', `56', `78', `765',
     `4321'.  This uses a group of 4 digits at the end, preceded by a
     group of 3 digits, preceded by groups of 2 digits (as many as
     needed).  With a separator of `,', the number would be printed as
     `12,34,56,78,765,4321'.

     A value of `"\03"' indicates repeated groups of three digits, as
     normally used in the U.S.

     In the standard `C' locale, both `grouping' and `mon_grouping'
     have a value of `""'.  This value specifies no grouping at all.

`char int_frac_digits'
`char frac_digits'
     These are small integers indicating how many fractional digits (to
     the right of the decimal point) should be displayed in a monetary
     value in international and local formats, respectively.  (Most
     often, both members have the same value.)

     In the standard `C' locale, both of these members have the value
     `CHAR_MAX', meaning "unspecified".  The ISO standard doesn't say
     what to do when you find this value; we recommend printing no
     fractional digits.  (This locale also specifies the empty string
     for `mon_decimal_point', so printing any fractional digits would be
     confusing!)

File: libc.info,  Node: Currency Symbol,  Next: Sign of Money Amount,  Prev: General Numeric,  Up: The Lame Way to Locale Data

7.7.1.2 Printing the Currency Symbol
....................................

These members of the `struct lconv' structure specify how to print the
symbol to identify a monetary value--the international analog of `$'
for US dollars.

   Each country has two standard currency symbols.  The "local currency
symbol" is used commonly within the country, while the "international
currency symbol" is used internationally to refer to that country's
currency when it is necessary to indicate the country unambiguously.

   For example, many countries use the dollar as their monetary unit,
and when dealing with international currencies it's important to specify
that one is dealing with (say) Canadian dollars instead of U.S. dollars
or Australian dollars.  But when the context is known to be Canada,
there is no need to make this explicit--dollar amounts are implicitly
assumed to be in Canadian dollars.

`char *currency_symbol'
     The local currency symbol for the selected locale.

     In the standard `C' locale, this member has a value of `""' (the
     empty string), meaning "unspecified".  The ISO standard doesn't
     say what to do when you find this value; we recommend you simply
     print the empty string as you would print any other string pointed
     to by this variable.

`char *int_curr_symbol'
     The international currency symbol for the selected locale.

     The value of `int_curr_symbol' should normally consist of a
     three-letter abbreviation determined by the international standard
     `ISO 4217 Codes for the Representation of Currency and Funds',
     followed by a one-character separator (often a space).

     In the standard `C' locale, this member has a value of `""' (the
     empty string), meaning "unspecified".  We recommend you simply
     print the empty string as you would print any other string pointed
     to by this variable.

`char p_cs_precedes'
`char n_cs_precedes'
`char int_p_cs_precedes'
`char int_n_cs_precedes'
     These members are `1' if the `currency_symbol' or
     `int_curr_symbol' strings should precede the value of a monetary
     amount, or `0' if the strings should follow the value.  The
     `p_cs_precedes' and `int_p_cs_precedes' members apply to positive
     amounts (or zero), and the `n_cs_precedes' and `int_n_cs_precedes'
     members apply to negative amounts.

     In the standard `C' locale, all of these members have a value of
     `CHAR_MAX', meaning "unspecified".  The ISO standard doesn't say
     what to do when you find this value.  We recommend printing the
     currency symbol before the amount, which is right for most
     countries.  In other words, treat all nonzero values alike in
     these members.

     The members with the `int_' prefix apply to the `int_curr_symbol'
     while the other two apply to `currency_symbol'.

`char p_sep_by_space'
`char n_sep_by_space'
`char int_p_sep_by_space'
`char int_n_sep_by_space'
     These members are `1' if a space should appear between the
     `currency_symbol' or `int_curr_symbol' strings and the amount, or
     `0' if no space should appear.  The `p_sep_by_space' and
     `int_p_sep_by_space' members apply to positive amounts (or zero),
     and the `n_sep_by_space' and `int_n_sep_by_space' members apply to
     negative amounts.

     In the standard `C' locale, all of these members have a value of
     `CHAR_MAX', meaning "unspecified".  The ISO standard doesn't say
     what you should do when you find this value; we suggest you treat
     it as 1 (print a space).  In other words, treat all nonzero values
     alike in these members.

     The members with the `int_' prefix apply to the `int_curr_symbol'
     while the other two apply to `currency_symbol'.  There is one
     specialty with the `int_curr_symbol', though.  Since all legal
     values contain a space at the end the string one either printf
     this space (if the currency symbol must appear in front and must
     be separated) or one has to avoid printing this character at all
     (especially when at the end of the string).

File: libc.info,  Node: Sign of Money Amount,  Prev: Currency Symbol,  Up: The Lame Way to Locale Data

7.7.1.3 Printing the Sign of a Monetary Amount
..............................................

These members of the `struct lconv' structure specify how to print the
sign (if any) of a monetary value.

`char *positive_sign'
`char *negative_sign'
     These are strings used to indicate positive (or zero) and negative
     monetary quantities, respectively.

     In the standard `C' locale, both of these members have a value of
     `""' (the empty string), meaning "unspecified".

     The ISO standard doesn't say what to do when you find this value;
     we recommend printing `positive_sign' as you find it, even if it is
     empty.  For a negative value, print `negative_sign' as you find it
     unless both it and `positive_sign' are empty, in which case print
     `-' instead.  (Failing to indicate the sign at all seems rather
     unreasonable.)

`char p_sign_posn'
`char n_sign_posn'
`char int_p_sign_posn'
`char int_n_sign_posn'
     These members are small integers that indicate how to position the
     sign for nonnegative and negative monetary quantities,
     respectively.  (The string used by the sign is what was specified
     with `positive_sign' or `negative_sign'.)  The possible values are
     as follows:

    `0'
          The currency symbol and quantity should be surrounded by
          parentheses.

    `1'
          Print the sign string before the quantity and currency symbol.

    `2'
          Print the sign string after the quantity and currency symbol.

    `3'
          Print the sign string right before the currency symbol.

    `4'
          Print the sign string right after the currency symbol.

    `CHAR_MAX'
          "Unspecified".  Both members have this value in the standard
          `C' locale.

     The ISO standard doesn't say what you should do when the value is
     `CHAR_MAX'.  We recommend you print the sign after the currency
     symbol.

     The members with the `int_' prefix apply to the `int_curr_symbol'
     while the other two apply to `currency_symbol'.

File: libc.info,  Node: The Elegant and Fast Way,  Prev: The Lame Way to Locale Data,  Up: Locale Information

7.7.2 Pinpoint Access to Locale Data
------------------------------------

When writing the X/Open Portability Guide the authors realized that the
`localeconv' function is not enough to provide reasonable access to
locale information.  The information which was meant to be available in
the locale (as later specified in the POSIX.1 standard) requires more
ways to access it.  Therefore the `nl_langinfo' function was introduced.

 -- Function: char * nl_langinfo (nl_item ITEM)
     The `nl_langinfo' function can be used to access individual
     elements of the locale categories.  Unlike the `localeconv'
     function, which returns all the information, `nl_langinfo' lets
     the caller select what information it requires.  This is very fast
     and it is not a problem to call this function multiple times.

     A second advantage is that in addition to the numeric and monetary
     formatting information, information from the `LC_TIME' and
     `LC_MESSAGES' categories is available.

     The type `nl_type' is defined in `nl_types.h'.  The argument ITEM
     is a numeric value defined in the header `langinfo.h'.  The X/Open
     standard defines the following values:

    `CODESET'
          `nl_langinfo' returns a string with the name of the coded
          character set used in the selected locale.

    `ABDAY_1'
    `ABDAY_2'
    `ABDAY_3'
    `ABDAY_4'
    `ABDAY_5'
    `ABDAY_6'
    `ABDAY_7'
          `nl_langinfo' returns the abbreviated weekday name.  `ABDAY_1'
          corresponds to Sunday.

    `DAY_1'
    `DAY_2'
    `DAY_3'
    `DAY_4'
    `DAY_5'
    `DAY_6'
    `DAY_7'
          Similar to `ABDAY_1' etc., but here the return value is the
          unabbreviated weekday name.

    `ABMON_1'
    `ABMON_2'
    `ABMON_3'
    `ABMON_4'
    `ABMON_5'
    `ABMON_6'
    `ABMON_7'
    `ABMON_8'
    `ABMON_9'
    `ABMON_10'
    `ABMON_11'
    `ABMON_12'
          The return value is abbreviated name of the month.  `ABMON_1'
          corresponds to January.

    `MON_1'
    `MON_2'
    `MON_3'
    `MON_4'
    `MON_5'
    `MON_6'
    `MON_7'
    `MON_8'
    `MON_9'
    `MON_10'
    `MON_11'
    `MON_12'
          Similar to `ABMON_1' etc., but here the month names are not
          abbreviated.  Here the first value `MON_1' also corresponds
          to January.

    `AM_STR'
    `PM_STR'
          The return values are strings which can be used in the
          representation of time as an hour from 1 to 12 plus an am/pm
          specifier.

          Note that in locales which do not use this time representation
          these strings might be empty, in which case the am/pm format
          cannot be used at all.

    `D_T_FMT'
          The return value can be used as a format string for
          `strftime' to represent time and date in a locale-specific
          way.

    `D_FMT'
          The return value can be used as a format string for
          `strftime' to represent a date in a locale-specific way.

    `T_FMT'
          The return value can be used as a format string for
          `strftime' to represent time in a locale-specific way.

    `T_FMT_AMPM'
          The return value can be used as a format string for
          `strftime' to represent time in the am/pm format.

          Note that if the am/pm format does not make any sense for the
          selected locale, the return value might be the same as the
          one for `T_FMT'.

    `ERA'
          The return value represents the era used in the current
          locale.

          Most locales do not define this value.  An example of a
          locale which does define this value is the Japanese one.  In
          Japan, the traditional representation of dates includes the
          name of the era corresponding to the then-emperor's reign.

          Normally it should not be necessary to use this value
          directly.  Specifying the `E' modifier in their format
          strings causes the `strftime' functions to use this
          information.  The format of the returned string is not
          specified, and therefore you should not assume knowledge of
          it on different systems.

    `ERA_YEAR'
          The return value gives the year in the relevant era of the
          locale.  As for `ERA' it should not be necessary to use this
          value directly.

    `ERA_D_T_FMT'
          This return value can be used as a format string for
          `strftime' to represent dates and times in a locale-specific
          era-based way.

    `ERA_D_FMT'
          This return value can be used as a format string for
          `strftime' to represent a date in a locale-specific era-based
          way.

    `ERA_T_FMT'
          This return value can be used as a format string for
          `strftime' to represent time in a locale-specific era-based
          way.

    `ALT_DIGITS'
          The return value is a representation of up to 100 values used
          to represent the values 0 to 99.  As for `ERA' this value is
          not intended to be used directly, but instead indirectly
          through the `strftime' function.  When the modifier `O' is
          used in a format which would otherwise use numerals to
          represent hours, minutes, seconds, weekdays, months, or
          weeks, the appropriate value for the locale is used instead.

    `INT_CURR_SYMBOL'
          The same as the value returned by `localeconv' in the
          `int_curr_symbol' element of the `struct lconv'.

    `CURRENCY_SYMBOL'
    `CRNCYSTR'
          The same as the value returned by `localeconv' in the
          `currency_symbol' element of the `struct lconv'.

          `CRNCYSTR' is a deprecated alias still required by Unix98.

    `MON_DECIMAL_POINT'
          The same as the value returned by `localeconv' in the
          `mon_decimal_point' element of the `struct lconv'.

    `MON_THOUSANDS_SEP'
          The same as the value returned by `localeconv' in the
          `mon_thousands_sep' element of the `struct lconv'.

    `MON_GROUPING'
          The same as the value returned by `localeconv' in the
          `mon_grouping' element of the `struct lconv'.

    `POSITIVE_SIGN'
          The same as the value returned by `localeconv' in the
          `positive_sign' element of the `struct lconv'.

    `NEGATIVE_SIGN'
          The same as the value returned by `localeconv' in the
          `negative_sign' element of the `struct lconv'.

    `INT_FRAC_DIGITS'
          The same as the value returned by `localeconv' in the
          `int_frac_digits' element of the `struct lconv'.

    `FRAC_DIGITS'
          The same as the value returned by `localeconv' in the
          `frac_digits' element of the `struct lconv'.

    `P_CS_PRECEDES'
          The same as the value returned by `localeconv' in the
          `p_cs_precedes' element of the `struct lconv'.

    `P_SEP_BY_SPACE'
          The same as the value returned by `localeconv' in the
          `p_sep_by_space' element of the `struct lconv'.

    `N_CS_PRECEDES'
          The same as the value returned by `localeconv' in the
          `n_cs_precedes' element of the `struct lconv'.

    `N_SEP_BY_SPACE'
          The same as the value returned by `localeconv' in the
          `n_sep_by_space' element of the `struct lconv'.

    `P_SIGN_POSN'
          The same as the value returned by `localeconv' in the
          `p_sign_posn' element of the `struct lconv'.

    `N_SIGN_POSN'
          The same as the value returned by `localeconv' in the
          `n_sign_posn' element of the `struct lconv'.

    `INT_P_CS_PRECEDES'
          The same as the value returned by `localeconv' in the
          `int_p_cs_precedes' element of the `struct lconv'.

    `INT_P_SEP_BY_SPACE'
          The same as the value returned by `localeconv' in the
          `int_p_sep_by_space' element of the `struct lconv'.

    `INT_N_CS_PRECEDES'
          The same as the value returned by `localeconv' in the
          `int_n_cs_precedes' element of the `struct lconv'.

    `INT_N_SEP_BY_SPACE'
          The same as the value returned by `localeconv' in the
          `int_n_sep_by_space' element of the `struct lconv'.

    `INT_P_SIGN_POSN'
          The same as the value returned by `localeconv' in the
          `int_p_sign_posn' element of the `struct lconv'.

    `INT_N_SIGN_POSN'
          The same as the value returned by `localeconv' in the
          `int_n_sign_posn' element of the `struct lconv'.

    `DECIMAL_POINT'
    `RADIXCHAR'
          The same as the value returned by `localeconv' in the
          `decimal_point' element of the `struct lconv'.

          The name `RADIXCHAR' is a deprecated alias still used in
          Unix98.

    `THOUSANDS_SEP'
    `THOUSEP'
          The same as the value returned by `localeconv' in the
          `thousands_sep' element of the `struct lconv'.

          The name `THOUSEP' is a deprecated alias still used in Unix98.

    `GROUPING'
          The same as the value returned by `localeconv' in the
          `grouping' element of the `struct lconv'.

    `YESEXPR'
          The return value is a regular expression which can be used
          with the `regex' function to recognize a positive response to
          a yes/no question.  The GNU C library provides the `rpmatch'
          function for easier handling in applications.

    `NOEXPR'
          The return value is a regular expression which can be used
          with the `regex' function to recognize a negative response to
          a yes/no question.

    `YESSTR'
          The return value is a locale-specific translation of the
          positive response to a yes/no question.

          Using this value is deprecated since it is a very special
          case of message translation, and is better handled by the
          message translation functions (*note Message Translation::).

          The use of this symbol is deprecated.  Instead message
          translation should be used.

    `NOSTR'
          The return value is a locale-specific translation of the
          negative response to a yes/no question.  What is said for
          `YESSTR' is also true here.

          The use of this symbol is deprecated.  Instead message
          translation should be used.

     The file `langinfo.h' defines a lot more symbols but none of them
     is official.  Using them is not portable, and the format of the
     return values might change.  Therefore we recommended you not use
     them.

     Note that the return value for any valid argument can be used for
     in all situations (with the possible exception of the am/pm time
     formatting codes).  If the user has not selected any locale for the
     appropriate category, `nl_langinfo' returns the information from
     the `"C"' locale.  It is therefore possible to use this function as
     shown in the example below.

     If the argument ITEM is not valid, a pointer to an empty string is
     returned.

   An example of `nl_langinfo' usage is a function which has to print a
given date and time in a locale-specific way.  At first one might think
that, since `strftime' internally uses the locale information, writing
something like the following is enough:

     size_t
     i18n_time_n_data (char *s, size_t len, const struct tm *tp)
     {
       return strftime (s, len, "%X %D", tp);
     }

   The format contains no weekday or month names and therefore is
internationally usable.  Wrong!  The output produced is something like
`"hh:mm:ss MM/DD/YY"'.  This format is only recognizable in the USA.
Other countries use different formats.  Therefore the function should
be rewritten like this:

     size_t
     i18n_time_n_data (char *s, size_t len, const struct tm *tp)
     {
       return strftime (s, len, nl_langinfo (D_T_FMT), tp);
     }

   Now it uses the date and time format of the locale selected when the
program runs.  If the user selects the locale correctly there should
never be a misunderstanding over the time and date format.

File: libc.info,  Node: Formatting Numbers,  Next: Yes-or-No Questions,  Prev: Locale Information,  Up: Locales

7.8 A dedicated function to format numbers
==========================================

We have seen that the structure returned by `localeconv' as well as the
values given to `nl_langinfo' allow you to retrieve the various pieces
of locale-specific information to format numbers and monetary amounts.
We have also seen that the underlying rules are quite complex.

   Therefore the X/Open standards introduce a function which uses such
locale information, making it easier for the user to format numbers
according to these rules.

 -- Function: ssize_t strfmon (char *S, size_t MAXSIZE, const char
          *FORMAT, ...)
     The `strfmon' function is similar to the `strftime' function in
     that it takes a buffer, its size, a format string, and values to
     write into the buffer as text in a form specified by the format
     string.  Like `strftime', the function also returns the number of
     bytes written into the buffer.

     There are two differences: `strfmon' can take more than one
     argument, and, of course, the format specification is different.
     Like `strftime', the format string consists of normal text, which
     is output as is, and format specifiers, which are indicated by a
     `%'.  Immediately after the `%', you can optionally specify
     various flags and formatting information before the main
     formatting character, in a similar way to `printf':

        * Immediately following the `%' there can be one or more of the
          following flags:
         `=F'
               The single byte character F is used for this field as
               the numeric fill character.  By default this character
               is a space character.  Filling with this character is
               only performed if a left precision is specified.  It is
               not just to fill to the given field width.

         `^'
               The number is printed without grouping the digits
               according to the rules of the current locale.  By
               default grouping is enabled.

         `+', `('
               At most one of these flags can be used.  They select
               which format to represent the sign of a currency amount.
               By default, and if `+' is given, the locale equivalent
               of +/- is used.  If `(' is given, negative amounts are
               enclosed in parentheses.  The exact format is determined
               by the values of the `LC_MONETARY' category of the
               locale selected at program runtime.

         `!'
               The output will not contain the currency symbol.

         `-'
               The output will be formatted left-justified instead of
               right-justified if it does not fill the entire field
               width.

     The next part of a specification is an optional field width.  If no
     width is specified 0 is taken.  During output, the function first
     determines how much space is required.  If it requires at least as
     many characters as given by the field width, it is output using as
     much space as necessary.  Otherwise, it is extended to use the
     full width by filling with the space character.  The presence or
     absence of the `-' flag determines the side at which such padding
     occurs.  If present, the spaces are added at the right making the
     output left-justified, and vice versa.

     So far the format looks familiar, being similar to the `printf' and
     `strftime' formats.  However, the next two optional fields
     introduce something new.  The first one is a `#' character followed
     by a decimal digit string.  The value of the digit string
     specifies the number of _digit_ positions to the left of the
     decimal point (or equivalent).  This does _not_ include the
     grouping character when the `^' flag is not given.  If the space
     needed to print the number does not fill the whole width, the
     field is padded at the left side with the fill character, which
     can be selected using the `=' flag and by default is a space.  For
     example, if the field width is selected as 6 and the number is
     123, the fill character is `*' the result will be `***123'.

     The second optional field starts with a `.' (period) and consists
     of another decimal digit string.  Its value describes the number of
     characters printed after the decimal point.  The default is
     selected from the current locale (`frac_digits',
     `int_frac_digits', see *note General Numeric::).  If the exact
     representation needs more digits than given by the field width,
     the displayed value is rounded.  If the number of fractional
     digits is selected to be zero, no decimal point is printed.

     As a GNU extension, the `strfmon' implementation in the GNU libc
     allows an optional `L' next as a format modifier.  If this modifier
     is given, the argument is expected to be a `long double' instead of
     a `double' value.

     Finally, the last component is a format specifier.  There are three
     specifiers defined:

    `i'
          Use the locale's rules for formatting an international
          currency value.

    `n'
          Use the locale's rules for formatting a national currency
          value.

    `%'
          Place a `%' in the output.  There must be no flag, width
          specifier or modifier given, only `%%' is allowed.

     As for `printf', the function reads the format string from left to
     right and uses the values passed to the function following the
     format string.  The values are expected to be either of type
     `double' or `long double', depending on the presence of the
     modifier `L'.  The result is stored in the buffer pointed to by S.
     At most MAXSIZE characters are stored.

     The return value of the function is the number of characters
     stored in S, including the terminating `NULL' byte.  If the number
     of characters stored would exceed MAXSIZE, the function returns -1
     and the content of the buffer S is unspecified.  In this case
     `errno' is set to `E2BIG'.

   A few examples should make clear how the function works.  It is
assumed that all the following pieces of code are executed in a program
which uses the USA locale (`en_US').  The simplest form of the format
is this:

     strfmon (buf, 100, "@%n@%n@%n@", 123.45, -567.89, 12345.678);

The output produced is
     "@$123.45@-$567.89@$12,345.68@"

   We can notice several things here.  First, the widths of the output
numbers are different.  We have not specified a width in the format
string, and so this is no wonder.  Second, the third number is printed
using thousands separators.  The thousands separator for the `en_US'
locale is a comma.  The number is also rounded.  .678 is rounded to .68
since the format does not specify a precision and the default value in
the locale is 2.  Finally, note that the national currency symbol is
printed since `%n' was used, not `i'.  The next example shows how we
can align the output.

     strfmon (buf, 100, "@%=*11n@%=*11n@%=*11n@", 123.45, -567.89, 12345.678);

The output this time is:

     "@    $123.45@   -$567.89@ $12,345.68@"

   Two things stand out.  Firstly, all fields have the same width
(eleven characters) since this is the width given in the format and
since no number required more characters to be printed.  The second
important point is that the fill character is not used.  This is
correct since the white space was not used to achieve a precision given
by a `#' modifier, but instead to fill to the given width.  The
difference becomes obvious if we now add a width specification.

     strfmon (buf, 100, "@%=*11#5n@%=*11#5n@%=*11#5n@",
              123.45, -567.89, 12345.678);

The output is

     "@ $***123.45@-$***567.89@ $12,456.68@"

   Here we can see that all the currency symbols are now aligned, and
that the space between the currency sign and the number is filled with
the selected fill character.  Note that although the width is selected
to be 5 and 123.45 has three digits left of the decimal point, the
space is filled with three asterisks.  This is correct since, as
explained above, the width does not include the positions used to store
thousands separators.  One last example should explain the remaining
functionality.

     strfmon (buf, 100, "@%=0(16#5.3i@%=0(16#5.3i@%=0(16#5.3i@",
              123.45, -567.89, 12345.678);

This rather complex format string produces the following output:

     "@ USD 000123,450 @(USD 000567.890)@ USD 12,345.678 @"

   The most noticeable change is the alternative way of representing
negative numbers.  In financial circles this is often done using
parentheses, and this is what the `(' flag selected.  The fill
character is now `0'.  Note that this `0' character is not regarded as
a numeric zero, and therefore the first and second numbers are not
printed using a thousands separator.  Since we used the format
specifier `i' instead of `n', the international form of the currency
symbol is used.  This is a four letter string, in this case `"USD "'.
The last point is that since the precision right of the decimal point
is selected to be three, the first and second numbers are printed with
an extra zero at the end and the third number is printed without
rounding.

File: libc.info,  Node: Yes-or-No Questions,  Prev: Formatting Numbers,  Up: Locales

7.9 Yes-or-No Questions
=======================

Some non GUI programs ask a yes-or-no question.  If the messages
(especially the questions) are translated into foreign languages, be
sure that you localize the answers too.  It would be very bad habit to
ask a question in one language and request the answer in another, often
English.

   The GNU C library contains `rpmatch' to give applications easy
access to the corresponding locale definitions.

 -- Function: int rpmatch (const char *RESPONSE)
     The function `rpmatch' checks the string in RESPONSE whether or
     not it is a correct yes-or-no answer and if yes, which one.  The
     check uses the `YESEXPR' and `NOEXPR' data in the `LC_MESSAGES'
     category of the currently selected locale.  The return value is as
     follows:

    `1'
          The user entered an affirmative answer.

    `0'
          The user entered a negative answer.

    `-1'
          The answer matched neither the `YESEXPR' nor the `NOEXPR'
          regular expression.

     This function is not standardized but available beside in GNU libc
     at least also in the IBM AIX library.

This function would normally be used like this:

       ...
       /* Use a safe default.  */
       _Bool doit = false;

       fputs (gettext ("Do you really want to do this? "), stdout);
       fflush (stdout);
       /* Prepare the `getline' call.  */
       line = NULL;
       len = 0;
       while (getline (&line, &len, stdout) >= 0)
         {
           /* Check the response.  */
           int res = rpmatch (line);
           if (res >= 0)
             {
               /* We got a definitive answer.  */
               if (res > 0)
                 doit = true;
               break;
             }
         }
       /* Free what `getline' allocated.  */
       free (line);

   Note that the loop continues until an read error is detected or
until a definitive (positive or negative) answer is read.

File: libc.info,  Node: Message Translation,  Next: Searching and Sorting,  Prev: Locales,  Up: Top

8 Message Translation
*********************

The program's interface with the human should be designed in a way to
ease the human the task.  One of the possibilities is to use messages in
whatever language the user prefers.

   Printing messages in different languages can be implemented in
different ways.  One could add all the different languages in the
source code and add among the variants every time a message has to be
printed.  This is certainly no good solution since extending the set of
languages is difficult (the code must be changed) and the code itself
can become really big with dozens of message sets.

   A better solution is to keep the message sets for each language are
kept in separate files which are loaded at runtime depending on the
language selection of the user.

   The GNU C Library provides two different sets of functions to support
message translation.  The problem is that neither of the interfaces is
officially defined by the POSIX standard.  The `catgets' family of
functions is defined in the X/Open standard but this is derived from
industry decisions and therefore not necessarily based on reasonable
decisions.

   As mentioned above the message catalog handling provides easy
extendibility by using external data files which contain the message
translations.  I.e., these files contain for each of the messages used
in the program a translation for the appropriate language.  So the tasks
of the message handling functions are

   * locate the external data file with the appropriate translations.

   * load the data and make it possible to address the messages

   * map a given key to the translated message

   The two approaches mainly differ in the implementation of this last
step.  The design decisions made for this influences the whole rest.

* Menu:

* Message catalogs a la X/Open::  The `catgets' family of functions.
* The Uniforum approach::         The `gettext' family of functions.

File: libc.info,  Node: Message catalogs a la X/Open,  Next: The Uniforum approach,  Up: Message Translation

8.1 X/Open Message Catalog Handling
===================================

The `catgets' functions are based on the simple scheme:

     Associate every message to translate in the source code with a
     unique identifier.  To retrieve a message from a catalog file
     solely the identifier is used.

   This means for the author of the program that s/he will have to make
sure the meaning of the identifier in the program code and in the
message catalogs are always the same.

   Before a message can be translated the catalog file must be located.
The user of the program must be able to guide the responsible function
to find whatever catalog the user wants.  This is separated from what
the programmer had in mind.

   All the types, constants and functions for the `catgets' functions
are defined/declared in the `nl_types.h' header file.

* Menu:

* The catgets Functions::      The `catgets' function family.
* The message catalog files::  Format of the message catalog files.
* The gencat program::         How to generate message catalogs files which
                                can be used by the functions.
* Common Usage::               How to use the `catgets' interface.

File: libc.info,  Node: The catgets Functions,  Next: The message catalog files,  Up: Message catalogs a la X/Open

8.1.1 The `catgets' function family
-----------------------------------

 -- Function: nl_catd catopen (const char *CAT_NAME, int FLAG)
     The `catgets' function tries to locate the message data file names
     CAT_NAME and loads it when found.  The return value is of an
     opaque type and can be used in calls to the other functions to
     refer to this loaded catalog.

     The return value is `(nl_catd) -1' in case the function failed and
     no catalog was loaded.  The global variable ERRNO contains a code
     for the error causing the failure.  But even if the function call
     succeeded this does not mean that all messages can be translated.

     Locating the catalog file must happen in a way which lets the user
     of the program influence the decision.  It is up to the user to
     decide about the language to use and sometimes it is useful to use
     alternate catalog files.  All this can be specified by the user by
     setting some environment variables.

     The first problem is to find out where all the message catalogs are
     stored.  Every program could have its own place to keep all the
     different files but usually the catalog files are grouped by
     languages and the catalogs for all programs are kept in the same
     place.

     To tell the `catopen' function where the catalog for the program
     can be found the user can set the environment variable `NLSPATH' to
     a value which describes her/his choice.  Since this value must be
     usable for different languages and locales it cannot be a simple
     string.  Instead it is a format string (similar to `printf''s).
     An example is

          /usr/share/locale/%L/%N:/usr/share/locale/%L/LC_MESSAGES/%N

     First one can see that more than one directory can be specified
     (with the usual syntax of separating them by colons).  The next
     things to observe are the format string, `%L' and `%N' in this
     case.  The `catopen' function knows about several of them and the
     replacement for all of them is of course different.

    `%N'
          This format element is substituted with the name of the
          catalog file.  This is the value of the CAT_NAME argument
          given to `catgets'.

    `%L'
          This format element is substituted with the name of the
          currently selected locale for translating messages.  How this
          is determined is explained below.

    `%l'
          (This is the lowercase ell.) This format element is
          substituted with the language element of the locale name.
          The string describing the selected locale is expected to have
          the form `LANG[_TERR[.CODESET]]' and this format uses the
          first part LANG.

    `%t'
          This format element is substituted by the territory part TERR
          of the name of the currently selected locale.  See the
          explanation of the format above.

    `%c'
          This format element is substituted by the codeset part
          CODESET of the name of the currently selected locale.  See
          the explanation of the format above.

    `%%'
          Since `%' is used in a meta character there must be a way to
          express the `%' character in the result itself.  Using `%%'
          does this just like it works for `printf'.

     Using `NLSPATH' allows arbitrary directories to be searched for
     message catalogs while still allowing different languages to be
     used.  If the `NLSPATH' environment variable is not set, the
     default value is

          PREFIX/share/locale/%L/%N:PREFIX/share/locale/%L/LC_MESSAGES/%N

     where PREFIX is given to `configure' while installing the GNU C
     Library (this value is in many cases `/usr' or the empty string).

     The remaining problem is to decide which must be used.  The value
     decides about the substitution of the format elements mentioned
     above.  First of all the user can specify a path in the message
     catalog name (i.e., the name contains a slash character).  In this
     situation the `NLSPATH' environment variable is not used.  The
     catalog must exist as specified in the program, perhaps relative
     to the current working directory.  This situation in not desirable
     and catalogs names never should be written this way.  Beside this,
     this behavior is not portable to all other platforms providing the
     `catgets' interface.

     Otherwise the values of environment variables from the standard
     environment are examined (*note Standard Environment::).  Which
     variables are examined is decided by the FLAG parameter of
     `catopen'.  If the value is `NL_CAT_LOCALE' (which is defined in
     `nl_types.h') then the `catopen' function use the name of the
     locale currently selected for the `LC_MESSAGES' category.

     If FLAG is zero the `LANG' environment variable is examined.  This
     is a left-over from the early days where the concept of the locales
     had not even reached the level of POSIX locales.

     The environment variable and the locale name should have a value
     of the form `LANG[_TERR[.CODESET]]' as explained above.  If no
     environment variable is set the `"C"' locale is used which
     prevents any translation.

     The return value of the function is in any case a valid string.
     Either it is a translation from a message catalog or it is the
     same as the STRING parameter.  So a piece of code to decide
     whether a translation actually happened must look like this:

          {
            char *trans = catgets (desc, set, msg, input_string);
            if (trans == input_string)
              {
                /* Something went wrong.  */
              }
          }

     When an error occurred the global variable ERRNO is set to

    EBADF
          The catalog does not exist.

    ENOMSG
          The set/message tuple does not name an existing element in the
          message catalog.

     While it sometimes can be useful to test for errors programs
     normally will avoid any test.  If the translation is not available
     it is no big problem if the original, untranslated message is
     printed.  Either the user understands this as well or s/he will
     look for the reason why the messages are not translated.

   Please note that the currently selected locale does not depend on a
call to the `setlocale' function.  It is not necessary that the locale
data files for this locale exist and calling `setlocale' succeeds.  The
`catopen' function directly reads the values of the environment
variables.

 -- Function: char * catgets (nl_catd CATALOG_DESC, int SET, int
          MESSAGE, const char *STRING)
     The function `catgets' has to be used to access the massage catalog
     previously opened using the `catopen' function.  The CATALOG_DESC
     parameter must be a value previously returned by `catopen'.

     The next two parameters, SET and MESSAGE, reflect the internal
     organization of the message catalog files.  This will be explained
     in detail below.  For now it is interesting to know that a catalog
     can consists of several set and the messages in each thread are
     individually numbered using numbers.  Neither the set number nor
     the message number must be consecutive.  They can be arbitrarily
     chosen.  But each message (unless equal to another one) must have
     its own unique pair of set and message number.

     Since it is not guaranteed that the message catalog for the
     language selected by the user exists the last parameter STRING
     helps to handle this case gracefully.  If no matching string can
     be found STRING is returned.  This means for the programmer that

        * the STRING parameters should contain reasonable text (this
          also helps to understand the program seems otherwise there
          would be no hint on the string which is expected to be
          returned.

        * all STRING arguments should be written in the same language.

   It is somewhat uncomfortable to write a program using the `catgets'
functions if no supporting functionality is available.  Since each
set/message number tuple must be unique the programmer must keep lists
of the messages at the same time the code is written.  And the work
between several people working on the same project must be coordinated.
We will see some how these problems can be relaxed a bit (*note Common
Usage::).

 -- Function: int catclose (nl_catd CATALOG_DESC)
     The `catclose' function can be used to free the resources
     associated with a message catalog which previously was opened by a
     call to `catopen'.  If the resources can be successfully freed the
     function returns `0'.  Otherwise it return `-1' and the global
     variable ERRNO is set.  Errors can occur if the catalog descriptor
     CATALOG_DESC is not valid in which case ERRNO is set to `EBADF'.

File: libc.info,  Node: The message catalog files,  Next: The gencat program,  Prev: The catgets Functions,  Up: Message catalogs a la X/Open

8.1.2 Format of the message catalog files
-----------------------------------------

The only reasonable way the translate all the messages of a function and
store the result in a message catalog file which can be read by the
`catopen' function is to write all the message text to the translator
and let her/him translate them all.  I.e., we must have a file with
entries which associate the set/message tuple with a specific
translation.  This file format is specified in the X/Open standard and
is as follows:

   * Lines containing only whitespace characters or empty lines are
     ignored.

   * Lines which contain as the first non-whitespace character a `$'
     followed by a whitespace character are comment and are also
     ignored.

   * If a line contains as the first non-whitespace characters the
     sequence `$set' followed by a whitespace character an additional
     argument is required to follow.  This argument can either be:

        - a number.  In this case the value of this number determines
          the set to which the following messages are added.

        - an identifier consisting of alphanumeric characters plus the
          underscore character.  In this case the set get automatically
          a number assigned.  This value is one added to the largest
          set number which so far appeared.

          How to use the symbolic names is explained in section *note
          Common Usage::.

          It is an error if a symbol name appears more than once.  All
          following messages are placed in a set with this number.

   * If a line contains as the first non-whitespace characters the
     sequence `$delset' followed by a whitespace character an
     additional argument is required to follow.  This argument can
     either be:

        - a number.  In this case the value of this number determines
          the set which will be deleted.

        - an identifier consisting of alphanumeric characters plus the
          underscore character.  This symbolic identifier must match a
          name for a set which previously was defined.  It is an error
          if the name is unknown.

     In both cases all messages in the specified set will be removed.
     They will not appear in the output.  But if this set is later
     again selected with a `$set' command again messages could be added
     and these messages will appear in the output.

   * If a line contains after leading whitespaces the sequence
     `$quote', the quoting character used for this input file is
     changed to the first non-whitespace character following the
     `$quote'.  If no non-whitespace character is present before the
     line ends quoting is disable.

     By default no quoting character is used.  In this mode strings are
     terminated with the first unescaped line break.  If there is a
     `$quote' sequence present newline need not be escaped.  Instead a
     string is terminated with the first unescaped appearance of the
     quote character.

     A common usage of this feature would be to set the quote character
     to `"'.  Then any appearance of the `"' in the strings must be
     escaped using the backslash (i.e., `\"' must be written).

   * Any other line must start with a number or an alphanumeric
     identifier (with the underscore character included).  The
     following characters (starting after the first whitespace
     character) will form the string which gets associated with the
     currently selected set and the message number represented by the
     number and identifier respectively.

     If the start of the line is a number the message number is
     obvious.  It is an error if the same message number already
     appeared for this set.

     If the leading token was an identifier the message number gets
     automatically assigned.  The value is the current maximum messages
     number for this set plus one.  It is an error if the identifier was
     already used for a message in this set.  It is OK to reuse the
     identifier for a message in another thread.  How to use the
     symbolic identifiers will be explained below (*note Common
     Usage::).  There is one limitation with the identifier: it must
     not be `Set'.  The reason will be explained below.

     The text of the messages can contain escape characters.  The usual
     bunch of characters known from the ISO C language are recognized
     (`\n', `\t', `\v', `\b', `\r', `\f', `\\', and `\NNN', where NNN
     is the octal coding of a character code).

   *Important:* The handling of identifiers instead of numbers for the
set and messages is a GNU extension.  Systems strictly following the
X/Open specification do not have this feature.  An example for a message
catalog file is this:

     $ This is a leading comment.
     $quote "

     $set SetOne
     1 Message with ID 1.
     two "   Message with ID \"two\", which gets the value 2 assigned"

     $set SetTwo
     $ Since the last set got the number 1 assigned this set has number 2.
     4000 "The numbers can be arbitrary, they need not start at one."

   This small example shows various aspects:
   * Lines 1 and 9 are comments since they start with `$' followed by a
     whitespace.

   * The quoting character is set to `"'.  Otherwise the quotes in the
     message definition would have to be left away and in this case the
     message with the identifier `two' would loose its leading
     whitespace.

   * Mixing numbered messages with message having symbolic names is no
     problem and the numbering happens automatically.

   While this file format is pretty easy it is not the best possible for
use in a running program.  The `catopen' function would have to parser
the file and handle syntactic errors gracefully.  This is not so easy
and the whole process is pretty slow.  Therefore the `catgets'
functions expect the data in another more compact and ready-to-use file
format.  There is a special program `gencat' which is explained in
detail in the next section.

   Files in this other format are not human readable.  To be easy to
use by programs it is a binary file.  But the format is byte order
independent so translation files can be shared by systems of arbitrary
architecture (as long as they use the GNU C Library).

   Details about the binary file format are not important to know since
these files are always created by the `gencat' program.  The sources of
the GNU C Library also provide the sources for the `gencat' program and
so the interested reader can look through these source files to learn
about the file format.

File: libc.info,  Node: The gencat program,  Next: Common Usage,  Prev: The message catalog files,  Up: Message catalogs a la X/Open

8.1.3 Generate Message Catalogs files
-------------------------------------

The `gencat' program is specified in the X/Open standard and the GNU
implementation follows this specification and so processes all
correctly formed input files.  Additionally some extension are
implemented which help to work in a more reasonable way with the
`catgets' functions.

   The `gencat' program can be invoked in two ways:

     `gencat [OPTION]... [OUTPUT-FILE [INPUT-FILE]...]`

   This is the interface defined in the X/Open standard.  If no
INPUT-FILE parameter is given input will be read from standard input.
Multiple input files will be read as if they are concatenated.  If
OUTPUT-FILE is also missing, the output will be written to standard
output.  To provide the interface one is used to from other programs a
second interface is provided.

     `gencat [OPTION]... -o OUTPUT-FILE [INPUT-FILE]...`

   The option `-o' is used to specify the output file and all file
arguments are used as input files.

   Beside this one can use `-' or `/dev/stdin' for INPUT-FILE to denote
the standard input.  Corresponding one can use `-' and `/dev/stdout'
for OUTPUT-FILE to denote standard output.  Using `-' as a file name is
allowed in X/Open while using the device names is a GNU extension.

   The `gencat' program works by concatenating all input files and then
*merge* the resulting collection of message sets with a possibly
existing output file.  This is done by removing all messages with
set/message number tuples matching any of the generated messages from
the output file and then adding all the new messages.  To regenerate a
catalog file while ignoring the old contents therefore requires to
remove the output file if it exists.  If the output is written to
standard output no merging takes place.

The following table shows the options understood by the `gencat'
program.  The X/Open standard does not specify any option for the
program so all of these are GNU extensions.

`-V'
`--version'
     Print the version information and exit.

`-h'
`--help'
     Print a usage message listing all available options, then exit
     successfully.

`--new'
     Do never merge the new messages from the input files with the old
     content of the output files.  The old content of the output file
     is discarded.

`-H'
`--header=name'
     This option is used to emit the symbolic names given to sets and
     messages in the input files for use in the program.  Details about
     how to use this are given in the next section.  The NAME parameter
     to this option specifies the name of the output file.  It will
     contain a number of C preprocessor `#define's to associate a name
     with a number.

     Please note that the generated file only contains the symbols from
     the input files.  If the output is merged with the previous
     content of the output file the possibly existing symbols from the
     file(s) which generated the old output files are not in the
     generated header file.

File: libc.info,  Node: Common Usage,  Prev: The gencat program,  Up: Message catalogs a la X/Open

8.1.4 How to use the `catgets' interface
----------------------------------------

The `catgets' functions can be used in two different ways.  By
following slavishly the X/Open specs and not relying on the extension
and by using the GNU extensions.  We will take a look at the former
method first to understand the benefits of extensions.

8.1.4.1 Not using symbolic names
................................

Since the X/Open format of the message catalog files does not allow
symbol names we have to work with numbers all the time.  When we start
writing a program we have to replace all appearances of translatable
strings with something like

     catgets (catdesc, set, msg, "string")

CATGETS is retrieved from a call to `catopen' which is normally done
once at the program start.  The `"string"' is the string we want to
translate.  The problems start with the set and message numbers.

   In a bigger program several programmers usually work at the same
time on the program and so coordinating the number allocation is
crucial.  Though no two different strings must be indexed by the same
tuple of numbers it is highly desirable to reuse the numbers for equal
strings with equal translations (please note that there might be
strings which are equal in one language but have different translations
due to difference contexts).

   The allocation process can be relaxed a bit by different set numbers
for different parts of the program.  So the number of developers who
have to coordinate the allocation can be reduced.  But still lists must
be keep track of the allocation and errors can easily happen.  These
errors cannot be discovered by the compiler or the `catgets' functions.
Only the user of the program might see wrong messages printed.  In the
worst cases the messages are so irritating that they cannot be
recognized as wrong.  Think about the translations for `"true"' and
`"false"' being exchanged.  This could result in a disaster.

8.1.4.2 Using symbolic names
............................

The problems mentioned in the last section derive from the fact that:

  1. the numbers are allocated once and due to the possibly frequent
     use of them it is difficult to change a number later.

  2. the numbers do not allow to guess anything about the string and
     therefore collisions can easily happen.

   By constantly using symbolic names and by providing a method which
maps the string content to a symbolic name (however this will happen)
one can prevent both problems above.  The cost of this is that the
programmer has to write a complete message catalog file while s/he is
writing the program itself.

   This is necessary since the symbolic names must be mapped to numbers
before the program sources can be compiled.  In the last section it was
described how to generate a header containing the mapping of the names.
E.g., for the example message file given in the last section we could
call the `gencat' program as follow (assume `ex.msg' contains the
sources).

     gencat -H ex.h -o ex.cat ex.msg

This generates a header file with the following content:

     #define SetTwoSet 0x2   /* ex.msg:8 */

     #define SetOneSet 0x1   /* ex.msg:4 */
     #define SetOnetwo 0x2   /* ex.msg:6 */

   As can be seen the various symbols given in the source file are
mangled to generate unique identifiers and these identifiers get numbers
assigned.  Reading the source file and knowing about the rules will
allow to predict the content of the header file (it is deterministic)
but this is not necessary.  The `gencat' program can take care for
everything.  All the programmer has to do is to put the generated header
file in the dependency list of the source files of her/his project and
to add a rules to regenerate the header of any of the input files
change.

   One word about the symbol mangling.  Every symbol consists of two
parts: the name of the message set plus the name of the message or the
special string `Set'.  So `SetOnetwo' means this macro can be used to
access the translation with identifier `two' in the message set
`SetOne'.

   The other names denote the names of the message sets.  The special
string `Set' is used in the place of the message identifier.

   If in the code the second string of the set `SetOne' is used the C
code should look like this:

     catgets (catdesc, SetOneSet, SetOnetwo,
              "   Message with ID \"two\", which gets the value 2 assigned")

   Writing the function this way will allow to change the message number
and even the set number without requiring any change in the C source
code.  (The text of the string is normally not the same; this is only
for this example.)

8.1.4.3 How does to this allow to develop
.........................................

To illustrate the usual way to work with the symbolic version numbers
here is a little example.  Assume we want to write the very complex and
famous greeting program.  We start by writing the code as usual:

     #include <stdio.h>
     int
     main (void)
     {
       printf ("Hello, world!\n");
       return 0;
     }

   Now we want to internationalize the message and therefore replace the
message with whatever the user wants.

     #include <nl_types.h>
     #include <stdio.h>
     #include "msgnrs.h"
     int
     main (void)
     {
       nl_catd catdesc = catopen ("hello.cat", NL_CAT_LOCALE);
       printf (catgets (catdesc, SetMainSet, SetMainHello,
                        "Hello, world!\n"));
       catclose (catdesc);
       return 0;
     }

   We see how the catalog object is opened and the returned descriptor
used in the other function calls.  It is not really necessary to check
for failure of any of the functions since even in these situations the
functions will behave reasonable.  They simply will be return a
translation.

   What remains unspecified here are the constants `SetMainSet' and
`SetMainHello'.  These are the symbolic names describing the message.
To get the actual definitions which match the information in the
catalog file we have to create the message catalog source file and
process it using the `gencat' program.

     $ Messages for the famous greeting program.
     $quote "

     $set Main
     Hello "Hallo, Welt!\n"

   Now we can start building the program (assume the message catalog
source file is named `hello.msg' and the program source file `hello.c'):

     % gencat -H msgnrs.h -o hello.cat hello.msg
     % cat msgnrs.h
     #define MainSet 0x1     /* hello.msg:4 */
     #define MainHello 0x1   /* hello.msg:5 */
     % gcc -o hello hello.c -I.
     % cp hello.cat /usr/share/locale/de/LC_MESSAGES
     % echo $LC_ALL
     de
     % ./hello
     Hallo, Welt!
     %

   The call of the `gencat' program creates the missing header file
`msgnrs.h' as well as the message catalog binary.  The former is used
in the compilation of `hello.c' while the later is placed in a
directory in which the `catopen' function will try to locate it.
Please check the `LC_ALL' environment variable and the default path for
`catopen' presented in the description above.

File: libc.info,  Node: The Uniforum approach,  Prev: Message catalogs a la X/Open,  Up: Message Translation

8.2 The Uniforum approach to Message Translation
================================================

Sun Microsystems tried to standardize a different approach to message
translation in the Uniforum group.  There never was a real standard
defined but still the interface was used in Sun's operation systems.
Since this approach fits better in the development process of free
software it is also used throughout the GNU project and the GNU
`gettext' package provides support for this outside the GNU C Library.

   The code of the `libintl' from GNU `gettext' is the same as the code
in the GNU C Library.  So the documentation in the GNU `gettext' manual
is also valid for the functionality here.  The following text will
describe the library functions in detail.  But the numerous helper
programs are not described in this manual.  Instead people should read
the GNU `gettext' manual (*note GNU gettext utilities: (gettext)Top.).
We will only give a short overview.

   Though the `catgets' functions are available by default on more
systems the `gettext' interface is at least as portable as the former.
The GNU `gettext' package can be used wherever the functions are not
available.

* Menu:

* Message catalogs with gettext::  The `gettext' family of functions.
* Helper programs for gettext::    Programs to handle message catalogs
                                    for `gettext'.

File: libc.info,  Node: Message catalogs with gettext,  Next: Helper programs for gettext,  Up: The Uniforum approach

8.2.1 The `gettext' family of functions
---------------------------------------

The paradigms underlying the `gettext' approach to message translations
is different from that of the `catgets' functions the basic
functionally is equivalent.  There are functions of the following
categories:

* Menu:

* Translation with gettext::       What has to be done to translate a message.
* Locating gettext catalog::       How to determine which catalog to be used.
* Advanced gettext functions::     Additional functions for more complicated
                                    situations.
* Charset conversion in gettext::  How to specify the output character set
                                    `gettext' uses.
* GUI program problems::           How to use `gettext' in GUI programs.
* Using gettextized software::     The possibilities of the user to influence
                                    the way `gettext' works.

File: libc.info,  Node: Translation with gettext,  Next: Locating gettext catalog,  Up: Message catalogs with gettext

8.2.1.1 What has to be done to translate a message?
...................................................

The `gettext' functions have a very simple interface.  The most basic
function just takes the string which shall be translated as the
argument and it returns the translation.  This is fundamentally
different from the `catgets' approach where an extra key is necessary
and the original string is only used for the error case.

   If the string which has to be translated is the only argument this of
course means the string itself is the key.  I.e., the translation will
be selected based on the original string.  The message catalogs must
therefore contain the original strings plus one translation for any such
string.  The task of the `gettext' function is it to compare the
argument string with the available strings in the catalog and return the
appropriate translation.  Of course this process is optimized so that
this process is not more expensive than an access using an atomic key
like in `catgets'.

   The `gettext' approach has some advantages but also some
disadvantages.  Please see the GNU `gettext' manual for a detailed
discussion of the pros and cons.

   All the definitions and declarations for `gettext' can be found in
the `libintl.h' header file.  On systems where these functions are not
part of the C library they can be found in a separate library named
`libintl.a' (or accordingly different for shared libraries).

 -- Function: char * gettext (const char *MSGID)
     The `gettext' function searches the currently selected message
     catalogs for a string which is equal to MSGID.  If there is such a
     string available it is returned.  Otherwise the argument string
     MSGID is returned.

     Please note that all though the return value is `char *' the
     returned string must not be changed.  This broken type results
     from the history of the function and does not reflect the way the
     function should be used.

     Please note that above we wrote "message catalogs" (plural).  This
     is a specialty of the GNU implementation of these functions and we
     will say more about this when we talk about the ways message
     catalogs are selected (*note Locating gettext catalog::).

     The `gettext' function does not modify the value of the global
     ERRNO variable.  This is necessary to make it possible to write
     something like

            printf (gettext ("Operation failed: %m\n"));

     Here the ERRNO value is used in the `printf' function while
     processing the `%m' format element and if the `gettext' function
     would change this value (it is called before `printf' is called)
     we would get a wrong message.

     So there is no easy way to detect a missing message catalog beside
     comparing the argument string with the result.  But it is normally
     the task of the user to react on missing catalogs.  The program
     cannot guess when a message catalog is really necessary since for
     a user who speaks the language the program was developed in does
     not need any translation.

   The remaining two functions to access the message catalog add some
functionality to select a message catalog which is not the default one.
This is important if parts of the program are developed independently.
Every part can have its own message catalog and all of them can be used
at the same time.  The C library itself is an example: internally it
uses the `gettext' functions but since it must not depend on a
currently selected default message catalog it must specify all ambiguous
information.

 -- Function: char * dgettext (const char *DOMAINNAME, const char
          *MSGID)
     The `dgettext' functions acts just like the `gettext' function.
     It only takes an additional first argument DOMAINNAME which guides
     the selection of the message catalogs which are searched for the
     translation.  If the DOMAINNAME parameter is the null pointer the
     `dgettext' function is exactly equivalent to `gettext' since the
     default value for the domain name is used.

     As for `gettext' the return value type is `char *' which is an
     anachronism.  The returned string must never be modified.

 -- Function: char * dcgettext (const char *DOMAINNAME, const char
          *MSGID, int CATEGORY)
     The `dcgettext' adds another argument to those which `dgettext'
     takes.  This argument CATEGORY specifies the last piece of
     information needed to localize the message catalog.  I.e., the
     domain name and the locale category exactly specify which message
     catalog has to be used (relative to a given directory, see below).

     The `dgettext' function can be expressed in terms of `dcgettext'
     by using

          dcgettext (domain, string, LC_MESSAGES)

     instead of

          dgettext (domain, string)

     This also shows which values are expected for the third parameter.
     One has to use the available selectors for the categories
     available in `locale.h'.  Normally the available values are
     `LC_CTYPE', `LC_COLLATE', `LC_MESSAGES', `LC_MONETARY',
     `LC_NUMERIC', and `LC_TIME'.  Please note that `LC_ALL' must not
     be used and even though the names might suggest this, there is no
     relation to the environments variables of this name.

     The `dcgettext' function is only implemented for compatibility with
     other systems which have `gettext' functions.  There is not really
     any situation where it is necessary (or useful) to use a different
     value but `LC_MESSAGES' in for the CATEGORY parameter.  We are
     dealing with messages here and any other choice can only be
     irritating.

     As for `gettext' the return value type is `char *' which is an
     anachronism.  The returned string must never be modified.

   When using the three functions above in a program it is a frequent
case that the MSGID argument is a constant string.  So it is worth to
optimize this case.  Thinking shortly about this one will realize that
as long as no new message catalog is loaded the translation of a message
will not change.  This optimization is actually implemented by the
`gettext', `dgettext' and `dcgettext' functions.

File: libc.info,  Node: Locating gettext catalog,  Next: Advanced gettext functions,  Prev: Translation with gettext,  Up: Message catalogs with gettext

8.2.1.2 How to determine which catalog to be used
.................................................

The functions to retrieve the translations for a given message have a
remarkable simple interface.  But to provide the user of the program
still the opportunity to select exactly the translation s/he wants and
also to provide the programmer the possibility to influence the way to
locate the search for catalogs files there is a quite complicated
underlying mechanism which controls all this.  The code is complicated
the use is easy.

   Basically we have two different tasks to perform which can also be
performed by the `catgets' functions:

  1. Locate the set of message catalogs.  There are a number of files
     for different languages and which all belong to the package.
     Usually they are all stored in the filesystem below a certain
     directory.

     There can be arbitrary many packages installed and they can follow
     different guidelines for the placement of their files.

  2. Relative to the location specified by the package the actual
     translation files must be searched, based on the wishes of the
     user.  I.e., for each language the user selects the program should
     be able to locate the appropriate file.

   This is the functionality required by the specifications for
`gettext' and this is also what the `catgets' functions are able to do.
But there are some problems unresolved:

   * The language to be used can be specified in several different ways.
     There is no generally accepted standard for this and the user
     always expects the program understand what s/he means.  E.g., to
     select the German translation one could write `de', `german', or
     `deutsch' and the program should always react the same.

   * Sometimes the specification of the user is too detailed.  If s/he,
     e.g., specifies `de_DE.ISO-8859-1' which means German, spoken in
     Germany, coded using the ISO 8859-1 character set there is the
     possibility that a message catalog matching this exactly is not
     available.  But there could be a catalog matching `de' and if the
     character set used on the machine is always ISO 8859-1 there is no
     reason why this later message catalog should not be used.  (We
     call this "message inheritance".)

   * If a catalog for a wanted language is not available it is not
     always the second best choice to fall back on the language of the
     developer and simply not translate any message.  Instead a user
     might be better able to read the messages in another language and
     so the user of the program should be able to define an precedence
     order of languages.

   We can divide the configuration actions in two parts: the one is
performed by the programmer, the other by the user.  We will start with
the functions the programmer can use since the user configuration will
be based on this.

   As the functions described in the last sections already mention
separate sets of messages can be selected by a "domain name".  This is a
simple string which should be unique for each program part with uses a
separate domain.  It is possible to use in one program arbitrary many
domains at the same time.  E.g., the GNU C Library itself uses a domain
named `libc' while the program using the C Library could use a domain
named `foo'.  The important point is that at any time exactly one
domain is active.  This is controlled with the following function.

 -- Function: char * textdomain (const char *DOMAINNAME)
     The `textdomain' function sets the default domain, which is used in
     all future `gettext' calls, to DOMAINNAME.  Please note that
     `dgettext' and `dcgettext' calls are not influenced if the
     DOMAINNAME parameter of these functions is not the null pointer.

     Before the first call to `textdomain' the default domain is
     `messages'.  This is the name specified in the specification of
     the `gettext' API.  This name is as good as any other name.  No
     program should ever really use a domain with this name since this
     can only lead to problems.

     The function returns the value which is from now on taken as the
     default domain.  If the system went out of memory the returned
     value is `NULL' and the global variable ERRNO is set to `ENOMEM'.
     Despite the return value type being `char *' the return string must
     not be changed.  It is allocated internally by the `textdomain'
     function.

     If the DOMAINNAME parameter is the null pointer no new default
     domain is set.  Instead the currently selected default domain is
     returned.

     If the DOMAINNAME parameter is the empty string the default domain
     is reset to its initial value, the domain with the name `messages'.
     This possibility is questionable to use since the domain `messages'
     really never should be used.

 -- Function: char * bindtextdomain (const char *DOMAINNAME, const char
          *DIRNAME)
     The `bindtextdomain' function can be used to specify the directory
     which contains the message catalogs for domain DOMAINNAME for the
     different languages.  To be correct, this is the directory where
     the hierarchy of directories is expected.  Details are explained
     below.

     For the programmer it is important to note that the translations
     which come with the program have be placed in a directory
     hierarchy starting at, say, `/foo/bar'.  Then the program should
     make a `bindtextdomain' call to bind the domain for the current
     program to this directory.  So it is made sure the catalogs are
     found.  A correctly running program does not depend on the user
     setting an environment variable.

     The `bindtextdomain' function can be used several times and if the
     DOMAINNAME argument is different the previously bound domains will
     not be overwritten.

     If the program which wish to use `bindtextdomain' at some point of
     time use the `chdir' function to change the current working
     directory it is important that the DIRNAME strings ought to be an
     absolute pathname.  Otherwise the addressed directory might vary
     with the time.

     If the DIRNAME parameter is the null pointer `bindtextdomain'
     returns the currently selected directory for the domain with the
     name DOMAINNAME.

     The `bindtextdomain' function returns a pointer to a string
     containing the name of the selected directory name.  The string is
     allocated internally in the function and must not be changed by the
     user.  If the system went out of core during the execution of
     `bindtextdomain' the return value is `NULL' and the global
     variable ERRNO is set accordingly.

File: libc.info,  Node: Advanced gettext functions,  Next: Charset conversion in gettext,  Prev: Locating gettext catalog,  Up: Message catalogs with gettext

8.2.1.3 Additional functions for more complicated situations
............................................................

The functions of the `gettext' family described so far (and all the
`catgets' functions as well) have one problem in the real world which
have been neglected completely in all existing approaches.  What is
meant here is the handling of plural forms.

   Looking through Unix source code before the time anybody thought
about internationalization (and, sadly, even afterwards) one can often
find code similar to the following:

        printf ("%d file%s deleted", n, n == 1 ? "" : "s");

After the first complaints from people internationalizing the code
people either completely avoided formulations like this or used strings
like `"file(s)"'.  Both look unnatural and should be avoided.  First
tries to solve the problem correctly looked like this:

        if (n == 1)
          printf ("%d file deleted", n);
        else
          printf ("%d files deleted", n);

   But this does not solve the problem.  It helps languages where the
plural form of a noun is not simply constructed by adding an `s' but
that is all.  Once again people fell into the trap of believing the
rules their language is using are universal.  But the handling of plural
forms differs widely between the language families.  There are two
things we can differ between (and even inside language families);

   * The form how plural forms are build differs.  This is a problem
     with language which have many irregularities.  German, for
     instance, is a drastic case.  Though English and German are part
     of the same language family (Germanic), the almost regular forming
     of plural noun forms (appending an `s') is hardly found in German.

   * The number of plural forms differ.  This is somewhat surprising for
     those who only have experiences with Romanic and Germanic languages
     since here the number is the same (there are two).

     But other language families have only one form or many forms.  More
     information on this in an extra section.

   The consequence of this is that application writers should not try to
solve the problem in their code.  This would be localization since it is
only usable for certain, hardcoded language environments.  Instead the
extended `gettext' interface should be used.

   These extra functions are taking instead of the one key string two
strings and an numerical argument.  The idea behind this is that using
the numerical argument and the first string as a key, the implementation
can select using rules specified by the translator the right plural
form.  The two string arguments then will be used to provide a return
value in case no message catalog is found (similar to the normal
`gettext' behavior).  In this case the rules for Germanic language is
used and it is assumed that the first string argument is the singular
form, the second the plural form.

   This has the consequence that programs without language catalogs can
display the correct strings only if the program itself is written using
a Germanic language.  This is a limitation but since the GNU C library
(as well as the GNU `gettext' package) are written as part of the GNU
package and the coding standards for the GNU project require program
being written in English, this solution nevertheless fulfills its
purpose.

 -- Function: char * ngettext (const char *MSGID1, const char *MSGID2,
          unsigned long int N)
     The `ngettext' function is similar to the `gettext' function as it
     finds the message catalogs in the same way.  But it takes two
     extra arguments.  The MSGID1 parameter must contain the singular
     form of the string to be converted.  It is also used as the key
     for the search in the catalog.  The MSGID2 parameter is the plural
     form.  The parameter N is used to determine the plural form.  If no
     message catalog is found MSGID1 is returned if `n == 1', otherwise
     `msgid2'.

     An example for the us of this function is:

            printf (ngettext ("%d file removed", "%d files removed", n), n);

     Please note that the numeric value N has to be passed to the
     `printf' function as well.  It is not sufficient to pass it only to
     `ngettext'.

 -- Function: char * dngettext (const char *DOMAIN, const char *MSGID1,
          const char *MSGID2, unsigned long int N)
     The `dngettext' is similar to the `dgettext' function in the way
     the message catalog is selected.  The difference is that it takes
     two extra parameter to provide the correct plural form.  These two
     parameters are handled in the same way `ngettext' handles them.

 -- Function: char * dcngettext (const char *DOMAIN, const char
          *MSGID1, const char *MSGID2, unsigned long int N, int
          CATEGORY)
     The `dcngettext' is similar to the `dcgettext' function in the way
     the message catalog is selected.  The difference is that it takes
     two extra parameter to provide the correct plural form.  These two
     parameters are handled in the same way `ngettext' handles them.

The problem of plural forms
...........................

A description of the problem can be found at the beginning of the last
section.  Now there is the question how to solve it.  Without the input
of linguists (which was not available) it was not possible to determine
whether there are only a few different forms in which plural forms are
formed or whether the number can increase with every new supported
language.

   Therefore the solution implemented is to allow the translator to
specify the rules of how to select the plural form.  Since the formula
varies with every language this is the only viable solution except for
hardcoding the information in the code (which still would require the
possibility of extensions to not prevent the use of new languages).  The
details are explained in the GNU `gettext' manual.  Here only a bit of
information is provided.

   The information about the plural form selection has to be stored in
the header entry (the one with the empty (`msgid' string).  It looks
like this:

     Plural-Forms: nplurals=2; plural=n == 1 ? 0 : 1;

   The `nplurals' value must be a decimal number which specifies how
many different plural forms exist for this language.  The string
following `plural' is an expression which is using the C language
syntax.  Exceptions are that no negative number are allowed, numbers
must be decimal, and the only variable allowed is `n'.  This expression
will be evaluated whenever one of the functions `ngettext',
`dngettext', or `dcngettext' is called.  The numeric value passed to
these functions is then substituted for all uses of the variable `n' in
the expression.  The resulting value then must be greater or equal to
zero and smaller than the value given as the value of `nplurals'.

The following rules are known at this point.  The language with families
are listed.  But this does not necessarily mean the information can be
generalized for the whole family (as can be easily seen in the table
below).(1)

Only one form:
     Some languages only require one single form.  There is no
     distinction between the singular and plural form.  An appropriate
     header entry would look like this:

          Plural-Forms: nplurals=1; plural=0;

     Languages with this property include:

    Finno-Ugric family
          Hungarian

    Asian family
          Japanese, Korean

    Turkic/Altaic family
          Turkish

Two forms, singular used for one only
     This is the form used in most existing programs since it is what
     English is using.  A header entry would look like this:

          Plural-Forms: nplurals=2; plural=n != 1;

     (Note: this uses the feature of C expressions that boolean
     expressions have to value zero or one.)

     Languages with this property include:

    Germanic family
          Danish, Dutch, English, German, Norwegian, Swedish

    Finno-Ugric family
          Estonian, Finnish

    Latin/Greek family
          Greek

    Semitic family
          Hebrew

    Romance family
          Italian, Portuguese, Spanish

    Artificial
          Esperanto

Two forms, singular used for zero and one
     Exceptional case in the language family.  The header entry would
     be:

          Plural-Forms: nplurals=2; plural=n>1;

     Languages with this property include:

    Romanic family
          French, Brazilian Portuguese

Three forms, special case for zero
     The header entry would be:

          Plural-Forms: nplurals=3; plural=n%10==1 && n%100!=11 ? 0 : n != 0 ? 1 : 2;

     Languages with this property include:

    Baltic family
          Latvian

Three forms, special cases for one and two
     The header entry would be:

          Plural-Forms: nplurals=3; plural=n==1 ? 0 : n==2 ? 1 : 2;

     Languages with this property include:

    Celtic
          Gaeilge (Irish)

Three forms, special case for numbers ending in 1[2-9]
     The header entry would look like this:

          Plural-Forms: nplurals=3; \
              plural=n%10==1 && n%100!=11 ? 0 : \
                     n%10>=2 && (n%100<10 || n%100>=20) ? 1 : 2;

     Languages with this property include:

    Baltic family
          Lithuanian

Three forms, special cases for numbers ending in 1 and 2, 3, 4, except those ending in 1[1-4]
     The header entry would look like this:

          Plural-Forms: nplurals=3; \
              plural=n%100/10==1 ? 2 : n%10==1 ? 0 : (n+9)%10>3 ? 2 : 1;

     Languages with this property include:

    Slavic family
          Croatian, Czech, Russian, Ukrainian

Three forms, special cases for 1 and 2, 3, 4
     The header entry would look like this:

          Plural-Forms: nplurals=3; \
              plural=(n==1) ? 1 : (n>=2 && n<=4) ? 2 : 0;

     Languages with this property include:

    Slavic family
          Slovak

Three forms, special case for one and some numbers ending in 2, 3, or 4
     The header entry would look like this:

          Plural-Forms: nplurals=3; \
              plural=n==1 ? 0 : \
                     n%10>=2 && n%10<=4 && (n%100<10 || n%100>=20) ? 1 : 2;

     Languages with this property include:

    Slavic family
          Polish

Four forms, special case for one and all numbers ending in 02, 03, or 04
     The header entry would look like this:

          Plural-Forms: nplurals=4; \
              plural=n%100==1 ? 0 : n%100==2 ? 1 : n%100==3 || n%100==4 ? 2 : 3;

     Languages with this property include:

    Slavic family
          Slovenian

   ---------- Footnotes ----------

   (1) Additions are welcome.  Send appropriate information to
<bug-glibc-manualATgnu.org>.

File: libc.info,  Node: Charset conversion in gettext,  Next: GUI program problems,  Prev: Advanced gettext functions,  Up: Message catalogs with gettext

8.2.1.4 How to specify the output character set `gettext' uses
..............................................................

`gettext' not only looks up a translation in a message catalog.  It
also converts the translation on the fly to the desired output character
set.  This is useful if the user is working in a different character set
than the translator who created the message catalog, because it avoids
distributing variants of message catalogs which differ only in the
character set.

   The output character set is, by default, the value of `nl_langinfo
(CODESET)', which depends on the `LC_CTYPE' part of the current locale.
But programs which store strings in a locale independent way (e.g.
UTF-8) can request that `gettext' and related functions return the
translations in that encoding, by use of the `bind_textdomain_codeset'
function.

   Note that the MSGID argument to `gettext' is not subject to
character set conversion.  Also, when `gettext' does not find a
translation for MSGID, it returns MSGID unchanged - independently of
the current output character set.  It is therefore recommended that all
MSGIDs be US-ASCII strings.

 -- Function: char * bind_textdomain_codeset (const char *DOMAINNAME,
          const char *CODESET)
     The `bind_textdomain_codeset' function can be used to specify the
     output character set for message catalogs for domain DOMAINNAME.
     The CODESET argument must be a valid codeset name which can be used
     for the `iconv_open' function, or a null pointer.

     If the CODESET parameter is the null pointer,
     `bind_textdomain_codeset' returns the currently selected codeset
     for the domain with the name DOMAINNAME. It returns `NULL' if no
     codeset has yet been selected.

     The `bind_textdomain_codeset' function can be used several times.
     If used multiple times with the same DOMAINNAME argument, the
     later call overrides the settings made by the earlier one.

     The `bind_textdomain_codeset' function returns a pointer to a
     string containing the name of the selected codeset.  The string is
     allocated internally in the function and must not be changed by the
     user.  If the system went out of core during the execution of
     `bind_textdomain_codeset', the return value is `NULL' and the
     global variable ERRNO is set accordingly.

File: libc.info,  Node: GUI program problems,  Next: Using gettextized software,  Prev: Charset conversion in gettext,  Up: Message catalogs with gettext

8.2.1.5 How to use `gettext' in GUI programs
............................................

One place where the `gettext' functions, if used normally, have big
problems is within programs with graphical user interfaces (GUIs).  The
problem is that many of the strings which have to be translated are very
short.  They have to appear in pull-down menus which restricts the
length.  But strings which are not containing entire sentences or at
least large fragments of a sentence may appear in more than one
situation in the program but might have different translations.  This is
especially true for the one-word strings which are frequently used in
GUI programs.

   As a consequence many people say that the `gettext' approach is
wrong and instead `catgets' should be used which indeed does not have
this problem.  But there is a very simple and powerful method to handle
these kind of problems with the `gettext' functions.

As an example consider the following fictional situation.  A GUI program
has a menu bar with the following entries:

     +------------+------------+--------------------------------------+
     | File       | Printer    |                                      |
     +------------+------------+--------------------------------------+
     | Open     | | Select   |
     | New      | | Open     |
     +----------+ | Connect  |
                  +----------+

   To have the strings `File', `Printer', `Open', `New', `Select', and
`Connect' translated there has to be at some point in the code a call
to a function of the `gettext' family.  But in two places the string
passed into the function would be `Open'.  The translations might not
be the same and therefore we are in the dilemma described above.

   One solution to this problem is to artificially enlengthen the
strings to make them unambiguous.  But what would the program do if no
translation is available?  The enlengthened string is not what should be
printed.  So we should use a little bit modified version of the
functions.

   To enlengthen the strings a uniform method should be used.  E.g., in
the example above the strings could be chosen as

     Menu|File
     Menu|Printer
     Menu|File|Open
     Menu|File|New
     Menu|Printer|Select
     Menu|Printer|Open
     Menu|Printer|Connect

   Now all the strings are different and if now instead of `gettext'
the following little wrapper function is used, everything works just
fine:

       char *
       sgettext (const char *msgid)
       {
         char *msgval = gettext (msgid);
         if (msgval == msgid)
           msgval = strrchr (msgid, '|') + 1;
         return msgval;
       }

   What this little function does is to recognize the case when no
translation is available.  This can be done very efficiently by a
pointer comparison since the return value is the input value.  If there
is no translation we know that the input string is in the format we used
for the Menu entries and therefore contains a `|' character.  We simply
search for the last occurrence of this character and return a pointer
to the character following it.  That's it!

   If one now consistently uses the enlengthened string form and
replaces the `gettext' calls with calls to `sgettext' (this is normally
limited to very few places in the GUI implementation) then it is
possible to produce a program which can be internationalized.

   With advanced compilers (such as GNU C) one can write the `sgettext'
functions as an inline function or as a macro like this:

     #define sgettext(msgid) \
       ({ const char *__msgid = (msgid);            \
          char *__msgstr = gettext (__msgid);       \
          if (__msgval == __msgid)                  \
            __msgval = strrchr (__msgid, '|') + 1;  \
          __msgval; })

   The other `gettext' functions (`dgettext', `dcgettext' and the
`ngettext' equivalents) can and should have corresponding functions as
well which look almost identical, except for the parameters and the
call to the underlying function.

   Now there is of course the question why such functions do not exist
in the GNU C library?  There are two parts of the answer to this
question.

   * They are easy to write and therefore can be provided by the
     project they are used in.  This is not an answer by itself and
     must be seen together with the second part which is:

   * There is no way the C library can contain a version which can work
     everywhere.  The problem is the selection of the character to
     separate the prefix from the actual string in the enlenghtened
     string.  The examples above used `|' which is a quite good choice
     because it resembles a notation frequently used in this context
     and it also is a character not often used in message strings.

     But what if the character is used in message strings.  Or if the
     chose character is not available in the character set on the
     machine one compiles (e.g., `|' is not required to exist for
     ISO C; this is why the `iso646.h' file exists in ISO C programming
     environments).

   There is only one more comment to make left.  The wrapper function
above require that the translations strings are not enlengthened
themselves.  This is only logical.  There is no need to disambiguate
the strings (since they are never used as keys for a search) and one
also saves quite some memory and disk space by doing this.

File: libc.info,  Node: Using gettextized software,  Prev: GUI program problems,  Up: Message catalogs with gettext

8.2.1.6 User influence on `gettext'
...................................

The last sections described what the programmer can do to
internationalize the messages of the program.  But it is finally up to
the user to select the message s/he wants to see.  S/He must understand
them.

   The POSIX locale model uses the environment variables `LC_COLLATE',
`LC_CTYPE', `LC_MESSAGES', `LC_MONETARY', `LC_NUMERIC', and `LC_TIME'
to select the locale which is to be used.  This way the user can
influence lots of functions.  As we mentioned above the `gettext'
functions also take advantage of this.

   To understand how this happens it is necessary to take a look at the
various components of the filename which gets computed to locate a
message catalog.  It is composed as follows:

     DIR_NAME/LOCALE/LC_CATEGORY/DOMAIN_NAME.mo

   The default value for DIR_NAME is system specific.  It is computed
from the value given as the prefix while configuring the C library.
This value normally is `/usr' or `/'.  For the former the complete
DIR_NAME is:

     /usr/share/locale

   We can use `/usr/share' since the `.mo' files containing the message
catalogs are system independent, so all systems can use the same files.
If the program executed the `bindtextdomain' function for the message
domain that is currently handled, the `dir_name' component is exactly
the value which was given to the function as the second parameter.
I.e., `bindtextdomain' allows overwriting the only system dependent and
fixed value to make it possible to address files anywhere in the
filesystem.

   The CATEGORY is the name of the locale category which was selected
in the program code.  For `gettext' and `dgettext' this is always
`LC_MESSAGES', for `dcgettext' this is selected by the value of the
third parameter.  As said above it should be avoided to ever use a
category other than `LC_MESSAGES'.

   The LOCALE component is computed based on the category used.  Just
like for the `setlocale' function here comes the user selection into
the play.  Some environment variables are examined in a fixed order and
the first environment variable set determines the return value of the
lookup process.  In detail, for the category `LC_xxx' the following
variables in this order are examined:

`LANGUAGE'

`LC_ALL'

`LC_xxx'

`LANG'

   This looks very familiar.  With the exception of the `LANGUAGE'
environment variable this is exactly the lookup order the `setlocale'
function uses.  But why introducing the `LANGUAGE' variable?

   The reason is that the syntax of the values these variables can have
is different to what is expected by the `setlocale' function.  If we
would set `LC_ALL' to a value following the extended syntax that would
mean the `setlocale' function will never be able to use the value of
this variable as well.  An additional variable removes this problem
plus we can select the language independently of the locale setting
which sometimes is useful.

   While for the `LC_xxx' variables the value should consist of exactly
one specification of a locale the `LANGUAGE' variable's value can
consist of a colon separated list of locale names.  The attentive
reader will realize that this is the way we manage to implement one of
our additional demands above: we want to be able to specify an ordered
list of language.

   Back to the constructed filename we have only one component missing.
The DOMAIN_NAME part is the name which was either registered using the
`textdomain' function or which was given to `dgettext' or `dcgettext'
as the first parameter.  Now it becomes obvious that a good choice for
the domain name in the program code is a string which is closely
related to the program/package name.  E.g., for the GNU C Library the
domain name is `libc'.

A limit piece of example code should show how the programmer is supposed
to work:

     {
       setlocale (LC_ALL, "");
       textdomain ("test-package");
       bindtextdomain ("test-package", "/usr/local/share/locale");
       puts (gettext ("Hello, world!"));
     }

   At the program start the default domain is `messages', and the
default locale is "C".  The `setlocale' call sets the locale according
to the user's environment variables; remember that correct functioning
of `gettext' relies on the correct setting of the `LC_MESSAGES' locale
(for looking up the message catalog) and of the `LC_CTYPE' locale (for
the character set conversion).  The `textdomain' call changes the
default domain to `test-package'.  The `bindtextdomain' call specifies
that the message catalogs for the domain `test-package' can be found
below the directory `/usr/local/share/locale'.

   If now the user set in her/his environment the variable `LANGUAGE'
to `de' the `gettext' function will try to use the translations from
the file

     /usr/local/share/locale/de/LC_MESSAGES/test-package.mo

   From the above descriptions it should be clear which component of
this filename is determined by which source.

   In the above example we assumed that the `LANGUAGE' environment
variable to `de'.  This might be an appropriate selection but what
happens if the user wants to use `LC_ALL' because of the wider
usability and here the required value is `de_DE.ISO-8859-1'?  We
already mentioned above that a situation like this is not infrequent.
E.g., a person might prefer reading a dialect and if this is not
available fall back on the standard language.

   The `gettext' functions know about situations like this and can
handle them gracefully.  The functions recognize the format of the value
of the environment variable.  It can split the value is different pieces
and by leaving out the only or the other part it can construct new
values.  This happens of course in a predictable way.  To understand
this one must know the format of the environment variable value.  There
is one more or less standardized form, originally from the X/Open
specification:

   `language[_territory[.codeset]][@modifier]'

   Less specific locale names will be stripped of in the order of the
following list:

  1. `codeset'

  2. `normalized codeset'

  3. `territory'

  4. `modifier'

   The `language' field will never be dropped for obvious reasons.

   The only new thing is the `normalized codeset' entry.  This is
another goodie which is introduced to help reducing the chaos which
derives from the inability of the people to standardize the names of
character sets.  Instead of ISO-8859-1 one can often see 8859-1, 88591,
iso8859-1, or iso_8859-1.  The `normalized codeset' value is generated
from the user-provided character set name by applying the following
rules:

  1. Remove all characters beside numbers and letters.

  2. Fold letters to lowercase.

  3. If the same only contains digits prepend the string `"iso"'.

So all of the above name will be normalized to `iso88591'.  This allows
the program user much more freely choosing the locale name.

   Even this extended functionality still does not help to solve the
problem that completely different names can be used to denote the same
locale (e.g., `de' and `german').  To be of help in this situation the
locale implementation and also the `gettext' functions know about
aliases.

   The file `/usr/share/locale/locale.alias' (replace `/usr' with
whatever prefix you used for configuring the C library) contains a
mapping of alternative names to more regular names.  The system manager
is free to add new entries to fill her/his own needs.  The selected
locale from the environment is compared with the entries in the first
column of this file ignoring the case.  If they match the value of the
second column is used instead for the further handling.

   In the description of the format of the environment variables we
already mentioned the character set as a factor in the selection of the
message catalog.  In fact, only catalogs which contain text written
using the character set of the system/program can be used (directly;
there will come a solution for this some day).  This means for the user
that s/he will always have to take care for this.  If in the collection
of the message catalogs there are files for the same language but coded
using different character sets the user has to be careful.

File: libc.info,  Node: Helper programs for gettext,  Prev: Message catalogs with gettext,  Up: The Uniforum approach

8.2.2 Programs to handle message catalogs for `gettext'
-------------------------------------------------------

The GNU C Library does not contain the source code for the programs to
handle message catalogs for the `gettext' functions.  As part of the
GNU project the GNU gettext package contains everything the developer
needs.  The functionality provided by the tools in this package by far
exceeds the abilities of the `gencat' program described above for the
`catgets' functions.

   There is a program `msgfmt' which is the equivalent program to the
`gencat' program.  It generates from the human-readable and -editable
form of the message catalog a binary file which can be used by the
`gettext' functions.  But there are several more programs available.

   The `xgettext' program can be used to automatically extract the
translatable messages from a source file.  I.e., the programmer need not
take care for the translations and the list of messages which have to be
translated.  S/He will simply wrap the translatable string in calls to
`gettext' et.al and the rest will be done by `xgettext'.  This program
has a lot of option which help to customize the output or do help to
understand the input better.

   Other programs help to manage development cycle when new messages
appear in the source files or when a new translation of the messages
appear.  Here it should only be noted that using all the tools in GNU
gettext it is possible to _completely_ automate the handling of message
catalog.  Beside marking the translatable string in the source code and
generating the translations the developers do not have anything to do
themselves.

File: libc.info,  Node: Searching and Sorting,  Next: Pattern Matching,  Prev: Message Translation,  Up: Top

9 Searching and Sorting
***********************

This chapter describes functions for searching and sorting arrays of
arbitrary objects.  You pass the appropriate comparison function to be
applied as an argument, along with the size of the objects in the array
and the total number of elements.

* Menu:

* Comparison Functions::        Defining how to compare two objects.
				 Since the sort and search facilities
                                 are general, you have to specify the
                                 ordering.
* Array Search Function::       The `bsearch' function.
* Array Sort Function::         The `qsort' function.
* Search/Sort Example::         An example program.
* Hash Search Function::        The `hsearch' function.
* Tree Search Function::        The `tsearch' function.

File: libc.info,  Node: Comparison Functions,  Next: Array Search Function,  Up: Searching and Sorting

9.1 Defining the Comparison Function
====================================

In order to use the sorted array library functions, you have to describe
how to compare the elements of the array.

   To do this, you supply a comparison function to compare two elements
of the array.  The library will call this function, passing as arguments
pointers to two array elements to be compared.  Your comparison function
should return a value the way `strcmp' (*note String/Array
Comparison::) does: negative if the first argument is "less" than the
second, zero if they are "equal", and positive if the first argument is
"greater".

   Here is an example of a comparison function which works with an
array of numbers of type `double':

     int
     compare_doubles (const void *a, const void *b)
     {
       const double *da = (const double *) a;
       const double *db = (const double *) b;

       return (*da > *db) - (*da < *db);
     }

   The header file `stdlib.h' defines a name for the data type of
comparison functions.  This type is a GNU extension.

     int comparison_fn_t (const void *, const void *);

File: libc.info,  Node: Array Search Function,  Next: Array Sort Function,  Prev: Comparison Functions,  Up: Searching and Sorting

9.2 Array Search Function
=========================

Generally searching for a specific element in an array means that
potentially all elements must be checked.  The GNU C library contains
functions to perform linear search.  The prototypes for the following
two functions can be found in `search.h'.

 -- Function: void * lfind (const void *KEY, void *BASE, size_t *NMEMB,
          size_t SIZE, comparison_fn_t COMPAR)
     The `lfind' function searches in the array with `*NMEMB' elements
     of SIZE bytes pointed to by BASE for an element which matches the
     one pointed to by KEY.  The function pointed to by COMPAR is used
     decide whether two elements match.

     The return value is a pointer to the matching element in the array
     starting at BASE if it is found.  If no matching element is
     available `NULL' is returned.

     The mean runtime of this function is `*NMEMB'/2.  This function
     should only be used if elements often get added to or deleted from
     the array in which case it might not be useful to sort the array
     before searching.

 -- Function: void * lsearch (const void *KEY, void *BASE, size_t
          *NMEMB, size_t SIZE, comparison_fn_t COMPAR)
     The `lsearch' function is similar to the `lfind' function.  It
     searches the given array for an element and returns it if found.
     The difference is that if no matching element is found the
     `lsearch' function adds the object pointed to by KEY (with a size
     of SIZE bytes) at the end of the array and it increments the value
     of `*NMEMB' to reflect this addition.

     This means for the caller that if it is not sure that the array
     contains the element one is searching for the memory allocated for
     the array starting at BASE must have room for at least SIZE more
     bytes.  If one is sure the element is in the array it is better to
     use `lfind' so having more room in the array is always necessary
     when calling `lsearch'.

   To search a sorted array for an element matching the key, use the
`bsearch' function.  The prototype for this function is in the header
file `stdlib.h'.

 -- Function: void * bsearch (const void *KEY, const void *ARRAY,
          size_t COUNT, size_t SIZE, comparison_fn_t COMPARE)
     The `bsearch' function searches the sorted array ARRAY for an
     object that is equivalent to KEY.  The array contains COUNT
     elements, each of which is of size SIZE bytes.

     The COMPARE function is used to perform the comparison.  This
     function is called with two pointer arguments and should return an
     integer less than, equal to, or greater than zero corresponding to
     whether its first argument is considered less than, equal to, or
     greater than its second argument.  The elements of the ARRAY must
     already be sorted in ascending order according to this comparison
     function.

     The return value is a pointer to the matching array element, or a
     null pointer if no match is found.  If the array contains more
     than one element that matches, the one that is returned is
     unspecified.

     This function derives its name from the fact that it is implemented
     using the binary search algorithm.

File: libc.info,  Node: Array Sort Function,  Next: Search/Sort Example,  Prev: Array Search Function,  Up: Searching and Sorting

9.3 Array Sort Function
=======================

To sort an array using an arbitrary comparison function, use the
`qsort' function.  The prototype for this function is in `stdlib.h'.

 -- Function: void qsort (void *ARRAY, size_t COUNT, size_t SIZE,
          comparison_fn_t COMPARE)
     The QSORT function sorts the array ARRAY.  The array contains
     COUNT elements, each of which is of size SIZE.

     The COMPARE function is used to perform the comparison on the
     array elements.  This function is called with two pointer
     arguments and should return an integer less than, equal to, or
     greater than zero corresponding to whether its first argument is
     considered less than, equal to, or greater than its second
     argument.

     *Warning:* If two objects compare as equal, their order after
     sorting is unpredictable.  That is to say, the sorting is not
     stable.  This can make a difference when the comparison considers
     only part of the elements.  Two elements with the same sort key
     may differ in other respects.

     If you want the effect of a stable sort, you can get this result by
     writing the comparison function so that, lacking other reason
     distinguish between two elements, it compares them by their
     addresses.  Note that doing this may make the sorting algorithm
     less efficient, so do it only if necessary.

     Here is a simple example of sorting an array of doubles in
     numerical order, using the comparison function defined above
     (*note Comparison Functions::):

          {
            double *array;
            int size;
            ...
            qsort (array, size, sizeof (double), compare_doubles);
          }

     The `qsort' function derives its name from the fact that it was
     originally implemented using the "quick sort" algorithm.

     The implementation of `qsort' in this library might not be an
     in-place sort and might thereby use an extra amount of memory to
     store the array.

File: libc.info,  Node: Search/Sort Example,  Next: Hash Search Function,  Prev: Array Sort Function,  Up: Searching and Sorting

9.4 Searching and Sorting Example
=================================

Here is an example showing the use of `qsort' and `bsearch' with an
array of structures.  The objects in the array are sorted by comparing
their `name' fields with the `strcmp' function.  Then, we can look up
individual objects based on their names.

     #include <stdlib.h>
     #include <stdio.h>
     #include <string.h>

     /* Define an array of critters to sort. */

     struct critter
       {
         const char *name;
         const char *species;
       };

     struct critter muppets[] =
       {
         {"Kermit", "frog"},
         {"Piggy", "pig"},
         {"Gonzo", "whatever"},
         {"Fozzie", "bear"},
         {"Sam", "eagle"},
         {"Robin", "frog"},
         {"Animal", "animal"},
         {"Camilla", "chicken"},
         {"Sweetums", "monster"},
         {"Dr. Strangepork", "pig"},
         {"Link Hogthrob", "pig"},
         {"Zoot", "human"},
         {"Dr. Bunsen Honeydew", "human"},
         {"Beaker", "human"},
         {"Swedish Chef", "human"}
       };

     int count = sizeof (muppets) / sizeof (struct critter);



     /* This is the comparison function used for sorting and searching. */

     int
     critter_cmp (const struct critter *c1, const struct critter *c2)
     {
       return strcmp (c1->name, c2->name);
     }


     /* Print information about a critter. */

     void
     print_critter (const struct critter *c)
     {
       printf ("%s, the %s\n", c->name, c->species);
     }


     /* Do the lookup into the sorted array. */

     void
     find_critter (const char *name)
     {
       struct critter target, *result;
       target.name = name;
       result = bsearch (&target, muppets, count, sizeof (struct critter),
                         critter_cmp);
       if (result)
         print_critter (result);
       else
         printf ("Couldn't find %s.\n", name);
     }

     /* Main program. */

     int
     main (void)
     {
       int i;

       for (i = 0; i < count; i++)
         print_critter (&muppets[i]);
       printf ("\n");

       qsort (muppets, count, sizeof (struct critter), critter_cmp);

       for (i = 0; i < count; i++)
         print_critter (&muppets[i]);
       printf ("\n");

       find_critter ("Kermit");
       find_critter ("Gonzo");
       find_critter ("Janice");

       return 0;
     }

   The output from this program looks like:

     Kermit, the frog
     Piggy, the pig
     Gonzo, the whatever
     Fozzie, the bear
     Sam, the eagle
     Robin, the frog
     Animal, the animal
     Camilla, the chicken
     Sweetums, the monster
     Dr. Strangepork, the pig
     Link Hogthrob, the pig
     Zoot, the human
     Dr. Bunsen Honeydew, the human
     Beaker, the human
     Swedish Chef, the human

     Animal, the animal
     Beaker, the human
     Camilla, the chicken
     Dr. Bunsen Honeydew, the human
     Dr. Strangepork, the pig
     Fozzie, the bear
     Gonzo, the whatever
     Kermit, the frog
     Link Hogthrob, the pig
     Piggy, the pig
     Robin, the frog
     Sam, the eagle
     Swedish Chef, the human
     Sweetums, the monster
     Zoot, the human

     Kermit, the frog
     Gonzo, the whatever
     Couldn't find Janice.

File: libc.info,  Node: Hash Search Function,  Next: Tree Search Function,  Prev: Search/Sort Example,  Up: Searching and Sorting

9.5 The `hsearch' function.
===========================

The functions mentioned so far in this chapter are for searching in a
sorted or unsorted array.  There are other methods to organize
information which later should be searched.  The costs of insert,
delete and search differ.  One possible implementation is using hashing
tables.  The following functions are declared in the header file
`search.h'.

 -- Function: int hcreate (size_t NEL)
     The `hcreate' function creates a hashing table which can contain at
     least NEL elements.  There is no possibility to grow this table so
     it is necessary to choose the value for NEL wisely.  The method
     used to implement this function might make it necessary to make the
     number of elements in the hashing table larger than the expected
     maximal number of elements.  Hashing tables usually work
     inefficiently if they are filled 80% or more.  The constant access
     time guaranteed by hashing can only be achieved if few collisions
     exist.  See Knuth's "The Art of Computer Programming, Part 3:
     Searching and Sorting" for more information.

     The weakest aspect of this function is that there can be at most
     one hashing table used through the whole program.  The table is
     allocated in local memory out of control of the programmer.  As an
     extension the GNU C library provides an additional set of
     functions with an reentrant interface which provide a similar
     interface but which allow to keep arbitrarily many hashing tables.

     It is possible to use more than one hashing table in the program
     run if the former table is first destroyed by a call to `hdestroy'.

     The function returns a non-zero value if successful.  If it return
     zero something went wrong.  This could either mean there is
     already a hashing table in use or the program runs out of memory.

 -- Function: void hdestroy (void)
     The `hdestroy' function can be used to free all the resources
     allocated in a previous call of `hcreate'.  After a call to this
     function it is again possible to call `hcreate' and allocate a new
     table with possibly different size.

     It is important to remember that the elements contained in the
     hashing table at the time `hdestroy' is called are _not_ freed by
     this function.  It is the responsibility of the program code to
     free those strings (if necessary at all).  Freeing all the element
     memory is not possible without extra, separately kept information
     since there is no function to iterate through all available
     elements in the hashing table.  If it is really necessary to free
     a table and all elements the programmer has to keep a list of all
     table elements and before calling `hdestroy' s/he has to free all
     element's data using this list.  This is a very unpleasant
     mechanism and it also shows that this kind of hashing tables is
     mainly meant for tables which are created once and used until the
     end of the program run.

   Entries of the hashing table and keys for the search are defined
using this type:

 -- Data type: struct ENTRY
     Both elements of this structure are pointers to zero-terminated
     strings.  This is a limiting restriction of the functionality of
     the `hsearch' functions.  They can only be used for data sets
     which use the NUL character always and solely to terminate the
     records.  It is not possible to handle general binary data.

    `char *key'
          Pointer to a zero-terminated string of characters describing
          the key for the search or the element in the hashing table.

    `char *data'
          Pointer to a zero-terminated string of characters describing
          the data.  If the functions will be called only for searching
          an existing entry this element might stay undefined since it
          is not used.

 -- Function: ENTRY * hsearch (ENTRY ITEM, ACTION ACTION)
     To search in a hashing table created using `hcreate' the `hsearch'
     function must be used.  This function can perform simple search
     for an element (if ACTION has the `FIND') or it can alternatively
     insert the key element into the hashing table.  Entries are never
     replaced.

     The key is denoted by a pointer to an object of type `ENTRY'.  For
     locating the corresponding position in the hashing table only the
     `key' element of the structure is used.

     If an entry with matching key is found the ACTION parameter is
     irrelevant.  The found entry is returned.  If no matching entry is
     found and the ACTION parameter has the value `FIND' the function
     returns a `NULL' pointer.  If no entry is found and the ACTION
     parameter has the value `ENTER' a new entry is added to the
     hashing table which is initialized with the parameter ITEM.  A
     pointer to the newly added entry is returned.

   As mentioned before the hashing table used by the functions
described so far is global and there can be at any time at most one
hashing table in the program.  A solution is to use the following
functions which are a GNU extension.  All have in common that they
operate on a hashing table which is described by the content of an
object of the type `struct hsearch_data'.  This type should be treated
as opaque, none of its members should be changed directly.

 -- Function: int hcreate_r (size_t NEL, struct hsearch_data *HTAB)
     The `hcreate_r' function initializes the object pointed to by HTAB
     to contain a hashing table with at least NEL elements.  So this
     function is equivalent to the `hcreate' function except that the
     initialized data structure is controlled by the user.

     This allows having more than one hashing table at one time.  The
     memory necessary for the `struct hsearch_data' object can be
     allocated dynamically.  It must be initialized with zero before
     calling this function.

     The return value is non-zero if the operation was successful.  If
     the return value is zero, something went wrong, which probably
     means the programs ran out of memory.

 -- Function: void hdestroy_r (struct hsearch_data *HTAB)
     The `hdestroy_r' function frees all resources allocated by the
     `hcreate_r' function for this very same object HTAB.  As for
     `hdestroy' it is the programs responsibility to free the strings
     for the elements of the table.

 -- Function: int hsearch_r (ENTRY ITEM, ACTION ACTION, ENTRY **RETVAL,
          struct hsearch_data *HTAB)
     The `hsearch_r' function is equivalent to `hsearch'.  The meaning
     of the first two arguments is identical.  But instead of operating
     on a single global hashing table the function works on the table
     described by the object pointed to by HTAB (which is initialized
     by a call to `hcreate_r').

     Another difference to `hcreate' is that the pointer to the found
     entry in the table is not the return value of the functions.  It is
     returned by storing it in a pointer variables pointed to by the
     RETVAL parameter.  The return value of the function is an integer
     value indicating success if it is non-zero and failure if it is
     zero.  In the latter case the global variable ERRNO signals the
     reason for the failure.

    `ENOMEM'
          The table is filled and `hsearch_r' was called with an so far
          unknown key and ACTION set to `ENTER'.

    `ESRCH'
          The ACTION parameter is `FIND' and no corresponding element
          is found in the table.

File: libc.info,  Node: Tree Search Function,  Prev: Hash Search Function,  Up: Searching and Sorting

9.6 The `tsearch' function.
===========================

Another common form to organize data for efficient search is to use
trees.  The `tsearch' function family provides a nice interface to
functions to organize possibly large amounts of data by providing a mean
access time proportional to the logarithm of the number of elements.
The GNU C library implementation even guarantees that this bound is
never exceeded even for input data which cause problems for simple
binary tree implementations.

   The functions described in the chapter are all described in the
System V and X/Open specifications and are therefore quite portable.

   In contrast to the `hsearch' functions the `tsearch' functions can
be used with arbitrary data and not only zero-terminated strings.

   The `tsearch' functions have the advantage that no function to
initialize data structures is necessary.  A simple pointer of type
`void *' initialized to `NULL' is a valid tree and can be extended or
searched.  The prototypes for these functions can be found in the
header file `search.h'.

 -- Function: void * tsearch (const void *KEY, void **ROOTP,
          comparison_fn_t COMPAR)
     The `tsearch' function searches in the tree pointed to by `*ROOTP'
     for an element matching KEY.  The function pointed to by COMPAR is
     used to determine whether two elements match.  *Note Comparison
     Functions::, for a specification of the functions which can be
     used for the COMPAR parameter.

     If the tree does not contain a matching entry the KEY value will
     be added to the tree.  `tsearch' does not make a copy of the object
     pointed to by KEY (how could it since the size is unknown).
     Instead it adds a reference to this object which means the object
     must be available as long as the tree data structure is used.

     The tree is represented by a pointer to a pointer since it is
     sometimes necessary to change the root node of the tree.  So it
     must not be assumed that the variable pointed to by ROOTP has the
     same value after the call.  This also shows that it is not safe to
     call the `tsearch' function more than once at the same time using
     the same tree.  It is no problem to run it more than once at a
     time on different trees.

     The return value is a pointer to the matching element in the tree.
     If a new element was created the pointer points to the new data
     (which is in fact KEY).  If an entry had to be created and the
     program ran out of space `NULL' is returned.

 -- Function: void * tfind (const void *KEY, void *const *ROOTP,
          comparison_fn_t COMPAR)
     The `tfind' function is similar to the `tsearch' function.  It
     locates an element matching the one pointed to by KEY and returns
     a pointer to this element.  But if no matching element is
     available no new element is entered (note that the ROOTP parameter
     points to a constant pointer).  Instead the function returns
     `NULL'.

   Another advantage of the `tsearch' function in contrast to the
`hsearch' functions is that there is an easy way to remove elements.

 -- Function: void * tdelete (const void *KEY, void **ROOTP,
          comparison_fn_t COMPAR)
     To remove a specific element matching KEY from the tree `tdelete'
     can be used.  It locates the matching element using the same
     method as `tfind'.  The corresponding element is then removed and
     a pointer to the parent of the deleted node is returned by the
     function.  If there is no matching entry in the tree nothing can be
     deleted and the function returns `NULL'.  If the root of the tree
     is deleted `tdelete' returns some unspecified value not equal to
     `NULL'.

 -- Function: void tdestroy (void *VROOT, __free_fn_t FREEFCT)
     If the complete search tree has to be removed one can use
     `tdestroy'.  It frees all resources allocated by the `tsearch'
     function to generate the tree pointed to by VROOT.

     For the data in each tree node the function FREEFCT is called.
     The pointer to the data is passed as the argument to the function.
     If no such work is necessary FREEFCT must point to a function doing
     nothing.  It is called in any case.

     This function is a GNU extension and not covered by the System V or
     X/Open specifications.

   In addition to the function to create and destroy the tree data
structure, there is another function which allows you to apply a
function to all elements of the tree.  The function must have this type:

     void __action_fn_t (const void *nodep, VISIT value, int level);

   The NODEP is the data value of the current node (once given as the
KEY argument to `tsearch').  LEVEL is a numeric value which corresponds
to the depth of the current node in the tree.  The root node has the
depth 0 and its children have a depth of 1 and so on.  The `VISIT' type
is an enumeration type.

 -- Data Type: VISIT
     The `VISIT' value indicates the status of the current node in the
     tree and how the function is called.  The status of a node is
     either `leaf' or `internal node'.  For each leaf node the function
     is called exactly once, for each internal node it is called three
     times: before the first child is processed, after the first child
     is processed and after both children are processed.  This makes it
     possible to handle all three methods of tree traversal (or even a
     combination of them).

    `preorder'
          The current node is an internal node and the function is
          called before the first child was processed.

    `postorder'
          The current node is an internal node and the function is
          called after the first child was processed.

    `endorder'
          The current node is an internal node and the function is
          called after the second child was processed.

    `leaf'
          The current node is a leaf.

 -- Function: void twalk (const void *ROOT, __action_fn_t ACTION)
     For each node in the tree with a node pointed to by ROOT, the
     `twalk' function calls the function provided by the parameter
     ACTION.  For leaf nodes the function is called exactly once with
     VALUE set to `leaf'.  For internal nodes the function is called
     three times, setting the VALUE parameter or ACTION to the
     appropriate value.  The LEVEL argument for the ACTION function is
     computed while descending the tree with increasing the value by
     one for the descend to a child, starting with the value 0 for the
     root node.

     Since the functions used for the ACTION parameter to `twalk' must
     not modify the tree data, it is safe to run `twalk' in more than
     one thread at the same time, working on the same tree.  It is also
     safe to call `tfind' in parallel.  Functions which modify the tree
     must not be used, otherwise the behavior is undefined.

File: libc.info,  Node: Pattern Matching,  Next: I/O Overview,  Prev: Searching and Sorting,  Up: Top

10 Pattern Matching
*******************

The GNU C Library provides pattern matching facilities for two kinds of
patterns: regular expressions and file-name wildcards.  The library also
provides a facility for expanding variable and command references and
parsing text into words in the way the shell does.

* Menu:

* Wildcard Matching::    Matching a wildcard pattern against a single string.
* Globbing::             Finding the files that match a wildcard pattern.
* Regular Expressions::  Matching regular expressions against strings.
* Word Expansion::       Expanding shell variables, nested commands,
			    arithmetic, and wildcards.
			    This is what the shell does with shell commands.

File: libc.info,  Node: Wildcard Matching,  Next: Globbing,  Up: Pattern Matching

10.1 Wildcard Matching
======================

This section describes how to match a wildcard pattern against a
particular string.  The result is a yes or no answer: does the string
fit the pattern or not.  The symbols described here are all declared in
`fnmatch.h'.

 -- Function: int fnmatch (const char *PATTERN, const char *STRING, int
          FLAGS)
     This function tests whether the string STRING matches the pattern
     PATTERN.  It returns `0' if they do match; otherwise, it returns
     the nonzero value `FNM_NOMATCH'.  The arguments PATTERN and STRING
     are both strings.

     The argument FLAGS is a combination of flag bits that alter the
     details of matching.  See below for a list of the defined flags.

     In the GNU C Library, `fnmatch' cannot experience an "error"--it
     always returns an answer for whether the match succeeds.  However,
     other implementations of `fnmatch' might sometimes report "errors".
     They would do so by returning nonzero values that are not equal to
     `FNM_NOMATCH'.

   These are the available flags for the FLAGS argument:

`FNM_FILE_NAME'
     Treat the `/' character specially, for matching file names.  If
     this flag is set, wildcard constructs in PATTERN cannot match `/'
     in STRING.  Thus, the only way to match `/' is with an explicit
     `/' in PATTERN.

`FNM_PATHNAME'
     This is an alias for `FNM_FILE_NAME'; it comes from POSIX.2.  We
     don't recommend this name because we don't use the term "pathname"
     for file names.

`FNM_PERIOD'
     Treat the `.' character specially if it appears at the beginning of
     STRING.  If this flag is set, wildcard constructs in PATTERN
     cannot match `.' as the first character of STRING.

     If you set both `FNM_PERIOD' and `FNM_FILE_NAME', then the special
     treatment applies to `.' following `/' as well as to `.' at the
     beginning of STRING.  (The shell uses the `FNM_PERIOD' and
     `FNM_FILE_NAME' flags together for matching file names.)

`FNM_NOESCAPE'
     Don't treat the `\' character specially in patterns.  Normally,
     `\' quotes the following character, turning off its special meaning
     (if any) so that it matches only itself.  When quoting is enabled,
     the pattern `\?' matches only the string `?', because the question
     mark in the pattern acts like an ordinary character.

     If you use `FNM_NOESCAPE', then `\' is an ordinary character.

`FNM_LEADING_DIR'
     Ignore a trailing sequence of characters starting with a `/' in
     STRING; that is to say, test whether STRING starts with a
     directory name that PATTERN matches.

     If this flag is set, either `foo*' or `foobar' as a pattern would
     match the string `foobar/frobozz'.

`FNM_CASEFOLD'
     Ignore case in comparing STRING to PATTERN.

`FNM_EXTMATCH'
     Recognize beside the normal patterns also the extended patterns
     introduced in `ksh'.  The patterns are written in the form
     explained in the following table where PATTERN-LIST is a `|'
     separated list of patterns.

    `?(PATTERN-LIST)'
          The pattern matches if zero or one occurrences of any of the
          patterns in the PATTERN-LIST allow matching the input string.

    `*(PATTERN-LIST)'
          The pattern matches if zero or more occurrences of any of the
          patterns in the PATTERN-LIST allow matching the input string.

    `+(PATTERN-LIST)'
          The pattern matches if one or more occurrences of any of the
          patterns in the PATTERN-LIST allow matching the input string.

    `@(PATTERN-LIST)'
          The pattern matches if exactly one occurrence of any of the
          patterns in the PATTERN-LIST allows matching the input string.

    `!(PATTERN-LIST)'
          The pattern matches if the input string cannot be matched
          with any of the patterns in the PATTERN-LIST.

File: libc.info,  Node: Globbing,  Next: Regular Expressions,  Prev: Wildcard Matching,  Up: Pattern Matching

10.2 Globbing
=============

The archetypal use of wildcards is for matching against the files in a
directory, and making a list of all the matches.  This is called
"globbing".

   You could do this using `fnmatch', by reading the directory entries
one by one and testing each one with `fnmatch'.  But that would be slow
(and complex, since you would have to handle subdirectories by hand).

   The library provides a function `glob' to make this particular use
of wildcards convenient.  `glob' and the other symbols in this section
are declared in `glob.h'.

* Menu:

* Calling Glob::             Basic use of `glob'.
* Flags for Globbing::       Flags that enable various options in `glob'.
* More Flags for Globbing::  GNU specific extensions to `glob'.

File: libc.info,  Node: Calling Glob,  Next: Flags for Globbing,  Up: Globbing

10.2.1 Calling `glob'
---------------------

The result of globbing is a vector of file names (strings).  To return
this vector, `glob' uses a special data type, `glob_t', which is a
structure.  You pass `glob' the address of the structure, and it fills
in the structure's fields to tell you about the results.

 -- Data Type: glob_t
     This data type holds a pointer to a word vector.  More precisely,
     it records both the address of the word vector and its size.  The
     GNU implementation contains some more fields which are non-standard
     extensions.

    `gl_pathc'
          The number of elements in the vector, excluding the initial
          null entries if the GLOB_DOOFFS flag is used (see gl_offs
          below).

    `gl_pathv'
          The address of the vector.  This field has type `char **'.

    `gl_offs'
          The offset of the first real element of the vector, from its
          nominal address in the `gl_pathv' field.  Unlike the other
          fields, this is always an input to `glob', rather than an
          output from it.

          If you use a nonzero offset, then that many elements at the
          beginning of the vector are left empty.  (The `glob' function
          fills them with null pointers.)

          The `gl_offs' field is meaningful only if you use the
          `GLOB_DOOFFS' flag.  Otherwise, the offset is always zero
          regardless of what is in this field, and the first real
          element comes at the beginning of the vector.

    `gl_closedir'
          The address of an alternative implementation of the `closedir'
          function.  It is used if the `GLOB_ALTDIRFUNC' bit is set in
          the flag parameter.  The type of this field is
          `void (*) (void *)'.

          This is a GNU extension.

    `gl_readdir'
          The address of an alternative implementation of the `readdir'
          function used to read the contents of a directory.  It is
          used if the `GLOB_ALTDIRFUNC' bit is set in the flag
          parameter.  The type of this field is
          `struct dirent *(*) (void *)'.

          This is a GNU extension.

    `gl_opendir'
          The address of an alternative implementation of the `opendir'
          function.  It is used if the `GLOB_ALTDIRFUNC' bit is set in
          the flag parameter.  The type of this field is
          `void *(*) (const char *)'.

          This is a GNU extension.

    `gl_stat'
          The address of an alternative implementation of the `stat'
          function to get information about an object in the
          filesystem.  It is used if the `GLOB_ALTDIRFUNC' bit is set
          in the flag parameter.  The type of this field is
          `int (*) (const char *, struct stat *)'.

          This is a GNU extension.

    `gl_lstat'
          The address of an alternative implementation of the `lstat'
          function to get information about an object in the
          filesystems, not following symbolic links.  It is used if the
          `GLOB_ALTDIRFUNC' bit is set in the flag parameter.  The type
          of this field is `int (*) (const char *, struct stat *)'.

          This is a GNU extension.

   For use in the `glob64' function `glob.h' contains another
definition for a very similar type.  `glob64_t' differs from `glob_t'
only in the types of the members `gl_readdir', `gl_stat', and
`gl_lstat'.

 -- Data Type: glob64_t
     This data type holds a pointer to a word vector.  More precisely,
     it records both the address of the word vector and its size.  The
     GNU implementation contains some more fields which are non-standard
     extensions.

    `gl_pathc'
          The number of elements in the vector, excluding the initial
          null entries if the GLOB_DOOFFS flag is used (see gl_offs
          below).

    `gl_pathv'
          The address of the vector.  This field has type `char **'.

    `gl_offs'
          The offset of the first real element of the vector, from its
          nominal address in the `gl_pathv' field.  Unlike the other
          fields, this is always an input to `glob', rather than an
          output from it.

          If you use a nonzero offset, then that many elements at the
          beginning of the vector are left empty.  (The `glob' function
          fills them with null pointers.)

          The `gl_offs' field is meaningful only if you use the
          `GLOB_DOOFFS' flag.  Otherwise, the offset is always zero
          regardless of what is in this field, and the first real
          element comes at the beginning of the vector.

    `gl_closedir'
          The address of an alternative implementation of the `closedir'
          function.  It is used if the `GLOB_ALTDIRFUNC' bit is set in
          the flag parameter.  The type of this field is
          `void (*) (void *)'.

          This is a GNU extension.

    `gl_readdir'
          The address of an alternative implementation of the
          `readdir64' function used to read the contents of a
          directory.  It is used if the `GLOB_ALTDIRFUNC' bit is set in
          the flag parameter.  The type of this field is
          `struct dirent64 *(*) (void *)'.

          This is a GNU extension.

    `gl_opendir'
          The address of an alternative implementation of the `opendir'
          function.  It is used if the `GLOB_ALTDIRFUNC' bit is set in
          the flag parameter.  The type of this field is
          `void *(*) (const char *)'.

          This is a GNU extension.

    `gl_stat'
          The address of an alternative implementation of the `stat64'
          function to get information about an object in the
          filesystem.  It is used if the `GLOB_ALTDIRFUNC' bit is set
          in the flag parameter.  The type of this field is
          `int (*) (const char *, struct stat64 *)'.

          This is a GNU extension.

    `gl_lstat'
          The address of an alternative implementation of the `lstat64'
          function to get information about an object in the
          filesystems, not following symbolic links.  It is used if the
          `GLOB_ALTDIRFUNC' bit is set in the flag parameter.  The type
          of this field is `int (*) (const char *, struct stat64 *)'.

          This is a GNU extension.

 -- Function: int glob (const char *PATTERN, int FLAGS, int (*ERRFUNC)
          (const char *FILENAME, int ERROR-CODE), glob_t *VECTOR-PTR)
     The function `glob' does globbing using the pattern PATTERN in the
     current directory.  It puts the result in a newly allocated
     vector, and stores the size and address of this vector into
     `*VECTOR-PTR'.  The argument FLAGS is a combination of bit flags;
     see *note Flags for Globbing::, for details of the flags.

     The result of globbing is a sequence of file names.  The function
     `glob' allocates a string for each resulting word, then allocates
     a vector of type `char **' to store the addresses of these
     strings.  The last element of the vector is a null pointer.  This
     vector is called the "word vector".

     To return this vector, `glob' stores both its address and its
     length (number of elements, not counting the terminating null
     pointer) into `*VECTOR-PTR'.

     Normally, `glob' sorts the file names alphabetically before
     returning them.  You can turn this off with the flag `GLOB_NOSORT'
     if you want to get the information as fast as possible.  Usually
     it's a good idea to let `glob' sort them--if you process the files
     in alphabetical order, the users will have a feel for the rate of
     progress that your application is making.

     If `glob' succeeds, it returns 0.  Otherwise, it returns one of
     these error codes:

    `GLOB_ABORTED'
          There was an error opening a directory, and you used the flag
          `GLOB_ERR' or your specified ERRFUNC returned a nonzero value.
          *Note Flags for Globbing::, for an explanation of the
          `GLOB_ERR' flag and ERRFUNC.

    `GLOB_NOMATCH'
          The pattern didn't match any existing files.  If you use the
          `GLOB_NOCHECK' flag, then you never get this error code,
          because that flag tells `glob' to _pretend_ that the pattern
          matched at least one file.

    `GLOB_NOSPACE'
          It was impossible to allocate memory to hold the result.

     In the event of an error, `glob' stores information in
     `*VECTOR-PTR' about all the matches it has found so far.

     It is important to notice that the `glob' function will not fail if
     it encounters directories or files which cannot be handled without
     the LFS interfaces.  The implementation of `glob' is supposed to
     use these functions internally.  This at least is the assumptions
     made by the Unix standard.  The GNU extension of allowing the user
     to provide own directory handling and `stat' functions complicates
     things a bit.  If these callback functions are used and a large
     file or directory is encountered `glob' _can_ fail.

 -- Function: int glob64 (const char *PATTERN, int FLAGS, int
          (*ERRFUNC) (const char *FILENAME, int ERROR-CODE), glob64_t
          *VECTOR-PTR)
     The `glob64' function was added as part of the Large File Summit
     extensions but is not part of the original LFS proposal.  The
     reason for this is simple: it is not necessary.  The necessity for
     a `glob64' function is added by the extensions of the GNU `glob'
     implementation which allows the user to provide own directory
     handling and `stat' functions.  The `readdir' and `stat' functions
     do depend on the choice of `_FILE_OFFSET_BITS' since the definition
     of the types `struct dirent' and `struct stat' will change
     depending on the choice.

     Beside this difference the `glob64' works just like `glob' in all
     aspects.

     This function is a GNU extension.

File: libc.info,  Node: Flags for Globbing,  Next: More Flags for Globbing,  Prev: Calling Glob,  Up: Globbing

10.2.2 Flags for Globbing
-------------------------

This section describes the flags that you can specify in the FLAGS
argument to `glob'.  Choose the flags you want, and combine them with
the C bitwise OR operator `|'.

`GLOB_APPEND'
     Append the words from this expansion to the vector of words
     produced by previous calls to `glob'.  This way you can
     effectively expand several words as if they were concatenated with
     spaces between them.

     In order for appending to work, you must not modify the contents
     of the word vector structure between calls to `glob'.  And, if you
     set `GLOB_DOOFFS' in the first call to `glob', you must also set
     it when you append to the results.

     Note that the pointer stored in `gl_pathv' may no longer be valid
     after you call `glob' the second time, because `glob' might have
     relocated the vector.  So always fetch `gl_pathv' from the
     `glob_t' structure after each `glob' call; *never* save the
     pointer across calls.

`GLOB_DOOFFS'
     Leave blank slots at the beginning of the vector of words.  The
     `gl_offs' field says how many slots to leave.  The blank slots
     contain null pointers.

`GLOB_ERR'
     Give up right away and report an error if there is any difficulty
     reading the directories that must be read in order to expand
     PATTERN fully.  Such difficulties might include a directory in
     which you don't have the requisite access.  Normally, `glob' tries
     its best to keep on going despite any errors, reading whatever
     directories it can.

     You can exercise even more control than this by specifying an
     error-handler function ERRFUNC when you call `glob'.  If ERRFUNC
     is not a null pointer, then `glob' doesn't give up right away when
     it can't read a directory; instead, it calls ERRFUNC with two
     arguments, like this:

          (*ERRFUNC) (FILENAME, ERROR-CODE)

     The argument FILENAME is the name of the directory that `glob'
     couldn't open or couldn't read, and ERROR-CODE is the `errno'
     value that was reported to `glob'.

     If the error handler function returns nonzero, then `glob' gives up
     right away.  Otherwise, it continues.

`GLOB_MARK'
     If the pattern matches the name of a directory, append `/' to the
     directory's name when returning it.

`GLOB_NOCHECK'
     If the pattern doesn't match any file names, return the pattern
     itself as if it were a file name that had been matched.
     (Normally, when the pattern doesn't match anything, `glob' returns
     that there were no matches.)

`GLOB_NOSORT'
     Don't sort the file names; return them in no particular order.
     (In practice, the order will depend on the order of the entries in
     the directory.)  The only reason _not_ to sort is to save time.

`GLOB_NOESCAPE'
     Don't treat the `\' character specially in patterns.  Normally,
     `\' quotes the following character, turning off its special meaning
     (if any) so that it matches only itself.  When quoting is enabled,
     the pattern `\?' matches only the string `?', because the question
     mark in the pattern acts like an ordinary character.

     If you use `GLOB_NOESCAPE', then `\' is an ordinary character.

     `glob' does its work by calling the function `fnmatch' repeatedly.
     It handles the flag `GLOB_NOESCAPE' by turning on the
     `FNM_NOESCAPE' flag in calls to `fnmatch'.

File: libc.info,  Node: More Flags for Globbing,  Prev: Flags for Globbing,  Up: Globbing

10.2.3 More Flags for Globbing
------------------------------

Beside the flags described in the last section, the GNU implementation
of `glob' allows a few more flags which are also defined in the
`glob.h' file.  Some of the extensions implement functionality which is
available in modern shell implementations.

`GLOB_PERIOD'
     The `.' character (period) is treated special.  It cannot be
     matched by wildcards.  *Note Wildcard Matching::, `FNM_PERIOD'.

`GLOB_MAGCHAR'
     The `GLOB_MAGCHAR' value is not to be given to `glob' in the FLAGS
     parameter.  Instead, `glob' sets this bit in the GL_FLAGS element
     of the GLOB_T structure provided as the result if the pattern used
     for matching contains any wildcard character.

`GLOB_ALTDIRFUNC'
     Instead of the using the using the normal functions for accessing
     the filesystem the `glob' implementation uses the user-supplied
     functions specified in the structure pointed to by PGLOB
     parameter.  For more information about the functions refer to the
     sections about directory handling see *note Accessing
     Directories::, and *note Reading Attributes::.

`GLOB_BRACE'
     If this flag is given the handling of braces in the pattern is
     changed.  It is now required that braces appear correctly grouped.
     I.e., for each opening brace there must be a closing one.  Braces
     can be used recursively.  So it is possible to define one brace
     expression in another one.  It is important to note that the range
     of each brace expression is completely contained in the outer
     brace expression (if there is one).

     The string between the matching braces is separated into single
     expressions by splitting at `,' (comma) characters.  The commas
     themselves are discarded.  Please note what we said above about
     recursive brace expressions.  The commas used to separate the
     subexpressions must be at the same level.  Commas in brace
     subexpressions are not matched.  They are used during expansion of
     the brace expression of the deeper level.  The example below shows
     this

          glob ("{foo/{,bar,biz},baz}", GLOB_BRACE, NULL, &result)

     is equivalent to the sequence

          glob ("foo/", GLOB_BRACE, NULL, &result)
          glob ("foo/bar", GLOB_BRACE|GLOB_APPEND, NULL, &result)
          glob ("foo/biz", GLOB_BRACE|GLOB_APPEND, NULL, &result)
          glob ("baz", GLOB_BRACE|GLOB_APPEND, NULL, &result)

     if we leave aside error handling.

`GLOB_NOMAGIC'
     If the pattern contains no wildcard constructs (it is a literal
     file name), return it as the sole "matching" word, even if no file
     exists by that name.

`GLOB_TILDE'
     If this flag is used the character `~' (tilde) is handled special
     if it appears at the beginning of the pattern.  Instead of being
     taken verbatim it is used to represent the home directory of a
     known user.

     If `~' is the only character in pattern or it is followed by a `/'
     (slash), the home directory of the process owner is substituted.
     Using `getlogin' and `getpwnam' the information is read from the
     system databases.  As an example take user `bart' with his home
     directory at `/home/bart'.  For him a call like

          glob ("~/bin/*", GLOB_TILDE, NULL, &result)

     would return the contents of the directory `/home/bart/bin'.
     Instead of referring to the own home directory it is also possible
     to name the home directory of other users.  To do so one has to
     append the user name after the tilde character.  So the contents
     of user `homer''s `bin' directory can be retrieved by

          glob ("~homer/bin/*", GLOB_TILDE, NULL, &result)

     If the user name is not valid or the home directory cannot be
     determined for some reason the pattern is left untouched and
     itself used as the result.  I.e., if in the last example `home' is
     not available the tilde expansion yields to `"~homer/bin/*"' and
     `glob' is not looking for a directory named `~homer'.

     This functionality is equivalent to what is available in C-shells
     if the `nonomatch' flag is set.

`GLOB_TILDE_CHECK'
     If this flag is used `glob' behaves like as if `GLOB_TILDE' is
     given.  The only difference is that if the user name is not
     available or the home directory cannot be determined for other
     reasons this leads to an error.  `glob' will return `GLOB_NOMATCH'
     instead of using the pattern itself as the name.

     This functionality is equivalent to what is available in C-shells
     if `nonomatch' flag is not set.

`GLOB_ONLYDIR'
     If this flag is used the globbing function takes this as a *hint*
     that the caller is only interested in directories matching the
     pattern.  If the information about the type of the file is easily
     available non-directories will be rejected but no extra work will
     be done to determine the information for each file.  I.e., the
     caller must still be able to filter directories out.

     This functionality is only available with the GNU `glob'
     implementation.  It is mainly used internally to increase the
     performance but might be useful for a user as well and therefore is
     documented here.

   Calling `glob' will in most cases allocate resources which are used
to represent the result of the function call.  If the same object of
type `glob_t' is used in multiple call to `glob' the resources are
freed or reused so that no leaks appear.  But this does not include the
time when all `glob' calls are done.

 -- Function: void globfree (glob_t *PGLOB)
     The `globfree' function frees all resources allocated by previous
     calls to `glob' associated with the object pointed to by PGLOB.
     This function should be called whenever the currently used
     `glob_t' typed object isn't used anymore.

 -- Function: void globfree64 (glob64_t *PGLOB)
     This function is equivalent to `globfree' but it frees records of
     type `glob64_t' which were allocated by `glob64'.

File: libc.info,  Node: Regular Expressions,  Next: Word Expansion,  Prev: Globbing,  Up: Pattern Matching

10.3 Regular Expression Matching
================================

The GNU C library supports two interfaces for matching regular
expressions.  One is the standard POSIX.2 interface, and the other is
what the GNU system has had for many years.

   Both interfaces are declared in the header file `regex.h'.  If you
define `_POSIX_C_SOURCE', then only the POSIX.2 functions, structures,
and constants are declared.

* Menu:

* POSIX Regexp Compilation::    Using `regcomp' to prepare to match.
* Flags for POSIX Regexps::     Syntax variations for `regcomp'.
* Matching POSIX Regexps::      Using `regexec' to match the compiled
				   pattern that you get from `regcomp'.
* Regexp Subexpressions::       Finding which parts of the string were matched.
* Subexpression Complications:: Find points of which parts were matched.
* Regexp Cleanup::		Freeing storage; reporting errors.

File: libc.info,  Node: POSIX Regexp Compilation,  Next: Flags for POSIX Regexps,  Up: Regular Expressions

10.3.1 POSIX Regular Expression Compilation
-------------------------------------------

Before you can actually match a regular expression, you must "compile"
it.  This is not true compilation--it produces a special data
structure, not machine instructions.  But it is like ordinary
compilation in that its purpose is to enable you to "execute" the
pattern fast.  (*Note Matching POSIX Regexps::, for how to use the
compiled regular expression for matching.)

   There is a special data type for compiled regular expressions:

 -- Data Type: regex_t
     This type of object holds a compiled regular expression.  It is
     actually a structure.  It has just one field that your programs
     should look at:

    `re_nsub'
          This field holds the number of parenthetical subexpressions
          in the regular expression that was compiled.

     There are several other fields, but we don't describe them here,
     because only the functions in the library should use them.

   After you create a `regex_t' object, you can compile a regular
expression into it by calling `regcomp'.

 -- Function: int regcomp (regex_t *restrict COMPILED, const char
          *restrict PATTERN, int CFLAGS)
     The function `regcomp' "compiles" a regular expression into a data
     structure that you can use with `regexec' to match against a
     string.  The compiled regular expression format is designed for
     efficient matching.  `regcomp' stores it into `*COMPILED'.

     It's up to you to allocate an object of type `regex_t' and pass its
     address to `regcomp'.

     The argument CFLAGS lets you specify various options that control
     the syntax and semantics of regular expressions.  *Note Flags for
     POSIX Regexps::.

     If you use the flag `REG_NOSUB', then `regcomp' omits from the
     compiled regular expression the information necessary to record
     how subexpressions actually match.  In this case, you might as well
     pass `0' for the MATCHPTR and NMATCH arguments when you call
     `regexec'.

     If you don't use `REG_NOSUB', then the compiled regular expression
     does have the capacity to record how subexpressions match.  Also,
     `regcomp' tells you how many subexpressions PATTERN has, by
     storing the number in `COMPILED->re_nsub'.  You can use that value
     to decide how long an array to allocate to hold information about
     subexpression matches.

     `regcomp' returns `0' if it succeeds in compiling the regular
     expression; otherwise, it returns a nonzero error code (see the
     table below).  You can use `regerror' to produce an error message
     string describing the reason for a nonzero value; see *note Regexp
     Cleanup::.


   Here are the possible nonzero values that `regcomp' can return:

`REG_BADBR'
     There was an invalid `\{...\}' construct in the regular
     expression.  A valid `\{...\}' construct must contain either a
     single number, or two numbers in increasing order separated by a
     comma.

`REG_BADPAT'
     There was a syntax error in the regular expression.

`REG_BADRPT'
     A repetition operator such as `?' or `*' appeared in a bad
     position (with no preceding subexpression to act on).

`REG_ECOLLATE'
     The regular expression referred to an invalid collating element
     (one not defined in the current locale for string collation).
     *Note Locale Categories::.

`REG_ECTYPE'
     The regular expression referred to an invalid character class name.

`REG_EESCAPE'
     The regular expression ended with `\'.

`REG_ESUBREG'
     There was an invalid number in the `\DIGIT' construct.

`REG_EBRACK'
     There were unbalanced square brackets in the regular expression.

`REG_EPAREN'
     An extended regular expression had unbalanced parentheses, or a
     basic regular expression had unbalanced `\(' and `\)'.

`REG_EBRACE'
     The regular expression had unbalanced `\{' and `\}'.

`REG_ERANGE'
     One of the endpoints in a range expression was invalid.

`REG_ESPACE'
     `regcomp' ran out of memory.

File: libc.info,  Node: Flags for POSIX Regexps,  Next: Matching POSIX Regexps,  Prev: POSIX Regexp Compilation,  Up: Regular Expressions

10.3.2 Flags for POSIX Regular Expressions
------------------------------------------

These are the bit flags that you can use in the CFLAGS operand when
compiling a regular expression with `regcomp'.

`REG_EXTENDED'
     Treat the pattern as an extended regular expression, rather than
     as a basic regular expression.

`REG_ICASE'
     Ignore case when matching letters.

`REG_NOSUB'
     Don't bother storing the contents of the MATCHES-PTR array.

`REG_NEWLINE'
     Treat a newline in STRING as dividing STRING into multiple lines,
     so that `$' can match before the newline and `^' can match after.
     Also, don't permit `.' to match a newline, and don't permit
     `[^...]' to match a newline.

     Otherwise, newline acts like any other ordinary character.

File: libc.info,  Node: Matching POSIX Regexps,  Next: Regexp Subexpressions,  Prev: Flags for POSIX Regexps,  Up: Regular Expressions

10.3.3 Matching a Compiled POSIX Regular Expression
---------------------------------------------------

Once you have compiled a regular expression, as described in *note
POSIX Regexp Compilation::, you can match it against strings using
`regexec'.  A match anywhere inside the string counts as success,
unless the regular expression contains anchor characters (`^' or `$').

 -- Function: int regexec (const regex_t *restrict COMPILED, const char
          *restrict STRING, size_t NMATCH, regmatch_t
          MATCHPTR[restrict], int EFLAGS)
     This function tries to match the compiled regular expression
     `*COMPILED' against STRING.

     `regexec' returns `0' if the regular expression matches;
     otherwise, it returns a nonzero value.  See the table below for
     what nonzero values mean.  You can use `regerror' to produce an
     error message string describing the reason for a nonzero value;
     see *note Regexp Cleanup::.

     The argument EFLAGS is a word of bit flags that enable various
     options.

     If you want to get information about what part of STRING actually
     matched the regular expression or its subexpressions, use the
     arguments MATCHPTR and NMATCH.  Otherwise, pass `0' for NMATCH,
     and `NULL' for MATCHPTR.  *Note Regexp Subexpressions::.

   You must match the regular expression with the same set of current
locales that were in effect when you compiled the regular expression.

   The function `regexec' accepts the following flags in the EFLAGS
argument:

`REG_NOTBOL'
     Do not regard the beginning of the specified string as the
     beginning of a line; more generally, don't make any assumptions
     about what text might precede it.

`REG_NOTEOL'
     Do not regard the end of the specified string as the end of a
     line; more generally, don't make any assumptions about what text
     might follow it.

   Here are the possible nonzero values that `regexec' can return:

`REG_NOMATCH'
     The pattern didn't match the string.  This isn't really an error.

`REG_ESPACE'
     `regexec' ran out of memory.

File: libc.info,  Node: Regexp Subexpressions,  Next: Subexpression Complications,  Prev: Matching POSIX Regexps,  Up: Regular Expressions

10.3.4 Match Results with Subexpressions
----------------------------------------

When `regexec' matches parenthetical subexpressions of PATTERN, it
records which parts of STRING they match.  It returns that information
by storing the offsets into an array whose elements are structures of
type `regmatch_t'.  The first element of the array (index `0') records
the part of the string that matched the entire regular expression.
Each other element of the array records the beginning and end of the
part that matched a single parenthetical subexpression.

 -- Data Type: regmatch_t
     This is the data type of the MATCHARRAY array that you pass to
     `regexec'.  It contains two structure fields, as follows:

    `rm_so'
          The offset in STRING of the beginning of a substring.  Add
          this value to STRING to get the address of that part.

    `rm_eo'
          The offset in STRING of the end of the substring.

 -- Data Type: regoff_t
     `regoff_t' is an alias for another signed integer type.  The
     fields of `regmatch_t' have type `regoff_t'.

   The `regmatch_t' elements correspond to subexpressions positionally;
the first element (index `1') records where the first subexpression
matched, the second element records the second subexpression, and so
on.  The order of the subexpressions is the order in which they begin.

   When you call `regexec', you specify how long the MATCHPTR array is,
with the NMATCH argument.  This tells `regexec' how many elements to
store.  If the actual regular expression has more than NMATCH
subexpressions, then you won't get offset information about the rest of
them.  But this doesn't alter whether the pattern matches a particular
string or not.

   If you don't want `regexec' to return any information about where
the subexpressions matched, you can either supply `0' for NMATCH, or
use the flag `REG_NOSUB' when you compile the pattern with `regcomp'.

File: libc.info,  Node: Subexpression Complications,  Next: Regexp Cleanup,  Prev: Regexp Subexpressions,  Up: Regular Expressions

10.3.5 Complications in Subexpression Matching
----------------------------------------------

Sometimes a subexpression matches a substring of no characters.  This
happens when `f\(o*\)' matches the string `fum'.  (It really matches
just the `f'.)  In this case, both of the offsets identify the point in
the string where the null substring was found.  In this example, the
offsets are both `1'.

   Sometimes the entire regular expression can match without using some
of its subexpressions at all--for example, when `ba\(na\)*' matches the
string `ba', the parenthetical subexpression is not used.  When this
happens, `regexec' stores `-1' in both fields of the element for that
subexpression.

   Sometimes matching the entire regular expression can match a
particular subexpression more than once--for example, when `ba\(na\)*'
matches the string `bananana', the parenthetical subexpression matches
three times.  When this happens, `regexec' usually stores the offsets
of the last part of the string that matched the subexpression.  In the
case of `bananana', these offsets are `6' and `8'.

   But the last match is not always the one that is chosen.  It's more
accurate to say that the last _opportunity_ to match is the one that
takes precedence.  What this means is that when one subexpression
appears within another, then the results reported for the inner
subexpression reflect whatever happened on the last match of the outer
subexpression.  For an example, consider `\(ba\(na\)*s \)*' matching
the string `bananas bas '.  The last time the inner expression actually
matches is near the end of the first word.  But it is _considered_
again in the second word, and fails to match there.  `regexec' reports
nonuse of the "na" subexpression.

   Another place where this rule applies is when the regular expression
     \(ba\(na\)*s \|nefer\(ti\)* \)*
   matches `bananas nefertiti'.  The "na" subexpression does match in
the first word, but it doesn't match in the second word because the
other alternative is used there.  Once again, the second repetition of
the outer subexpression overrides the first, and within that second
repetition, the "na" subexpression is not used.  So `regexec' reports
nonuse of the "na" subexpression.

File: libc.info,  Node: Regexp Cleanup,  Prev: Subexpression Complications,  Up: Regular Expressions

10.3.6 POSIX Regexp Matching Cleanup
------------------------------------

When you are finished using a compiled regular expression, you can free
the storage it uses by calling `regfree'.

 -- Function: void regfree (regex_t *COMPILED)
     Calling `regfree' frees all the storage that `*COMPILED' points
     to.  This includes various internal fields of the `regex_t'
     structure that aren't documented in this manual.

     `regfree' does not free the object `*COMPILED' itself.

   You should always free the space in a `regex_t' structure with
`regfree' before using the structure to compile another regular
expression.

   When `regcomp' or `regexec' reports an error, you can use the
function `regerror' to turn it into an error message string.

 -- Function: size_t regerror (int ERRCODE, const regex_t *restrict
          COMPILED, char *restrict BUFFER, size_t LENGTH)
     This function produces an error message string for the error code
     ERRCODE, and stores the string in LENGTH bytes of memory starting
     at BUFFER.  For the COMPILED argument, supply the same compiled
     regular expression structure that `regcomp' or `regexec' was
     working with when it got the error.  Alternatively, you can supply
     `NULL' for COMPILED; you will still get a meaningful error
     message, but it might not be as detailed.

     If the error message can't fit in LENGTH bytes (including a
     terminating null character), then `regerror' truncates it.  The
     string that `regerror' stores is always null-terminated even if it
     has been truncated.

     The return value of `regerror' is the minimum length needed to
     store the entire error message.  If this is less than LENGTH, then
     the error message was not truncated, and you can use it.
     Otherwise, you should call `regerror' again with a larger buffer.

     Here is a function which uses `regerror', but always dynamically
     allocates a buffer for the error message:

          char *get_regerror (int errcode, regex_t *compiled)
          {
            size_t length = regerror (errcode, compiled, NULL, 0);
            char *buffer = xmalloc (length);
            (void) regerror (errcode, compiled, buffer, length);
            return buffer;
          }

File: libc.info,  Node: Word Expansion,  Prev: Regular Expressions,  Up: Pattern Matching

10.4 Shell-Style Word Expansion
===============================

"Word expansion" means the process of splitting a string into "words"
and substituting for variables, commands, and wildcards just as the
shell does.

   For example, when you write `ls -l foo.c', this string is split into
three separate words--`ls', `-l' and `foo.c'.  This is the most basic
function of word expansion.

   When you write `ls *.c', this can become many words, because the
word `*.c' can be replaced with any number of file names.  This is
called "wildcard expansion", and it is also a part of word expansion.

   When you use `echo $PATH' to print your path, you are taking
advantage of "variable substitution", which is also part of word
expansion.

   Ordinary programs can perform word expansion just like the shell by
calling the library function `wordexp'.

* Menu:

* Expansion Stages::            What word expansion does to a string.
* Calling Wordexp::             How to call `wordexp'.
* Flags for Wordexp::           Options you can enable in `wordexp'.
* Wordexp Example::             A sample program that does word expansion.
* Tilde Expansion::             Details of how tilde expansion works.
* Variable Substitution::       Different types of variable substitution.

File: libc.info,  Node: Expansion Stages,  Next: Calling Wordexp,  Up: Word Expansion

10.4.1 The Stages of Word Expansion
-----------------------------------

When word expansion is applied to a sequence of words, it performs the
following transformations in the order shown here:

  1. "Tilde expansion": Replacement of `~foo' with the name of the home
     directory of `foo'.

  2. Next, three different transformations are applied in the same step,
     from left to right:

        * "Variable substitution": Environment variables are
          substituted for references such as `$foo'.

        * "Command substitution": Constructs such as ``cat foo`' and
          the equivalent `$(cat foo)' are replaced with the output from
          the inner command.

        * "Arithmetic expansion": Constructs such as `$(($x-1))' are
          replaced with the result of the arithmetic computation.

  3. "Field splitting": subdivision of the text into "words".

  4. "Wildcard expansion": The replacement of a construct such as `*.c'
     with a list of `.c' file names.  Wildcard expansion applies to an
     entire word at a time, and replaces that word with 0 or more file
     names that are themselves words.

  5. "Quote removal": The deletion of string-quotes, now that they have
     done their job by inhibiting the above transformations when
     appropriate.

   For the details of these transformations, and how to write the
constructs that use them, see `The BASH Manual' (to appear).

File: libc.info,  Node: Calling Wordexp,  Next: Flags for Wordexp,  Prev: Expansion Stages,  Up: Word Expansion

10.4.2 Calling `wordexp'
------------------------

All the functions, constants and data types for word expansion are
declared in the header file `wordexp.h'.

   Word expansion produces a vector of words (strings).  To return this
vector, `wordexp' uses a special data type, `wordexp_t', which is a
structure.  You pass `wordexp' the address of the structure, and it
fills in the structure's fields to tell you about the results.

 -- Data Type: wordexp_t
     This data type holds a pointer to a word vector.  More precisely,
     it records both the address of the word vector and its size.

    `we_wordc'
          The number of elements in the vector.

    `we_wordv'
          The address of the vector.  This field has type `char **'.

    `we_offs'
          The offset of the first real element of the vector, from its
          nominal address in the `we_wordv' field.  Unlike the other
          fields, this is always an input to `wordexp', rather than an
          output from it.

          If you use a nonzero offset, then that many elements at the
          beginning of the vector are left empty.  (The `wordexp'
          function fills them with null pointers.)

          The `we_offs' field is meaningful only if you use the
          `WRDE_DOOFFS' flag.  Otherwise, the offset is always zero
          regardless of what is in this field, and the first real
          element comes at the beginning of the vector.

 -- Function: int wordexp (const char *WORDS, wordexp_t
          *WORD-VECTOR-PTR, int FLAGS)
     Perform word expansion on the string WORDS, putting the result in
     a newly allocated vector, and store the size and address of this
     vector into `*WORD-VECTOR-PTR'.  The argument FLAGS is a
     combination of bit flags; see *note Flags for Wordexp::, for
     details of the flags.

     You shouldn't use any of the characters `|&;<>' in the string
     WORDS unless they are quoted; likewise for newline.  If you use
     these characters unquoted, you will get the `WRDE_BADCHAR' error
     code.  Don't use parentheses or braces unless they are quoted or
     part of a word expansion construct.  If you use quotation
     characters `'"`', they should come in pairs that balance.

     The results of word expansion are a sequence of words.  The
     function `wordexp' allocates a string for each resulting word, then
     allocates a vector of type `char **' to store the addresses of
     these strings.  The last element of the vector is a null pointer.
     This vector is called the "word vector".

     To return this vector, `wordexp' stores both its address and its
     length (number of elements, not counting the terminating null
     pointer) into `*WORD-VECTOR-PTR'.

     If `wordexp' succeeds, it returns 0.  Otherwise, it returns one of
     these error codes:

    `WRDE_BADCHAR'
          The input string WORDS contains an unquoted invalid character
          such as `|'.

    `WRDE_BADVAL'
          The input string refers to an undefined shell variable, and
          you used the flag `WRDE_UNDEF' to forbid such references.

    `WRDE_CMDSUB'
          The input string uses command substitution, and you used the
          flag `WRDE_NOCMD' to forbid command substitution.

    `WRDE_NOSPACE'
          It was impossible to allocate memory to hold the result.  In
          this case, `wordexp' can store part of the results--as much
          as it could allocate room for.

    `WRDE_SYNTAX'
          There was a syntax error in the input string.  For example,
          an unmatched quoting character is a syntax error.

 -- Function: void wordfree (wordexp_t *WORD-VECTOR-PTR)
     Free the storage used for the word-strings and vector that
     `*WORD-VECTOR-PTR' points to.  This does not free the structure
     `*WORD-VECTOR-PTR' itself--only the other data it points to.

File: libc.info,  Node: Flags for Wordexp,  Next: Wordexp Example,  Prev: Calling Wordexp,  Up: Word Expansion

10.4.3 Flags for Word Expansion
-------------------------------

This section describes the flags that you can specify in the FLAGS
argument to `wordexp'.  Choose the flags you want, and combine them
with the C operator `|'.

`WRDE_APPEND'
     Append the words from this expansion to the vector of words
     produced by previous calls to `wordexp'.  This way you can
     effectively expand several words as if they were concatenated with
     spaces between them.

     In order for appending to work, you must not modify the contents
     of the word vector structure between calls to `wordexp'.  And, if
     you set `WRDE_DOOFFS' in the first call to `wordexp', you must also
     set it when you append to the results.

`WRDE_DOOFFS'
     Leave blank slots at the beginning of the vector of words.  The
     `we_offs' field says how many slots to leave.  The blank slots
     contain null pointers.

`WRDE_NOCMD'
     Don't do command substitution; if the input requests command
     substitution, report an error.

`WRDE_REUSE'
     Reuse a word vector made by a previous call to `wordexp'.  Instead
     of allocating a new vector of words, this call to `wordexp' will
     use the vector that already exists (making it larger if necessary).

     Note that the vector may move, so it is not safe to save an old
     pointer and use it again after calling `wordexp'.  You must fetch
     `we_pathv' anew after each call.

`WRDE_SHOWERR'
     Do show any error messages printed by commands run by command
     substitution.  More precisely, allow these commands to inherit the
     standard error output stream of the current process.  By default,
     `wordexp' gives these commands a standard error stream that
     discards all output.

`WRDE_UNDEF'
     If the input refers to a shell variable that is not defined,
     report an error.

File: libc.info,  Node: Wordexp Example,  Next: Tilde Expansion,  Prev: Flags for Wordexp,  Up: Word Expansion

10.4.4 `wordexp' Example
------------------------

Here is an example of using `wordexp' to expand several strings and use
the results to run a shell command.  It also shows the use of
`WRDE_APPEND' to concatenate the expansions and of `wordfree' to free
the space allocated by `wordexp'.

     int
     expand_and_execute (const char *program, const char **options)
     {
       wordexp_t result;
       pid_t pid
       int status, i;

       /* Expand the string for the program to run.  */
       switch (wordexp (program, &result, 0))
         {
         case 0:			/* Successful.  */
           break;
         case WRDE_NOSPACE:
           /* If the error was `WRDE_NOSPACE',
              then perhaps part of the result was allocated.  */
           wordfree (&result);
         default:                    /* Some other error.  */
           return -1;
         }

       /* Expand the strings specified for the arguments.  */
       for (i = 0; options[i] != NULL; i++)
         {
           if (wordexp (options[i], &result, WRDE_APPEND))
             {
               wordfree (&result);
               return -1;
             }
         }

       pid = fork ();
       if (pid == 0)
         {
           /* This is the child process.  Execute the command. */
           execv (result.we_wordv[0], result.we_wordv);
           exit (EXIT_FAILURE);
         }
       else if (pid < 0)
         /* The fork failed.  Report failure.  */
         status = -1;
       else
         /* This is the parent process.  Wait for the child to complete.  */
         if (waitpid (pid, &status, 0) != pid)
           status = -1;

       wordfree (&result);
       return status;
     }

File: libc.info,  Node: Tilde Expansion,  Next: Variable Substitution,  Prev: Wordexp Example,  Up: Word Expansion

10.4.5 Details of Tilde Expansion
---------------------------------

It's a standard part of shell syntax that you can use `~' at the
beginning of a file name to stand for your own home directory.  You can
use `~USER' to stand for USER's home directory.

   "Tilde expansion" is the process of converting these abbreviations
to the directory names that they stand for.

   Tilde expansion applies to the `~' plus all following characters up
to whitespace or a slash.  It takes place only at the beginning of a
word, and only if none of the characters to be transformed is quoted in
any way.

   Plain `~' uses the value of the environment variable `HOME' as the
proper home directory name.  `~' followed by a user name uses
`getpwname' to look up that user in the user database, and uses
whatever directory is recorded there.  Thus, `~' followed by your own
name can give different results from plain `~', if the value of `HOME'
is not really your home directory.

File: libc.info,  Node: Variable Substitution,  Prev: Tilde Expansion,  Up: Word Expansion

10.4.6 Details of Variable Substitution
---------------------------------------

Part of ordinary shell syntax is the use of `$VARIABLE' to substitute
the value of a shell variable into a command.  This is called "variable
substitution", and it is one part of doing word expansion.

   There are two basic ways you can write a variable reference for
substitution:

`${VARIABLE}'
     If you write braces around the variable name, then it is completely
     unambiguous where the variable name ends.  You can concatenate
     additional letters onto the end of the variable value by writing
     them immediately after the close brace.  For example, `${foo}s'
     expands into `tractors'.

`$VARIABLE'
     If you do not put braces around the variable name, then the
     variable name consists of all the alphanumeric characters and
     underscores that follow the `$'.  The next punctuation character
     ends the variable name.  Thus, `$foo-bar' refers to the variable
     `foo' and expands into `tractor-bar'.

   When you use braces, you can also use various constructs to modify
the value that is substituted, or test it in various ways.

`${VARIABLE:-DEFAULT}'
     Substitute the value of VARIABLE, but if that is empty or
     undefined, use DEFAULT instead.

`${VARIABLE:=DEFAULT}'
     Substitute the value of VARIABLE, but if that is empty or
     undefined, use DEFAULT instead and set the variable to DEFAULT.

`${VARIABLE:?MESSAGE}'
     If VARIABLE is defined and not empty, substitute its value.

     Otherwise, print MESSAGE as an error message on the standard error
     stream, and consider word expansion a failure.

`${VARIABLE:+REPLACEMENT}'
     Substitute REPLACEMENT, but only if VARIABLE is defined and
     nonempty.  Otherwise, substitute nothing for this construct.

`${#VARIABLE}'
     Substitute a numeral which expresses in base ten the number of
     characters in the value of VARIABLE.  `${#foo}' stands for `7',
     because `tractor' is seven characters.

   These variants of variable substitution let you remove part of the
variable's value before substituting it.  The PREFIX and SUFFIX are not
mere strings; they are wildcard patterns, just like the patterns that
you use to match multiple file names.  But in this context, they match
against parts of the variable value rather than against file names.

`${VARIABLE%%SUFFIX}'
     Substitute the value of VARIABLE, but first discard from that
     variable any portion at the end that matches the pattern SUFFIX.

     If there is more than one alternative for how to match against
     SUFFIX, this construct uses the longest possible match.

     Thus, `${foo%%r*}' substitutes `t', because the largest match for
     `r*' at the end of `tractor' is `ractor'.

`${VARIABLE%SUFFIX}'
     Substitute the value of VARIABLE, but first discard from that
     variable any portion at the end that matches the pattern SUFFIX.

     If there is more than one alternative for how to match against
     SUFFIX, this construct uses the shortest possible alternative.

     Thus, `${foo%r*}' substitutes `tracto', because the shortest match
     for `r*' at the end of `tractor' is just `r'.

`${VARIABLE##PREFIX}'
     Substitute the value of VARIABLE, but first discard from that
     variable any portion at the beginning that matches the pattern
     PREFIX.

     If there is more than one alternative for how to match against
     PREFIX, this construct uses the longest possible match.

     Thus, `${foo##*t}' substitutes `or', because the largest match for
     `*t' at the beginning of `tractor' is `tract'.

`${VARIABLE#PREFIX}'
     Substitute the value of VARIABLE, but first discard from that
     variable any portion at the beginning that matches the pattern
     PREFIX.

     If there is more than one alternative for how to match against
     PREFIX, this construct uses the shortest possible alternative.

     Thus, `${foo#*t}' substitutes `ractor', because the shortest match
     for `*t' at the beginning of `tractor' is just `t'.


File: libc.info,  Node: I/O Overview,  Next: I/O on Streams,  Prev: Pattern Matching,  Up: Top

11 Input/Output Overview
************************

Most programs need to do either input (reading data) or output (writing
data), or most frequently both, in order to do anything useful.  The GNU
C library provides such a large selection of input and output functions
that the hardest part is often deciding which function is most
appropriate!

   This chapter introduces concepts and terminology relating to input
and output.  Other chapters relating to the GNU I/O facilities are:

   * *note I/O on Streams::, which covers the high-level functions that
     operate on streams, including formatted input and output.

   * *note Low-Level I/O::, which covers the basic I/O and control
     functions on file descriptors.

   * *note File System Interface::, which covers functions for
     operating on directories and for manipulating file attributes such
     as access modes and ownership.

   * *note Pipes and FIFOs::, which includes information on the basic
     interprocess communication facilities.

   * *note Sockets::, which covers a more complicated interprocess
     communication facility with support for networking.

   * *note Low-Level Terminal Interface::, which covers functions for
     changing how input and output to terminals or other serial devices
     are processed.

* Menu:

* I/O Concepts::       Some basic information and terminology.
* File Names::         How to refer to a file.

File: libc.info,  Node: I/O Concepts,  Next: File Names,  Up: I/O Overview

11.1 Input/Output Concepts
==========================

Before you can read or write the contents of a file, you must establish
a connection or communications channel to the file.  This process is
called "opening" the file.  You can open a file for reading, writing,
or both.

   The connection to an open file is represented either as a stream or
as a file descriptor.  You pass this as an argument to the functions
that do the actual read or write operations, to tell them which file to
operate on.  Certain functions expect streams, and others are designed
to operate on file descriptors.

   When you have finished reading to or writing from the file, you can
terminate the connection by "closing" the file.  Once you have closed a
stream or file descriptor, you cannot do any more input or output
operations on it.

* Menu:

* Streams and File Descriptors::    The GNU Library provides two ways
			             to access the contents of files.
* File Position::                   The number of bytes from the
                                     beginning of the file.

File: libc.info,  Node: Streams and File Descriptors,  Next: File Position,  Up: I/O Concepts

11.1.1 Streams and File Descriptors
-----------------------------------

When you want to do input or output to a file, you have a choice of two
basic mechanisms for representing the connection between your program
and the file: file descriptors and streams.  File descriptors are
represented as objects of type `int', while streams are represented as
`FILE *' objects.

   File descriptors provide a primitive, low-level interface to input
and output operations.  Both file descriptors and streams can represent
a connection to a device (such as a terminal), or a pipe or socket for
communicating with another process, as well as a normal file.  But, if
you want to do control operations that are specific to a particular kind
of device, you must use a file descriptor; there are no facilities to
use streams in this way.  You must also use file descriptors if your
program needs to do input or output in special modes, such as
nonblocking (or polled) input (*note File Status Flags::).

   Streams provide a higher-level interface, layered on top of the
primitive file descriptor facilities.  The stream interface treats all
kinds of files pretty much alike--the sole exception being the three
styles of buffering that you can choose (*note Stream Buffering::).

   The main advantage of using the stream interface is that the set of
functions for performing actual input and output operations (as opposed
to control operations) on streams is much richer and more powerful than
the corresponding facilities for file descriptors.  The file descriptor
interface provides only simple functions for transferring blocks of
characters, but the stream interface also provides powerful formatted
input and output functions (`printf' and `scanf') as well as functions
for character- and line-oriented input and output.

   Since streams are implemented in terms of file descriptors, you can
extract the file descriptor from a stream and perform low-level
operations directly on the file descriptor.  You can also initially open
a connection as a file descriptor and then make a stream associated with
that file descriptor.

   In general, you should stick with using streams rather than file
descriptors, unless there is some specific operation you want to do that
can only be done on a file descriptor.  If you are a beginning
programmer and aren't sure what functions to use, we suggest that you
concentrate on the formatted input functions (*note Formatted Input::)
and formatted output functions (*note Formatted Output::).

   If you are concerned about portability of your programs to systems
other than GNU, you should also be aware that file descriptors are not
as portable as streams.  You can expect any system running ISO C to
support streams, but non-GNU systems may not support file descriptors at
all, or may only implement a subset of the GNU functions that operate on
file descriptors.  Most of the file descriptor functions in the GNU
library are included in the POSIX.1 standard, however.

File: libc.info,  Node: File Position,  Prev: Streams and File Descriptors,  Up: I/O Concepts

11.1.2 File Position
--------------------

One of the attributes of an open file is its "file position" that keeps
track of where in the file the next character is to be read or written.
In the GNU system, and all POSIX.1 systems, the file position is simply
an integer representing the number of bytes from the beginning of the
file.

   The file position is normally set to the beginning of the file when
it is opened, and each time a character is read or written, the file
position is incremented.  In other words, access to the file is normally
"sequential".

   Ordinary files permit read or write operations at any position within
the file.  Some other kinds of files may also permit this.  Files which
do permit this are sometimes referred to as "random-access" files.  You
can change the file position using the `fseek' function on a stream
(*note File Positioning::) or the `lseek' function on a file descriptor
(*note I/O Primitives::).  If you try to change the file position on a
file that doesn't support random access, you get the `ESPIPE' error.

   Streams and descriptors that are opened for "append access" are
treated specially for output: output to such files is _always_ appended
sequentially to the _end_ of the file, regardless of the file position.
However, the file position is still used to control where in the file
reading is done.

   If you think about it, you'll realize that several programs can read
a given file at the same time.  In order for each program to be able to
read the file at its own pace, each program must have its own file
pointer, which is not affected by anything the other programs do.

   In fact, each opening of a file creates a separate file position.
Thus, if you open a file twice even in the same program, you get two
streams or descriptors with independent file positions.

   By contrast, if you open a descriptor and then duplicate it to get
another descriptor, these two descriptors share the same file position:
changing the file position of one descriptor will affect the other.

File: libc.info,  Node: File Names,  Prev: I/O Concepts,  Up: I/O Overview

11.2 File Names
===============

In order to open a connection to a file, or to perform other operations
such as deleting a file, you need some way to refer to the file.  Nearly
all files have names that are strings--even files which are actually
devices such as tape drives or terminals.  These strings are called
"file names".  You specify the file name to say which file you want to
open or operate on.

   This section describes the conventions for file names and how the
operating system works with them.

* Menu:

* Directories::                 Directories contain entries for files.
* File Name Resolution::        A file name specifies how to look up a file.
* File Name Errors::            Error conditions relating to file names.
* File Name Portability::       File name portability and syntax issues.

File: libc.info,  Node: Directories,  Next: File Name Resolution,  Up: File Names

11.2.1 Directories
------------------

In order to understand the syntax of file names, you need to understand
how the file system is organized into a hierarchy of directories.

   A "directory" is a file that contains information to associate other
files with names; these associations are called "links" or "directory
entries".  Sometimes, people speak of "files in a directory", but in
reality, a directory only contains pointers to files, not the files
themselves.

   The name of a file contained in a directory entry is called a "file
name component".  In general, a file name consists of a sequence of one
or more such components, separated by the slash character (`/').  A
file name which is just one component names a file with respect to its
directory.  A file name with multiple components names a directory, and
then a file in that directory, and so on.

   Some other documents, such as the POSIX standard, use the term
"pathname" for what we call a file name, and either "filename" or
"pathname component" for what this manual calls a file name component.
We don't use this terminology because a "path" is something completely
different (a list of directories to search), and we think that
"pathname" used for something else will confuse users.  We always use
"file name" and "file name component" (or sometimes just "component",
where the context is obvious) in GNU documentation.  Some macros use
the POSIX terminology in their names, such as `PATH_MAX'.  These macros
are defined by the POSIX standard, so we cannot change their names.

   You can find more detailed information about operations on
directories in *note File System Interface::.

File: libc.info,  Node: File Name Resolution,  Next: File Name Errors,  Prev: Directories,  Up: File Names

11.2.2 File Name Resolution
---------------------------

A file name consists of file name components separated by slash (`/')
characters.  On the systems that the GNU C library supports, multiple
successive `/' characters are equivalent to a single `/' character.

   The process of determining what file a file name refers to is called
"file name resolution".  This is performed by examining the components
that make up a file name in left-to-right order, and locating each
successive component in the directory named by the previous component.
Of course, each of the files that are referenced as directories must
actually exist, be directories instead of regular files, and have the
appropriate permissions to be accessible by the process; otherwise the
file name resolution fails.

   If a file name begins with a `/', the first component in the file
name is located in the "root directory" of the process (usually all
processes on the system have the same root directory).  Such a file name
is called an "absolute file name".

   Otherwise, the first component in the file name is located in the
current working directory (*note Working Directory::).  This kind of
file name is called a "relative file name".

   The file name components `.' ("dot") and `..' ("dot-dot") have
special meanings.  Every directory has entries for these file name
components.  The file name component `.' refers to the directory
itself, while the file name component `..' refers to its "parent
directory" (the directory that contains the link for the directory in
question).  As a special case, `..' in the root directory refers to the
root directory itself, since it has no parent; thus `/..' is the same
as `/'.

   Here are some examples of file names:

`/a'
     The file named `a', in the root directory.

`/a/b'
     The file named `b', in the directory named `a' in the root
     directory.

`a'
     The file named `a', in the current working directory.

`/a/./b'
     This is the same as `/a/b'.

`./a'
     The file named `a', in the current working directory.

`../a'
     The file named `a', in the parent directory of the current working
     directory.

   A file name that names a directory may optionally end in a `/'.  You
can specify a file name of `/' to refer to the root directory, but the
empty string is not a meaningful file name.  If you want to refer to
the current working directory, use a file name of `.' or `./'.

   Unlike some other operating systems, the GNU system doesn't have any
built-in support for file types (or extensions) or file versions as part
of its file name syntax.  Many programs and utilities use conventions
for file names--for example, files containing C source code usually
have names suffixed with `.c'--but there is nothing in the file system
itself that enforces this kind of convention.

File: libc.info,  Node: File Name Errors,  Next: File Name Portability,  Prev: File Name Resolution,  Up: File Names

11.2.3 File Name Errors
-----------------------

Functions that accept file name arguments usually detect these `errno'
error conditions relating to the file name syntax or trouble finding
the named file.  These errors are referred to throughout this manual as
the "usual file name errors".

`EACCES'
     The process does not have search permission for a directory
     component of the file name.

`ENAMETOOLONG'
     This error is used when either the total length of a file name is
     greater than `PATH_MAX', or when an individual file name component
     has a length greater than `NAME_MAX'.  *Note Limits for Files::.

     In the GNU system, there is no imposed limit on overall file name
     length, but some file systems may place limits on the length of a
     component.

`ENOENT'
     This error is reported when a file referenced as a directory
     component in the file name doesn't exist, or when a component is a
     symbolic link whose target file does not exist.  *Note Symbolic
     Links::.

`ENOTDIR'
     A file that is referenced as a directory component in the file name
     exists, but it isn't a directory.

`ELOOP'
     Too many symbolic links were resolved while trying to look up the
     file name.  The system has an arbitrary limit on the number of
     symbolic links that may be resolved in looking up a single file
     name, as a primitive way to detect loops.  *Note Symbolic Links::.

File: libc.info,  Node: File Name Portability,  Prev: File Name Errors,  Up: File Names

11.2.4 Portability of File Names
--------------------------------

The rules for the syntax of file names discussed in *note File Names::,
are the rules normally used by the GNU system and by other POSIX
systems.  However, other operating systems may use other conventions.

   There are two reasons why it can be important for you to be aware of
file name portability issues:

   * If your program makes assumptions about file name syntax, or
     contains embedded literal file name strings, it is more difficult
     to get it to run under other operating systems that use different
     syntax conventions.

   * Even if you are not concerned about running your program on
     machines that run other operating systems, it may still be
     possible to access files that use different naming conventions.
     For example, you may be able to access file systems on another
     computer running a different operating system over a network, or
     read and write disks in formats used by other operating systems.

   The ISO C standard says very little about file name syntax, only that
file names are strings.  In addition to varying restrictions on the
length of file names and what characters can validly appear in a file
name, different operating systems use different conventions and syntax
for concepts such as structured directories and file types or
extensions.  Some concepts such as file versions might be supported in
some operating systems and not by others.

   The POSIX.1 standard allows implementations to put additional
restrictions on file name syntax, concerning what characters are
permitted in file names and on the length of file name and file name
component strings.  However, in the GNU system, you do not need to worry
about these restrictions; any character except the null character is
permitted in a file name string, and there are no limits on the length
of file name strings.

File: libc.info,  Node: I/O on Streams,  Next: Low-Level I/O,  Prev: I/O Overview,  Up: Top

12 Input/Output on Streams
**************************

This chapter describes the functions for creating streams and performing
input and output operations on them.  As discussed in *note I/O
Overview::, a stream is a fairly abstract, high-level concept
representing a communications channel to a file, device, or process.

* Menu:

* Streams::                     About the data type representing a stream.
* Standard Streams::            Streams to the standard input and output
                                 devices are created for you.
* Opening Streams::             How to create a stream to talk to a file.
* Closing Streams::             Close a stream when you are finished with it.
* Streams and Threads::         Issues with streams in threaded programs.
* Streams and I18N::            Streams in internationalized applications.
* Simple Output::               Unformatted output by characters and lines.
* Character Input::             Unformatted input by characters and words.
* Line Input::                  Reading a line or a record from a stream.
* Unreading::                   Peeking ahead/pushing back input just read.
* Block Input/Output::          Input and output operations on blocks of data.
* Formatted Output::            `printf' and related functions.
* Customizing Printf::          You can define new conversion specifiers for
                                 `printf' and friends.
* Formatted Input::             `scanf' and related functions.
* EOF and Errors::              How you can tell if an I/O error happens.
* Error Recovery::		What you can do about errors.
* Binary Streams::              Some systems distinguish between text files
                                 and binary files.
* File Positioning::            About random-access streams.
* Portable Positioning::        Random access on peculiar ISO C systems.
* Stream Buffering::            How to control buffering of streams.
* Other Kinds of Streams::      Streams that do not necessarily correspond
                                 to an open file.
* Formatted Messages::          Print strictly formatted messages.

File: libc.info,  Node: Streams,  Next: Standard Streams,  Up: I/O on Streams

12.1 Streams
============

For historical reasons, the type of the C data structure that represents
a stream is called `FILE' rather than "stream".  Since most of the
library functions deal with objects of type `FILE *', sometimes the
term "file pointer" is also used to mean "stream".  This leads to
unfortunate confusion over terminology in many books on C.  This
manual, however, is careful to use the terms "file" and "stream" only
in the technical sense.

   The `FILE' type is declared in the header file `stdio.h'.

 -- Data Type: FILE
     This is the data type used to represent stream objects.  A `FILE'
     object holds all of the internal state information about the
     connection to the associated file, including such things as the
     file position indicator and buffering information.  Each stream
     also has error and end-of-file status indicators that can be
     tested with the `ferror' and `feof' functions; see *note EOF and
     Errors::.

   `FILE' objects are allocated and managed internally by the
input/output library functions.  Don't try to create your own objects of
type `FILE'; let the library do it.  Your programs should deal only
with pointers to these objects (that is, `FILE *' values) rather than
the objects themselves.

File: libc.info,  Node: Standard Streams,  Next: Opening Streams,  Prev: Streams,  Up: I/O on Streams

12.2 Standard Streams
=====================

When the `main' function of your program is invoked, it already has
three predefined streams open and available for use.  These represent
the "standard" input and output channels that have been established for
the process.

   These streams are declared in the header file `stdio.h'.

 -- Variable: FILE * stdin
     The "standard input" stream, which is the normal source of input
     for the program.

 -- Variable: FILE * stdout
     The "standard output" stream, which is used for normal output from
     the program.

 -- Variable: FILE * stderr
     The "standard error" stream, which is used for error messages and
     diagnostics issued by the program.

   In the GNU system, you can specify what files or processes
correspond to these streams using the pipe and redirection facilities
provided by the shell.  (The primitives shells use to implement these
facilities are described in *note File System Interface::.)  Most other
operating systems provide similar mechanisms, but the details of how to
use them can vary.

   In the GNU C library, `stdin', `stdout', and `stderr' are normal
variables which you can set just like any others.  For example, to
redirect the standard output to a file, you could do:

     fclose (stdout);
     stdout = fopen ("standard-output-file", "w");

   Note however, that in other systems `stdin', `stdout', and `stderr'
are macros that you cannot assign to in the normal way.  But you can
use `freopen' to get the effect of closing one and reopening it.  *Note
Opening Streams::.

   The three streams `stdin', `stdout', and `stderr' are not unoriented
at program start (*note Streams and I18N::).

File: libc.info,  Node: Opening Streams,  Next: Closing Streams,  Prev: Standard Streams,  Up: I/O on Streams

12.3 Opening Streams
====================

Opening a file with the `fopen' function creates a new stream and
establishes a connection between the stream and a file.  This may
involve creating a new file.

   Everything described in this section is declared in the header file
`stdio.h'.

 -- Function: FILE * fopen (const char *FILENAME, const char *OPENTYPE)
     The `fopen' function opens a stream for I/O to the file FILENAME,
     and returns a pointer to the stream.

     The OPENTYPE argument is a string that controls how the file is
     opened and specifies attributes of the resulting stream.  It must
     begin with one of the following sequences of characters:

    `r'
          Open an existing file for reading only.

    `w'
          Open the file for writing only.  If the file already exists,
          it is truncated to zero length.  Otherwise a new file is
          created.

    `a'
          Open a file for append access; that is, writing at the end of
          file only.  If the file already exists, its initial contents
          are unchanged and output to the stream is appended to the end
          of the file.  Otherwise, a new, empty file is created.

    `r+'
          Open an existing file for both reading and writing.  The
          initial contents of the file are unchanged and the initial
          file position is at the beginning of the file.

    `w+'
          Open a file for both reading and writing.  If the file
          already exists, it is truncated to zero length.  Otherwise, a
          new file is created.

    `a+'
          Open or create file for both reading and appending.  If the
          file exists, its initial contents are unchanged.  Otherwise,
          a new file is created.  The initial file position for reading
          is at the beginning of the file, but output is always
          appended to the end of the file.

     As you can see, `+' requests a stream that can do both input and
     output.  The ISO standard says that when using such a stream, you
     must call `fflush' (*note Stream Buffering::) or a file positioning
     function such as `fseek' (*note File Positioning::) when switching
     from reading to writing or vice versa.  Otherwise, internal buffers
     might not be emptied properly.  The GNU C library does not have
     this limitation; you can do arbitrary reading and writing
     operations on a stream in whatever order.

     Additional characters may appear after these to specify flags for
     the call.  Always put the mode (`r', `w+', etc.) first; that is
     the only part you are guaranteed will be understood by all systems.

     The GNU C library defines one additional character for use in
     OPENTYPE: the character `x' insists on creating a new file--if a
     file FILENAME already exists, `fopen' fails rather than opening
     it.  If you use `x' you are guaranteed that you will not clobber
     an existing file.  This is equivalent to the `O_EXCL' option to
     the `open' function (*note Opening and Closing Files::).

     The character `b' in OPENTYPE has a standard meaning; it requests
     a binary stream rather than a text stream.  But this makes no
     difference in POSIX systems (including the GNU system).  If both
     `+' and `b' are specified, they can appear in either order.  *Note
     Binary Streams::.

     If the OPENTYPE string contains the sequence `,ccs=STRING' then
     STRING is taken as the name of a coded character set and `fopen'
     will mark the stream as wide-oriented which appropriate conversion
     functions in place to convert from and to the character set STRING
     is place.  Any other stream is opened initially unoriented and the
     orientation is decided with the first file operation.  If the
     first operation is a wide character operation, the stream is not
     only marked as wide-oriented, also the conversion functions to
     convert to the coded character set used for the current locale are
     loaded.  This will not change anymore from this point on even if
     the locale selected for the `LC_CTYPE' category is changed.

     Any other characters in OPENTYPE are simply ignored.  They may be
     meaningful in other systems.

     If the open fails, `fopen' returns a null pointer.

     When the sources are compiling with `_FILE_OFFSET_BITS == 64' on a
     32 bit machine this function is in fact `fopen64' since the LFS
     interface replaces transparently the old interface.

   You can have multiple streams (or file descriptors) pointing to the
same file open at the same time.  If you do only input, this works
straightforwardly, but you must be careful if any output streams are
included.  *Note Stream/Descriptor Precautions::.  This is equally true
whether the streams are in one program (not usual) or in several
programs (which can easily happen).  It may be advantageous to use the
file locking facilities to avoid simultaneous access.  *Note File
Locks::.

 -- Function: FILE * fopen64 (const char *FILENAME, const char
          *OPENTYPE)
     This function is similar to `fopen' but the stream it returns a
     pointer for is opened using `open64'.  Therefore this stream can be
     used even on files larger then 2^31 bytes on 32 bit machines.

     Please note that the return type is still `FILE *'.  There is no
     special `FILE' type for the LFS interface.

     If the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32
     bits machine this function is available under the name `fopen' and
     so transparently replaces the old interface.

 -- Macro: int FOPEN_MAX
     The value of this macro is an integer constant expression that
     represents the minimum number of streams that the implementation
     guarantees can be open simultaneously.  You might be able to open
     more than this many streams, but that is not guaranteed.  The
     value of this constant is at least eight, which includes the three
     standard streams `stdin', `stdout', and `stderr'.  In POSIX.1
     systems this value is determined by the `OPEN_MAX' parameter;
     *note General Limits::.  In BSD and GNU, it is controlled by the
     `RLIMIT_NOFILE' resource limit; *note Limits on Resources::.

 -- Function: FILE * freopen (const char *FILENAME, const char
          *OPENTYPE, FILE *STREAM)
     This function is like a combination of `fclose' and `fopen'.  It
     first closes the stream referred to by STREAM, ignoring any errors
     that are detected in the process.  (Because errors are ignored,
     you should not use `freopen' on an output stream if you have
     actually done any output using the stream.)  Then the file named by
     FILENAME is opened with mode OPENTYPE as for `fopen', and
     associated with the same stream object STREAM.

     If the operation fails, a null pointer is returned; otherwise,
     `freopen' returns STREAM.

     `freopen' has traditionally been used to connect a standard stream
     such as `stdin' with a file of your own choice.  This is useful in
     programs in which use of a standard stream for certain purposes is
     hard-coded.  In the GNU C library, you can simply close the
     standard streams and open new ones with `fopen'.  But other
     systems lack this ability, so using `freopen' is more portable.

     When the sources are compiling with `_FILE_OFFSET_BITS == 64' on a
     32 bit machine this function is in fact `freopen64' since the LFS
     interface replaces transparently the old interface.

 -- Function: FILE * freopen64 (const char *FILENAME, const char
          *OPENTYPE, FILE *STREAM)
     This function is similar to `freopen'.  The only difference is that
     on 32 bit machine the stream returned is able to read beyond the
     2^31 bytes limits imposed by the normal interface.  It should be
     noted that the stream pointed to by STREAM need not be opened
     using `fopen64' or `freopen64' since its mode is not important for
     this function.

     If the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32
     bits machine this function is available under the name `freopen'
     and so transparently replaces the old interface.

   In some situations it is useful to know whether a given stream is
available for reading or writing.  This information is normally not
available and would have to be remembered separately.  Solaris
introduced a few functions to get this information from the stream
descriptor and these functions are also available in the GNU C library.

 -- Function: int __freadable (FILE *STREAM)
     The `__freadable' function determines whether the stream STREAM
     was opened to allow reading.  In this case the return value is
     nonzero.  For write-only streams the function returns zero.

     This function is declared in `stdio_ext.h'.

 -- Function: int __fwritable (FILE *STREAM)
     The `__fwritable' function determines whether the stream STREAM
     was opened to allow writing.  In this case the return value is
     nonzero.  For read-only streams the function returns zero.

     This function is declared in `stdio_ext.h'.

   For slightly different kind of problems there are two more functions.
They provide even finer-grained information.

 -- Function: int __freading (FILE *STREAM)
     The `__freading' function determines whether the stream STREAM was
     last read from or whether it is opened read-only.  In this case
     the return value is nonzero, otherwise it is zero.  Determining
     whether a stream opened for reading and writing was last used for
     writing allows to draw conclusions about the content about the
     buffer, among other things.

     This function is declared in `stdio_ext.h'.

 -- Function: int __fwriting (FILE *STREAM)
     The `__fwriting' function determines whether the stream STREAM was
     last written to or whether it is opened write-only.  In this case
     the return value is nonzero, otherwise it is zero.

     This function is declared in `stdio_ext.h'.

File: libc.info,  Node: Closing Streams,  Next: Streams and Threads,  Prev: Opening Streams,  Up: I/O on Streams

12.4 Closing Streams
====================

When a stream is closed with `fclose', the connection between the
stream and the file is canceled.  After you have closed a stream, you
cannot perform any additional operations on it.

 -- Function: int fclose (FILE *STREAM)
     This function causes STREAM to be closed and the connection to the
     corresponding file to be broken.  Any buffered output is written
     and any buffered input is discarded.  The `fclose' function returns
     a value of `0' if the file was closed successfully, and `EOF' if
     an error was detected.

     It is important to check for errors when you call `fclose' to close
     an output stream, because real, everyday errors can be detected at
     this time.  For example, when `fclose' writes the remaining
     buffered output, it might get an error because the disk is full.
     Even if you know the buffer is empty, errors can still occur when
     closing a file if you are using NFS.

     The function `fclose' is declared in `stdio.h'.

   To close all streams currently available the GNU C Library provides
another function.

 -- Function: int fcloseall (void)
     This function causes all open streams of the process to be closed
     and the connection to corresponding files to be broken.  All
     buffered data is written and any buffered input is discarded.  The
     `fcloseall' function returns a value of `0' if all the files were
     closed successfully, and `EOF' if an error was detected.

     This function should be used only in special situations, e.g.,
     when an error occurred and the program must be aborted.  Normally
     each single stream should be closed separately so that problems
     with individual streams can be identified.  It is also problematic
     since the standard streams (*note Standard Streams::) will also be
     closed.

     The function `fcloseall' is declared in `stdio.h'.

   If the `main' function to your program returns, or if you call the
`exit' function (*note Normal Termination::), all open streams are
automatically closed properly.  If your program terminates in any other
manner, such as by calling the `abort' function (*note Aborting a
Program::) or from a fatal signal (*note Signal Handling::), open
streams might not be closed properly.  Buffered output might not be
flushed and files may be incomplete.  For more information on buffering
of streams, see *note Stream Buffering::.

File: libc.info,  Node: Streams and Threads,  Next: Streams and I18N,  Prev: Closing Streams,  Up: I/O on Streams

12.5 Streams and Threads
========================

Streams can be used in multi-threaded applications in the same way they
are used in single-threaded applications.  But the programmer must be
aware of the possible complications.  It is important to know about
these also if the program one writes never use threads since the design
and implementation of many stream functions is heavily influenced by the
requirements added by multi-threaded programming.

   The POSIX standard requires that by default the stream operations are
atomic.  I.e., issuing two stream operations for the same stream in two
threads at the same time will cause the operations to be executed as if
they were issued sequentially.  The buffer operations performed while
reading or writing are protected from other uses of the same stream.  To
do this each stream has an internal lock object which has to be
(implicitly) acquired before any work can be done.

   But there are situations where this is not enough and there are also
situations where this is not wanted.  The implicit locking is not enough
if the program requires more than one stream function call to happen
atomically.  One example would be if an output line a program wants to
generate is created by several function calls.  The functions by
themselves would ensure only atomicity of their own operation, but not
atomicity over all the function calls.  For this it is necessary to
perform the stream locking in the application code.

 -- Function: void flockfile (FILE *STREAM)
     The `flockfile' function acquires the internal locking object
     associated with the stream STREAM.  This ensures that no other
     thread can explicitly through `flockfile'/`ftrylockfile' or
     implicit through a call of a stream function lock the stream.  The
     thread will block until the lock is acquired.  An explicit call to
     `funlockfile' has to be used to release the lock.

 -- Function: int ftrylockfile (FILE *STREAM)
     The `ftrylockfile' function tries to acquire the internal locking
     object associated with the stream STREAM just like `flockfile'.
     But unlike `flockfile' this function does not block if the lock is
     not available.  `ftrylockfile' returns zero if the lock was
     successfully acquired.  Otherwise the stream is locked by another
     thread.

 -- Function: void funlockfile (FILE *STREAM)
     The `funlockfile' function releases the internal locking object of
     the stream STREAM. The stream must have been locked before by a
     call to `flockfile' or a successful call of `ftrylockfile'.  The
     implicit locking performed by the stream operations do not count.
     The `funlockfile' function does not return an error status and the
     behavior of a call for a stream which is not locked by the current
     thread is undefined.

   The following example shows how the functions above can be used to
generate an output line atomically even in multi-threaded applications
(yes, the same job could be done with one `fprintf' call but it is
sometimes not possible):

     FILE *fp;
     {
        ...
        flockfile (fp);
        fputs ("This is test number ", fp);
        fprintf (fp, "%d\n", test);
        funlockfile (fp)
     }

   Without the explicit locking it would be possible for another thread
to use the stream FP after the `fputs' call return and before `fprintf'
was called with the result that the number does not follow the word
`number'.

   From this description it might already be clear that the locking
objects in streams are no simple mutexes.  Since locking the same
stream twice in the same thread is allowed the locking objects must be
equivalent to recursive mutexes.  These mutexes keep track of the owner
and the number of times the lock is acquired.  The same number of
`funlockfile' calls by the same threads is necessary to unlock the
stream completely.  For instance:

     void
     foo (FILE *fp)
     {
       ftrylockfile (fp);
       fputs ("in foo\n", fp);
       /* This is very wrong!!!  */
       funlockfile (fp);
     }

   It is important here that the `funlockfile' function is only called
if the `ftrylockfile' function succeeded in locking the stream.  It is
therefore always wrong to ignore the result of `ftrylockfile'.  And it
makes no sense since otherwise one would use `flockfile'.  The result
of code like that above is that either `funlockfile' tries to free a
stream that hasn't been locked by the current thread or it frees the
stream prematurely.  The code should look like this:

     void
     foo (FILE *fp)
     {
       if (ftrylockfile (fp) == 0)
         {
           fputs ("in foo\n", fp);
           funlockfile (fp);
         }
     }

   Now that we covered why it is necessary to have these locking it is
necessary to talk about situations when locking is unwanted and what can
be done.  The locking operations (explicit or implicit) don't come for
free.  Even if a lock is not taken the cost is not zero.  The operations
which have to be performed require memory operations that are safe in
multi-processor environments.  With the many local caches involved in
such systems this is quite costly.  So it is best to avoid the locking
completely if it is not needed - because the code in question is never
used in a context where two or more threads may use a stream at a time.
This can be determined most of the time for application code; for
library code which can be used in many contexts one should default to be
conservative and use locking.

   There are two basic mechanisms to avoid locking.  The first is to use
the `_unlocked' variants of the stream operations.  The POSIX standard
defines quite a few of those and the GNU library adds a few more.
These variants of the functions behave just like the functions with the
name without the suffix except that they do not lock the stream.  Using
these functions is very desirable since they are potentially much
faster.  This is not only because the locking operation itself is
avoided.  More importantly, functions like `putc' and `getc' are very
simple and traditionally (before the introduction of threads) were
implemented as macros which are very fast if the buffer is not empty.
With the addition of locking requirements these functions are no longer
implemented as macros since they would expand to too much code.  But
these macros are still available with the same functionality under the
new names `putc_unlocked' and `getc_unlocked'.  This possibly huge
difference of speed also suggests the use of the `_unlocked' functions
even if locking is required.  The difference is that the locking then
has to be performed in the program:

     void
     foo (FILE *fp, char *buf)
     {
       flockfile (fp);
       while (*buf != '/')
         putc_unlocked (*buf++, fp);
       funlockfile (fp);
     }

   If in this example the `putc' function would be used and the
explicit locking would be missing the `putc' function would have to
acquire the lock in every call, potentially many times depending on when
the loop terminates.  Writing it the way illustrated above allows the
`putc_unlocked' macro to be used which means no locking and direct
manipulation of the buffer of the stream.

   A second way to avoid locking is by using a non-standard function
which was introduced in Solaris and is available in the GNU C library
as well.

 -- Function: int __fsetlocking (FILE *STREAM, int TYPE)
     The `__fsetlocking' function can be used to select whether the
     stream operations will implicitly acquire the locking object of the
     stream STREAM.  By default this is done but it can be disabled and
     reinstated using this function.  There are three values defined
     for the TYPE parameter.

    `FSETLOCKING_INTERNAL'
          The stream `stream' will from now on use the default internal
          locking.  Every stream operation with exception of the
          `_unlocked' variants will implicitly lock the stream.

    `FSETLOCKING_BYCALLER'
          After the `__fsetlocking' function returns the user is
          responsible for locking the stream.  None of the stream
          operations will implicitly do this anymore until the state is
          set back to `FSETLOCKING_INTERNAL'.

    `FSETLOCKING_QUERY'
          `__fsetlocking' only queries the current locking state of the
          stream.  The return value will be `FSETLOCKING_INTERNAL' or
          `FSETLOCKING_BYCALLER' depending on the state.

     The return value of `__fsetlocking' is either
     `FSETLOCKING_INTERNAL' or `FSETLOCKING_BYCALLER' depending on the
     state of the stream before the call.

     This function and the values for the TYPE parameter are declared
     in `stdio_ext.h'.

   This function is especially useful when program code has to be used
which is written without knowledge about the `_unlocked' functions (or
if the programmer was too lazy to use them).

File: libc.info,  Node: Streams and I18N,  Next: Simple Output,  Prev: Streams and Threads,  Up: I/O on Streams

12.6 Streams in Internationalized Applications
==============================================

ISO C90 introduced the new type `wchar_t' to allow handling larger
character sets.  What was missing was a possibility to output strings
of `wchar_t' directly.  One had to convert them into multibyte strings
using `mbstowcs' (there was no `mbsrtowcs' yet) and then use the normal
stream functions.  While this is doable it is very cumbersome since
performing the conversions is not trivial and greatly increases program
complexity and size.

   The Unix standard early on (I think in XPG4.2) introduced two
additional format specifiers for the `printf' and `scanf' families of
functions.  Printing and reading of single wide characters was made
possible using the `%C' specifier and wide character strings can be
handled with `%S'.  These modifiers behave just like `%c' and `%s' only
that they expect the corresponding argument to have the wide character
type and that the wide character and string are transformed into/from
multibyte strings before being used.

   This was a beginning but it is still not good enough.  Not always is
it desirable to use `printf' and `scanf'.  The other, smaller and
faster functions cannot handle wide characters.  Second, it is not
possible to have a format string for `printf' and `scanf' consisting of
wide characters.  The result is that format strings would have to be
generated if they have to contain non-basic characters.

   In the Amendment 1 to ISO C90 a whole new set of functions was added
to solve the problem.  Most of the stream functions got a counterpart
which take a wide character or wide character string instead of a
character or string respectively.  The new functions operate on the
same streams (like `stdout').  This is different from the model of the
C++ runtime library where separate streams for wide and normal I/O are
used.

   Being able to use the same stream for wide and normal operations
comes with a restriction: a stream can be used either for wide
operations or for normal operations.  Once it is decided there is no
way back.  Only a call to `freopen' or `freopen64' can reset the
"orientation".  The orientation can be decided in three ways:

   * If any of the normal character functions is used (this includes the
     `fread' and `fwrite' functions) the stream is marked as not wide
     oriented.

   * If any of the wide character functions is used the stream is
     marked as wide oriented.

   * The `fwide' function can be used to set the orientation either way.

   It is important to never mix the use of wide and not wide operations
on a stream.  There are no diagnostics issued.  The application behavior
will simply be strange or the application will simply crash.  The
`fwide' function can help avoiding this.

 -- Function: int fwide (FILE *STREAM, int MODE)
     The `fwide' function can be used to set and query the state of the
     orientation of the stream STREAM.  If the MODE parameter has a
     positive value the streams get wide oriented, for negative values
     narrow oriented.  It is not possible to overwrite previous
     orientations with `fwide'.  I.e., if the stream STREAM was already
     oriented before the call nothing is done.

     If MODE is zero the current orientation state is queried and
     nothing is changed.

     The `fwide' function returns a negative value, zero, or a positive
     value if the stream is narrow, not at all, or wide oriented
     respectively.

     This function was introduced in Amendment 1 to ISO C90 and is
     declared in `wchar.h'.

   It is generally a good idea to orient a stream as early as possible.
This can prevent surprise especially for the standard streams `stdin',
`stdout', and `stderr'.  If some library function in some situations
uses one of these streams and this use orients the stream in a
different way the rest of the application expects it one might end up
with hard to reproduce errors.  Remember that no errors are signal if
the streams are used incorrectly.  Leaving a stream unoriented after
creation is normally only necessary for library functions which create
streams which can be used in different contexts.

   When writing code which uses streams and which can be used in
different contexts it is important to query the orientation of the
stream before using it (unless the rules of the library interface
demand a specific orientation).  The following little, silly function
illustrates this.

     void
     print_f (FILE *fp)
     {
       if (fwide (fp, 0) > 0)
         /* Positive return value means wide orientation.  */
         fputwc (L'f', fp);
       else
         fputc ('f', fp);
     }

   Note that in this case the function `print_f' decides about the
orientation of the stream if it was unoriented before (will not happen
if the advise above is followed).

   The encoding used for the `wchar_t' values is unspecified and the
user must not make any assumptions about it.  For I/O of `wchar_t'
values this means that it is impossible to write these values directly
to the stream.  This is not what follows from the ISO C locale model
either.  What happens instead is that the bytes read from or written to
the underlying media are first converted into the internal encoding
chosen by the implementation for `wchar_t'.  The external encoding is
determined by the `LC_CTYPE' category of the current locale or by the
`ccs' part of the mode specification given to `fopen', `fopen64',
`freopen', or `freopen64'.  How and when the conversion happens is
unspecified and it happens invisible to the user.

   Since a stream is created in the unoriented state it has at that
point no conversion associated with it.  The conversion which will be
used is determined by the `LC_CTYPE' category selected at the time the
stream is oriented.  If the locales are changed at the runtime this
might produce surprising results unless one pays attention.  This is
just another good reason to orient the stream explicitly as soon as
possible, perhaps with a call to `fwide'.

File: libc.info,  Node: Simple Output,  Next: Character Input,  Prev: Streams and I18N,  Up: I/O on Streams

12.7 Simple Output by Characters or Lines
=========================================

This section describes functions for performing character- and
line-oriented output.

   These narrow streams functions are declared in the header file
`stdio.h' and the wide stream functions in `wchar.h'.

 -- Function: int fputc (int C, FILE *STREAM)
     The `fputc' function converts the character C to type `unsigned
     char', and writes it to the stream STREAM.  `EOF' is returned if a
     write error occurs; otherwise the character C is returned.

 -- Function: wint_t fputwc (wchar_t WC, FILE *STREAM)
     The `fputwc' function writes the wide character WC to the stream
     STREAM.  `WEOF' is returned if a write error occurs; otherwise the
     character WC is returned.

 -- Function: int fputc_unlocked (int C, FILE *STREAM)
     The `fputc_unlocked' function is equivalent to the `fputc'
     function except that it does not implicitly lock the stream.

 -- Function: wint_t fputwc_unlocked (wint_t WC, FILE *STREAM)
     The `fputwc_unlocked' function is equivalent to the `fputwc'
     function except that it does not implicitly lock the stream.

     This function is a GNU extension.

 -- Function: int putc (int C, FILE *STREAM)
     This is just like `fputc', except that most systems implement it as
     a macro, making it faster.  One consequence is that it may
     evaluate the STREAM argument more than once, which is an exception
     to the general rule for macros.  `putc' is usually the best
     function to use for writing a single character.

 -- Function: wint_t putwc (wchar_t WC, FILE *STREAM)
     This is just like `fputwc', except that it can be implement as a
     macro, making it faster.  One consequence is that it may evaluate
     the STREAM argument more than once, which is an exception to the
     general rule for macros.  `putwc' is usually the best function to
     use for writing a single wide character.

 -- Function: int putc_unlocked (int C, FILE *STREAM)
     The `putc_unlocked' function is equivalent to the `putc' function
     except that it does not implicitly lock the stream.

 -- Function: wint_t putwc_unlocked (wchar_t WC, FILE *STREAM)
     The `putwc_unlocked' function is equivalent to the `putwc'
     function except that it does not implicitly lock the stream.

     This function is a GNU extension.

 -- Function: int putchar (int C)
     The `putchar' function is equivalent to `putc' with `stdout' as
     the value of the STREAM argument.

 -- Function: wint_t putwchar (wchar_t WC)
     The `putwchar' function is equivalent to `putwc' with `stdout' as
     the value of the STREAM argument.

 -- Function: int putchar_unlocked (int C)
     The `putchar_unlocked' function is equivalent to the `putchar'
     function except that it does not implicitly lock the stream.

 -- Function: wint_t putwchar_unlocked (wchar_t WC)
     The `putwchar_unlocked' function is equivalent to the `putwchar'
     function except that it does not implicitly lock the stream.

     This function is a GNU extension.

 -- Function: int fputs (const char *S, FILE *STREAM)
     The function `fputs' writes the string S to the stream STREAM.
     The terminating null character is not written.  This function does
     _not_ add a newline character, either.  It outputs only the
     characters in the string.

     This function returns `EOF' if a write error occurs, and otherwise
     a non-negative value.

     For example:

          fputs ("Are ", stdout);
          fputs ("you ", stdout);
          fputs ("hungry?\n", stdout);

     outputs the text `Are you hungry?' followed by a newline.

 -- Function: int fputws (const wchar_t *WS, FILE *STREAM)
     The function `fputws' writes the wide character string WS to the
     stream STREAM.  The terminating null character is not written.
     This function does _not_ add a newline character, either.  It
     outputs only the characters in the string.

     This function returns `WEOF' if a write error occurs, and otherwise
     a non-negative value.

 -- Function: int fputs_unlocked (const char *S, FILE *STREAM)
     The `fputs_unlocked' function is equivalent to the `fputs'
     function except that it does not implicitly lock the stream.

     This function is a GNU extension.

 -- Function: int fputws_unlocked (const wchar_t *WS, FILE *STREAM)
     The `fputws_unlocked' function is equivalent to the `fputws'
     function except that it does not implicitly lock the stream.

     This function is a GNU extension.

 -- Function: int puts (const char *S)
     The `puts' function writes the string S to the stream `stdout'
     followed by a newline.  The terminating null character of the
     string is not written.  (Note that `fputs' does _not_ write a
     newline as this function does.)

     `puts' is the most convenient function for printing simple
     messages.  For example:

          puts ("This is a message.");

     outputs the text `This is a message.' followed by a newline.

 -- Function: int putw (int W, FILE *STREAM)
     This function writes the word W (that is, an `int') to STREAM.  It
     is provided for compatibility with SVID, but we recommend you use
     `fwrite' instead (*note Block Input/Output::).

File: libc.info,  Node: Character Input,  Next: Line Input,  Prev: Simple Output,  Up: I/O on Streams

12.8 Character Input
====================

This section describes functions for performing character-oriented
input.  These narrow streams functions are declared in the header file
`stdio.h' and the wide character functions are declared in `wchar.h'.

   These functions return an `int' or `wint_t' value (for narrow and
wide stream functions respectively) that is either a character of
input, or the special value `EOF'/`WEOF' (usually -1).  For the narrow
stream functions it is important to store the result of these functions
in a variable of type `int' instead of `char', even when you plan to
use it only as a character.  Storing `EOF' in a `char' variable
truncates its value to the size of a character, so that it is no longer
distinguishable from the valid character `(char) -1'.  So always use an
`int' for the result of `getc' and friends, and check for `EOF' after
the call; once you've verified that the result is not `EOF', you can be
sure that it will fit in a `char' variable without loss of information.

 -- Function: int fgetc (FILE *STREAM)
     This function reads the next character as an `unsigned char' from
     the stream STREAM and returns its value, converted to an `int'.
     If an end-of-file condition or read error occurs, `EOF' is
     returned instead.

 -- Function: wint_t fgetwc (FILE *STREAM)
     This function reads the next wide character from the stream STREAM
     and returns its value.  If an end-of-file condition or read error
     occurs, `WEOF' is returned instead.

 -- Function: int fgetc_unlocked (FILE *STREAM)
     The `fgetc_unlocked' function is equivalent to the `fgetc'
     function except that it does not implicitly lock the stream.

 -- Function: wint_t fgetwc_unlocked (FILE *STREAM)
     The `fgetwc_unlocked' function is equivalent to the `fgetwc'
     function except that it does not implicitly lock the stream.

     This function is a GNU extension.

 -- Function: int getc (FILE *STREAM)
     This is just like `fgetc', except that it is permissible (and
     typical) for it to be implemented as a macro that evaluates the
     STREAM argument more than once.  `getc' is often highly optimized,
     so it is usually the best function to use to read a single
     character.

 -- Function: wint_t getwc (FILE *STREAM)
     This is just like `fgetwc', except that it is permissible for it to
     be implemented as a macro that evaluates the STREAM argument more
     than once.  `getwc' can be highly optimized, so it is usually the
     best function to use to read a single wide character.

 -- Function: int getc_unlocked (FILE *STREAM)
     The `getc_unlocked' function is equivalent to the `getc' function
     except that it does not implicitly lock the stream.

 -- Function: wint_t getwc_unlocked (FILE *STREAM)
     The `getwc_unlocked' function is equivalent to the `getwc'
     function except that it does not implicitly lock the stream.

     This function is a GNU extension.

 -- Function: int getchar (void)
     The `getchar' function is equivalent to `getc' with `stdin' as the
     value of the STREAM argument.

 -- Function: wint_t getwchar (void)
     The `getwchar' function is equivalent to `getwc' with `stdin' as
     the value of the STREAM argument.

 -- Function: int getchar_unlocked (void)
     The `getchar_unlocked' function is equivalent to the `getchar'
     function except that it does not implicitly lock the stream.

 -- Function: wint_t getwchar_unlocked (void)
     The `getwchar_unlocked' function is equivalent to the `getwchar'
     function except that it does not implicitly lock the stream.

     This function is a GNU extension.

   Here is an example of a function that does input using `fgetc'.  It
would work just as well using `getc' instead, or using `getchar ()'
instead of `fgetc (stdin)'.  The code would also work the same for the
wide character stream functions.

     int
     y_or_n_p (const char *question)
     {
       fputs (question, stdout);
       while (1)
         {
           int c, answer;
           /* Write a space to separate answer from question. */
           fputc (' ', stdout);
           /* Read the first character of the line.
              This should be the answer character, but might not be. */
           c = tolower (fgetc (stdin));
           answer = c;
           /* Discard rest of input line. */
           while (c != '\n' && c != EOF)
             c = fgetc (stdin);
           /* Obey the answer if it was valid. */
           if (answer == 'y')
             return 1;
           if (answer == 'n')
             return 0;
           /* Answer was invalid: ask for valid answer. */
           fputs ("Please answer y or n:", stdout);
         }
     }

 -- Function: int getw (FILE *STREAM)
     This function reads a word (that is, an `int') from STREAM.  It's
     provided for compatibility with SVID.  We recommend you use
     `fread' instead (*note Block Input/Output::).  Unlike `getc', any
     `int' value could be a valid result.  `getw' returns `EOF' when it
     encounters end-of-file or an error, but there is no way to
     distinguish this from an input word with value -1.

File: libc.info,  Node: Line Input,  Next: Unreading,  Prev: Character Input,  Up: I/O on Streams

12.9 Line-Oriented Input
========================

Since many programs interpret input on the basis of lines, it is
convenient to have functions to read a line of text from a stream.

   Standard C has functions to do this, but they aren't very safe: null
characters and even (for `gets') long lines can confuse them.  So the
GNU library provides the nonstandard `getline' function that makes it
easy to read lines reliably.

   Another GNU extension, `getdelim', generalizes `getline'.  It reads
a delimited record, defined as everything through the next occurrence
of a specified delimiter character.

   All these functions are declared in `stdio.h'.

 -- Function: ssize_t getline (char **LINEPTR, size_t *N, FILE *STREAM)
     This function reads an entire line from STREAM, storing the text
     (including the newline and a terminating null character) in a
     buffer and storing the buffer address in `*LINEPTR'.

     Before calling `getline', you should place in `*LINEPTR' the
     address of a buffer `*N' bytes long, allocated with `malloc'.  If
     this buffer is long enough to hold the line, `getline' stores the
     line in this buffer.  Otherwise, `getline' makes the buffer bigger
     using `realloc', storing the new buffer address back in `*LINEPTR'
     and the increased size back in `*N'.  *Note Unconstrained
     Allocation::.

     If you set `*LINEPTR' to a null pointer, and `*N' to zero, before
     the call, then `getline' allocates the initial buffer for you by
     calling `malloc'.

     In either case, when `getline' returns,  `*LINEPTR' is a `char *'
     which points to the text of the line.

     When `getline' is successful, it returns the number of characters
     read (including the newline, but not including the terminating
     null).  This value enables you to distinguish null characters that
     are part of the line from the null character inserted as a
     terminator.

     This function is a GNU extension, but it is the recommended way to
     read lines from a stream.  The alternative standard functions are
     unreliable.

     If an error occurs or end of file is reached without any bytes
     read, `getline' returns `-1'.

 -- Function: ssize_t getdelim (char **LINEPTR, size_t *N, int
          DELIMITER, FILE *STREAM)
     This function is like `getline' except that the character which
     tells it to stop reading is not necessarily newline.  The argument
     DELIMITER specifies the delimiter character; `getdelim' keeps
     reading until it sees that character (or end of file).

     The text is stored in LINEPTR, including the delimiter character
     and a terminating null.  Like `getline', `getdelim' makes LINEPTR
     bigger if it isn't big enough.

     `getline' is in fact implemented in terms of `getdelim', just like
     this:

          ssize_t
          getline (char **lineptr, size_t *n, FILE *stream)
          {
            return getdelim (lineptr, n, '\n', stream);
          }

 -- Function: char * fgets (char *S, int COUNT, FILE *STREAM)
     The `fgets' function reads characters from the stream STREAM up to
     and including a newline character and stores them in the string S,
     adding a null character to mark the end of the string.  You must
     supply COUNT characters worth of space in S, but the number of
     characters read is at most COUNT - 1.  The extra character space
     is used to hold the null character at the end of the string.

     If the system is already at end of file when you call `fgets', then
     the contents of the array S are unchanged and a null pointer is
     returned.  A null pointer is also returned if a read error occurs.
     Otherwise, the return value is the pointer S.

     *Warning:*  If the input data has a null character, you can't tell.
     So don't use `fgets' unless you know the data cannot contain a
     null.  Don't use it to read files edited by the user because, if
     the user inserts a null character, you should either handle it
     properly or print a clear error message.  We recommend using
     `getline' instead of `fgets'.

 -- Function: wchar_t * fgetws (wchar_t *WS, int COUNT, FILE *STREAM)
     The `fgetws' function reads wide characters from the stream STREAM
     up to and including a newline character and stores them in the
     string WS, adding a null wide character to mark the end of the
     string.  You must supply COUNT wide characters worth of space in
     WS, but the number of characters read is at most COUNT - 1.  The
     extra character space is used to hold the null wide character at
     the end of the string.

     If the system is already at end of file when you call `fgetws',
     then the contents of the array WS are unchanged and a null pointer
     is returned.  A null pointer is also returned if a read error
     occurs.  Otherwise, the return value is the pointer WS.

     *Warning:* If the input data has a null wide character (which are
     null bytes in the input stream), you can't tell.  So don't use
     `fgetws' unless you know the data cannot contain a null.  Don't use
     it to read files edited by the user because, if the user inserts a
     null character, you should either handle it properly or print a
     clear error message.

 -- Function: char * fgets_unlocked (char *S, int COUNT, FILE *STREAM)
     The `fgets_unlocked' function is equivalent to the `fgets'
     function except that it does not implicitly lock the stream.

     This function is a GNU extension.

 -- Function: wchar_t * fgetws_unlocked (wchar_t *WS, int COUNT, FILE
          *STREAM)
     The `fgetws_unlocked' function is equivalent to the `fgetws'
     function except that it does not implicitly lock the stream.

     This function is a GNU extension.

 -- Deprecated function: char * gets (char *S)
     The function `gets' reads characters from the stream `stdin' up to
     the next newline character, and stores them in the string S.  The
     newline character is discarded (note that this differs from the
     behavior of `fgets', which copies the newline character into the
     string).  If `gets' encounters a read error or end-of-file, it
     returns a null pointer; otherwise it returns S.

     *Warning:* The `gets' function is *very dangerous* because it
     provides no protection against overflowing the string S.  The GNU
     library includes it for compatibility only.  You should *always*
     use `fgets' or `getline' instead.  To remind you of this, the
     linker (if using GNU `ld') will issue a warning whenever you use
     `gets'.

File: libc.info,  Node: Unreading,  Next: Block Input/Output,  Prev: Line Input,  Up: I/O on Streams

12.10 Unreading
===============

In parser programs it is often useful to examine the next character in
the input stream without removing it from the stream.  This is called
"peeking ahead" at the input because your program gets a glimpse of the
input it will read next.

   Using stream I/O, you can peek ahead at input by first reading it and
then "unreading" it (also called  "pushing it back" on the stream).
Unreading a character makes it available to be input again from the
stream, by  the next call to `fgetc' or other input function on that
stream.

* Menu:

* Unreading Idea::              An explanation of unreading with pictures.
* How Unread::                  How to call `ungetc' to do unreading.

File: libc.info,  Node: Unreading Idea,  Next: How Unread,  Up: Unreading

12.10.1 What Unreading Means
----------------------------

Here is a pictorial explanation of unreading.  Suppose you have a
stream reading a file that contains just six characters, the letters
`foobar'.  Suppose you have read three characters so far.  The
situation looks like this:

     f  o  o  b  a  r
              ^

so the next input character will be `b'.

   If instead of reading `b' you unread the letter `o', you get a
situation like this:

     f  o  o  b  a  r
              |
           o--
           ^

so that the next input characters will be `o' and `b'.

   If you unread `9' instead of `o', you get this situation:

     f  o  o  b  a  r
              |
           9--
           ^

so that the next input characters will be `9' and `b'.

File: libc.info,  Node: How Unread,  Prev: Unreading Idea,  Up: Unreading

12.10.2 Using `ungetc' To Do Unreading
--------------------------------------

The function to unread a character is called `ungetc', because it
reverses the action of `getc'.

 -- Function: int ungetc (int C, FILE *STREAM)
     The `ungetc' function pushes back the character C onto the input
     stream STREAM.  So the next input from STREAM will read C before
     anything else.

     If C is `EOF', `ungetc' does nothing and just returns `EOF'.  This
     lets you call `ungetc' with the return value of `getc' without
     needing to check for an error from `getc'.

     The character that you push back doesn't have to be the same as
     the last character that was actually read from the stream.  In
     fact, it isn't necessary to actually read any characters from the
     stream before unreading them with `ungetc'!  But that is a strange
     way to write a program; usually `ungetc' is used only to unread a
     character that was just read from the same stream.  The GNU C
     library supports this even on files opened in binary mode, but
     other systems might not.

     The GNU C library only supports one character of pushback--in other
     words, it does not work to call `ungetc' twice without doing input
     in between.  Other systems might let you push back multiple
     characters; then reading from the stream retrieves the characters
     in the reverse order that they were pushed.

     Pushing back characters doesn't alter the file; only the internal
     buffering for the stream is affected.  If a file positioning
     function (such as `fseek', `fseeko' or `rewind'; *note File
     Positioning::) is called, any pending pushed-back characters are
     discarded.

     Unreading a character on a stream that is at end of file clears the
     end-of-file indicator for the stream, because it makes the
     character of input available.  After you read that character,
     trying to read again will encounter end of file.

 -- Function: wint_t ungetwc (wint_t WC, FILE *STREAM)
     The `ungetwc' function behaves just like `ungetc' just that it
     pushes back a wide character.

   Here is an example showing the use of `getc' and `ungetc' to skip
over whitespace characters.  When this function reaches a
non-whitespace character, it unreads that character to be seen again on
the next read operation on the stream.

     #include <stdio.h>
     #include <ctype.h>

     void
     skip_whitespace (FILE *stream)
     {
       int c;
       do
         /* No need to check for `EOF' because it is not
            `isspace', and `ungetc' ignores `EOF'.  */
         c = getc (stream);
       while (isspace (c));
       ungetc (c, stream);
     }

File: libc.info,  Node: Block Input/Output,  Next: Formatted Output,  Prev: Unreading,  Up: I/O on Streams

12.11 Block Input/Output
========================

This section describes how to do input and output operations on blocks
of data.  You can use these functions to read and write binary data, as
well as to read and write text in fixed-size blocks instead of by
characters or lines.

   Binary files are typically used to read and write blocks of data in
the same format as is used to represent the data in a running program.
In other words, arbitrary blocks of memory--not just character or string
objects--can be written to a binary file, and meaningfully read in
again by the same program.

   Storing data in binary form is often considerably more efficient than
using the formatted I/O functions.  Also, for floating-point numbers,
the binary form avoids possible loss of precision in the conversion
process.  On the other hand, binary files can't be examined or modified
easily using many standard file utilities (such as text editors), and
are not portable between different implementations of the language, or
different kinds of computers.

   These functions are declared in `stdio.h'.

 -- Function: size_t fread (void *DATA, size_t SIZE, size_t COUNT, FILE
          *STREAM)
     This function reads up to COUNT objects of size SIZE into the
     array DATA, from the stream STREAM.  It returns the number of
     objects actually read, which might be less than COUNT if a read
     error occurs or the end of the file is reached.  This function
     returns a value of zero (and doesn't read anything) if either SIZE
     or COUNT is zero.

     If `fread' encounters end of file in the middle of an object, it
     returns the number of complete objects read, and discards the
     partial object.  Therefore, the stream remains at the actual end
     of the file.

 -- Function: size_t fread_unlocked (void *DATA, size_t SIZE, size_t
          COUNT, FILE *STREAM)
     The `fread_unlocked' function is equivalent to the `fread'
     function except that it does not implicitly lock the stream.

     This function is a GNU extension.

 -- Function: size_t fwrite (const void *DATA, size_t SIZE, size_t
          COUNT, FILE *STREAM)
     This function writes up to COUNT objects of size SIZE from the
     array DATA, to the stream STREAM.  The return value is normally
     COUNT, if the call succeeds.  Any other value indicates some sort
     of error, such as running out of space.

 -- Function: size_t fwrite_unlocked (const void *DATA, size_t SIZE,
          size_t COUNT, FILE *STREAM)
     The `fwrite_unlocked' function is equivalent to the `fwrite'
     function except that it does not implicitly lock the stream.

     This function is a GNU extension.

File: libc.info,  Node: Formatted Output,  Next: Customizing Printf,  Prev: Block Input/Output,  Up: I/O on Streams

12.12 Formatted Output
======================

The functions described in this section (`printf' and related
functions) provide a convenient way to perform formatted output.  You
call `printf' with a "format string" or "template string" that
specifies how to format the values of the remaining arguments.

   Unless your program is a filter that specifically performs line- or
character-oriented processing, using `printf' or one of the other
related functions described in this section is usually the easiest and
most concise way to perform output.  These functions are especially
useful for printing error messages, tables of data, and the like.

* Menu:

* Formatted Output Basics::     Some examples to get you started.
* Output Conversion Syntax::    General syntax of conversion
                                 specifications.
* Table of Output Conversions:: Summary of output conversions and
                                 what they do.
* Integer Conversions::         Details about formatting of integers.
* Floating-Point Conversions::  Details about formatting of
                                 floating-point numbers.
* Other Output Conversions::    Details about formatting of strings,
                                 characters, pointers, and the like.
* Formatted Output Functions::  Descriptions of the actual functions.
* Dynamic Output::		Functions that allocate memory for the output.
* Variable Arguments Output::   `vprintf' and friends.
* Parsing a Template String::   What kinds of args does a given template
                                 call for?
* Example of Parsing::          Sample program using `parse_printf_format'.

File: libc.info,  Node: Formatted Output Basics,  Next: Output Conversion Syntax,  Up: Formatted Output

12.12.1 Formatted Output Basics
-------------------------------

The `printf' function can be used to print any number of arguments.
The template string argument you supply in a call provides information
not only about the number of additional arguments, but also about their
types and what style should be used for printing them.

   Ordinary characters in the template string are simply written to the
output stream as-is, while "conversion specifications" introduced by a
`%' character in the template cause subsequent arguments to be
formatted and written to the output stream.  For example,

     int pct = 37;
     char filename[] = "foo.txt";
     printf ("Processing of `%s' is %d%% finished.\nPlease be patient.\n",
             filename, pct);

produces output like

     Processing of `foo.txt' is 37% finished.
     Please be patient.

   This example shows the use of the `%d' conversion to specify that an
`int' argument should be printed in decimal notation, the `%s'
conversion to specify printing of a string argument, and the `%%'
conversion to print a literal `%' character.

   There are also conversions for printing an integer argument as an
unsigned value in octal, decimal, or hexadecimal radix (`%o', `%u', or
`%x', respectively); or as a character value (`%c').

   Floating-point numbers can be printed in normal, fixed-point notation
using the `%f' conversion or in exponential notation using the `%e'
conversion.  The `%g' conversion uses either `%e' or `%f' format,
depending on what is more appropriate for the magnitude of the
particular number.

   You can control formatting more precisely by writing "modifiers"
between the `%' and the character that indicates which conversion to
apply.  These slightly alter the ordinary behavior of the conversion.
For example, most conversion specifications permit you to specify a
minimum field width and a flag indicating whether you want the result
left- or right-justified within the field.

   The specific flags and modifiers that are permitted and their
interpretation vary depending on the particular conversion.  They're all
described in more detail in the following sections.  Don't worry if this
all seems excessively complicated at first; you can almost always get
reasonable free-format output without using any of the modifiers at all.
The modifiers are mostly used to make the output look "prettier" in
tables.

File: libc.info,  Node: Output Conversion Syntax,  Next: Table of Output Conversions,  Prev: Formatted Output Basics,  Up: Formatted Output

12.12.2 Output Conversion Syntax
--------------------------------

This section provides details about the precise syntax of conversion
specifications that can appear in a `printf' template string.

   Characters in the template string that are not part of a conversion
specification are printed as-is to the output stream.  Multibyte
character sequences (*note Character Set Handling::) are permitted in a
template string.

   The conversion specifications in a `printf' template string have the
general form:

     % [ PARAM-NO $] FLAGS WIDTH [ . PRECISION ] TYPE CONVERSION

or

     % [ PARAM-NO $] FLAGS WIDTH . * [ PARAM-NO $] TYPE CONVERSION

   For example, in the conversion specifier `%-10.8ld', the `-' is a
flag, `10' specifies the field width, the precision is `8', the letter
`l' is a type modifier, and `d' specifies the conversion style.  (This
particular type specifier says to print a `long int' argument in
decimal notation, with a minimum of 8 digits left-justified in a field
at least 10 characters wide.)

   In more detail, output conversion specifications consist of an
initial `%' character followed in sequence by:

   * An optional specification of the parameter used for this format.
     Normally the parameters to the `printf' function are assigned to
     the formats in the order of appearance in the format string.  But
     in some situations (such as message translation) this is not
     desirable and this extension allows an explicit parameter to be
     specified.

     The PARAM-NO parts of the format must be integers in the range of
     1 to the maximum number of arguments present to the function call.
     Some implementations limit this number to a certainly upper bound.
     The exact limit can be retrieved by the following constant.

      -- Macro: NL_ARGMAX
          The value of `NL_ARGMAX' is the maximum value allowed for the
          specification of an positional parameter in a `printf' call.
          The actual value in effect at runtime can be retrieved by
          using `sysconf' using the `_SC_NL_ARGMAX' parameter *note
          Sysconf Definition::.

          Some system have a quite low limit such as 9 for System V
          systems.  The GNU C library has no real limit.

     If any of the formats has a specification for the parameter
     position all of them in the format string shall have one.
     Otherwise the behavior is undefined.

   * Zero or more "flag characters" that modify the normal behavior of
     the conversion specification.

   * An optional decimal integer specifying the "minimum field width".
     If the normal conversion produces fewer characters than this, the
     field is padded with spaces to the specified width.  This is a
     _minimum_ value; if the normal conversion produces more characters
     than this, the field is _not_ truncated.  Normally, the output is
     right-justified within the field.

     You can also specify a field width of `*'.  This means that the
     next argument in the argument list (before the actual value to be
     printed) is used as the field width.  The value must be an `int'.
     If the value is negative, this means to set the `-' flag (see
     below) and to use the absolute value as the field width.

   * An optional "precision" to specify the number of digits to be
     written for the numeric conversions.  If the precision is
     specified, it consists of a period (`.') followed optionally by a
     decimal integer (which defaults to zero if omitted).

     You can also specify a precision of `*'.  This means that the next
     argument in the argument list (before the actual value to be
     printed) is used as the precision.  The value must be an `int',
     and is ignored if it is negative.  If you specify `*' for both the
     field width and precision, the field width argument precedes the
     precision argument.  Other C library versions may not recognize
     this syntax.

   * An optional "type modifier character", which is used to specify the
     data type of the corresponding argument if it differs from the
     default type.  (For example, the integer conversions assume a type
     of `int', but you can specify `h', `l', or `L' for other integer
     types.)

   * A character that specifies the conversion to be applied.

   The exact options that are permitted and how they are interpreted
vary between the different conversion specifiers.  See the descriptions
of the individual conversions for information about the particular
options that they use.

   With the `-Wformat' option, the GNU C compiler checks calls to
`printf' and related functions.  It examines the format string and
verifies that the correct number and types of arguments are supplied.
There is also a GNU C syntax to tell the compiler that a function you
write uses a `printf'-style format string.  *Note Declaring Attributes
of Functions: (gcc.info)Function Attributes, for more information.

File: libc.info,  Node: Table of Output Conversions,  Next: Integer Conversions,  Prev: Output Conversion Syntax,  Up: Formatted Output

12.12.3 Table of Output Conversions
-----------------------------------

Here is a table summarizing what all the different conversions do:

`%d', `%i'
     Print an integer as a signed decimal number.  *Note Integer
     Conversions::, for details.  `%d' and `%i' are synonymous for
     output, but are different when used with `scanf' for input (*note
     Table of Input Conversions::).

`%o'
     Print an integer as an unsigned octal number.  *Note Integer
     Conversions::, for details.

`%u'
     Print an integer as an unsigned decimal number.  *Note Integer
     Conversions::, for details.

`%x', `%X'
     Print an integer as an unsigned hexadecimal number.  `%x' uses
     lower-case letters and `%X' uses upper-case.  *Note Integer
     Conversions::, for details.

`%f'
     Print a floating-point number in normal (fixed-point) notation.
     *Note Floating-Point Conversions::, for details.

`%e', `%E'
     Print a floating-point number in exponential notation.  `%e' uses
     lower-case letters and `%E' uses upper-case.  *Note Floating-Point
     Conversions::, for details.

`%g', `%G'
     Print a floating-point number in either normal or exponential
     notation, whichever is more appropriate for its magnitude.  `%g'
     uses lower-case letters and `%G' uses upper-case.  *Note
     Floating-Point Conversions::, for details.

`%a', `%A'
     Print a floating-point number in a hexadecimal fractional notation
     which the exponent to base 2 represented in decimal digits.  `%a'
     uses lower-case letters and `%A' uses upper-case.  *Note
     Floating-Point Conversions::, for details.

`%c'
     Print a single character.  *Note Other Output Conversions::.

`%C'
     This is an alias for `%lc' which is supported for compatibility
     with the Unix standard.

`%s'
     Print a string.  *Note Other Output Conversions::.

`%S'
     This is an alias for `%ls' which is supported for compatibility
     with the Unix standard.

`%p'
     Print the value of a pointer.  *Note Other Output Conversions::.

`%n'
     Get the number of characters printed so far.  *Note Other Output
     Conversions::.  Note that this conversion specification never
     produces any output.

`%m'
     Print the string corresponding to the value of `errno'.  (This is
     a GNU extension.)  *Note Other Output Conversions::.

`%%'
     Print a literal `%' character.  *Note Other Output Conversions::.

   If the syntax of a conversion specification is invalid, unpredictable
things will happen, so don't do this.  If there aren't enough function
arguments provided to supply values for all the conversion
specifications in the template string, or if the arguments are not of
the correct types, the results are unpredictable.  If you supply more
arguments than conversion specifications, the extra argument values are
simply ignored; this is sometimes useful.

File: libc.info,  Node: Integer Conversions,  Next: Floating-Point Conversions,  Prev: Table of Output Conversions,  Up: Formatted Output

12.12.4 Integer Conversions
---------------------------

This section describes the options for the `%d', `%i', `%o', `%u',
`%x', and `%X' conversion specifications.  These conversions print
integers in various formats.

   The `%d' and `%i' conversion specifications both print an `int'
argument as a signed decimal number; while `%o', `%u', and `%x' print
the argument as an unsigned octal, decimal, or hexadecimal number
(respectively).  The `%X' conversion specification is just like `%x'
except that it uses the characters `ABCDEF' as digits instead of
`abcdef'.

   The following flags are meaningful:

`-'
     Left-justify the result in the field (instead of the normal
     right-justification).

`+'
     For the signed `%d' and `%i' conversions, print a plus sign if the
     value is positive.

` '
     For the signed `%d' and `%i' conversions, if the result doesn't
     start with a plus or minus sign, prefix it with a space character
     instead.  Since the `+' flag ensures that the result includes a
     sign, this flag is ignored if you supply both of them.

`#'
     For the `%o' conversion, this forces the leading digit to be `0',
     as if by increasing the precision.  For `%x' or `%X', this
     prefixes a leading `0x' or `0X' (respectively) to the result.
     This doesn't do anything useful for the `%d', `%i', or `%u'
     conversions.  Using this flag produces output which can be parsed
     by the `strtoul' function (*note Parsing of Integers::) and
     `scanf' with the `%i' conversion (*note Numeric Input
     Conversions::).

`''
     Separate the digits into groups as specified by the locale
     specified for the `LC_NUMERIC' category; *note General Numeric::.
     This flag is a GNU extension.

`0'
     Pad the field with zeros instead of spaces.  The zeros are placed
     after any indication of sign or base.  This flag is ignored if the
     `-' flag is also specified, or if a precision is specified.

   If a precision is supplied, it specifies the minimum number of
digits to appear; leading zeros are produced if necessary.  If you
don't specify a precision, the number is printed with as many digits as
it needs.  If you convert a value of zero with an explicit precision of
zero, then no characters at all are produced.

   Without a type modifier, the corresponding argument is treated as an
`int' (for the signed conversions `%i' and `%d') or `unsigned int' (for
the unsigned conversions `%o', `%u', `%x', and `%X').  Recall that
since `printf' and friends are variadic, any `char' and `short'
arguments are automatically converted to `int' by the default argument
promotions.  For arguments of other integer types, you can use these
modifiers:

`hh'
     Specifies that the argument is a `signed char' or `unsigned char',
     as appropriate.  A `char' argument is converted to an `int' or
     `unsigned int' by the default argument promotions anyway, but the
     `h' modifier says to convert it back to a `char' again.

     This modifier was introduced in ISO C99.

`h'
     Specifies that the argument is a `short int' or `unsigned short
     int', as appropriate.  A `short' argument is converted to an `int'
     or `unsigned int' by the default argument promotions anyway, but
     the `h' modifier says to convert it back to a `short' again.

`j'
     Specifies that the argument is a `intmax_t' or `uintmax_t', as
     appropriate.

     This modifier was introduced in ISO C99.

`l'
     Specifies that the argument is a `long int' or `unsigned long
     int', as appropriate.  Two `l' characters is like the `L'
     modifier, below.

     If used with `%c' or `%s' the corresponding parameter is
     considered as a wide character or wide character string
     respectively.  This use of `l' was introduced in Amendment 1 to
     ISO C90.

`L'
`ll'
`q'
     Specifies that the argument is a `long long int'.  (This type is
     an extension supported by the GNU C compiler.  On systems that
     don't support extra-long integers, this is the same as `long int'.)

     The `q' modifier is another name for the same thing, which comes
     from 4.4 BSD; a `long long int' is sometimes called a "quad" `int'.

`t'
     Specifies that the argument is a `ptrdiff_t'.

     This modifier was introduced in ISO C99.

`z'
`Z'
     Specifies that the argument is a `size_t'.

     `z' was introduced in ISO C99.  `Z' is a GNU extension predating
     this addition and should not be used in new code.

   Here is an example.  Using the template string:

     "|%5d|%-5d|%+5d|%+-5d|% 5d|%05d|%5.0d|%5.2d|%d|\n"

to print numbers using the different options for the `%d' conversion
gives results like:

     |    0|0    |   +0|+0   |    0|00000|     |   00|0|
     |    1|1    |   +1|+1   |    1|00001|    1|   01|1|
     |   -1|-1   |   -1|-1   |   -1|-0001|   -1|  -01|-1|
     |100000|100000|+100000|+100000| 100000|100000|100000|100000|100000|

   In particular, notice what happens in the last case where the number
is too large to fit in the minimum field width specified.

   Here are some more examples showing how unsigned integers print under
various format options, using the template string:

     "|%5u|%5o|%5x|%5X|%#5o|%#5x|%#5X|%#10.8x|\n"

     |    0|    0|    0|    0|    0|    0|    0|  00000000|
     |    1|    1|    1|    1|   01|  0x1|  0X1|0x00000001|
     |100000|303240|186a0|186A0|0303240|0x186a0|0X186A0|0x000186a0|

File: libc.info,  Node: Floating-Point Conversions,  Next: Other Output Conversions,  Prev: Integer Conversions,  Up: Formatted Output

12.12.5 Floating-Point Conversions
----------------------------------

This section discusses the conversion specifications for floating-point
numbers: the `%f', `%e', `%E', `%g', and `%G' conversions.

   The `%f' conversion prints its argument in fixed-point notation,
producing output of the form [`-']DDD`.'DDD, where the number of digits
following the decimal point is controlled by the precision you specify.

   The `%e' conversion prints its argument in exponential notation,
producing output of the form [`-']D`.'DDD`e'[`+'|`-']DD.  Again, the
number of digits following the decimal point is controlled by the
precision.  The exponent always contains at least two digits.  The `%E'
conversion is similar but the exponent is marked with the letter `E'
instead of `e'.

   The `%g' and `%G' conversions print the argument in the style of
`%e' or `%E' (respectively) if the exponent would be less than -4 or
greater than or equal to the precision; otherwise they use the `%f'
style.  A precision of `0', is taken as 1.  Trailing zeros are removed
from the fractional portion of the result and a decimal-point character
appears only if it is followed by a digit.

   The `%a' and `%A' conversions are meant for representing
floating-point numbers exactly in textual form so that they can be
exchanged as texts between different programs and/or machines.  The
numbers are represented is the form [`-']`0x'H`.'HHH`p'[`+'|`-']DD.  At
the left of the decimal-point character exactly one digit is print.
This character is only `0' if the number is denormalized.  Otherwise
the value is unspecified; it is implementation dependent how many bits
are used.  The number of hexadecimal digits on the right side of the
decimal-point character is equal to the precision.  If the precision is
zero it is determined to be large enough to provide an exact
representation of the number (or it is large enough to distinguish two
adjacent values if the `FLT_RADIX' is not a power of 2, *note Floating
Point Parameters::).  For the `%a' conversion lower-case characters are
used to represent the hexadecimal number and the prefix and exponent
sign are printed as `0x' and `p' respectively.  Otherwise upper-case
characters are used and `0X' and `P' are used for the representation of
prefix and exponent string.  The exponent to the base of two is printed
as a decimal number using at least one digit but at most as many digits
as necessary to represent the value exactly.

   If the value to be printed represents infinity or a NaN, the output
is [`-']`inf' or `nan' respectively if the conversion specifier is
`%a', `%e', `%f', or `%g' and it is [`-']`INF' or `NAN' respectively if
the conversion is `%A', `%E', or `%G'.

   The following flags can be used to modify the behavior:

`-'
     Left-justify the result in the field.  Normally the result is
     right-justified.

`+'
     Always include a plus or minus sign in the result.

` '
     If the result doesn't start with a plus or minus sign, prefix it
     with a space instead.  Since the `+' flag ensures that the result
     includes a sign, this flag is ignored if you supply both of them.

`#'
     Specifies that the result should always include a decimal point,
     even if no digits follow it.  For the `%g' and `%G' conversions,
     this also forces trailing zeros after the decimal point to be left
     in place where they would otherwise be removed.

`''
     Separate the digits of the integer part of the result into groups
     as specified by the locale specified for the `LC_NUMERIC' category;
     *note General Numeric::.  This flag is a GNU extension.

`0'
     Pad the field with zeros instead of spaces; the zeros are placed
     after any sign.  This flag is ignored if the `-' flag is also
     specified.

   The precision specifies how many digits follow the decimal-point
character for the `%f', `%e', and `%E' conversions.  For these
conversions, the default precision is `6'.  If the precision is
explicitly `0', this suppresses the decimal point character entirely.
For the `%g' and `%G' conversions, the precision specifies how many
significant digits to print.  Significant digits are the first digit
before the decimal point, and all the digits after it.  If the
precision is `0' or not specified for `%g' or `%G', it is treated like
a value of `1'.  If the value being printed cannot be expressed
accurately in the specified number of digits, the value is rounded to
the nearest number that fits.

   Without a type modifier, the floating-point conversions use an
argument of type `double'.  (By the default argument promotions, any
`float' arguments are automatically converted to `double'.)  The
following type modifier is supported:

`L'
     An uppercase `L' specifies that the argument is a `long double'.

   Here are some examples showing how numbers print using the various
floating-point conversions.  All of the numbers were printed using this
template string:

     "|%13.4a|%13.4f|%13.4e|%13.4g|\n"

   Here is the output:

     |  0x0.0000p+0|       0.0000|   0.0000e+00|            0|
     |  0x1.0000p-1|       0.5000|   5.0000e-01|          0.5|
     |  0x1.0000p+0|       1.0000|   1.0000e+00|            1|
     | -0x1.0000p+0|      -1.0000|  -1.0000e+00|           -1|
     |  0x1.9000p+6|     100.0000|   1.0000e+02|          100|
     |  0x1.f400p+9|    1000.0000|   1.0000e+03|         1000|
     | 0x1.3880p+13|   10000.0000|   1.0000e+04|        1e+04|
     | 0x1.81c8p+13|   12345.0000|   1.2345e+04|    1.234e+04|
     | 0x1.86a0p+16|  100000.0000|   1.0000e+05|        1e+05|
     | 0x1.e240p+16|  123456.0000|   1.2346e+05|    1.235e+05|

   Notice how the `%g' conversion drops trailing zeros.

File: libc.info,  Node: Other Output Conversions,  Next: Formatted Output Functions,  Prev: Floating-Point Conversions,  Up: Formatted Output

12.12.6 Other Output Conversions
--------------------------------

This section describes miscellaneous conversions for `printf'.

   The `%c' conversion prints a single character.  In case there is no
`l' modifier the `int' argument is first converted to an `unsigned
char'.  Then, if used in a wide stream function, the character is
converted into the corresponding wide character.  The `-' flag can be
used to specify left-justification in the field, but no other flags are
defined, and no precision or type modifier can be given.  For example:

     printf ("%c%c%c%c%c", 'h', 'e', 'l', 'l', 'o');

prints `hello'.

   If there is a `l' modifier present the argument is expected to be of
type `wint_t'.  If used in a multibyte function the wide character is
converted into a multibyte character before being added to the output.
In this case more than one output byte can be produced.

   The `%s' conversion prints a string.  If no `l' modifier is present
the corresponding argument must be of type `char *' (or `const char
*').  If used in a wide stream function the string is first converted
in a wide character string.  A precision can be specified to indicate
the maximum number of characters to write; otherwise characters in the
string up to but not including the terminating null character are
written to the output stream.  The `-' flag can be used to specify
left-justification in the field, but no other flags or type modifiers
are defined for this conversion.  For example:

     printf ("%3s%-6s", "no", "where");

prints ` nowhere '.

   If there is a `l' modifier present the argument is expected to be of
type `wchar_t' (or `const wchar_t *').

   If you accidentally pass a null pointer as the argument for a `%s'
conversion, the GNU library prints it as `(null)'.  We think this is
more useful than crashing.  But it's not good practice to pass a null
argument intentionally.

   The `%m' conversion prints the string corresponding to the error
code in `errno'.  *Note Error Messages::.  Thus:

     fprintf (stderr, "can't open `%s': %m\n", filename);

is equivalent to:

     fprintf (stderr, "can't open `%s': %s\n", filename, strerror (errno));

The `%m' conversion is a GNU C library extension.

   The `%p' conversion prints a pointer value.  The corresponding
argument must be of type `void *'.  In practice, you can use any type
of pointer.

   In the GNU system, non-null pointers are printed as unsigned
integers, as if a `%#x' conversion were used.  Null pointers print as
`(nil)'.  (Pointers might print differently in other systems.)

   For example:

     printf ("%p", "testing");

prints `0x' followed by a hexadecimal number--the address of the string
constant `"testing"'.  It does not print the word `testing'.

   You can supply the `-' flag with the `%p' conversion to specify
left-justification, but no other flags, precision, or type modifiers
are defined.

   The `%n' conversion is unlike any of the other output conversions.
It uses an argument which must be a pointer to an `int', but instead of
printing anything it stores the number of characters printed so far by
this call at that location.  The `h' and `l' type modifiers are
permitted to specify that the argument is of type `short int *' or
`long int *' instead of `int *', but no flags, field width, or
precision are permitted.

   For example,

     int nchar;
     printf ("%d %s%n\n", 3, "bears", &nchar);

prints:

     3 bears

and sets `nchar' to `7', because `3 bears' is seven characters.

   The `%%' conversion prints a literal `%' character.  This conversion
doesn't use an argument, and no flags, field width, precision, or type
modifiers are permitted.

File: libc.info,  Node: Formatted Output Functions,  Next: Dynamic Output,  Prev: Other Output Conversions,  Up: Formatted Output

12.12.7 Formatted Output Functions
----------------------------------

This section describes how to call `printf' and related functions.
Prototypes for these functions are in the header file `stdio.h'.
Because these functions take a variable number of arguments, you _must_
declare prototypes for them before using them.  Of course, the easiest
way to make sure you have all the right prototypes is to just include
`stdio.h'.

 -- Function: int printf (const char *TEMPLATE, ...)
     The `printf' function prints the optional arguments under the
     control of the template string TEMPLATE to the stream `stdout'.
     It returns the number of characters printed, or a negative value
     if there was an output error.

 -- Function: int wprintf (const wchar_t *TEMPLATE, ...)
     The `wprintf' function prints the optional arguments under the
     control of the wide template string TEMPLATE to the stream
     `stdout'.  It returns the number of wide characters printed, or a
     negative value if there was an output error.

 -- Function: int fprintf (FILE *STREAM, const char *TEMPLATE, ...)
     This function is just like `printf', except that the output is
     written to the stream STREAM instead of `stdout'.

 -- Function: int fwprintf (FILE *STREAM, const wchar_t *TEMPLATE, ...)
     This function is just like `wprintf', except that the output is
     written to the stream STREAM instead of `stdout'.

 -- Function: int sprintf (char *S, const char *TEMPLATE, ...)
     This is like `printf', except that the output is stored in the
     character array S instead of written to a stream.  A null
     character is written to mark the end of the string.

     The `sprintf' function returns the number of characters stored in
     the array S, not including the terminating null character.

     The behavior of this function is undefined if copying takes place
     between objects that overlap--for example, if S is also given as
     an argument to be printed under control of the `%s' conversion.
     *Note Copying and Concatenation::.

     *Warning:* The `sprintf' function can be *dangerous* because it
     can potentially output more characters than can fit in the
     allocation size of the string S.  Remember that the field width
     given in a conversion specification is only a _minimum_ value.

     To avoid this problem, you can use `snprintf' or `asprintf',
     described below.

 -- Function: int swprintf (wchar_t *S, size_t SIZE, const wchar_t
          *TEMPLATE, ...)
     This is like `wprintf', except that the output is stored in the
     wide character array WS instead of written to a stream.  A null
     wide character is written to mark the end of the string.  The SIZE
     argument specifies the maximum number of characters to produce.
     The trailing null character is counted towards this limit, so you
     should allocate at least SIZE wide characters for the string WS.

     The return value is the number of characters generated for the
     given input, excluding the trailing null.  If not all output fits
     into the provided buffer a negative value is returned.  You should
     try again with a bigger output string.  _Note:_ this is different
     from how `snprintf' handles this situation.

     Note that the corresponding narrow stream function takes fewer
     parameters.  `swprintf' in fact corresponds to the `snprintf'
     function.  Since the `sprintf' function can be dangerous and should
     be avoided the ISO C committee refused to make the same mistake
     again and decided to not define an function exactly corresponding
     to `sprintf'.

 -- Function: int snprintf (char *S, size_t SIZE, const char *TEMPLATE,
          ...)
     The `snprintf' function is similar to `sprintf', except that the
     SIZE argument specifies the maximum number of characters to
     produce.  The trailing null character is counted towards this
     limit, so you should allocate at least SIZE characters for the
     string S.

     The return value is the number of characters which would be
     generated for the given input, excluding the trailing null.  If
     this value is greater or equal to SIZE, not all characters from
     the result have been stored in S.  You should try again with a
     bigger output string.  Here is an example of doing this:

          /* Construct a message describing the value of a variable
             whose name is NAME and whose value is VALUE. */
          char *
          make_message (char *name, char *value)
          {
            /* Guess we need no more than 100 chars of space. */
            int size = 100;
            char *buffer = (char *) xmalloc (size);
            int nchars;
            if (buffer == NULL)
              return NULL;

           /* Try to print in the allocated space. */
            nchars = snprintf (buffer, size, "value of %s is %s",
                               name, value);
            if (nchars >= size)
              {
                /* Reallocate buffer now that we know
                   how much space is needed. */
                size = nchars + 1;
                buffer = (char *) xrealloc (buffer, size);

                if (buffer != NULL)
                  /* Try again. */
                  snprintf (buffer, size, "value of %s is %s",
                            name, value);
              }
            /* The last call worked, return the string. */
            return buffer;
          }

     In practice, it is often easier just to use `asprintf', below.

     *Attention:* In versions of the GNU C library prior to 2.1 the
     return value is the number of characters stored, not including the
     terminating null; unless there was not enough space in S to store
     the result in which case `-1' is returned.  This was changed in
     order to comply with the ISO C99 standard.

File: libc.info,  Node: Dynamic Output,  Next: Variable Arguments Output,  Prev: Formatted Output Functions,  Up: Formatted Output

12.12.8 Dynamically Allocating Formatted Output
-----------------------------------------------

The functions in this section do formatted output and place the results
in dynamically allocated memory.

 -- Function: int asprintf (char **PTR, const char *TEMPLATE, ...)
     This function is similar to `sprintf', except that it dynamically
     allocates a string (as with `malloc'; *note Unconstrained
     Allocation::) to hold the output, instead of putting the output in
     a buffer you allocate in advance.  The PTR argument should be the
     address of a `char *' object, and a successful call to `asprintf'
     stores a pointer to the newly allocated string at that location.

     The return value is the number of characters allocated for the
     buffer, or less than zero if an error occurred. Usually this means
     that the buffer could not be allocated.

     Here is how to use `asprintf' to get the same result as the
     `snprintf' example, but more easily:

          /* Construct a message describing the value of a variable
             whose name is NAME and whose value is VALUE. */
          char *
          make_message (char *name, char *value)
          {
            char *result;
            if (asprintf (&result, "value of %s is %s", name, value) < 0)
              return NULL;
            return result;
          }

 -- Function: int obstack_printf (struct obstack *OBSTACK, const char
          *TEMPLATE, ...)
     This function is similar to `asprintf', except that it uses the
     obstack OBSTACK to allocate the space.  *Note Obstacks::.

     The characters are written onto the end of the current object.  To
     get at them, you must finish the object with `obstack_finish'
     (*note Growing Objects::).

File: libc.info,  Node: Variable Arguments Output,  Next: Parsing a Template String,  Prev: Dynamic Output,  Up: Formatted Output

12.12.9 Variable Arguments Output Functions
-------------------------------------------

The functions `vprintf' and friends are provided so that you can define
your own variadic `printf'-like functions that make use of the same
internals as the built-in formatted output functions.

   The most natural way to define such functions would be to use a
language construct to say, "Call `printf' and pass this template plus
all of my arguments after the first five."  But there is no way to do
this in C, and it would be hard to provide a way, since at the C
language level there is no way to tell how many arguments your function
received.

   Since that method is impossible, we provide alternative functions,
the `vprintf' series, which lets you pass a `va_list' to describe "all
of my arguments after the first five."

   When it is sufficient to define a macro rather than a real function,
the GNU C compiler provides a way to do this much more easily with
macros.  For example:

     #define myprintf(a, b, c, d, e, rest...) \
                 printf (mytemplate , ## rest)

*Note Variadic Macros: (cpp)Variadic Macros, for details.  But this is
limited to macros, and does not apply to real functions at all.

   Before calling `vprintf' or the other functions listed in this
section, you _must_ call `va_start' (*note Variadic Functions::) to
initialize a pointer to the variable arguments.  Then you can call
`va_arg' to fetch the arguments that you want to handle yourself.  This
advances the pointer past those arguments.

   Once your `va_list' pointer is pointing at the argument of your
choice, you are ready to call `vprintf'.  That argument and all
subsequent arguments that were passed to your function are used by
`vprintf' along with the template that you specified separately.

   In some other systems, the `va_list' pointer may become invalid
after the call to `vprintf', so you must not use `va_arg' after you
call `vprintf'.  Instead, you should call `va_end' to retire the
pointer from service.  However, you can safely call `va_start' on
another pointer variable and begin fetching the arguments again through
that pointer.  Calling `vprintf' does not destroy the argument list of
your function, merely the particular pointer that you passed to it.

   GNU C does not have such restrictions.  You can safely continue to
fetch arguments from a `va_list' pointer after passing it to `vprintf',
and `va_end' is a no-op.  (Note, however, that subsequent `va_arg'
calls will fetch the same arguments which `vprintf' previously used.)

   Prototypes for these functions are declared in `stdio.h'.

 -- Function: int vprintf (const char *TEMPLATE, va_list AP)
     This function is similar to `printf' except that, instead of taking
     a variable number of arguments directly, it takes an argument list
     pointer AP.

 -- Function: int vwprintf (const wchar_t *TEMPLATE, va_list AP)
     This function is similar to `wprintf' except that, instead of
     taking a variable number of arguments directly, it takes an
     argument list pointer AP.

 -- Function: int vfprintf (FILE *STREAM, const char *TEMPLATE, va_list
          AP)
     This is the equivalent of `fprintf' with the variable argument list
     specified directly as for `vprintf'.

 -- Function: int vfwprintf (FILE *STREAM, const wchar_t *TEMPLATE,
          va_list AP)
     This is the equivalent of `fwprintf' with the variable argument
     list specified directly as for `vwprintf'.

 -- Function: int vsprintf (char *S, const char *TEMPLATE, va_list AP)
     This is the equivalent of `sprintf' with the variable argument list
     specified directly as for `vprintf'.

 -- Function: int vswprintf (wchar_t *S, size_t SIZE, const wchar_t
          *TEMPLATE, va_list AP)
     This is the equivalent of `swprintf' with the variable argument
     list specified directly as for `vwprintf'.

 -- Function: int vsnprintf (char *S, size_t SIZE, const char
          *TEMPLATE, va_list AP)
     This is the equivalent of `snprintf' with the variable argument
     list specified directly as for `vprintf'.

 -- Function: int vasprintf (char **PTR, const char *TEMPLATE, va_list
          AP)
     The `vasprintf' function is the equivalent of `asprintf' with the
     variable argument list specified directly as for `vprintf'.

 -- Function: int obstack_vprintf (struct obstack *OBSTACK, const char
          *TEMPLATE, va_list AP)
     The `obstack_vprintf' function is the equivalent of
     `obstack_printf' with the variable argument list specified directly
     as for `vprintf'.

   Here's an example showing how you might use `vfprintf'.  This is a
function that prints error messages to the stream `stderr', along with
a prefix indicating the name of the program (*note Error Messages::,
for a description of `program_invocation_short_name').

     #include <stdio.h>
     #include <stdarg.h>

     void
     eprintf (const char *template, ...)
     {
       va_list ap;
       extern char *program_invocation_short_name;

       fprintf (stderr, "%s: ", program_invocation_short_name);
       va_start (ap, template);
       vfprintf (stderr, template, ap);
       va_end (ap);
     }

You could call `eprintf' like this:

     eprintf ("file `%s' does not exist\n", filename);

   In GNU C, there is a special construct you can use to let the
compiler know that a function uses a `printf'-style format string.
Then it can check the number and types of arguments in each call to the
function, and warn you when they do not match the format string.  For
example, take this declaration of `eprintf':

     void eprintf (const char *template, ...)
             __attribute__ ((format (printf, 1, 2)));

This tells the compiler that `eprintf' uses a format string like
`printf' (as opposed to `scanf'; *note Formatted Input::); the format
string appears as the first argument; and the arguments to satisfy the
format begin with the second.  *Note Declaring Attributes of Functions:
(gcc.info)Function Attributes, for more information.

File: libc.info,  Node: Parsing a Template String,  Next: Example of Parsing,  Prev: Variable Arguments Output,  Up: Formatted Output

12.12.10 Parsing a Template String
----------------------------------

You can use the function `parse_printf_format' to obtain information
about the number and types of arguments that are expected by a given
template string.  This function permits interpreters that provide
interfaces to `printf' to avoid passing along invalid arguments from
the user's program, which could cause a crash.

   All the symbols described in this section are declared in the header
file `printf.h'.

 -- Function: size_t parse_printf_format (const char *TEMPLATE, size_t
          N, int *ARGTYPES)
     This function returns information about the number and types of
     arguments expected by the `printf' template string TEMPLATE.  The
     information is stored in the array ARGTYPES; each element of this
     array describes one argument.  This information is encoded using
     the various `PA_' macros, listed below.

     The argument N specifies the number of elements in the array
     ARGTYPES.  This is the maximum number of elements that
     `parse_printf_format' will try to write.

     `parse_printf_format' returns the total number of arguments
     required by TEMPLATE.  If this number is greater than N, then the
     information returned describes only the first N arguments.  If you
     want information about additional arguments, allocate a bigger
     array and call `parse_printf_format' again.

   The argument types are encoded as a combination of a basic type and
modifier flag bits.

 -- Macro: int PA_FLAG_MASK
     This macro is a bitmask for the type modifier flag bits.  You can
     write the expression `(argtypes[i] & PA_FLAG_MASK)' to extract
     just the flag bits for an argument, or `(argtypes[i] &
     ~PA_FLAG_MASK)' to extract just the basic type code.

   Here are symbolic constants that represent the basic types; they
stand for integer values.

`PA_INT'
     This specifies that the base type is `int'.

`PA_CHAR'
     This specifies that the base type is `int', cast to `char'.

`PA_STRING'
     This specifies that the base type is `char *', a null-terminated
     string.

`PA_POINTER'
     This specifies that the base type is `void *', an arbitrary
     pointer.

`PA_FLOAT'
     This specifies that the base type is `float'.

`PA_DOUBLE'
     This specifies that the base type is `double'.

`PA_LAST'
     You can define additional base types for your own programs as
     offsets from `PA_LAST'.  For example, if you have data types `foo'
     and `bar' with their own specialized `printf' conversions, you
     could define encodings for these types as:

          #define PA_FOO  PA_LAST
          #define PA_BAR  (PA_LAST + 1)

   Here are the flag bits that modify a basic type.  They are combined
with the code for the basic type using inclusive-or.

`PA_FLAG_PTR'
     If this bit is set, it indicates that the encoded type is a
     pointer to the base type, rather than an immediate value.  For
     example, `PA_INT|PA_FLAG_PTR' represents the type `int *'.

`PA_FLAG_SHORT'
     If this bit is set, it indicates that the base type is modified
     with `short'.  (This corresponds to the `h' type modifier.)

`PA_FLAG_LONG'
     If this bit is set, it indicates that the base type is modified
     with `long'.  (This corresponds to the `l' type modifier.)

`PA_FLAG_LONG_LONG'
     If this bit is set, it indicates that the base type is modified
     with `long long'.  (This corresponds to the `L' type modifier.)

`PA_FLAG_LONG_DOUBLE'
     This is a synonym for `PA_FLAG_LONG_LONG', used by convention with
     a base type of `PA_DOUBLE' to indicate a type of `long double'.

   For an example of using these facilities, see *note Example of
Parsing::.

File: libc.info,  Node: Example of Parsing,  Prev: Parsing a Template String,  Up: Formatted Output

12.12.11 Example of Parsing a Template String
---------------------------------------------

Here is an example of decoding argument types for a format string.  We
assume this is part of an interpreter which contains arguments of type
`NUMBER', `CHAR', `STRING' and `STRUCTURE' (and perhaps others which
are not valid here).

     /* Test whether the NARGS specified objects
        in the vector ARGS are valid
        for the format string FORMAT:
        if so, return 1.
        If not, return 0 after printing an error message.  */

     int
     validate_args (char *format, int nargs, OBJECT *args)
     {
       int *argtypes;
       int nwanted;

       /* Get the information about the arguments.
          Each conversion specification must be at least two characters
          long, so there cannot be more specifications than half the
          length of the string.  */

       argtypes = (int *) alloca (strlen (format) / 2 * sizeof (int));
       nwanted = parse_printf_format (string, nelts, argtypes);

       /* Check the number of arguments.  */
       if (nwanted > nargs)
         {
           error ("too few arguments (at least %d required)", nwanted);
           return 0;
         }

       /* Check the C type wanted for each argument
          and see if the object given is suitable.  */
       for (i = 0; i < nwanted; i++)
         {
           int wanted;

           if (argtypes[i] & PA_FLAG_PTR)
             wanted = STRUCTURE;
           else
             switch (argtypes[i] & ~PA_FLAG_MASK)
               {
               case PA_INT:
               case PA_FLOAT:
               case PA_DOUBLE:
                 wanted = NUMBER;
                 break;
               case PA_CHAR:
                 wanted = CHAR;
                 break;
               case PA_STRING:
                 wanted = STRING;
                 break;
               case PA_POINTER:
                 wanted = STRUCTURE;
                 break;
               }
           if (TYPE (args[i]) != wanted)
             {
               error ("type mismatch for arg number %d", i);
               return 0;
             }
         }
       return 1;
     }

File: libc.info,  Node: Customizing Printf,  Next: Formatted Input,  Prev: Formatted Output,  Up: I/O on Streams

12.13 Customizing `printf'
==========================

The GNU C library lets you define your own custom conversion specifiers
for `printf' template strings, to teach `printf' clever ways to print
the important data structures of your program.

   The way you do this is by registering the conversion with the
function `register_printf_function'; see *note Registering New
Conversions::.  One of the arguments you pass to this function is a
pointer to a handler function that produces the actual output; see
*note Defining the Output Handler::, for information on how to write
this function.

   You can also install a function that just returns information about
the number and type of arguments expected by the conversion specifier.
*Note Parsing a Template String::, for information about this.

   The facilities of this section are declared in the header file
`printf.h'.

* Menu:

* Registering New Conversions::         Using `register_printf_function'
                                         to register a new output conversion.
* Conversion Specifier Options::        The handler must be able to get
                                         the options specified in the
                                         template when it is called.
* Defining the Output Handler::         Defining the handler and arginfo
                                         functions that are passed as arguments
                                         to `register_printf_function'.
* Printf Extension Example::            How to define a `printf'
                                         handler function.
* Predefined Printf Handlers::          Predefined `printf' handlers.

   *Portability Note:* The ability to extend the syntax of `printf'
template strings is a GNU extension.  ISO standard C has nothing
similar.

File: libc.info,  Node: Registering New Conversions,  Next: Conversion Specifier Options,  Up: Customizing Printf

12.13.1 Registering New Conversions
-----------------------------------

The function to register a new output conversion is
`register_printf_function', declared in `printf.h'.

 -- Function: int register_printf_function (int SPEC, printf_function
          HANDLER-FUNCTION, printf_arginfo_function ARGINFO-FUNCTION)
     This function defines the conversion specifier character SPEC.
     Thus, if SPEC is `'Y'', it defines the conversion `%Y'.  You can
     redefine the built-in conversions like `%s', but flag characters
     like `#' and type modifiers like `l' can never be used as
     conversions; calling `register_printf_function' for those
     characters has no effect.  It is advisable not to use lowercase
     letters, since the ISO C standard warns that additional lowercase
     letters may be standardized in future editions of the standard.

     The HANDLER-FUNCTION is the function called by `printf' and
     friends when this conversion appears in a template string.  *Note
     Defining the Output Handler::, for information about how to define
     a function to pass as this argument.  If you specify a null
     pointer, any existing handler function for SPEC is removed.

     The ARGINFO-FUNCTION is the function called by
     `parse_printf_format' when this conversion appears in a template
     string.  *Note Parsing a Template String::, for information about
     this.

     *Attention:* In the GNU C library versions before 2.0 the
     ARGINFO-FUNCTION function did not need to be installed unless the
     user used the `parse_printf_format' function.  This has changed.
     Now a call to any of the `printf' functions will call this
     function when this format specifier appears in the format string.

     The return value is `0' on success, and `-1' on failure (which
     occurs if SPEC is out of range).

     You can redefine the standard output conversions, but this is
     probably not a good idea because of the potential for confusion.
     Library routines written by other people could break if you do
     this.

File: libc.info,  Node: Conversion Specifier Options,  Next: Defining the Output Handler,  Prev: Registering New Conversions,  Up: Customizing Printf

12.13.2 Conversion Specifier Options
------------------------------------

If you define a meaning for `%A', what if the template contains `%+23A'
or `%-#A'?  To implement a sensible meaning for these, the handler when
called needs to be able to get the options specified in the template.

   Both the HANDLER-FUNCTION and ARGINFO-FUNCTION accept an argument
that points to a `struct printf_info', which contains information about
the options appearing in an instance of the conversion specifier.  This
data type is declared in the header file `printf.h'.

 -- Type: struct printf_info
     This structure is used to pass information about the options
     appearing in an instance of a conversion specifier in a `printf'
     template string to the handler and arginfo functions for that
     specifier.  It contains the following members:

    `int prec'
          This is the precision specified.  The value is `-1' if no
          precision was specified.  If the precision was given as `*',
          the `printf_info' structure passed to the handler function
          contains the actual value retrieved from the argument list.
          But the structure passed to the arginfo function contains a
          value of `INT_MIN', since the actual value is not known.

    `int width'
          This is the minimum field width specified.  The value is `0'
          if no width was specified.  If the field width was given as
          `*', the `printf_info' structure passed to the handler
          function contains the actual value retrieved from the
          argument list.  But the structure passed to the arginfo
          function contains a value of `INT_MIN', since the actual
          value is not known.

    `wchar_t spec'
          This is the conversion specifier character specified.  It's
          stored in the structure so that you can register the same
          handler function for multiple characters, but still have a
          way to tell them apart when the handler function is called.

    `unsigned int is_long_double'
          This is a boolean that is true if the `L', `ll', or `q' type
          modifier was specified.  For integer conversions, this
          indicates `long long int', as opposed to `long double' for
          floating point conversions.

    `unsigned int is_char'
          This is a boolean that is true if the `hh' type modifier was
          specified.

    `unsigned int is_short'
          This is a boolean that is true if the `h' type modifier was
          specified.

    `unsigned int is_long'
          This is a boolean that is true if the `l' type modifier was
          specified.

    `unsigned int alt'
          This is a boolean that is true if the `#' flag was specified.

    `unsigned int space'
          This is a boolean that is true if the ` ' flag was specified.

    `unsigned int left'
          This is a boolean that is true if the `-' flag was specified.

    `unsigned int showsign'
          This is a boolean that is true if the `+' flag was specified.

    `unsigned int group'
          This is a boolean that is true if the `'' flag was specified.

    `unsigned int extra'
          This flag has a special meaning depending on the context.  It
          could be used freely by the user-defined handlers but when
          called from the `printf' function this variable always
          contains the value `0'.

    `unsigned int wide'
          This flag is set if the stream is wide oriented.

    `wchar_t pad'
          This is the character to use for padding the output to the
          minimum field width.  The value is `'0'' if the `0' flag was
          specified, and `' '' otherwise.

File: libc.info,  Node: Defining the Output Handler,  Next: Printf Extension Example,  Prev: Conversion Specifier Options,  Up: Customizing Printf

12.13.3 Defining the Output Handler
-----------------------------------

Now let's look at how to define the handler and arginfo functions which
are passed as arguments to `register_printf_function'.

   *Compatibility Note:* The interface changed in GNU libc version 2.0.
Previously the third argument was of type `va_list *'.

   You should define your handler functions with a prototype like:

     int FUNCTION (FILE *stream, const struct printf_info *info,
                         const void *const *args)

   The STREAM argument passed to the handler function is the stream to
which it should write output.

   The INFO argument is a pointer to a structure that contains
information about the various options that were included with the
conversion in the template string.  You should not modify this structure
inside your handler function.  *Note Conversion Specifier Options::, for
a description of this data structure.

   The ARGS is a vector of pointers to the arguments data.  The number
of arguments was determined by calling the argument information
function provided by the user.

   Your handler function should return a value just like `printf' does:
it should return the number of characters it has written, or a negative
value to indicate an error.

 -- Data Type: printf_function
     This is the data type that a handler function should have.

   If you are going to use `parse_printf_format' in your application,
you must also define a function to pass as the ARGINFO-FUNCTION
argument for each new conversion you install with
`register_printf_function'.

   You have to define these functions with a prototype like:

     int FUNCTION (const struct printf_info *info,
                         size_t n, int *argtypes)

   The return value from the function should be the number of arguments
the conversion expects.  The function should also fill in no more than
N elements of the ARGTYPES array with information about the types of
each of these arguments.  This information is encoded using the various
`PA_' macros.  (You will notice that this is the same calling
convention `parse_printf_format' itself uses.)

 -- Data Type: printf_arginfo_function
     This type is used to describe functions that return information
     about the number and type of arguments used by a conversion
     specifier.

File: libc.info,  Node: Printf Extension Example,  Next: Predefined Printf Handlers,  Prev: Defining the Output Handler,  Up: Customizing Printf

12.13.4 `printf' Extension Example
----------------------------------

Here is an example showing how to define a `printf' handler function.
This program defines a data structure called a `Widget' and defines the
`%W' conversion to print information about `Widget *' arguments,
including the pointer value and the name stored in the data structure.
The `%W' conversion supports the minimum field width and
left-justification options, but ignores everything else.

     #include <stdio.h>
     #include <stdlib.h>
     #include <printf.h>

     typedef struct
     {
       char *name;
     }
     Widget;

     int
     print_widget (FILE *stream,
                   const struct printf_info *info,
                   const void *const *args)
     {
       const Widget *w;
       char *buffer;
       int len;

       /* Format the output into a string. */
       w = *((const Widget **) (args[0]));
       len = asprintf (&buffer, "<Widget %p: %s>", w, w->name);
       if (len == -1)
         return -1;

       /* Pad to the minimum field width and print to the stream. */
       len = fprintf (stream, "%*s",
                      (info->left ? -info->width : info->width),
                      buffer);

       /* Clean up and return. */
       free (buffer);
       return len;
     }


     int
     print_widget_arginfo (const struct printf_info *info, size_t n,
                           int *argtypes)
     {
       /* We always take exactly one argument and this is a pointer to the
          structure.. */
       if (n > 0)
         argtypes[0] = PA_POINTER;
       return 1;
     }


     int
     main (void)
     {
       /* Make a widget to print. */
       Widget mywidget;
       mywidget.name = "mywidget";

       /* Register the print function for widgets. */
       register_printf_function ('W', print_widget, print_widget_arginfo);

       /* Now print the widget. */
       printf ("|%W|\n", &mywidget);
       printf ("|%35W|\n", &mywidget);
       printf ("|%-35W|\n", &mywidget);

       return 0;
     }

   The output produced by this program looks like:

     |<Widget 0xffeffb7c: mywidget>|
     |      <Widget 0xffeffb7c: mywidget>|
     |<Widget 0xffeffb7c: mywidget>      |

File: libc.info,  Node: Predefined Printf Handlers,  Prev: Printf Extension Example,  Up: Customizing Printf

12.13.5 Predefined `printf' Handlers
------------------------------------

The GNU libc also contains a concrete and useful application of the
`printf' handler extension.  There are two functions available which
implement a special way to print floating-point numbers.

 -- Function: int printf_size (FILE *FP, const struct printf_info
          *INFO, const void *const *ARGS)
     Print a given floating point number as for the format `%f' except
     that there is a postfix character indicating the divisor for the
     number to make this less than 1000.  There are two possible
     divisors: powers of 1024 or powers of 1000.  Which one is used
     depends on the format character specified while registered this
     handler.  If the character is of lower case, 1024 is used.  For
     upper case characters, 1000 is used.

     The postfix tag corresponds to bytes, kilobytes, megabytes,
     gigabytes, etc.  The full table is:

     +------+--------------+--------+--------+---------------+
     |low|Multiplier|From|Upper|Multiplier|
     |' '|1||' '|1|
     |k|2^10 (1024)|kilo|K|10^3 (1000)|
     |m|2^20|mega|M|10^6|
     |g|2^30|giga|G|10^9|
     |t|2^40|tera|T|10^12|
     |p|2^50|peta|P|10^15|
     |e|2^60|exa|E|10^18|
     |z|2^70|zetta|Z|10^21|
     |y|2^80|yotta|Y|10^24|

     The default precision is 3, i.e., 1024 is printed with a lower-case
     format character as if it were `%.3fk' and will yield `1.000k'.

   Due to the requirements of `register_printf_function' we must also
provide the function which returns information about the arguments.

 -- Function: int printf_size_info (const struct printf_info *INFO,
          size_t N, int *ARGTYPES)
     This function will return in ARGTYPES the information about the
     used parameters in the way the `vfprintf' implementation expects
     it.  The format always takes one argument.

   To use these functions both functions must be registered with a call
like

     register_printf_function ('B', printf_size, printf_size_info);

   Here we register the functions to print numbers as powers of 1000
since the format character `'B'' is an upper-case character.  If we
would additionally use `'b'' in a line like

     register_printf_function ('b', printf_size, printf_size_info);

we could also print using a power of 1024.  Please note that all that is
different in these two lines is the format specifier.  The
`printf_size' function knows about the difference between lower and
upper case format specifiers.

   The use of `'B'' and `'b'' is no coincidence.  Rather it is the
preferred way to use this functionality since it is available on some
other systems which also use format specifiers.

File: libc.info,  Node: Formatted Input,  Next: EOF and Errors,  Prev: Customizing Printf,  Up: I/O on Streams

12.14 Formatted Input
=====================

The functions described in this section (`scanf' and related functions)
provide facilities for formatted input analogous to the formatted
output facilities.  These functions provide a mechanism for reading
arbitrary values under the control of a "format string" or "template
string".

* Menu:

* Formatted Input Basics::      Some basics to get you started.
* Input Conversion Syntax::     Syntax of conversion specifications.
* Table of Input Conversions::  Summary of input conversions and what they do.
* Numeric Input Conversions::   Details of conversions for reading numbers.
* String Input Conversions::    Details of conversions for reading strings.
* Dynamic String Input::	String conversions that `malloc' the buffer.
* Other Input Conversions::     Details of miscellaneous other conversions.
* Formatted Input Functions::   Descriptions of the actual functions.
* Variable Arguments Input::    `vscanf' and friends.

File: libc.info,  Node: Formatted Input Basics,  Next: Input Conversion Syntax,  Up: Formatted Input

12.14.1 Formatted Input Basics
------------------------------

Calls to `scanf' are superficially similar to calls to `printf' in that
arbitrary arguments are read under the control of a template string.
While the syntax of the conversion specifications in the template is
very similar to that for `printf', the interpretation of the template
is oriented more towards free-format input and simple pattern matching,
rather than fixed-field formatting.  For example, most `scanf'
conversions skip over any amount of "white space" (including spaces,
tabs, and newlines) in the input file, and there is no concept of
precision for the numeric input conversions as there is for the
corresponding output conversions.  Ordinarily, non-whitespace
characters in the template are expected to match characters in the
input stream exactly, but a matching failure is distinct from an input
error on the stream.

   Another area of difference between `scanf' and `printf' is that you
must remember to supply pointers rather than immediate values as the
optional arguments to `scanf'; the values that are read are stored in
the objects that the pointers point to.  Even experienced programmers
tend to forget this occasionally, so if your program is getting strange
errors that seem to be related to `scanf', you might want to
double-check this.

   When a "matching failure" occurs, `scanf' returns immediately,
leaving the first non-matching character as the next character to be
read from the stream.  The normal return value from `scanf' is the
number of values that were assigned, so you can use this to determine if
a matching error happened before all the expected values were read.

   The `scanf' function is typically used for things like reading in
the contents of tables.  For example, here is a function that uses
`scanf' to initialize an array of `double':

     void
     readarray (double *array, int n)
     {
       int i;
       for (i=0; i<n; i++)
         if (scanf (" %lf", &(array[i])) != 1)
           invalid_input_error ();
     }

   The formatted input functions are not used as frequently as the
formatted output functions.  Partly, this is because it takes some care
to use them properly.  Another reason is that it is difficult to recover
from a matching error.

   If you are trying to read input that doesn't match a single, fixed
pattern, you may be better off using a tool such as Flex to generate a
lexical scanner, or Bison to generate a parser, rather than using
`scanf'.  For more information about these tools, see *note Top:
(flex.info)Top, and *note Top: (bison.info)Top.

File: libc.info,  Node: Input Conversion Syntax,  Next: Table of Input Conversions,  Prev: Formatted Input Basics,  Up: Formatted Input

12.14.2 Input Conversion Syntax
-------------------------------

A `scanf' template string is a string that contains ordinary multibyte
characters interspersed with conversion specifications that start with
`%'.

   Any whitespace character (as defined by the `isspace' function;
*note Classification of Characters::) in the template causes any number
of whitespace characters in the input stream to be read and discarded.
The whitespace characters that are matched need not be exactly the same
whitespace characters that appear in the template string.  For example,
write ` , ' in the template to recognize a comma with optional
whitespace before and after.

   Other characters in the template string that are not part of
conversion specifications must match characters in the input stream
exactly; if this is not the case, a matching failure occurs.

   The conversion specifications in a `scanf' template string have the
general form:

     % FLAGS WIDTH TYPE CONVERSION

   In more detail, an input conversion specification consists of an
initial `%' character followed in sequence by:

   * An optional "flag character" `*', which says to ignore the text
     read for this specification.  When `scanf' finds a conversion
     specification that uses this flag, it reads input as directed by
     the rest of the conversion specification, but it discards this
     input, does not use a pointer argument, and does not increment the
     count of successful assignments.

   * An optional flag character `a' (valid with string conversions only)
     which requests allocation of a buffer long enough to store the
     string in.  (This is a GNU extension.)  *Note Dynamic String
     Input::.

   * An optional decimal integer that specifies the "maximum field
     width".  Reading of characters from the input stream stops either
     when this maximum is reached or when a non-matching character is
     found, whichever happens first.  Most conversions discard initial
     whitespace characters (those that don't are explicitly
     documented), and these discarded characters don't count towards
     the maximum field width.  String input conversions store a null
     character to mark the end of the input; the maximum field width
     does not include this terminator.

   * An optional "type modifier character".  For example, you can
     specify a type modifier of `l' with integer conversions such as
     `%d' to specify that the argument is a pointer to a `long int'
     rather than a pointer to an `int'.

   * A character that specifies the conversion to be applied.

   The exact options that are permitted and how they are interpreted
vary between the different conversion specifiers.  See the descriptions
of the individual conversions for information about the particular
options that they allow.

   With the `-Wformat' option, the GNU C compiler checks calls to
`scanf' and related functions.  It examines the format string and
verifies that the correct number and types of arguments are supplied.
There is also a GNU C syntax to tell the compiler that a function you
write uses a `scanf'-style format string.  *Note Declaring Attributes
of Functions: (gcc.info)Function Attributes, for more information.

File: libc.info,  Node: Table of Input Conversions,  Next: Numeric Input Conversions,  Prev: Input Conversion Syntax,  Up: Formatted Input

12.14.3 Table of Input Conversions
----------------------------------

Here is a table that summarizes the various conversion specifications:

`%d'
     Matches an optionally signed integer written in decimal.  *Note
     Numeric Input Conversions::.

`%i'
     Matches an optionally signed integer in any of the formats that
     the C language defines for specifying an integer constant.  *Note
     Numeric Input Conversions::.

`%o'
     Matches an unsigned integer written in octal radix.  *Note Numeric
     Input Conversions::.

`%u'
     Matches an unsigned integer written in decimal radix.  *Note
     Numeric Input Conversions::.

`%x', `%X'
     Matches an unsigned integer written in hexadecimal radix.  *Note
     Numeric Input Conversions::.

`%e', `%f', `%g', `%E', `%G'
     Matches an optionally signed floating-point number.  *Note Numeric
     Input Conversions::.

`%s'
     Matches a string containing only non-whitespace characters.  *Note
     String Input Conversions::.  The presence of the `l' modifier
     determines whether the output is stored as a wide character string
     or a multibyte string.  If `%s' is used in a wide character
     function the string is converted as with multiple calls to
     `wcrtomb' into a multibyte string.  This means that the buffer
     must provide room for `MB_CUR_MAX' bytes for each wide character
     read.  In case `%ls' is used in a multibyte function the result is
     converted into wide characters as with multiple calls of `mbrtowc'
     before being stored in the user provided buffer.

`%S'
     This is an alias for `%ls' which is supported for compatibility
     with the Unix standard.

`%['
     Matches a string of characters that belong to a specified set.
     *Note String Input Conversions::.  The presence of the `l' modifier
     determines whether the output is stored as a wide character string
     or a multibyte string.  If `%[' is used in a wide character
     function the string is converted as with multiple calls to
     `wcrtomb' into a multibyte string.  This means that the buffer
     must provide room for `MB_CUR_MAX' bytes for each wide character
     read.  In case `%l[' is used in a multibyte function the result is
     converted into wide characters as with multiple calls of `mbrtowc'
     before being stored in the user provided buffer.

`%c'
     Matches a string of one or more characters; the number of
     characters read is controlled by the maximum field width given for
     the conversion.  *Note String Input Conversions::.

     If the `%c' is used in a wide stream function the read value is
     converted from a wide character to the corresponding multibyte
     character before storing it.  Note that this conversion can
     produce more than one byte of output and therefore the provided
     buffer be large enough for up to `MB_CUR_MAX' bytes for each
     character.  If `%lc' is used in a multibyte function the input is
     treated as a multibyte sequence (and not bytes) and the result is
     converted as with calls to `mbrtowc'.

`%C'
     This is an alias for `%lc' which is supported for compatibility
     with the Unix standard.

`%p'
     Matches a pointer value in the same implementation-defined format
     used by the `%p' output conversion for `printf'.  *Note Other
     Input Conversions::.

`%n'
     This conversion doesn't read any characters; it records the number
     of characters read so far by this call.  *Note Other Input
     Conversions::.

`%%'
     This matches a literal `%' character in the input stream.  No
     corresponding argument is used.  *Note Other Input Conversions::.

   If the syntax of a conversion specification is invalid, the behavior
is undefined.  If there aren't enough function arguments provided to
supply addresses for all the conversion specifications in the template
strings that perform assignments, or if the arguments are not of the
correct types, the behavior is also undefined.  On the other hand, extra
arguments are simply ignored.

File: libc.info,  Node: Numeric Input Conversions,  Next: String Input Conversions,  Prev: Table of Input Conversions,  Up: Formatted Input

12.14.4 Numeric Input Conversions
---------------------------------

This section describes the `scanf' conversions for reading numeric
values.

   The `%d' conversion matches an optionally signed integer in decimal
radix.  The syntax that is recognized is the same as that for the
`strtol' function (*note Parsing of Integers::) with the value `10' for
the BASE argument.

   The `%i' conversion matches an optionally signed integer in any of
the formats that the C language defines for specifying an integer
constant.  The syntax that is recognized is the same as that for the
`strtol' function (*note Parsing of Integers::) with the value `0' for
the BASE argument.  (You can print integers in this syntax with
`printf' by using the `#' flag character with the `%x', `%o', or `%d'
conversion.  *Note Integer Conversions::.)

   For example, any of the strings `10', `0xa', or `012' could be read
in as integers under the `%i' conversion.  Each of these specifies a
number with decimal value `10'.

   The `%o', `%u', and `%x' conversions match unsigned integers in
octal, decimal, and hexadecimal radices, respectively.  The syntax that
is recognized is the same as that for the `strtoul' function (*note
Parsing of Integers::) with the appropriate value (`8', `10', or `16')
for the BASE argument.

   The `%X' conversion is identical to the `%x' conversion.  They both
permit either uppercase or lowercase letters to be used as digits.

   The default type of the corresponding argument for the `%d' and `%i'
conversions is `int *', and `unsigned int *' for the other integer
conversions.  You can use the following type modifiers to specify other
sizes of integer:

`hh'
     Specifies that the argument is a `signed char *' or `unsigned char
     *'.

     This modifier was introduced in ISO C99.

`h'
     Specifies that the argument is a `short int *' or `unsigned short
     int *'.

`j'
     Specifies that the argument is a `intmax_t *' or `uintmax_t *'.

     This modifier was introduced in ISO C99.

`l'
     Specifies that the argument is a `long int *' or `unsigned long
     int *'.  Two `l' characters is like the `L' modifier, below.

     If used with `%c' or `%s' the corresponding parameter is
     considered as a pointer to a wide character or wide character
     string respectively.  This use of `l' was introduced in
     Amendment 1 to ISO C90.

`ll'
`L'
`q'
     Specifies that the argument is a `long long int *' or `unsigned
     long long int *'.  (The `long long' type is an extension supported
     by the GNU C compiler.  For systems that don't provide extra-long
     integers, this is the same as `long int'.)

     The `q' modifier is another name for the same thing, which comes
     from 4.4 BSD; a `long long int' is sometimes called a "quad" `int'.

`t'
     Specifies that the argument is a `ptrdiff_t *'.

     This modifier was introduced in ISO C99.

`z'
     Specifies that the argument is a `size_t *'.

     This modifier was introduced in ISO C99.

   All of the `%e', `%f', `%g', `%E', and `%G' input conversions are
interchangeable.  They all match an optionally signed floating point
number, in the same syntax as for the `strtod' function (*note Parsing
of Floats::).

   For the floating-point input conversions, the default argument type
is `float *'.  (This is different from the corresponding output
conversions, where the default type is `double'; remember that `float'
arguments to `printf' are converted to `double' by the default argument
promotions, but `float *' arguments are not promoted to `double *'.)
You can specify other sizes of float using these type modifiers:

`l'
     Specifies that the argument is of type `double *'.

`L'
     Specifies that the argument is of type `long double *'.

   For all the above number parsing formats there is an additional
optional flag `''.  When this flag is given the `scanf' function
expects the number represented in the input string to be formatted
according to the grouping rules of the currently selected locale (*note
General Numeric::).

   If the `"C"' or `"POSIX"' locale is selected there is no difference.
But for a locale which specifies values for the appropriate fields in
the locale the input must have the correct form in the input.
Otherwise the longest prefix with a correct form is processed.

File: libc.info,  Node: String Input Conversions,  Next: Dynamic String Input,  Prev: Numeric Input Conversions,  Up: Formatted Input

12.14.5 String Input Conversions
--------------------------------

This section describes the `scanf' input conversions for reading string
and character values: `%s', `%S', `%[', `%c', and `%C'.

   You have two options for how to receive the input from these
conversions:

   * Provide a buffer to store it in.  This is the default.  You should
     provide an argument of type `char *' or `wchar_t *' (the latter of
     the `l' modifier is present).

     *Warning:* To make a robust program, you must make sure that the
     input (plus its terminating null) cannot possibly exceed the size
     of the buffer you provide.  In general, the only way to do this is
     to specify a maximum field width one less than the buffer size.
     *If you provide the buffer, always specify a maximum field width
     to prevent overflow.*

   * Ask `scanf' to allocate a big enough buffer, by specifying the `a'
     flag character.  This is a GNU extension.  You should provide an
     argument of type `char **' for the buffer address to be stored in.
     *Note Dynamic String Input::.

   The `%c' conversion is the simplest: it matches a fixed number of
characters, always.  The maximum field width says how many characters to
read; if you don't specify the maximum, the default is 1.  This
conversion doesn't append a null character to the end of the text it
reads.  It also does not skip over initial whitespace characters.  It
reads precisely the next N characters, and fails if it cannot get that
many.  Since there is always a maximum field width with `%c' (whether
specified, or 1 by default), you can always prevent overflow by making
the buffer long enough.

   If the format is `%lc' or `%C' the function stores wide characters
which are converted using the conversion determined at the time the
stream was opened from the external byte stream.  The number of bytes
read from the medium is limited by `MB_CUR_LEN * N' but at most N wide
character get stored in the output string.

   The `%s' conversion matches a string of non-whitespace characters.
It skips and discards initial whitespace, but stops when it encounters
more whitespace after having read something.  It stores a null character
at the end of the text that it reads.

   For example, reading the input:

      hello, world

with the conversion `%10c' produces `" hello, wo"', but reading the
same input with the conversion `%10s' produces `"hello,"'.

   *Warning:* If you do not specify a field width for `%s', then the
number of characters read is limited only by where the next whitespace
character appears.  This almost certainly means that invalid input can
make your program crash--which is a bug.

   The `%ls' and `%S' format are handled just like `%s' except that the
external byte sequence is converted using the conversion associated
with the stream to wide characters with their own encoding.  A width or
precision specified with the format do not directly determine how many
bytes are read from the stream since they measure wide characters.  But
an upper limit can be computed by multiplying the value of the width or
precision by `MB_CUR_MAX'.

   To read in characters that belong to an arbitrary set of your choice,
use the `%[' conversion.  You specify the set between the `[' character
and a following `]' character, using the same syntax used in regular
expressions.  As special cases:

   * A literal `]' character can be specified as the first character of
     the set.

   * An embedded `-' character (that is, one that is not the first or
     last character of the set) is used to specify a range of
     characters.

   * If a caret character `^' immediately follows the initial `[', then
     the set of allowed input characters is the everything _except_ the
     characters listed.

   The `%[' conversion does not skip over initial whitespace characters.

   Here are some examples of `%[' conversions and what they mean:

`%25[1234567890]'
     Matches a string of up to 25 digits.

`%25[][]'
     Matches a string of up to 25 square brackets.

`%25[^ \f\n\r\t\v]'
     Matches a string up to 25 characters long that doesn't contain any
     of the standard whitespace characters.  This is slightly different
     from `%s', because if the input begins with a whitespace character,
     `%[' reports a matching failure while `%s' simply discards the
     initial whitespace.

`%25[a-z]'
     Matches up to 25 lowercase characters.

   As for `%c' and `%s' the `%[' format is also modified to produce
wide characters if the `l' modifier is present.  All what is said about
`%ls' above is true for `%l['.

   One more reminder: the `%s' and `%[' conversions are *dangerous* if
you don't specify a maximum width or use the `a' flag, because input
too long would overflow whatever buffer you have provided for it.  No
matter how long your buffer is, a user could supply input that is
longer.  A well-written program reports invalid input with a
comprehensible error message, not with a crash.

File: libc.info,  Node: Dynamic String Input,  Next: Other Input Conversions,  Prev: String Input Conversions,  Up: Formatted Input

12.14.6 Dynamically Allocating String Conversions
-------------------------------------------------

A GNU extension to formatted input lets you safely read a string with no
maximum size.  Using this feature, you don't supply a buffer; instead,
`scanf' allocates a buffer big enough to hold the data and gives you
its address.  To use this feature, write `a' as a flag character, as in
`%as' or `%a[0-9a-z]'.

   The pointer argument you supply for where to store the input should
have type `char **'.  The `scanf' function allocates a buffer and
stores its address in the word that the argument points to.  You should
free the buffer with `free' when you no longer need it.

   Here is an example of using the `a' flag with the `%[...]'
conversion specification to read a "variable assignment" of the form
`VARIABLE = VALUE'.

     {
       char *variable, *value;

       if (2 > scanf ("%a[a-zA-Z0-9] = %a[^\n]\n",
                      &variable, &value))
         {
           invalid_input_error ();
           return 0;
         }

       ...
     }

File: libc.info,  Node: Other Input Conversions,  Next: Formatted Input Functions,  Prev: Dynamic String Input,  Up: Formatted Input

12.14.7 Other Input Conversions
-------------------------------

This section describes the miscellaneous input conversions.

   The `%p' conversion is used to read a pointer value.  It recognizes
the same syntax used by the `%p' output conversion for `printf' (*note
Other Output Conversions::); that is, a hexadecimal number just as the
`%x' conversion accepts.  The corresponding argument should be of type
`void **'; that is, the address of a place to store a pointer.

   The resulting pointer value is not guaranteed to be valid if it was
not originally written during the same program execution that reads it
in.

   The `%n' conversion produces the number of characters read so far by
this call.  The corresponding argument should be of type `int *'.  This
conversion works in the same way as the `%n' conversion for `printf';
see *note Other Output Conversions::, for an example.

   The `%n' conversion is the only mechanism for determining the
success of literal matches or conversions with suppressed assignments.
If the `%n' follows the locus of a matching failure, then no value is
stored for it since `scanf' returns before processing the `%n'.  If you
store `-1' in that argument slot before calling `scanf', the presence
of `-1' after `scanf' indicates an error occurred before the `%n' was
reached.

   Finally, the `%%' conversion matches a literal `%' character in the
input stream, without using an argument.  This conversion does not
permit any flags, field width, or type modifier to be specified.

File: libc.info,  Node: Formatted Input Functions,  Next: Variable Arguments Input,  Prev: Other Input Conversions,  Up: Formatted Input

12.14.8 Formatted Input Functions
---------------------------------

Here are the descriptions of the functions for performing formatted
input.  Prototypes for these functions are in the header file `stdio.h'.

 -- Function: int scanf (const char *TEMPLATE, ...)
     The `scanf' function reads formatted input from the stream `stdin'
     under the control of the template string TEMPLATE.  The optional
     arguments are pointers to the places which receive the resulting
     values.

     The return value is normally the number of successful assignments.
     If an end-of-file condition is detected before any matches are
     performed, including matches against whitespace and literal
     characters in the template, then `EOF' is returned.

 -- Function: int wscanf (const wchar_t *TEMPLATE, ...)
     The `wscanf' function reads formatted input from the stream
     `stdin' under the control of the template string TEMPLATE.  The
     optional arguments are pointers to the places which receive the
     resulting values.

     The return value is normally the number of successful assignments.
     If an end-of-file condition is detected before any matches are
     performed, including matches against whitespace and literal
     characters in the template, then `WEOF' is returned.

 -- Function: int fscanf (FILE *STREAM, const char *TEMPLATE, ...)
     This function is just like `scanf', except that the input is read
     from the stream STREAM instead of `stdin'.

 -- Function: int fwscanf (FILE *STREAM, const wchar_t *TEMPLATE, ...)
     This function is just like `wscanf', except that the input is read
     from the stream STREAM instead of `stdin'.

 -- Function: int sscanf (const char *S, const char *TEMPLATE, ...)
     This is like `scanf', except that the characters are taken from the
     null-terminated string S instead of from a stream.  Reaching the
     end of the string is treated as an end-of-file condition.

     The behavior of this function is undefined if copying takes place
     between objects that overlap--for example, if S is also given as
     an argument to receive a string read under control of the `%s',
     `%S', or `%[' conversion.

 -- Function: int swscanf (const wchar_t *WS, const char *TEMPLATE, ...)
     This is like `wscanf', except that the characters are taken from
     the null-terminated string WS instead of from a stream.  Reaching
     the end of the string is treated as an end-of-file condition.

     The behavior of this function is undefined if copying takes place
     between objects that overlap--for example, if WS is also given as
     an argument to receive a string read under control of the `%s',
     `%S', or `%[' conversion.

File: libc.info,  Node: Variable Arguments Input,  Prev: Formatted Input Functions,  Up: Formatted Input

12.14.9 Variable Arguments Input Functions
------------------------------------------

The functions `vscanf' and friends are provided so that you can define
your own variadic `scanf'-like functions that make use of the same
internals as the built-in formatted output functions.  These functions
are analogous to the `vprintf' series of output functions.  *Note
Variable Arguments Output::, for important information on how to use
them.

   *Portability Note:* The functions listed in this section were
introduced in ISO C99 and were before available as GNU extensions.

 -- Function: int vscanf (const char *TEMPLATE, va_list AP)
     This function is similar to `scanf', but instead of taking a
     variable number of arguments directly, it takes an argument list
     pointer AP of type `va_list' (*note Variadic Functions::).

 -- Function: int vwscanf (const wchar_t *TEMPLATE, va_list AP)
     This function is similar to `wscanf', but instead of taking a
     variable number of arguments directly, it takes an argument list
     pointer AP of type `va_list' (*note Variadic Functions::).

 -- Function: int vfscanf (FILE *STREAM, const char *TEMPLATE, va_list
          AP)
     This is the equivalent of `fscanf' with the variable argument list
     specified directly as for `vscanf'.

 -- Function: int vfwscanf (FILE *STREAM, const wchar_t *TEMPLATE,
          va_list AP)
     This is the equivalent of `fwscanf' with the variable argument list
     specified directly as for `vwscanf'.

 -- Function: int vsscanf (const char *S, const char *TEMPLATE, va_list
          AP)
     This is the equivalent of `sscanf' with the variable argument list
     specified directly as for `vscanf'.

 -- Function: int vswscanf (const wchar_t *S, const wchar_t *TEMPLATE,
          va_list AP)
     This is the equivalent of `swscanf' with the variable argument list
     specified directly as for `vwscanf'.

   In GNU C, there is a special construct you can use to let the
compiler know that a function uses a `scanf'-style format string.  Then
it can check the number and types of arguments in each call to the
function, and warn you when they do not match the format string.  For
details, see *note Declaring Attributes of Functions:
(gcc.info)Function Attributes.

File: libc.info,  Node: EOF and Errors,  Next: Error Recovery,  Prev: Formatted Input,  Up: I/O on Streams

12.15 End-Of-File and Errors
============================

Many of the functions described in this chapter return the value of the
macro `EOF' to indicate unsuccessful completion of the operation.
Since `EOF' is used to report both end of file and random errors, it's
often better to use the `feof' function to check explicitly for end of
file and `ferror' to check for errors.  These functions check
indicators that are part of the internal state of the stream object,
indicators set if the appropriate condition was detected by a previous
I/O operation on that stream.

 -- Macro: int EOF
     This macro is an integer value that is returned by a number of
     narrow stream functions to indicate an end-of-file condition, or
     some other error situation.  With the GNU library, `EOF' is `-1'.
     In other libraries, its value may be some other negative number.

     This symbol is declared in `stdio.h'.

 -- Macro: int WEOF
     This macro is an integer value that is returned by a number of wide
     stream functions to indicate an end-of-file condition, or some
     other error situation.  With the GNU library, `WEOF' is `-1'.  In
     other libraries, its value may be some other negative number.

     This symbol is declared in `wchar.h'.

 -- Function: int feof (FILE *STREAM)
     The `feof' function returns nonzero if and only if the end-of-file
     indicator for the stream STREAM is set.

     This symbol is declared in `stdio.h'.

 -- Function: int feof_unlocked (FILE *STREAM)
     The `feof_unlocked' function is equivalent to the `feof' function
     except that it does not implicitly lock the stream.

     This function is a GNU extension.

     This symbol is declared in `stdio.h'.

 -- Function: int ferror (FILE *STREAM)
     The `ferror' function returns nonzero if and only if the error
     indicator for the stream STREAM is set, indicating that an error
     has occurred on a previous operation on the stream.

     This symbol is declared in `stdio.h'.

 -- Function: int ferror_unlocked (FILE *STREAM)
     The `ferror_unlocked' function is equivalent to the `ferror'
     function except that it does not implicitly lock the stream.

     This function is a GNU extension.

     This symbol is declared in `stdio.h'.

   In addition to setting the error indicator associated with the
stream, the functions that operate on streams also set `errno' in the
same way as the corresponding low-level functions that operate on file
descriptors.  For example, all of the functions that perform output to a
stream--such as `fputc', `printf', and `fflush'--are implemented in
terms of `write', and all of the `errno' error conditions defined for
`write' are meaningful for these functions.  For more information about
the descriptor-level I/O functions, see *note Low-Level I/O::.

File: libc.info,  Node: Error Recovery,  Next: Binary Streams,  Prev: EOF and Errors,  Up: I/O on Streams

12.16 Recovering from errors
============================

You may explicitly clear the error and EOF flags with the `clearerr'
function.

 -- Function: void clearerr (FILE *STREAM)
     This function clears the end-of-file and error indicators for the
     stream STREAM.

     The file positioning functions (*note File Positioning::) also
     clear the end-of-file indicator for the stream.

 -- Function: void clearerr_unlocked (FILE *STREAM)
     The `clearerr_unlocked' function is equivalent to the `clearerr'
     function except that it does not implicitly lock the stream.

     This function is a GNU extension.

   Note that it is _not_ correct to just clear the error flag and retry
a failed stream operation.  After a failed write, any number of
characters since the last buffer flush may have been committed to the
file, while some buffered data may have been discarded.  Merely retrying
can thus cause lost or repeated data.

   A failed read may leave the file pointer in an inappropriate
position for a second try.  In both cases, you should seek to a known
position before retrying.

   Most errors that can happen are not recoverable -- a second try will
always fail again in the same way.  So usually it is best to give up and
report the error to the user, rather than install complicated recovery
logic.

   One important exception is `EINTR' (*note Interrupted Primitives::).
Many stream I/O implementations will treat it as an ordinary error,
which can be quite inconvenient.  You can avoid this hassle by
installing all signals with the `SA_RESTART' flag.

   For similar reasons, setting nonblocking I/O on a stream's file
descriptor is not usually advisable.

File: libc.info,  Node: Binary Streams,  Next: File Positioning,  Prev: Error Recovery,  Up: I/O on Streams

12.17 Text and Binary Streams
=============================

The GNU system and other POSIX-compatible operating systems organize all
files as uniform sequences of characters.  However, some other systems
make a distinction between files containing text and files containing
binary data, and the input and output facilities of ISO C provide for
this distinction.  This section tells you how to write programs portable
to such systems.

   When you open a stream, you can specify either a "text stream" or a
"binary stream".  You indicate that you want a binary stream by
specifying the `b' modifier in the OPENTYPE argument to `fopen'; see
*note Opening Streams::.  Without this option, `fopen' opens the file
as a text stream.

   Text and binary streams differ in several ways:

   * The data read from a text stream is divided into "lines" which are
     terminated by newline (`'\n'') characters, while a binary stream is
     simply a long series of characters.  A text stream might on some
     systems fail to handle lines more than 254 characters long
     (including the terminating newline character).

   * On some systems, text files can contain only printing characters,
     horizontal tab characters, and newlines, and so text streams may
     not support other characters.  However, binary streams can handle
     any character value.

   * Space characters that are written immediately preceding a newline
     character in a text stream may disappear when the file is read in
     again.

   * More generally, there need not be a one-to-one mapping between
     characters that are read from or written to a text stream, and the
     characters in the actual file.

   Since a binary stream is always more capable and more predictable
than a text stream, you might wonder what purpose text streams serve.
Why not simply always use binary streams?  The answer is that on these
operating systems, text and binary streams use different file formats,
and the only way to read or write "an ordinary file of text" that can
work with other text-oriented programs is through a text stream.

   In the GNU library, and on all POSIX systems, there is no difference
between text streams and binary streams.  When you open a stream, you
get the same kind of stream regardless of whether you ask for binary.
This stream can handle any file content, and has none of the
restrictions that text streams sometimes have.

File: libc.info,  Node: File Positioning,  Next: Portable Positioning,  Prev: Binary Streams,  Up: I/O on Streams

12.18 File Positioning
======================

The "file position" of a stream describes where in the file the stream
is currently reading or writing.  I/O on the stream advances the file
position through the file.  In the GNU system, the file position is
represented as an integer, which counts the number of bytes from the
beginning of the file.  *Note File Position::.

   During I/O to an ordinary disk file, you can change the file position
whenever you wish, so as to read or write any portion of the file.  Some
other kinds of files may also permit this.  Files which support changing
the file position are sometimes referred to as "random-access" files.

   You can use the functions in this section to examine or modify the
file position indicator associated with a stream.  The symbols listed
below are declared in the header file `stdio.h'.

 -- Function: long int ftell (FILE *STREAM)
     This function returns the current file position of the stream
     STREAM.

     This function can fail if the stream doesn't support file
     positioning, or if the file position can't be represented in a
     `long int', and possibly for other reasons as well.  If a failure
     occurs, a value of `-1' is returned.

 -- Function: off_t ftello (FILE *STREAM)
     The `ftello' function is similar to `ftell', except that it
     returns a value of type `off_t'.  Systems which support this type
     use it to describe all file positions, unlike the POSIX
     specification which uses a long int.  The two are not necessarily
     the same size.  Therefore, using ftell can lead to problems if the
     implementation is written on top of a POSIX compliant low-level
     I/O implementation, and using `ftello' is preferable whenever it
     is available.

     If this function fails it returns `(off_t) -1'.  This can happen
     due to missing support for file positioning or internal errors.
     Otherwise the return value is the current file position.

     The function is an extension defined in the Unix Single
     Specification version 2.

     When the sources are compiled with `_FILE_OFFSET_BITS == 64' on a
     32 bit system this function is in fact `ftello64'.  I.e., the LFS
     interface transparently replaces the old interface.

 -- Function: off64_t ftello64 (FILE *STREAM)
     This function is similar to `ftello' with the only difference that
     the return value is of type `off64_t'.  This also requires that the
     stream STREAM was opened using either `fopen64', `freopen64', or
     `tmpfile64' since otherwise the underlying file operations to
     position the file pointer beyond the 2^31 bytes limit might fail.

     If the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32
     bits machine this function is available under the name `ftello'
     and so transparently replaces the old interface.

 -- Function: int fseek (FILE *STREAM, long int OFFSET, int WHENCE)
     The `fseek' function is used to change the file position of the
     stream STREAM.  The value of WHENCE must be one of the constants
     `SEEK_SET', `SEEK_CUR', or `SEEK_END', to indicate whether the
     OFFSET is relative to the beginning of the file, the current file
     position, or the end of the file, respectively.

     This function returns a value of zero if the operation was
     successful, and a nonzero value to indicate failure.  A successful
     call also clears the end-of-file indicator of STREAM and discards
     any characters that were "pushed back" by the use of `ungetc'.

     `fseek' either flushes any buffered output before setting the file
     position or else remembers it so it will be written later in its
     proper place in the file.

 -- Function: int fseeko (FILE *STREAM, off_t OFFSET, int WHENCE)
     This function is similar to `fseek' but it corrects a problem with
     `fseek' in a system with POSIX types.  Using a value of type `long
     int' for the offset is not compatible with POSIX.  `fseeko' uses
     the correct type `off_t' for the OFFSET parameter.

     For this reason it is a good idea to prefer `ftello' whenever it is
     available since its functionality is (if different at all) closer
     the underlying definition.

     The functionality and return value is the same as for `fseek'.

     The function is an extension defined in the Unix Single
     Specification version 2.

     When the sources are compiled with `_FILE_OFFSET_BITS == 64' on a
     32 bit system this function is in fact `fseeko64'.  I.e., the LFS
     interface transparently replaces the old interface.

 -- Function: int fseeko64 (FILE *STREAM, off64_t OFFSET, int WHENCE)
     This function is similar to `fseeko' with the only difference that
     the OFFSET parameter is of type `off64_t'.  This also requires
     that the stream STREAM was opened using either `fopen64',
     `freopen64', or `tmpfile64' since otherwise the underlying file
     operations to position the file pointer beyond the 2^31 bytes
     limit might fail.

     If the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32
     bits machine this function is available under the name `fseeko'
     and so transparently replaces the old interface.

   *Portability Note:* In non-POSIX systems, `ftell', `ftello', `fseek'
and `fseeko' might work reliably only on binary streams.  *Note Binary
Streams::.

   The following symbolic constants are defined for use as the WHENCE
argument to `fseek'.  They are also used with the `lseek' function
(*note I/O Primitives::) and to specify offsets for file locks (*note
Control Operations::).

 -- Macro: int SEEK_SET
     This is an integer constant which, when used as the WHENCE
     argument to the `fseek' or `fseeko' function, specifies that the
     offset provided is relative to the beginning of the file.

 -- Macro: int SEEK_CUR
     This is an integer constant which, when used as the WHENCE
     argument to the `fseek' or `fseeko' function, specifies that the
     offset provided is relative to the current file position.

 -- Macro: int SEEK_END
     This is an integer constant which, when used as the WHENCE
     argument to the `fseek' or `fseeko' function, specifies that the
     offset provided is relative to the end of the file.

 -- Function: void rewind (FILE *STREAM)
     The `rewind' function positions the stream STREAM at the beginning
     of the file.  It is equivalent to calling `fseek' or `fseeko' on
     the STREAM with an OFFSET argument of `0L' and a WHENCE argument
     of `SEEK_SET', except that the return value is discarded and the
     error indicator for the stream is reset.

   These three aliases for the `SEEK_...' constants exist for the sake
of compatibility with older BSD systems.  They are defined in two
different header files: `fcntl.h' and `sys/file.h'.

`L_SET'
     An alias for `SEEK_SET'.

`L_INCR'
     An alias for `SEEK_CUR'.

`L_XTND'
     An alias for `SEEK_END'.

File: libc.info,  Node: Portable Positioning,  Next: Stream Buffering,  Prev: File Positioning,  Up: I/O on Streams

12.19 Portable File-Position Functions
======================================

On the GNU system, the file position is truly a character count.  You
can specify any character count value as an argument to `fseek' or
`fseeko' and get reliable results for any random access file.  However,
some ISO C systems do not represent file positions in this way.

   On some systems where text streams truly differ from binary streams,
it is impossible to represent the file position of a text stream as a
count of characters from the beginning of the file.  For example, the
file position on some systems must encode both a record offset within
the file, and a character offset within the record.

   As a consequence, if you want your programs to be portable to these
systems, you must observe certain rules:

   * The value returned from `ftell' on a text stream has no predictable
     relationship to the number of characters you have read so far.
     The only thing you can rely on is that you can use it subsequently
     as the OFFSET argument to `fseek' or `fseeko' to move back to the
     same file position.

   * In a call to `fseek' or `fseeko' on a text stream, either the
     OFFSET must be zero, or WHENCE must be `SEEK_SET' and and the
     OFFSET must be the result of an earlier call to `ftell' on the
     same stream.

   * The value of the file position indicator of a text stream is
     undefined while there are characters that have been pushed back
     with `ungetc' that haven't been read or discarded.  *Note
     Unreading::.

   But even if you observe these rules, you may still have trouble for
long files, because `ftell' and `fseek' use a `long int' value to
represent the file position.  This type may not have room to encode all
the file positions in a large file.  Using the `ftello' and `fseeko'
functions might help here since the `off_t' type is expected to be able
to hold all file position values but this still does not help to handle
additional information which must be associated with a file position.

   So if you do want to support systems with peculiar encodings for the
file positions, it is better to use the functions `fgetpos' and
`fsetpos' instead.  These functions represent the file position using
the data type `fpos_t', whose internal representation varies from
system to system.

   These symbols are declared in the header file `stdio.h'.

 -- Data Type: fpos_t
     This is the type of an object that can encode information about the
     file position of a stream, for use by the functions `fgetpos' and
     `fsetpos'.

     In the GNU system, `fpos_t' is an opaque data structure that
     contains internal data to represent file offset and conversion
     state information.  In other systems, it might have a different
     internal representation.

     When compiling with `_FILE_OFFSET_BITS == 64' on a 32 bit machine
     this type is in fact equivalent to `fpos64_t' since the LFS
     interface transparently replaces the old interface.

 -- Data Type: fpos64_t
     This is the type of an object that can encode information about the
     file position of a stream, for use by the functions `fgetpos64' and
     `fsetpos64'.

     In the GNU system, `fpos64_t' is an opaque data structure that
     contains internal data to represent file offset and conversion
     state information.  In other systems, it might have a different
     internal representation.

 -- Function: int fgetpos (FILE *STREAM, fpos_t *POSITION)
     This function stores the value of the file position indicator for
     the stream STREAM in the `fpos_t' object pointed to by POSITION.
     If successful, `fgetpos' returns zero; otherwise it returns a
     nonzero value and stores an implementation-defined positive value
     in `errno'.

     When the sources are compiled with `_FILE_OFFSET_BITS == 64' on a
     32 bit system the function is in fact `fgetpos64'.  I.e., the LFS
     interface transparently replaces the old interface.

 -- Function: int fgetpos64 (FILE *STREAM, fpos64_t *POSITION)
     This function is similar to `fgetpos' but the file position is
     returned in a variable of type `fpos64_t' to which POSITION points.

     If the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32
     bits machine this function is available under the name `fgetpos'
     and so transparently replaces the old interface.

 -- Function: int fsetpos (FILE *STREAM, const fpos_t *POSITION)
     This function sets the file position indicator for the stream
     STREAM to the position POSITION, which must have been set by a
     previous call to `fgetpos' on the same stream.  If successful,
     `fsetpos' clears the end-of-file indicator on the stream, discards
     any characters that were "pushed back" by the use of `ungetc', and
     returns a value of zero.  Otherwise, `fsetpos' returns a nonzero
     value and stores an implementation-defined positive value in
     `errno'.

     When the sources are compiled with `_FILE_OFFSET_BITS == 64' on a
     32 bit system the function is in fact `fsetpos64'.  I.e., the LFS
     interface transparently replaces the old interface.

 -- Function: int fsetpos64 (FILE *STREAM, const fpos64_t *POSITION)
     This function is similar to `fsetpos' but the file position used
     for positioning is provided in a variable of type `fpos64_t' to
     which POSITION points.

     If the sources are compiled with `_FILE_OFFSET_BITS == 64' on a 32
     bits machine this function is available under the name `fsetpos'
     and so transparently replaces the old interface.

File: libc.info,  Node: Stream Buffering,  Next: Other Kinds of Streams,  Prev: Portable Positioning,  Up: I/O on Streams

12.20 Stream Buffering
======================

Characters that are written to a stream are normally accumulated and
transmitted asynchronously to the file in a block, instead of appearing
as soon as they are output by the application program.  Similarly,
streams often retrieve input from the host environment in blocks rather
than on a character-by-character basis.  This is called "buffering".

   If you are writing programs that do interactive input and output
using streams, you need to understand how buffering works when you
design the user interface to your program.  Otherwise, you might find
that output (such as progress or prompt messages) doesn't appear when
you intended it to, or displays some other unexpected behavior.

   This section deals only with controlling when characters are
transmitted between the stream and the file or device, and _not_ with
how things like echoing, flow control, and the like are handled on
specific classes of devices.  For information on common control
operations on terminal devices, see *note Low-Level Terminal
Interface::.

   You can bypass the stream buffering facilities altogether by using
the low-level input and output functions that operate on file
descriptors instead.  *Note Low-Level I/O::.

* Menu:

* Buffering Concepts::          Terminology is defined here.
* Flushing Buffers::            How to ensure that output buffers are flushed.
* Controlling Buffering::       How to specify what kind of buffering to use.

File: libc.info,  Node: Buffering Concepts,  Next: Flushing Buffers,  Up: Stream Buffering

12.20.1 Buffering Concepts
--------------------------

There are three different kinds of buffering strategies:

   * Characters written to or read from an "unbuffered" stream are
     transmitted individually to or from the file as soon as possible.

   * Characters written to a "line buffered" stream are transmitted to
     the file in blocks when a newline character is encountered.

   * Characters written to or read from a "fully buffered" stream are
     transmitted to or from the file in blocks of arbitrary size.

   Newly opened streams are normally fully buffered, with one
exception: a stream connected to an interactive device such as a
terminal is initially line buffered.  *Note Controlling Buffering::,
for information on how to select a different kind of buffering.
Usually the automatic selection gives you the most convenient kind of
buffering for the file or device you open.

   The use of line buffering for interactive devices implies that output
messages ending in a newline will appear immediately--which is usually
what you want.  Output that doesn't end in a newline might or might not
show up immediately, so if you want them to appear immediately, you
should flush buffered output explicitly with `fflush', as described in
*note Flushing Buffers::.

File: libc.info,  Node: Flushing Buffers,  Next: Controlling Buffering,  Prev: Buffering Concepts,  Up: Stream Buffering

12.20.2 Flushing Buffers
------------------------

"Flushing" output on a buffered stream means transmitting all
accumulated characters to the file.  There are many circumstances when
buffered output on a stream is flushed automatically:

   * When you try to do output and the output buffer is full.

   * When the stream is closed.  *Note Closing Streams::.

   * When the program terminates by calling `exit'.  *Note Normal
     Termination::.

   * When a newline is written, if the stream is line buffered.

   * Whenever an input operation on _any_ stream actually reads data
     from its file.

   If you want to flush the buffered output at another time, call
`fflush', which is declared in the header file `stdio.h'.

 -- Function: int fflush (FILE *STREAM)
     This function causes any buffered output on STREAM to be delivered
     to the file.  If STREAM is a null pointer, then `fflush' causes
     buffered output on _all_ open output streams to be flushed.

     This function returns `EOF' if a write error occurs, or zero
     otherwise.

 -- Function: int fflush_unlocked (FILE *STREAM)
     The `fflush_unlocked' function is equivalent to the `fflush'
     function except that it does not implicitly lock the stream.

   The `fflush' function can be used to flush all streams currently
opened.  While this is useful in some situations it does often more than
necessary since it might be done in situations when terminal input is
required and the program wants to be sure that all output is visible on
the terminal.  But this means that only line buffered streams have to be
flushed.  Solaris introduced a function especially for this.  It was
always available in the GNU C library in some form but never officially
exported.

 -- Function: void _flushlbf (void)
     The `_flushlbf' function flushes all line buffered streams
     currently opened.

     This function is declared in the `stdio_ext.h' header.

   *Compatibility Note:* Some brain-damaged operating systems have been
known to be so thoroughly fixated on line-oriented input and output
that flushing a line buffered stream causes a newline to be written!
Fortunately, this "feature" seems to be becoming less common.  You do
not need to worry about this in the GNU system.

   In some situations it might be useful to not flush the output pending
for a stream but instead simply forget it.  If transmission is costly
and the output is not needed anymore this is valid reasoning.  In this
situation a non-standard function introduced in Solaris and available in
the GNU C library can be used.

 -- Function: void __fpurge (FILE *STREAM)
     The `__fpurge' function causes the buffer of the stream STREAM to
     be emptied.  If the stream is currently in read mode all input in
     the buffer is lost.  If the stream is in output mode the buffered
     output is not written to the device (or whatever other underlying
     storage) and the buffer the cleared.

     This function is declared in `stdio_ext.h'.

File: libc.info,  Node: Controlling Buffering,  Prev: Flushing Buffers,  Up: Stream Buffering

12.20.3 Controlling Which Kind of Buffering
-------------------------------------------

After opening a stream (but before any other operations have been
performed on it), you can explicitly specify what kind of buffering you
want it to have using the `setvbuf' function.

   The facilities listed in this section are declared in the header
file `stdio.h'.

 -- Function: int setvbuf (FILE *STREAM, char *BUF, int MODE, size_t
          SIZE)
     This function is used to specify that the stream STREAM should
     have the buffering mode MODE, which can be either `_IOFBF' (for
     full buffering), `_IOLBF' (for line buffering), or `_IONBF' (for
     unbuffered input/output).

     If you specify a null pointer as the BUF argument, then `setvbuf'
     allocates a buffer itself using `malloc'.  This buffer will be
     freed when you close the stream.

     Otherwise, BUF should be a character array that can hold at least
     SIZE characters.  You should not free the space for this array as
     long as the stream remains open and this array remains its buffer.
     You should usually either allocate it statically, or `malloc'
     (*note Unconstrained Allocation::) the buffer.  Using an automatic
     array is not a good idea unless you close the file before exiting
     the block that declares the array.

     While the array remains a stream buffer, the stream I/O functions
     will use the buffer for their internal purposes.  You shouldn't
     try to access the values in the array directly while the stream is
     using it for buffering.

     The `setvbuf' function returns zero on success, or a nonzero value
     if the value of MODE is not valid or if the request could not be
     honored.

 -- Macro: int _IOFBF
     The value of this macro is an integer constant expression that can
     be used as the MODE argument to the `setvbuf' function to specify
     that the stream should be fully buffered.

 -- Macro: int _IOLBF
     The value of this macro is an integer constant expression that can
     be used as the MODE argument to the `setvbuf' function to specify
     that the stream should be line buffered.

 -- Macro: int _IONBF
     The value of this macro is an integer constant expression that can
     be used as the MODE argument to the `setvbuf' function to specify
     that the stream should be unbuffered.

 -- Macro: int BUFSIZ
     The value of this macro is an integer constant expression that is
     good to use for the SIZE argument to `setvbuf'.  This value is
     guaranteed to be at least `256'.

     The value of `BUFSIZ' is chosen on each system so as to make stream
     I/O efficient.  So it is a good idea to use `BUFSIZ' as the size
     for the buffer when you call `setvbuf'.

     Actually, you can get an even better value to use for the buffer
     size by means of the `fstat' system call: it is found in the
     `st_blksize' field of the file attributes.  *Note Attribute
     Meanings::.

     Sometimes people also use `BUFSIZ' as the allocation size of
     buffers used for related purposes, such as strings used to receive
     a line of input with `fgets' (*note Character Input::).  There is
     no particular reason to use `BUFSIZ' for this instead of any other
     integer, except that it might lead to doing I/O in chunks of an
     efficient size.

 -- Function: void setbuf (FILE *STREAM, char *BUF)
     If BUF is a null pointer, the effect of this function is
     equivalent to calling `setvbuf' with a MODE argument of `_IONBF'.
     Otherwise, it is equivalent to calling `setvbuf' with BUF, and a
     MODE of `_IOFBF' and a SIZE argument of `BUFSIZ'.

     The `setbuf' function is provided for compatibility with old code;
     use `setvbuf' in all new programs.

 -- Function: void setbuffer (FILE *STREAM, char *BUF, size_t SIZE)
     If BUF is a null pointer, this function makes STREAM unbuffered.
     Otherwise, it makes STREAM fully buffered using BUF as the buffer.
     The SIZE argument specifies the length of BUF.

     This function is provided for compatibility with old BSD code.  Use
     `setvbuf' instead.

 -- Function: void setlinebuf (FILE *STREAM)
     This function makes STREAM be line buffered, and allocates the
     buffer for you.

     This function is provided for compatibility with old BSD code.  Use
     `setvbuf' instead.

   It is possible to query whether a given stream is line buffered or
not using a non-standard function introduced in Solaris and available
in the GNU C library.

 -- Function: int __flbf (FILE *STREAM)
     The `__flbf' function will return a nonzero value in case the
     stream STREAM is line buffered.  Otherwise the return value is
     zero.

     This function is declared in the `stdio_ext.h' header.

   Two more extensions allow to determine the size of the buffer and how
much of it is used.  These functions were also introduced in Solaris.

 -- Function: size_t __fbufsize (FILE *STREAM)
     The `__fbufsize' function return the size of the buffer in the
     stream STREAM.  This value can be used to optimize the use of the
     stream.

     This function is declared in the `stdio_ext.h' header.

 -- Function: size_t __fpending (FILE *STREAM) The `__fpending'
     function returns the number of bytes currently in the output
     buffer.  For wide-oriented stream the measuring unit is wide
     characters.  This function should not be used on buffers in read
     mode or opened read-only.

     This function is declared in the `stdio_ext.h' header.

File: libc.info,  Node: Other Kinds of Streams,  Next: Formatted Messages,  Prev: Stream Buffering,  Up: I/O on Streams

12.21 Other Kinds of Streams
============================

The GNU library provides ways for you to define additional kinds of
streams that do not necessarily correspond to an open file.

   One such type of stream takes input from or writes output to a
string.  These kinds of streams are used internally to implement the
`sprintf' and `sscanf' functions.  You can also create such a stream
explicitly, using the functions described in *note String Streams::.

   More generally, you can define streams that do input/output to
arbitrary objects using functions supplied by your program.  This
protocol is discussed in *note Custom Streams::.

   *Portability Note:* The facilities described in this section are
specific to GNU.  Other systems or C implementations might or might not
provide equivalent functionality.

* Menu:

* String Streams::              Streams that get data from or put data in
                                 a string or memory buffer.
* Obstack Streams::		Streams that store data in an obstack.
* Custom Streams::              Defining your own streams with an arbitrary
                                 input data source and/or output data sink.

File: libc.info,  Node: String Streams,  Next: Obstack Streams,  Up: Other Kinds of Streams

12.21.1 String Streams
----------------------

The `fmemopen' and `open_memstream' functions allow you to do I/O to a
string or memory buffer.  These facilities are declared in `stdio.h'.

 -- Function: FILE * fmemopen (void *BUF, size_t SIZE, const char
          *OPENTYPE)
     This function opens a stream that allows the access specified by
     the OPENTYPE argument, that reads from or writes to the buffer
     specified by the argument BUF.  This array must be at least SIZE
     bytes long.

     If you specify a null pointer as the BUF argument, `fmemopen'
     dynamically allocates an array SIZE bytes long (as with `malloc';
     *note Unconstrained Allocation::).  This is really only useful if
     you are going to write things to the buffer and then read them back
     in again, because you have no way of actually getting a pointer to
     the buffer (for this, try `open_memstream', below).  The buffer is
     freed when the stream is closed.

     The argument OPENTYPE is the same as in `fopen' (*note Opening
     Streams::).  If the OPENTYPE specifies append mode, then the
     initial file position is set to the first null character in the
     buffer.  Otherwise the initial file position is at the beginning
     of the buffer.

     When a stream open for writing is flushed or closed, a null
     character (zero byte) is written at the end of the buffer if it
     fits.  You should add an extra byte to the SIZE argument to
     account for this.  Attempts to write more than SIZE bytes to the
     buffer result in an error.

     For a stream open for reading, null characters (zero bytes) in the
     buffer do not count as "end of file".  Read operations indicate
     end of file only when the file position advances past SIZE bytes.
     So, if you want to read characters from a null-terminated string,
     you should supply the length of the string as the SIZE argument.

   Here is an example of using `fmemopen' to create a stream for
reading from a string:

     #include <stdio.h>

     static char buffer[] = "foobar";

     int
     main (void)
     {
       int ch;
       FILE *stream;

       stream = fmemopen (buffer, strlen (buffer), "r");
       while ((ch = fgetc (stream)) != EOF)
         printf ("Got %c\n", ch);
       fclose (stream);

       return 0;
     }

   This program produces the following output:

     Got f
     Got o
     Got o
     Got b
     Got a
     Got r

 -- Function: FILE * open_memstream (char **PTR, size_t *SIZELOC)
     This function opens a stream for writing to a buffer.  The buffer
     is allocated dynamically and grown as necessary, using `malloc'.
     After you've closed the stream, this buffer is your responsibility
     to clean up using `free' or `realloc'.  *Note Unconstrained
     Allocation::.

     When the stream is closed with `fclose' or flushed with `fflush',
     the locations PTR and SIZELOC are updated to contain the pointer
     to the buffer and its size.  The values thus stored remain valid
     only as long as no further output on the stream takes place.  If
     you do more output, you must flush the stream again to store new
     values before you use them again.

     A null character is written at the end of the buffer.  This null
     character is _not_ included in the size value stored at SIZELOC.

     You can move the stream's file position with `fseek' or `fseeko'
     (*note File Positioning::).  Moving the file position past the end
     of the data already written fills the intervening space with
     zeroes.

   Here is an example of using `open_memstream':

     #include <stdio.h>

     int
     main (void)
     {
       char *bp;
       size_t size;
       FILE *stream;

       stream = open_memstream (&bp, &size);
       fprintf (stream, "hello");
       fflush (stream);
       printf ("buf = `%s', size = %d\n", bp, size);
       fprintf (stream, ", world");
       fclose (stream);
       printf ("buf = `%s', size = %d\n", bp, size);

       return 0;
     }

   This program produces the following output:

     buf = `hello', size = 5
     buf = `hello, world', size = 12

File: libc.info,  Node: Obstack Streams,  Next: Custom Streams,  Prev: String Streams,  Up: Other Kinds of Streams

12.21.2 Obstack Streams
-----------------------

You can open an output stream that puts it data in an obstack.  *Note
Obstacks::.

 -- Function: FILE * open_obstack_stream (struct obstack *OBSTACK)
     This function opens a stream for writing data into the obstack
     OBSTACK.  This starts an object in the obstack and makes it grow
     as data is written (*note Growing Objects::).

     Calling `fflush' on this stream updates the current size of the
     object to match the amount of data that has been written.  After a
     call to `fflush', you can examine the object temporarily.

     You can move the file position of an obstack stream with `fseek' or
     `fseeko' (*note File Positioning::).  Moving the file position past
     the end of the data written fills the intervening space with zeros.

     To make the object permanent, update the obstack with `fflush', and
     then use `obstack_finish' to finalize the object and get its
     address.  The following write to the stream starts a new object in
     the obstack, and later writes add to that object until you do
     another `fflush' and `obstack_finish'.

     But how do you find out how long the object is?  You can get the
     length in bytes by calling `obstack_object_size' (*note Status of
     an Obstack::), or you can null-terminate the object like this:

          obstack_1grow (OBSTACK, 0);

     Whichever one you do, you must do it _before_ calling
     `obstack_finish'.  (You can do both if you wish.)

   Here is a sample function that uses `open_obstack_stream':

     char *
     make_message_string (const char *a, int b)
     {
       FILE *stream = open_obstack_stream (&message_obstack);
       output_task (stream);
       fprintf (stream, ": ");
       fprintf (stream, a, b);
       fprintf (stream, "\n");
       fclose (stream);
       obstack_1grow (&message_obstack, 0);
       return obstack_finish (&message_obstack);
     }

File: libc.info,  Node: Custom Streams,  Prev: Obstack Streams,  Up: Other Kinds of Streams

12.21.3 Programming Your Own Custom Streams
-------------------------------------------

This section describes how you can make a stream that gets input from an
arbitrary data source or writes output to an arbitrary data sink
programmed by you.  We call these "custom streams".  The functions and
types described here are all GNU extensions.

* Menu:

* Streams and Cookies::         The "cookie" records where to fetch or
                                 store data that is read or written.
* Hook Functions::              How you should define the four "hook
                                 functions" that a custom stream needs.

File: libc.info,  Node: Streams and Cookies,  Next: Hook Functions,  Up: Custom Streams

12.21.3.1 Custom Streams and Cookies
....................................

Inside every custom stream is a special object called the "cookie".
This is an object supplied by you which records where to fetch or store
the data read or written.  It is up to you to define a data type to use
for the cookie.  The stream functions in the library never refer
directly to its contents, and they don't even know what the type is;
they record its address with type `void *'.

   To implement a custom stream, you must specify _how_ to fetch or
store the data in the specified place.  You do this by defining "hook
functions" to read, write, change "file position", and close the
stream.  All four of these functions will be passed the stream's cookie
so they can tell where to fetch or store the data.  The library
functions don't know what's inside the cookie, but your functions will
know.

   When you create a custom stream, you must specify the cookie pointer,
and also the four hook functions stored in a structure of type
`cookie_io_functions_t'.

   These facilities are declared in `stdio.h'.

 -- Data Type: cookie_io_functions_t
     This is a structure type that holds the functions that define the
     communications protocol between the stream and its cookie.  It has
     the following members:

    `cookie_read_function_t *read'
          This is the function that reads data from the cookie.  If the
          value is a null pointer instead of a function, then read
          operations on this stream always return `EOF'.

    `cookie_write_function_t *write'
          This is the function that writes data to the cookie.  If the
          value is a null pointer instead of a function, then data
          written to the stream is discarded.

    `cookie_seek_function_t *seek'
          This is the function that performs the equivalent of file
          positioning on the cookie.  If the value is a null pointer
          instead of a function, calls to `fseek' or `fseeko' on this
          stream can only seek to locations within the buffer; any
          attempt to seek outside the buffer will return an `ESPIPE'
          error.

    `cookie_close_function_t *close'
          This function performs any appropriate cleanup on the cookie
          when closing the stream.  If the value is a null pointer
          instead of a function, nothing special is done to close the
          cookie when the stream is closed.

 -- Function: FILE * fopencookie (void *COOKIE, const char *OPENTYPE,
          cookie_io_functions_t IO-FUNCTIONS)
     This function actually creates the stream for communicating with
     the COOKIE using the functions in the IO-FUNCTIONS argument.  The
     OPENTYPE argument is interpreted as for `fopen'; see *note Opening
     Streams::.  (But note that the "truncate on open" option is
     ignored.)  The new stream is fully buffered.

     The `fopencookie' function returns the newly created stream, or a
     null pointer in case of an error.

File: libc.info,  Node: Hook Functions,  Prev: Streams and Cookies,  Up: Custom Streams

12.21.3.2 Custom Stream Hook Functions
......................................

Here are more details on how you should define the four hook functions
that a custom stream needs.

   You should define the function to read data from the cookie as:

     ssize_t READER (void *COOKIE, char *BUFFER, size_t SIZE)

   This is very similar to the `read' function; see *note I/O
Primitives::.  Your function should transfer up to SIZE bytes into the
BUFFER, and return the number of bytes read, or zero to indicate
end-of-file.  You can return a value of `-1' to indicate an error.

   You should define the function to write data to the cookie as:

     ssize_t WRITER (void *COOKIE, const char *BUFFER, size_t SIZE)

   This is very similar to the `write' function; see *note I/O
Primitives::.  Your function should transfer up to SIZE bytes from the
buffer, and return the number of bytes written.  You can return a value
of `-1' to indicate an error.

   You should define the function to perform seek operations on the
cookie as:

     int SEEKER (void *COOKIE, off64_t *POSITION, int WHENCE)

   For this function, the POSITION and WHENCE arguments are interpreted
as for `fgetpos'; see *note Portable Positioning::.

   After doing the seek operation, your function should store the
resulting file position relative to the beginning of the file in
POSITION.  Your function should return a value of `0' on success and
`-1' to indicate an error.

   You should define the function to do cleanup operations on the cookie
appropriate for closing the stream as:

     int CLEANER (void *COOKIE)

   Your function should return `-1' to indicate an error, and `0'
otherwise.

 -- Data Type: cookie_read_function
     This is the data type that the read function for a custom stream
     should have.  If you declare the function as shown above, this is
     the type it will have.

 -- Data Type: cookie_write_function
     The data type of the write function for a custom stream.

 -- Data Type: cookie_seek_function
     The data type of the seek function for a custom stream.

 -- Data Type: cookie_close_function
     The data type of the close function for a custom stream.

File: libc.info,  Node: Formatted Messages,  Prev: Other Kinds of Streams,  Up: I/O on Streams

12.22 Formatted Messages
========================

On systems which are based on System V messages of programs (especially
the system tools) are printed in a strict form using the `fmtmsg'
function.  The uniformity sometimes helps the user to interpret messages
and the strictness tests of the `fmtmsg' function ensure that the
programmer follows some minimal requirements.

* Menu:

* Printing Formatted Messages::   The `fmtmsg' function.
* Adding Severity Classes::       Add more severity classes.
* Example::                       How to use `fmtmsg' and `addseverity'.

File: libc.info,  Node: Printing Formatted Messages,  Next: Adding Severity Classes,  Up: Formatted Messages

12.22.1 Printing Formatted Messages
-----------------------------------

Messages can be printed to standard error and/or to the console.  To
select the destination the programmer can use the following two values,
bitwise OR combined if wanted, for the CLASSIFICATION parameter of
`fmtmsg':

`MM_PRINT'
     Display the message in standard error.

`MM_CONSOLE'
     Display the message on the system console.

   The erroneous piece of the system can be signalled by exactly one of
the following values which also is bitwise ORed with the CLASSIFICATION
parameter to `fmtmsg':

`MM_HARD'
     The source of the condition is some hardware.

`MM_SOFT'
     The source of the condition is some software.

`MM_FIRM'
     The source of the condition is some firmware.

   A third component of the CLASSIFICATION parameter to `fmtmsg' can
describe the part of the system which detects the problem.  This is
done by using exactly one of the following values:

`MM_APPL'
     The erroneous condition is detected by the application.

`MM_UTIL'
     The erroneous condition is detected by a utility.

`MM_OPSYS'
     The erroneous condition is detected by the operating system.

   A last component of CLASSIFICATION can signal the results of this
message.  Exactly one of the following values can be used:

`MM_RECOVER'
     It is a recoverable error.

`MM_NRECOV'
     It is a non-recoverable error.

 -- Function: int fmtmsg (long int CLASSIFICATION, const char *LABEL,
          int SEVERITY, const char *TEXT, const char *ACTION, const
          char *TAG)
     Display a message described by its parameters on the device(s)
     specified in the CLASSIFICATION parameter.  The LABEL parameter
     identifies the source of the message.  The string should consist
     of two colon separated parts where the first part has not more
     than 10 and the second part not more than 14 characters.  The TEXT
     parameter describes the condition of the error, the ACTION
     parameter possible steps to recover from the error and the TAG
     parameter is a reference to the online documentation where more
     information can be found.  It should contain the LABEL value and a
     unique identification number.

     Each of the parameters can be a special value which means this
     value is to be omitted.  The symbolic names for these values are:

    `MM_NULLLBL'
          Ignore LABEL parameter.

    `MM_NULLSEV'
          Ignore SEVERITY parameter.

    `MM_NULLMC'
          Ignore CLASSIFICATION parameter.  This implies that nothing is
          actually printed.

    `MM_NULLTXT'
          Ignore TEXT parameter.

    `MM_NULLACT'
          Ignore ACTION parameter.

    `MM_NULLTAG'
          Ignore TAG parameter.

     There is another way certain fields can be omitted from the output
     to standard error.  This is described below in the description of
     environment variables influencing the behavior.

     The SEVERITY parameter can have one of the values in the following
     table:

    `MM_NOSEV'
          Nothing is printed, this value is the same as `MM_NULLSEV'.

    `MM_HALT'
          This value is printed as `HALT'.

    `MM_ERROR'
          This value is printed as `ERROR'.

    `MM_WARNING'
          This value is printed as `WARNING'.

    `MM_INFO'
          This value is printed as `INFO'.

     The numeric value of these five macros are between `0' and `4'.
     Using the environment variable `SEV_LEVEL' or using the
     `addseverity' function one can add more severity levels with their
     corresponding string to print.  This is described below (*note
     Adding Severity Classes::).

     If no parameter is ignored the output looks like this:

          LABEL: SEVERITY-STRING: TEXT
          TO FIX: ACTION TAG

     The colons, new line characters and the `TO FIX' string are
     inserted if necessary, i.e., if the corresponding parameter is not
     ignored.

     This function is specified in the X/Open Portability Guide.  It is
     also available on all systems derived from System V.

     The function returns the value `MM_OK' if no error occurred.  If
     only the printing to standard error failed, it returns `MM_NOMSG'.
     If printing to the console fails, it returns `MM_NOCON'.  If
     nothing is printed `MM_NOTOK' is returned.  Among situations where
     all outputs fail this last value is also returned if a parameter
     value is incorrect.

   There are two environment variables which influence the behavior of
`fmtmsg'.  The first is `MSGVERB'.  It is used to control the output
actually happening on standard error (_not_ the console output).  Each
of the five fields can explicitly be enabled.  To do this the user has
to put the `MSGVERB' variable with a format like the following in the
environment before calling the `fmtmsg' function the first time:

     MSGVERB=KEYWORD[:KEYWORD[:...]]

   Valid KEYWORDs are `label', `severity', `text', `action', and `tag'.
If the environment variable is not given or is the empty string, a not
supported keyword is given or the value is somehow else invalid, no
part of the message is masked out.

   The second environment variable which influences the behavior of
`fmtmsg' is `SEV_LEVEL'.  This variable and the change in the behavior
of `fmtmsg' is not specified in the X/Open Portability Guide.  It is
available in System V systems, though.  It can be used to introduce new
severity levels.  By default, only the five severity levels described
above are available.  Any other numeric value would make `fmtmsg' print
nothing.

   If the user puts `SEV_LEVEL' with a format like

     SEV_LEVEL=[DESCRIPTION[:DESCRIPTION[:...]]]

in the environment of the process before the first call to `fmtmsg',
where DESCRIPTION has a value of the form

     SEVERITY-KEYWORD,LEVEL,PRINTSTRING

   The SEVERITY-KEYWORD part is not used by `fmtmsg' but it has to be
present.  The LEVEL part is a string representation of a number.  The
numeric value must be a number greater than 4.  This value must be used
in the SEVERITY parameter of `fmtmsg' to select this class.  It is not
possible to overwrite any of the predefined classes.  The PRINTSTRING
is the string printed when a message of this class is processed by
`fmtmsg' (see above, `fmtsmg' does not print the numeric value but
instead the string representation).

File: libc.info,  Node: Adding Severity Classes,  Next: Example,  Prev: Printing Formatted Messages,  Up: Formatted Messages

12.22.2 Adding Severity Classes
-------------------------------

There is another possibility to introduce severity classes besides using
the environment variable `SEV_LEVEL'.  This simplifies the task of
introducing new classes in a running program.  One could use the
`setenv' or `putenv' function to set the environment variable, but this
is toilsome.

 -- Function: int addseverity (int SEVERITY, const char *STRING)
     This function allows the introduction of new severity classes
     which can be addressed by the SEVERITY parameter of the `fmtmsg'
     function.  The SEVERITY parameter of `addseverity' must match the
     value for the parameter with the same name of `fmtmsg', and STRING
     is the string printed in the actual messages instead of the numeric
     value.

     If STRING is `NULL' the severity class with the numeric value
     according to SEVERITY is removed.

     It is not possible to overwrite or remove one of the default
     severity classes.  All calls to `addseverity' with SEVERITY set to
     one of the values for the default classes will fail.

     The return value is `MM_OK' if the task was successfully performed.
     If the return value is `MM_NOTOK' something went wrong.  This could
     mean that no more memory is available or a class is not available
     when it has to be removed.

     This function is not specified in the X/Open Portability Guide
     although the `fmtsmg' function is.  It is available on System V
     systems.

File: libc.info,  Node: Example,  Prev: Adding Severity Classes,  Up: Formatted Messages

12.22.3 How to use `fmtmsg' and `addseverity'
---------------------------------------------

Here is a simple example program to illustrate the use of the both
functions described in this section.

     #include <fmtmsg.h>

     int
     main (void)
     {
       addseverity (5, "NOTE2");
       fmtmsg (MM_PRINT, "only1field", MM_INFO, "text2", "action2", "tag2");
       fmtmsg (MM_PRINT, "UX:cat", 5, "invalid syntax", "refer to manual",
               "UX:cat:001");
       fmtmsg (MM_PRINT, "label:foo", 6, "text", "action", "tag");
       return 0;
     }

   The second call to `fmtmsg' illustrates a use of this function as it
usually occurs on System V systems, which heavily use this function.
It seems worthwhile to give a short explanation here of how this system
works on System V.  The value of the LABEL field (`UX:cat') says that
the error occurred in the Unix program `cat'.  The explanation of the
error follows and the value for the ACTION parameter is `"refer to
manual"'.  One could be more specific here, if necessary.  The TAG
field contains, as proposed above, the value of the string given for
the LABEL parameter, and additionally a unique ID (`001' in this case).
For a GNU environment this string could contain a reference to the
corresponding node in the Info page for the program.

Running this program without specifying the `MSGVERB' and `SEV_LEVEL'
function produces the following output:

     UX:cat: NOTE2: invalid syntax
     TO FIX: refer to manual UX:cat:001

   We see the different fields of the message and how the extra glue
(the colons and the `TO FIX' string) are printed.  But only one of the
three calls to `fmtmsg' produced output.  The first call does not print
anything because the LABEL parameter is not in the correct form.  The
string must contain two fields, separated by a colon (*note Printing
Formatted Messages::).  The third `fmtmsg' call produced no output
since the class with the numeric value `6' is not defined.  Although a
class with numeric value `5' is also not defined by default, the call
to `addseverity' introduces it and the second call to `fmtmsg' produces
the above output.

   When we change the environment of the program to contain
`SEV_LEVEL=XXX,6,NOTE' when running it we get a different result:

     UX:cat: NOTE2: invalid syntax
     TO FIX: refer to manual UX:cat:001
     label:foo: NOTE: text
     TO FIX: action tag

   Now the third call to `fmtmsg' produced some output and we see how
the string `NOTE' from the environment variable appears in the message.

   Now we can reduce the output by specifying which fields we are
interested in.  If we additionally set the environment variable
`MSGVERB' to the value `severity:label:action' we get the following
output:

     UX:cat: NOTE2
     TO FIX: refer to manual
     label:foo: NOTE
     TO FIX: action

I.e., the output produced by the TEXT and the TAG parameters to
`fmtmsg' vanished.  Please also note that now there is no colon after
the `NOTE' and `NOTE2' strings in the output.  This is not necessary
since there is no more output on this line because the text is missing.

File: libc.info,  Node: Low-Level I/O,  Next: File System Interface,  Prev: I/O on Streams,  Up: Top

13 Low-Level Input/Output
*************************

This chapter describes functions for performing low-level input/output
operations on file descriptors.  These functions include the primitives
for the higher-level I/O functions described in *note I/O on Streams::,
as well as functions for performing low-level control operations for
which there are no equivalents on streams.

   Stream-level I/O is more flexible and usually more convenient;
therefore, programmers generally use the descriptor-level functions only
when necessary.  These are some of the usual reasons:

   * For reading binary files in large chunks.

   * For reading an entire file into core before parsing it.

   * To perform operations other than data transfer, which can only be
     done with a descriptor.  (You can use `fileno' to get the
     descriptor corresponding to a stream.)

   * To pass descriptors to a child process.  (The child can create its
     own stream to use a descriptor that it inherits, but cannot
     inherit a stream directly.)

* Menu:

* Opening and Closing Files::           How to open and close file
                                         descriptors.
* I/O Primitives::                      Reading and writing data.
* File Position Primitive::             Setting a descriptor's file
                                         position.
* Descriptors and Streams::             Converting descriptor to stream
                                         or vice-versa.
* Stream/Descriptor Precautions::       Precautions needed if you use both
                                         descriptors and streams.
* Scatter-Gather::                      Fast I/O to discontinuous buffers.
* Memory-mapped I/O::                   Using files like memory.
* Waiting for I/O::                     How to check for input or output
					 on multiple file descriptors.
* Synchronizing I/O::                   Making sure all I/O actions completed.
* Asynchronous I/O::                    Perform I/O in parallel.
* Control Operations::                  Various other operations on file
					 descriptors.
* Duplicating Descriptors::             Fcntl commands for duplicating
                                         file descriptors.
* Descriptor Flags::                    Fcntl commands for manipulating
                                         flags associated with file
                                         descriptors.
* File Status Flags::                   Fcntl commands for manipulating
                                         flags associated with open files.
* File Locks::                          Fcntl commands for implementing
                                         file locking.
* Interrupt Input::                     Getting an asynchronous signal when
                                         input arrives.
* IOCTLs::                              Generic I/O Control operations.

File: libc.info,  Node: Opening and Closing Files,  Next: I/O Primitives,  Up: Low-Level I/O

13.1 Opening and Closing Files
==============================

This section describes the primitives for opening and closing files
using file descriptors.  The `open' and `creat' functions are declared
in the header file `fcntl.h', while `close' is declared in `unistd.h'.

 -- Function: int open (const char *FILENAME, int FLAGS[, mode_t MODE])
     The `open' function creates and returns a new file descriptor for
     the file named by FILENAME.  Initially, the file position
     indicator for the file is at the beginning of the file.  The
     argument MODE is used only when a file is created, but it doesn't
     hurt to supply the argument in any case.

     The FLAGS argument controls how the file is to be opened.  This is
     a bit mask; you create the value by the bitwise OR of the
     appropriate parameters (using the `|' operator in C).  *Note File
     Status Flags::, for the parameters available.

     The normal return value from `open' is a non-negative integer file
     descriptor.  In the case of an error, a value of -1 is returned
     instead.  In addition to the usual file name errors (*note File
     Name Errors::), the following `errno' error conditions are defined
     for this function:

    `EACCES'
          The file exists but is not readable/writable as requested by
          the FLAGS argument, the file does not exist and the directory
          is unwritable so it cannot be created.

    `EEXIST'
          Both `O_CREAT' and `O_EXCL' are set, and the named file
          already exists.

    `EINTR'
          The `open' operation was interrupted by a signal.  *Note
          Interrupted Primitives::.

    `EISDIR'
          The FLAGS argument specified write access, and the file is a
          directory.

    `EMFILE'
          The process has too many files open.  The maximum number of
          file descriptors is controlled by the `RLIMIT_NOFILE'
          resource limit; *note Limits on Resources::.

    `ENFILE'
          The entire system, or perhaps the file system which contains
          the directory, cannot support any additional open files at
          the moment.  (This problem cannot happen on the GNU system.)

    `ENOENT'
          The named file does not exist, and `O_CREAT' is not specified.

    `ENOSPC'
          The directory or file system that would contain the new file
          cannot be extended, because there is no disk space left.

    `ENXIO'
          `O_NONBLOCK' and `O_WRONLY' are both set in the FLAGS
          argument, the file named by FILENAME is a FIFO (*note Pipes
          and FIFOs::), and no process has the file open for reading.

    `EROFS'
          The file resides on a read-only file system and any of
          `O_WRONLY', `O_RDWR', and `O_TRUNC' are set in the FLAGS
          argument, or `O_CREAT' is set and the file does not already
          exist.

     If on a 32 bit machine the sources are translated with
     `_FILE_OFFSET_BITS == 64' the function `open' returns a file
     descriptor opened in the large file mode which enables the file
     handling functions to use files up to 2^63 bytes in size and
     offset from -2^63 to 2^63.  This happens transparently for the user
     since all of the lowlevel file handling functions are equally
     replaced.

     This function is a cancellation point in multi-threaded programs.
     This is a problem if the thread allocates some resources (like
     memory, file descriptors, semaphores or whatever) at the time
     `open' is called.  If the thread gets canceled these resources
     stay allocated until the program ends.  To avoid this calls to
     `open' should be protected using cancellation handlers.

     The `open' function is the underlying primitive for the `fopen'
     and `freopen' functions, that create streams.

 -- Function: int open64 (const char *FILENAME, int FLAGS[, mode_t
          MODE])
     This function is similar to `open'.  It returns a file descriptor
     which can be used to access the file named by FILENAME.  The only
     difference is that on 32 bit systems the file is opened in the
     large file mode.  I.e., file length and file offsets can exceed 31
     bits.

     When the sources are translated with `_FILE_OFFSET_BITS == 64' this
     function is actually available under the name `open'.  I.e., the
     new, extended API using 64 bit file sizes and offsets transparently
     replaces the old API.

 -- Obsolete function: int creat (const char *FILENAME, mode_t MODE)
     This function is obsolete.  The call:

          creat (FILENAME, MODE)

     is equivalent to:

          open (FILENAME, O_WRONLY | O_CREAT | O_TRUNC, MODE)

     If on a 32 bit machine the sources are translated with
     `_FILE_OFFSET_BITS == 64' the function `creat' returns a file
     descriptor opened in the large file mode which enables the file
     handling functions to use files up to 2^63 in size and offset from
     -2^63 to 2^63.  This happens transparently for the user since all
     of the lowlevel file handling functions are equally replaced.

 -- Obsolete function: int creat64 (const char *FILENAME, mode_t MODE)
     This function is similar to `creat'.  It returns a file descriptor
     which can be used to access the file named by FILENAME.  The only
     the difference is that on 32 bit systems the file is opened in the
     large file mode.  I.e., file length and file offsets can exceed 31
     bits.

     To use this file descriptor one must not use the normal operations
     but instead the counterparts named `*64', e.g., `read64'.

     When the sources are translated with `_FILE_OFFSET_BITS == 64' this
     function is actually available under the name `open'.  I.e., the
     new, extended API using 64 bit file sizes and offsets transparently
     replaces the old API.

 -- Function: int close (int FILEDES)
     The function `close' closes the file descriptor FILEDES.  Closing
     a file has the following consequences:

        * The file descriptor is deallocated.

        * Any record locks owned by the process on the file are
          unlocked.

        * When all file descriptors associated with a pipe or FIFO have
          been closed, any unread data is discarded.

     This function is a cancellation point in multi-threaded programs.
     This is a problem if the thread allocates some resources (like
     memory, file descriptors, semaphores or whatever) at the time
     `close' is called.  If the thread gets canceled these resources
     stay allocated until the program ends.  To avoid this, calls to
     `close' should be protected using cancellation handlers.

     The normal return value from `close' is 0; a value of -1 is
     returned in case of failure.  The following `errno' error
     conditions are defined for this function:

    `EBADF'
          The FILEDES argument is not a valid file descriptor.

    `EINTR'
          The `close' call was interrupted by a signal.  *Note
          Interrupted Primitives::.  Here is an example of how to
          handle `EINTR' properly:

               TEMP_FAILURE_RETRY (close (desc));

    `ENOSPC'
    `EIO'
    `EDQUOT'
          When the file is accessed by NFS, these errors from `write'
          can sometimes not be detected until `close'.  *Note I/O
          Primitives::, for details on their meaning.

     Please note that there is _no_ separate `close64' function.  This
     is not necessary since this function does not determine nor depend
     on the mode of the file.  The kernel which performs the `close'
     operation knows which mode the descriptor is used for and can
     handle this situation.

   To close a stream, call `fclose' (*note Closing Streams::) instead
of trying to close its underlying file descriptor with `close'.  This
flushes any buffered output and updates the stream object to indicate
that it is closed.

File: libc.info,  Node: I/O Primitives,  Next: File Position Primitive,  Prev: Opening and Closing Files,  Up: Low-Level I/O

13.2 Input and Output Primitives
================================

This section describes the functions for performing primitive input and
output operations on file descriptors: `read', `write', and `lseek'.
These functions are declared in the header file `unistd.h'.

 -- Data Type: ssize_t
     This data type is used to represent the sizes of blocks that can be
     read or written in a single operation.  It is similar to `size_t',
     but must be a signed type.

 -- Function: ssize_t read (int FILEDES, void *BUFFER, size_t SIZE)
     The `read' function reads up to SIZE bytes from the file with
     descriptor FILEDES, storing the results in the BUFFER.  (This is
     not necessarily a character string, and no terminating null
     character is added.)

     The return value is the number of bytes actually read.  This might
     be less than SIZE; for example, if there aren't that many bytes
     left in the file or if there aren't that many bytes immediately
     available.  The exact behavior depends on what kind of file it is.
     Note that reading less than SIZE bytes is not an error.

     A value of zero indicates end-of-file (except if the value of the
     SIZE argument is also zero).  This is not considered an error.  If
     you keep calling `read' while at end-of-file, it will keep
     returning zero and doing nothing else.

     If `read' returns at least one character, there is no way you can
     tell whether end-of-file was reached.  But if you did reach the
     end, the next read will return zero.

     In case of an error, `read' returns -1.  The following `errno'
     error conditions are defined for this function:

    `EAGAIN'
          Normally, when no input is immediately available, `read'
          waits for some input.  But if the `O_NONBLOCK' flag is set
          for the file (*note File Status Flags::), `read' returns
          immediately without reading any data, and reports this error.

          *Compatibility Note:* Most versions of BSD Unix use a
          different error code for this: `EWOULDBLOCK'.  In the GNU
          library, `EWOULDBLOCK' is an alias for `EAGAIN', so it
          doesn't matter which name you use.

          On some systems, reading a large amount of data from a
          character special file can also fail with `EAGAIN' if the
          kernel cannot find enough physical memory to lock down the
          user's pages.  This is limited to devices that transfer with
          direct memory access into the user's memory, which means it
          does not include terminals, since they always use separate
          buffers inside the kernel.  This problem never happens in the
          GNU system.

          Any condition that could result in `EAGAIN' can instead
          result in a successful `read' which returns fewer bytes than
          requested.  Calling `read' again immediately would result in
          `EAGAIN'.

    `EBADF'
          The FILEDES argument is not a valid file descriptor, or is
          not open for reading.

    `EINTR'
          `read' was interrupted by a signal while it was waiting for
          input.  *Note Interrupted Primitives::.  A signal will not
          necessary cause `read' to return `EINTR'; it may instead
          result in a successful `read' which returns fewer bytes than
          requested.

    `EIO'
          For many devices, and for disk files, this error code
          indicates a hardware error.

          `EIO' also occurs when a background process tries to read
          from the controlling terminal, and the normal action of
          stopping the process by sending it a `SIGTTIN' signal isn't
          working.  This might happen if the signal is being blocked or
          ignored, or because the process group is orphaned.  *Note Job
          Control::, for more information about job control, and *note
          Signal Handling::, for information about signals.

    `EINVAL'
          In some systems, when reading from a character or block
          device, position and size offsets must be aligned to a
          particular block size.  This error indicates that the offsets
          were not properly aligned.

     Please note that there is no function named `read64'.  This is not
     necessary since this function does not directly modify or handle
     the possibly wide file offset.  Since the kernel handles this state
     internally, the `read' function can be used for all cases.

     This function is a cancellation point in multi-threaded programs.
     This is a problem if the thread allocates some resources (like
     memory, file descriptors, semaphores or whatever) at the time
     `read' is called.  If the thread gets canceled these resources
     stay allocated until the program ends.  To avoid this, calls to
     `read' should be protected using cancellation handlers.

     The `read' function is the underlying primitive for all of the
     functions that read from streams, such as `fgetc'.

 -- Function: ssize_t pread (int FILEDES, void *BUFFER, size_t SIZE,
          off_t OFFSET)
     The `pread' function is similar to the `read' function.  The first
     three arguments are identical, and the return values and error
     codes also correspond.

     The difference is the fourth argument and its handling.  The data
     block is not read from the current position of the file descriptor
     `filedes'.  Instead the data is read from the file starting at
     position OFFSET.  The position of the file descriptor itself is
     not affected by the operation.  The value is the same as before
     the call.

     When the source file is compiled with `_FILE_OFFSET_BITS == 64' the
     `pread' function is in fact `pread64' and the type `off_t' has 64
     bits, which makes it possible to handle files up to 2^63 bytes in
     length.

     The return value of `pread' describes the number of bytes read.
     In the error case it returns -1 like `read' does and the error
     codes are also the same, with these additions:

    `EINVAL'
          The value given for OFFSET is negative and therefore illegal.

    `ESPIPE'
          The file descriptor FILEDES is associate with a pipe or a
          FIFO and this device does not allow positioning of the file
          pointer.

     The function is an extension defined in the Unix Single
     Specification version 2.

 -- Function: ssize_t pread64 (int FILEDES, void *BUFFER, size_t SIZE,
          off64_t OFFSET)
     This function is similar to the `pread' function.  The difference
     is that the OFFSET parameter is of type `off64_t' instead of
     `off_t' which makes it possible on 32 bit machines to address
     files larger than 2^31 bytes and up to 2^63 bytes.  The file
     descriptor `filedes' must be opened using `open64' since otherwise
     the large offsets possible with `off64_t' will lead to errors with
     a descriptor in small file mode.

     When the source file is compiled with `_FILE_OFFSET_BITS == 64' on
     a 32 bit machine this function is actually available under the name
     `pread' and so transparently replaces the 32 bit interface.

 -- Function: ssize_t write (int FILEDES, const void *BUFFER, size_t
          SIZE)
     The `write' function writes up to SIZE bytes from BUFFER to the
     file with descriptor FILEDES.  The data in BUFFER is not
     necessarily a character string and a null character is output like
     any other character.

     The return value is the number of bytes actually written.  This
     may be SIZE, but can always be smaller.  Your program should
     always call `write' in a loop, iterating until all the data is
     written.

     Once `write' returns, the data is enqueued to be written and can be
     read back right away, but it is not necessarily written out to
     permanent storage immediately.  You can use `fsync' when you need
     to be sure your data has been permanently stored before
     continuing.  (It is more efficient for the system to batch up
     consecutive writes and do them all at once when convenient.
     Normally they will always be written to disk within a minute or
     less.)  Modern systems provide another function `fdatasync' which
     guarantees integrity only for the file data and is therefore
     faster.  You can use the `O_FSYNC' open mode to make `write' always
     store the data to disk before returning; *note Operating Modes::.

     In the case of an error, `write' returns -1.  The following
     `errno' error conditions are defined for this function:

    `EAGAIN'
          Normally, `write' blocks until the write operation is
          complete.  But if the `O_NONBLOCK' flag is set for the file
          (*note Control Operations::), it returns immediately without
          writing any data and reports this error.  An example of a
          situation that might cause the process to block on output is
          writing to a terminal device that supports flow control,
          where output has been suspended by receipt of a STOP
          character.

          *Compatibility Note:* Most versions of BSD Unix use a
          different error code for this: `EWOULDBLOCK'.  In the GNU
          library, `EWOULDBLOCK' is an alias for `EAGAIN', so it
          doesn't matter which name you use.

          On some systems, writing a large amount of data from a
          character special file can also fail with `EAGAIN' if the
          kernel cannot find enough physical memory to lock down the
          user's pages.  This is limited to devices that transfer with
          direct memory access into the user's memory, which means it
          does not include terminals, since they always use separate
          buffers inside the kernel.  This problem does not arise in the
          GNU system.

    `EBADF'
          The FILEDES argument is not a valid file descriptor, or is
          not open for writing.

    `EFBIG'
          The size of the file would become larger than the
          implementation can support.

    `EINTR'
          The `write' operation was interrupted by a signal while it was
          blocked waiting for completion.  A signal will not
          necessarily cause `write' to return `EINTR'; it may instead
          result in a successful `write' which writes fewer bytes than
          requested.  *Note Interrupted Primitives::.

    `EIO'
          For many devices, and for disk files, this error code
          indicates a hardware error.

    `ENOSPC'
          The device containing the file is full.

    `EPIPE'
          This error is returned when you try to write to a pipe or
          FIFO that isn't open for reading by any process.  When this
          happens, a `SIGPIPE' signal is also sent to the process; see
          *note Signal Handling::.

    `EINVAL'
          In some systems, when writing to a character or block device,
          position and size offsets must be aligned to a particular
          block size.  This error indicates that the offsets were not
          properly aligned.

     Unless you have arranged to prevent `EINTR' failures, you should
     check `errno' after each failing call to `write', and if the error
     was `EINTR', you should simply repeat the call.  *Note Interrupted
     Primitives::.  The easy way to do this is with the macro
     `TEMP_FAILURE_RETRY', as follows:

          nbytes = TEMP_FAILURE_RETRY (write (desc, buffer, count));

     Please note that there is no function named `write64'.  This is not
     necessary since this function does not directly modify or handle
     the possibly wide file offset.  Since the kernel handles this state
     internally the `write' function can be used for all cases.

     This function is a cancellation point in multi-threaded programs.
     This is a problem if the thread allocates some resources (like
     memory, file descriptors, semaphores or whatever) at the time
     `write' is called.  If the thread gets canceled these resources
     stay allocated until the program ends.  To avoid this, calls to
     `write' should be protected using cancellation handlers.

     The `write' function is the underlying primitive for all of the
     functions that write to streams, such as `fputc'.

 -- Function: ssize_t pwrite (int FILEDES, const void *BUFFER, size_t
          SIZE, off_t OFFSET)
     The `pwrite' function is similar to the `write' function.  The
     first three arguments are identical, and the return values and
     error codes also correspond.

     The difference is the fourth argument and its handling.  The data
     block is not written to the current position of the file descriptor
     `filedes'.  Instead the data is written to the file starting at
     position OFFSET.  The position of the file descriptor itself is
     not affected by the operation.  The value is the same as before
     the call.

     When the source file is compiled with `_FILE_OFFSET_BITS == 64' the
     `pwrite' function is in fact `pwrite64' and the type `off_t' has
     64 bits, which makes it possible to handle files up to 2^63 bytes
     in length.

     The return value of `pwrite' describes the number of written bytes.
     In the error case it returns -1 like `write' does and the error
     codes are also the same, with these additions:

    `EINVAL'
          The value given for OFFSET is negative and therefore illegal.

    `ESPIPE'
          The file descriptor FILEDES is associated with a pipe or a
          FIFO and this device does not allow positioning of the file
          pointer.

     The function is an extension defined in the Unix Single
     Specification version 2.

 -- Function: ssize_t pwrite64 (int FILEDES, const void *BUFFER, size_t
          SIZE, off64_t OFFSET)
     This function is similar to the `pwrite' function.  The difference
     is that the OFFSET parameter is of type `off64_t' instead of
     `off_t' which makes it possible on 32 bit machines to address
     files larger than 2^31 bytes and up to 2^63 bytes.  The file
     descriptor `filedes' must be opened using `open64' since otherwise
     the large offsets possible with `off64_t' will lead to errors with
     a descriptor in small file mode.

     When the source file is compiled using `_FILE_OFFSET_BITS == 64'
     on a 32 bit machine this function is actually available under the
     name `pwrite' and so transparently replaces the 32 bit interface.

File: libc.info,  Node: File Position Primitive,  Next: Descriptors and Streams,  Prev: I/O Primitives,  Up: Low-Level I/O

13.3 Setting the File Position of a Descriptor
==============================================

Just as you can set the file position of a stream with `fseek', you can
set the file position of a descriptor with `lseek'.  This specifies the
position in the file for the next `read' or `write' operation.  *Note
File Positioning::, for more information on the file position and what
it means.

   To read the current file position value from a descriptor, use
`lseek (DESC, 0, SEEK_CUR)'.

 -- Function: off_t lseek (int FILEDES, off_t OFFSET, int WHENCE)
     The `lseek' function is used to change the file position of the
     file with descriptor FILEDES.

     The WHENCE argument specifies how the OFFSET should be
     interpreted, in the same way as for the `fseek' function, and it
     must be one of the symbolic constants `SEEK_SET', `SEEK_CUR', or
     `SEEK_END'.

    `SEEK_SET'
          Specifies that WHENCE is a count of characters from the
          beginning of the file.

    `SEEK_CUR'
          Specifies that WHENCE is a count of characters from the
          current file position.  This count may be positive or
          negative.

    `SEEK_END'
          Specifies that WHENCE is a count of characters from the end of
          the file.  A negative count specifies a position within the
          current extent of the file; a positive count specifies a
          position past the current end.  If you set the position past
          the current end, and actually write data, you will extend the
          file with zeros up to that position.

     The return value from `lseek' is normally the resulting file
     position, measured in bytes from the beginning of the file.  You
     can use this feature together with `SEEK_CUR' to read the current
     file position.

     If you want to append to the file, setting the file position to the
     current end of file with `SEEK_END' is not sufficient.  Another
     process may write more data after you seek but before you write,
     extending the file so the position you write onto clobbers their
     data.  Instead, use the `O_APPEND' operating mode; *note Operating
     Modes::.

     You can set the file position past the current end of the file.
     This does not by itself make the file longer; `lseek' never
     changes the file.  But subsequent output at that position will
     extend the file.  Characters between the previous end of file and
     the new position are filled with zeros.  Extending the file in
     this way can create a "hole": the blocks of zeros are not actually
     allocated on disk, so the file takes up less space than it appears
     to; it is then called a "sparse file".

     If the file position cannot be changed, or the operation is in
     some way invalid, `lseek' returns a value of -1.  The following
     `errno' error conditions are defined for this function:

    `EBADF'
          The FILEDES is not a valid file descriptor.

    `EINVAL'
          The WHENCE argument value is not valid, or the resulting file
          offset is not valid.  A file offset is invalid.

    `ESPIPE'
          The FILEDES corresponds to an object that cannot be
          positioned, such as a pipe, FIFO or terminal device.
          (POSIX.1 specifies this error only for pipes and FIFOs, but
          in the GNU system, you always get `ESPIPE' if the object is
          not seekable.)

     When the source file is compiled with `_FILE_OFFSET_BITS == 64' the
     `lseek' function is in fact `lseek64' and the type `off_t' has 64
     bits which makes it possible to handle files up to 2^63 bytes in
     length.

     This function is a cancellation point in multi-threaded programs.
     This is a problem if the thread allocates some resources (like
     memory, file descriptors, semaphores or whatever) at the time
     `lseek' is called.  If the thread gets canceled these resources
     stay allocated until the program ends.  To avoid this calls to
     `lseek' should be protected using cancellation handlers.

     The `lseek' function is the underlying primitive for the `fseek',
     `fseeko', `ftell', `ftello' and `rewind' functions, which operate
     on streams instead of file descriptors.

 -- Function: off64_t lseek64 (int FILEDES, off64_t OFFSET, int WHENCE)
     This function is similar to the `lseek' function.  The difference
     is that the OFFSET parameter is of type `off64_t' instead of
     `off_t' which makes it possible on 32 bit machines to address
     files larger than 2^31 bytes and up to 2^63 bytes.  The file
     descriptor `filedes' must be opened using `open64' since otherwise
     the large offsets possible with `off64_t' will lead to errors with
     a descriptor in small file mode.

     When the source file is compiled with `_FILE_OFFSET_BITS == 64' on
     a 32 bits machine this function is actually available under the
     name `lseek' and so transparently replaces the 32 bit interface.

   You can have multiple descriptors for the same file if you open the
file more than once, or if you duplicate a descriptor with `dup'.
Descriptors that come from separate calls to `open' have independent
file positions; using `lseek' on one descriptor has no effect on the
other.  For example,

     {
       int d1, d2;
       char buf[4];
       d1 = open ("foo", O_RDONLY);
       d2 = open ("foo", O_RDONLY);
       lseek (d1, 1024, SEEK_SET);
       read (d2, buf, 4);
     }

will read the first four characters of the file `foo'.  (The
error-checking code necessary for a real program has been omitted here
for brevity.)

   By contrast, descriptors made by duplication share a common file
position with the original descriptor that was duplicated.  Anything
which alters the file position of one of the duplicates, including
reading or writing data, affects all of them alike.  Thus, for example,

     {
       int d1, d2, d3;
       char buf1[4], buf2[4];
       d1 = open ("foo", O_RDONLY);
       d2 = dup (d1);
       d3 = dup (d2);
       lseek (d3, 1024, SEEK_SET);
       read (d1, buf1, 4);
       read (d2, buf2, 4);
     }

will read four characters starting with the 1024'th character of `foo',
and then four more characters starting with the 1028'th character.

 -- Data Type: off_t
     This is an arithmetic data type used to represent file sizes.  In
     the GNU system, this is equivalent to `fpos_t' or `long int'.

     If the source is compiled with `_FILE_OFFSET_BITS == 64' this type
     is transparently replaced by `off64_t'.

 -- Data Type: off64_t
     This type is used similar to `off_t'.  The difference is that even
     on 32 bit machines, where the `off_t' type would have 32 bits,
     `off64_t' has 64 bits and so is able to address files up to 2^63
     bytes in length.

     When compiling with `_FILE_OFFSET_BITS == 64' this type is
     available under the name `off_t'.

   These aliases for the `SEEK_...' constants exist for the sake of
compatibility with older BSD systems.  They are defined in two
different header files: `fcntl.h' and `sys/file.h'.

`L_SET'
     An alias for `SEEK_SET'.

`L_INCR'
     An alias for `SEEK_CUR'.

`L_XTND'
     An alias for `SEEK_END'.

File: libc.info,  Node: Descriptors and Streams,  Next: Stream/Descriptor Precautions,  Prev: File Position Primitive,  Up: Low-Level I/O

13.4 Descriptors and Streams
============================

Given an open file descriptor, you can create a stream for it with the
`fdopen' function.  You can get the underlying file descriptor for an
existing stream with the `fileno' function.  These functions are
declared in the header file `stdio.h'.

 -- Function: FILE * fdopen (int FILEDES, const char *OPENTYPE)
     The `fdopen' function returns a new stream for the file descriptor
     FILEDES.

     The OPENTYPE argument is interpreted in the same way as for the
     `fopen' function (*note Opening Streams::), except that the `b'
     option is not permitted; this is because GNU makes no distinction
     between text and binary files.  Also, `"w"' and `"w+"' do not
     cause truncation of the file; these have an effect only when
     opening a file, and in this case the file has already been opened.
     You must make sure that the OPENTYPE argument matches the actual
     mode of the open file descriptor.

     The return value is the new stream.  If the stream cannot be
     created (for example, if the modes for the file indicated by the
     file descriptor do not permit the access specified by the OPENTYPE
     argument), a null pointer is returned instead.

     In some other systems, `fdopen' may fail to detect that the modes
     for file descriptor do not permit the access specified by
     `opentype'.  The GNU C library always checks for this.

   For an example showing the use of the `fdopen' function, see *note
Creating a Pipe::.

 -- Function: int fileno (FILE *STREAM)
     This function returns the file descriptor associated with the
     stream STREAM.  If an error is detected (for example, if the STREAM
     is not valid) or if STREAM does not do I/O to a file, `fileno'
     returns -1.

 -- Function: int fileno_unlocked (FILE *STREAM)
     The `fileno_unlocked' function is equivalent to the `fileno'
     function except that it does not implicitly lock the stream if the
     state is `FSETLOCKING_INTERNAL'.

     This function is a GNU extension.

   There are also symbolic constants defined in `unistd.h' for the file
descriptors belonging to the standard streams `stdin', `stdout', and
`stderr'; see *note Standard Streams::.

`STDIN_FILENO'
     This macro has value `0', which is the file descriptor for
     standard input.

`STDOUT_FILENO'
     This macro has value `1', which is the file descriptor for
     standard output.

`STDERR_FILENO'
     This macro has value `2', which is the file descriptor for
     standard error output.

File: libc.info,  Node: Stream/Descriptor Precautions,  Next: Scatter-Gather,  Prev: Descriptors and Streams,  Up: Low-Level I/O

13.5 Dangers of Mixing Streams and Descriptors
==============================================

You can have multiple file descriptors and streams (let's call both
streams and descriptors "channels" for short) connected to the same
file, but you must take care to avoid confusion between channels.  There
are two cases to consider: "linked" channels that share a single file
position value, and "independent" channels that have their own file
positions.

   It's best to use just one channel in your program for actual data
transfer to any given file, except when all the access is for input.
For example, if you open a pipe (something you can only do at the file
descriptor level), either do all I/O with the descriptor, or construct a
stream from the descriptor with `fdopen' and then do all I/O with the
stream.

* Menu:

* Linked Channels::	   Dealing with channels sharing a file position.
* Independent Channels::   Dealing with separately opened, unlinked channels.
* Cleaning Streams::	   Cleaning a stream makes it safe to use
                            another channel.

File: libc.info,  Node: Linked Channels,  Next: Independent Channels,  Up: Stream/Descriptor Precautions

13.5.1 Linked Channels
----------------------

Channels that come from a single opening share the same file position;
we call them "linked" channels.  Linked channels result when you make a
stream from a descriptor using `fdopen', when you get a descriptor from
a stream with `fileno', when you copy a descriptor with `dup' or
`dup2', and when descriptors are inherited during `fork'.  For files
that don't support random access, such as terminals and pipes, _all_
channels are effectively linked.  On random-access files, all
append-type output streams are effectively linked to each other.

   If you have been using a stream for I/O (or have just opened the
stream), and you want to do I/O using another channel (either a stream
or a descriptor) that is linked to it, you must first "clean up" the
stream that you have been using.  *Note Cleaning Streams::.

   Terminating a process, or executing a new program in the process,
destroys all the streams in the process.  If descriptors linked to these
streams persist in other processes, their file positions become
undefined as a result.  To prevent this, you must clean up the streams
before destroying them.

File: libc.info,  Node: Independent Channels,  Next: Cleaning Streams,  Prev: Linked Channels,  Up: Stream/Descriptor Precautions

13.5.2 Independent Channels
---------------------------

When you open channels (streams or descriptors) separately on a seekable
file, each channel has its own file position.  These are called
"independent channels".

   The system handles each channel independently.  Most of the time,
this is quite predictable and natural (especially for input): each
channel can read or write sequentially at its own place in the file.
However, if some of the channels are streams, you must take these
precautions:

   * You should clean an output stream after use, before doing anything
     else that might read or write from the same part of the file.

   * You should clean an input stream before reading data that may have
     been modified using an independent channel.  Otherwise, you might
     read obsolete data that had been in the stream's buffer.

   If you do output to one channel at the end of the file, this will
certainly leave the other independent channels positioned somewhere
before the new end.  You cannot reliably set their file positions to the
new end of file before writing, because the file can always be extended
by another process between when you set the file position and when you
write the data.  Instead, use an append-type descriptor or stream; they
always output at the current end of the file.  In order to make the
end-of-file position accurate, you must clean the output channel you
were using, if it is a stream.

   It's impossible for two channels to have separate file pointers for a
file that doesn't support random access.  Thus, channels for reading or
writing such files are always linked, never independent.  Append-type
channels are also always linked.  For these channels, follow the rules
for linked channels; see *note Linked Channels::.

File: libc.info,  Node: Cleaning Streams,  Prev: Independent Channels,  Up: Stream/Descriptor Precautions

13.5.3 Cleaning Streams
-----------------------

On the GNU system, you can clean up any stream with `fclean':

 -- Function: int fclean (FILE *STREAM)
     Clean up the stream STREAM so that its buffer is empty.  If STREAM
     is doing output, force it out.  If STREAM is doing input, give the
     data in the buffer back to the system, arranging to reread it.

   On other systems, you can use `fflush' to clean a stream in most
cases.

   You can skip the `fclean' or `fflush' if you know the stream is
already clean.  A stream is clean whenever its buffer is empty.  For
example, an unbuffered stream is always clean.  An input stream that is
at end-of-file is clean.  A line-buffered stream is clean when the last
character output was a newline.  However, a just-opened input stream
might not be clean, as its input buffer might not be empty.

   There is one case in which cleaning a stream is impossible on most
systems.  This is when the stream is doing input from a file that is not
random-access.  Such streams typically read ahead, and when the file is
not random access, there is no way to give back the excess data already
read.  When an input stream reads from a random-access file, `fflush'
does clean the stream, but leaves the file pointer at an unpredictable
place; you must set the file pointer before doing any further I/O.  On
the GNU system, using `fclean' avoids both of these problems.

   Closing an output-only stream also does `fflush', so this is a valid
way of cleaning an output stream.  On the GNU system, closing an input
stream does `fclean'.

   You need not clean a stream before using its descriptor for control
operations such as setting terminal modes; these operations don't affect
the file position and are not affected by it.  You can use any
descriptor for these operations, and all channels are affected
simultaneously.  However, text already "output" to a stream but still
buffered by the stream will be subject to the new terminal modes when
subsequently flushed.  To make sure "past" output is covered by the
terminal settings that were in effect at the time, flush the output
streams for that terminal before setting the modes.  *Note Terminal
Modes::.

File: libc.info,  Node: Scatter-Gather,  Next: Memory-mapped I/O,  Prev: Stream/Descriptor Precautions,  Up: Low-Level I/O

13.6 Fast Scatter-Gather I/O
============================

Some applications may need to read or write data to multiple buffers,
which are separated in memory.  Although this can be done easily enough
with multiple calls to `read' and `write', it is inefficient because
there is overhead associated with each kernel call.

   Instead, many platforms provide special high-speed primitives to
perform these "scatter-gather" operations in a single kernel call.  The
GNU C library will provide an emulation on any system that lacks these
primitives, so they are not a portability threat.  They are defined in
`sys/uio.h'.

   These functions are controlled with arrays of `iovec' structures,
which describe the location and size of each buffer.

 -- Data Type: struct iovec
     The `iovec' structure describes a buffer. It contains two fields:

    `void *iov_base'
          Contains the address of a buffer.

    `size_t iov_len'
          Contains the length of the buffer.


 -- Function: ssize_t readv (int FILEDES, const struct iovec *VECTOR,
          int COUNT)
     The `readv' function reads data from FILEDES and scatters it into
     the buffers described in VECTOR, which is taken to be COUNT
     structures long.  As each buffer is filled, data is sent to the
     next.

     Note that `readv' is not guaranteed to fill all the buffers.  It
     may stop at any point, for the same reasons `read' would.

     The return value is a count of bytes (_not_ buffers) read, 0
     indicating end-of-file, or -1 indicating an error.  The possible
     errors are the same as in `read'.


 -- Function: ssize_t writev (int FILEDES, const struct iovec *VECTOR,
          int COUNT)
     The `writev' function gathers data from the buffers described in
     VECTOR, which is taken to be COUNT structures long, and writes
     them to `filedes'.  As each buffer is written, it moves on to the
     next.

     Like `readv', `writev' may stop midstream under the same
     conditions `write' would.

     The return value is a count of bytes written, or -1 indicating an
     error.  The possible errors are the same as in `write'.


   Note that if the buffers are small (under about 1kB), high-level
streams may be easier to use than these functions.  However, `readv' and
`writev' are more efficient when the individual buffers themselves (as
opposed to the total output), are large.  In that case, a high-level
stream would not be able to cache the data effectively.

File: libc.info,  Node: Memory-mapped I/O,  Next: Waiting for I/O,  Prev: Scatter-Gather,  Up: Low-Level I/O

13.7 Memory-mapped I/O
======================

On modern operating systems, it is possible to "mmap" (pronounced
"em-map") a file to a region of memory.  When this is done, the file can
be accessed just like an array in the program.

   This is more efficient than `read' or `write', as only the regions
of the file that a program actually accesses are loaded.  Accesses to
not-yet-loaded parts of the mmapped region are handled in the same way
as swapped out pages.

   Since mmapped pages can be stored back to their file when physical
memory is low, it is possible to mmap files orders of magnitude larger
than both the physical memory _and_ swap space.  The only limit is
address space.  The theoretical limit is 4GB on a 32-bit machine -
however, the actual limit will be smaller since some areas will be
reserved for other purposes.  If the LFS interface is used the file size
on 32-bit systems is not limited to 2GB (offsets are signed which
reduces the addressable area of 4GB by half); the full 64-bit are
available.

   Memory mapping only works on entire pages of memory.  Thus, addresses
for mapping must be page-aligned, and length values will be rounded up.
To determine the size of a page the machine uses one should use

     size_t page_size = (size_t) sysconf (_SC_PAGESIZE);

These functions are declared in `sys/mman.h'.

 -- Function: void * mmap (void *ADDRESS, size_t LENGTH,int PROTECT,
          int FLAGS, int FILEDES, off_t OFFSET)
     The `mmap' function creates a new mapping, connected to bytes
     (OFFSET) to (OFFSET + LENGTH - 1) in the file open on FILEDES.  A
     new reference for the file specified by FILEDES is created, which
     is not removed by closing the file.

     ADDRESS gives a preferred starting address for the mapping.
     `NULL' expresses no preference. Any previous mapping at that
     address is automatically removed. The address you give may still be
     changed, unless you use the `MAP_FIXED' flag.

     PROTECT contains flags that control what kind of access is
     permitted.  They include `PROT_READ', `PROT_WRITE', and
     `PROT_EXEC', which permit reading, writing, and execution,
     respectively.  Inappropriate access will cause a segfault (*note
     Program Error Signals::).

     Note that most hardware designs cannot support write permission
     without read permission, and many do not distinguish read and
     execute permission.  Thus, you may receive wider permissions than
     you ask for, and mappings of write-only files may be denied even
     if you do not use `PROT_READ'.

     FLAGS contains flags that control the nature of the map.  One of
     `MAP_SHARED' or `MAP_PRIVATE' must be specified.

     They include:

    `MAP_PRIVATE'
          This specifies that writes to the region should never be
          written back to the attached file.  Instead, a copy is made
          for the process, and the region will be swapped normally if
          memory runs low.  No other process will see the changes.

          Since private mappings effectively revert to ordinary memory
          when written to, you must have enough virtual memory for a
          copy of the entire mmapped region if you use this mode with
          `PROT_WRITE'.

    `MAP_SHARED'
          This specifies that writes to the region will be written back
          to the file.  Changes made will be shared immediately with
          other processes mmaping the same file.

          Note that actual writing may take place at any time.  You
          need to use `msync', described below, if it is important that
          other processes using conventional I/O get a consistent view
          of the file.

    `MAP_FIXED'
          This forces the system to use the exact mapping address
          specified in ADDRESS and fail if it can't.

    `MAP_ANONYMOUS'
    `MAP_ANON'
          This flag tells the system to create an anonymous mapping,
          not connected to a file.  FILEDES and OFF are ignored, and
          the region is initialized with zeros.

          Anonymous maps are used as the basic primitive to extend the
          heap on some systems.  They are also useful to share data
          between multiple tasks without creating a file.

          On some systems using private anonymous mmaps is more
          efficient than using `malloc' for large blocks.  This is not
          an issue with the GNU C library, as the included `malloc'
          automatically uses `mmap' where appropriate.


     `mmap' returns the address of the new mapping, or -1 for an error.

     Possible errors include:

    `EINVAL'
          Either ADDRESS was unusable, or inconsistent FLAGS were given.

    `EACCES'
          FILEDES was not open for the type of access specified in
          PROTECT.

    `ENOMEM'
          Either there is not enough memory for the operation, or the
          process is out of address space.

    `ENODEV'
          This file is of a type that doesn't support mapping.

    `ENOEXEC'
          The file is on a filesystem that doesn't support mapping.



 -- Function: void * mmap64 (void *ADDRESS, size_t LENGTH,int PROTECT,
          int FLAGS, int FILEDES, off64_t OFFSET)
     The `mmap64' function is equivalent to the `mmap' function but the
     OFFSET parameter is of type `off64_t'.  On 32-bit systems this
     allows the file associated with the FILEDES descriptor to be
     larger than 2GB.  FILEDES must be a descriptor returned from a
     call to `open64' or `fopen64' and `freopen64' where the descriptor
     is retrieved with `fileno'.

     When the sources are translated with `_FILE_OFFSET_BITS == 64' this
     function is actually available under the name `mmap'.  I.e., the
     new, extended API using 64 bit file sizes and offsets transparently
     replaces the old API.

 -- Function: int munmap (void *ADDR, size_t LENGTH)
     `munmap' removes any memory maps from (ADDR) to (ADDR + LENGTH).
     LENGTH should be the length of the mapping.

     It is safe to unmap multiple mappings in one command, or include
     unmapped space in the range.  It is also possible to unmap only
     part of an existing mapping.  However, only entire pages can be
     removed.  If LENGTH is not an even number of pages, it will be
     rounded up.

     It returns 0 for success and -1 for an error.

     One error is possible:

    `EINVAL'
          The memory range given was outside the user mmap range or
          wasn't page aligned.



 -- Function: int msync (void *ADDRESS, size_t LENGTH, int FLAGS)
     When using shared mappings, the kernel can write the file at any
     time before the mapping is removed.  To be certain data has
     actually been written to the file and will be accessible to
     non-memory-mapped I/O, it is necessary to use this function.

     It operates on the region ADDRESS to (ADDRESS + LENGTH).  It may
     be used on part of a mapping or multiple mappings, however the
     region given should not contain any unmapped space.

     FLAGS can contain some options:

    `MS_SYNC'
          This flag makes sure the data is actually written _to disk_.
          Normally `msync' only makes sure that accesses to a file with
          conventional I/O reflect the recent changes.

    `MS_ASYNC'
          This tells `msync' to begin the synchronization, but not to
          wait for it to complete.


     `msync' returns 0 for success and -1 for error.  Errors include:

    `EINVAL'
          An invalid region was given, or the FLAGS were invalid.

    `EFAULT'
          There is no existing mapping in at least part of the given
          region.



 -- Function: void * mremap (void *ADDRESS, size_t LENGTH, size_t
          NEW_LENGTH, int FLAG)
     This function can be used to change the size of an existing memory
     area. ADDRESS and LENGTH must cover a region entirely mapped in
     the same `mmap' statement. A new mapping with the same
     characteristics will be returned with the length NEW_LENGTH.

     One option is possible, `MREMAP_MAYMOVE'. If it is given in FLAGS,
     the system may remove the existing mapping and create a new one of
     the desired length in another location.

     The address of the resulting mapping is returned, or -1. Possible
     error codes include:

    `EFAULT'
          There is no existing mapping in at least part of the original
          region, or the region covers two or more distinct mappings.

    `EINVAL'
          The address given is misaligned or inappropriate.

    `EAGAIN'
          The region has pages locked, and if extended it would exceed
          the process's resource limit for locked pages.  *Note Limits
          on Resources::.

    `ENOMEM'
          The region is private writable, and insufficient virtual
          memory is available to extend it.  Also, this error will
          occur if `MREMAP_MAYMOVE' is not given and the extension
          would collide with another mapped region.


   This function is only available on a few systems.  Except for
performing optional optimizations one should not rely on this function.

   Not all file descriptors may be mapped.  Sockets, pipes, and most
devices only allow sequential access and do not fit into the mapping
abstraction.  In addition, some regular files may not be mmapable, and
older kernels may not support mapping at all.  Thus, programs using
`mmap' should have a fallback method to use should it fail. *Note Mmap:
(standards)Mmap.

 -- Function: int madvise (void *ADDR, size_t LENGTH, int ADVICE)
     This function can be used to provide the system with ADVICE about
     the intended usage patterns of the memory region starting at ADDR
     and extending LENGTH bytes.

     The valid BSD values for ADVICE are:

    `MADV_NORMAL'
          The region should receive no further special treatment.

    `MADV_RANDOM'
          The region will be accessed via random page references. The
          kernel should page-in the minimal number of pages for each
          page fault.

    `MADV_SEQUENTIAL'
          The region will be accessed via sequential page references.
          This may cause the kernel to aggressively read-ahead,
          expecting further sequential references after any page fault
          within this region.

    `MADV_WILLNEED'
          The region will be needed.  The pages within this region may
          be pre-faulted in by the kernel.

    `MADV_DONTNEED'
          The region is no longer needed.  The kernel may free these
          pages, causing any changes to the pages to be lost, as well
          as swapped out pages to be discarded.


     The POSIX names are slightly different, but with the same meanings:

    `POSIX_MADV_NORMAL'
          This corresponds with BSD's `MADV_NORMAL'.

    `POSIX_MADV_RANDOM'
          This corresponds with BSD's `MADV_RANDOM'.

    `POSIX_MADV_SEQUENTIAL'
          This corresponds with BSD's `MADV_SEQUENTIAL'.

    `POSIX_MADV_WILLNEED'
          This corresponds with BSD's `MADV_WILLNEED'.

    `POSIX_MADV_DONTNEED'
          This corresponds with BSD's `MADV_DONTNEED'.


     `msync' returns 0 for success and -1 for error.  Errors include:
    `EINVAL'
          An invalid region was given, or the ADVICE was invalid.

    `EFAULT'
          There is no existing mapping in at least part of the given
          region.


File: libc.info,  Node: Waiting for I/O,  Next: Synchronizing I/O,  Prev: Memory-mapped I/O,  Up: Low-Level I/O

13.8 Waiting for Input or Output
================================

Sometimes a program needs to accept input on multiple input channels
whenever input arrives.  For example, some workstations may have devices
such as a digitizing tablet, function button box, or dial box that are
connected via normal asynchronous serial interfaces; good user interface
style requires responding immediately to input on any device.  Another
example is a program that acts as a server to several other processes
via pipes or sockets.

   You cannot normally use `read' for this purpose, because this blocks
the program until input is available on one particular file descriptor;
input on other channels won't wake it up.  You could set nonblocking
mode and poll each file descriptor in turn, but this is very
inefficient.

   A better solution is to use the `select' function.  This blocks the
program until input or output is ready on a specified set of file
descriptors, or until a timer expires, whichever comes first.  This
facility is declared in the header file `sys/types.h'.

   In the case of a server socket (*note Listening::), we say that
"input" is available when there are pending connections that could be
accepted (*note Accepting Connections::).  `accept' for server sockets
blocks and interacts with `select' just as `read' does for normal input.

   The file descriptor sets for the `select' function are specified as
`fd_set' objects.  Here is the description of the data type and some
macros for manipulating these objects.

 -- Data Type: fd_set
     The `fd_set' data type represents file descriptor sets for the
     `select' function.  It is actually a bit array.

 -- Macro: int FD_SETSIZE
     The value of this macro is the maximum number of file descriptors
     that a `fd_set' object can hold information about.  On systems
     with a fixed maximum number, `FD_SETSIZE' is at least that number.
     On some systems, including GNU, there is no absolute limit on the
     number of descriptors open, but this macro still has a constant
     value which controls the number of bits in an `fd_set'; if you get
     a file descriptor with a value as high as `FD_SETSIZE', you cannot
     put that descriptor into an `fd_set'.

 -- Macro: void FD_ZERO (fd_set *SET)
     This macro initializes the file descriptor set SET to be the empty
     set.

 -- Macro: void FD_SET (int FILEDES, fd_set *SET)
     This macro adds FILEDES to the file descriptor set SET.

     The FILEDES parameter must not have side effects since it is
     evaluated more than once.

 -- Macro: void FD_CLR (int FILEDES, fd_set *SET)
     This macro removes FILEDES from the file descriptor set SET.

     The FILEDES parameter must not have side effects since it is
     evaluated more than once.

 -- Macro: int FD_ISSET (int FILEDES, const fd_set *SET)
     This macro returns a nonzero value (true) if FILEDES is a member
     of the file descriptor set SET, and zero (false) otherwise.

     The FILEDES parameter must not have side effects since it is
     evaluated more than once.

   Next, here is the description of the `select' function itself.

 -- Function: int select (int NFDS, fd_set *READ-FDS, fd_set
          *WRITE-FDS, fd_set *EXCEPT-FDS, struct timeval *TIMEOUT)
     The `select' function blocks the calling process until there is
     activity on any of the specified sets of file descriptors, or
     until the timeout period has expired.

     The file descriptors specified by the READ-FDS argument are
     checked to see if they are ready for reading; the WRITE-FDS file
     descriptors are checked to see if they are ready for writing; and
     the EXCEPT-FDS file descriptors are checked for exceptional
     conditions.  You can pass a null pointer for any of these
     arguments if you are not interested in checking for that kind of
     condition.

     A file descriptor is considered ready for reading if a `read' call
     will not block.  This usually includes the read offset being at
     the end of the file or there is an error to report.  A server
     socket is considered ready for reading if there is a pending
     connection which can be accepted with `accept'; *note Accepting
     Connections::.  A client socket is ready for writing when its
     connection is fully established; *note Connecting::.

     "Exceptional conditions" does not mean errors--errors are reported
     immediately when an erroneous system call is executed, and do not
     constitute a state of the descriptor.  Rather, they include
     conditions such as the presence of an urgent message on a socket.
     (*Note Sockets::, for information on urgent messages.)

     The `select' function checks only the first NFDS file descriptors.
     The usual thing is to pass `FD_SETSIZE' as the value of this
     argument.

     The TIMEOUT specifies the maximum time to wait.  If you pass a
     null pointer for this argument, it means to block indefinitely
     until one of the file descriptors is ready.  Otherwise, you should
     provide the time in `struct timeval' format; see *note
     High-Resolution Calendar::.  Specify zero as the time (a `struct
     timeval' containing all zeros) if you want to find out which
     descriptors are ready without waiting if none are ready.

     The normal return value from `select' is the total number of ready
     file descriptors in all of the sets.  Each of the argument sets is
     overwritten with information about the descriptors that are ready
     for the corresponding operation.  Thus, to see if a particular
     descriptor DESC has input, use `FD_ISSET (DESC, READ-FDS)' after
     `select' returns.

     If `select' returns because the timeout period expires, it returns
     a value of zero.

     Any signal will cause `select' to return immediately.  So if your
     program uses signals, you can't rely on `select' to keep waiting
     for the full time specified.  If you want to be sure of waiting
     for a particular amount of time, you must check for `EINTR' and
     repeat the `select' with a newly calculated timeout based on the
     current time.  See the example below.  See also *note Interrupted
     Primitives::.

     If an error occurs, `select' returns `-1' and does not modify the
     argument file descriptor sets.  The following `errno' error
     conditions are defined for this function:

    `EBADF'
          One of the file descriptor sets specified an invalid file
          descriptor.

    `EINTR'
          The operation was interrupted by a signal.  *Note Interrupted
          Primitives::.

    `EINVAL'
          The TIMEOUT argument is invalid; one of the components is
          negative or too large.

   *Portability Note:*  The `select' function is a BSD Unix feature.

   Here is an example showing how you can use `select' to establish a
timeout period for reading from a file descriptor.  The `input_timeout'
function blocks the calling process until input is available on the
file descriptor, or until the timeout period expires.

     #include <errno.h>
     #include <stdio.h>
     #include <unistd.h>
     #include <sys/types.h>
     #include <sys/time.h>

     int
     input_timeout (int filedes, unsigned int seconds)
     {
       fd_set set;
       struct timeval timeout;

       /* Initialize the file descriptor set. */
       FD_ZERO (&set);
       FD_SET (filedes, &set);

       /* Initialize the timeout data structure. */
       timeout.tv_sec = seconds;
       timeout.tv_usec = 0;

       /* `select' returns 0 if timeout, 1 if input available, -1 if error. */
       return TEMP_FAILURE_RETRY (select (FD_SETSIZE,
                                          &set, NULL, NULL,
                                          &timeout));
     }

     int
     main (void)
     {
       fprintf (stderr, "select returned %d.\n",
                input_timeout (STDIN_FILENO, 5));
       return 0;
     }

   There is another example showing the use of `select' to multiplex
input from multiple sockets in *note Server Example::.

File: libc.info,  Node: Synchronizing I/O,  Next: Asynchronous I/O,  Prev: Waiting for I/O,  Up: Low-Level I/O

13.9 Synchronizing I/O operations
=================================

In most modern operating systems, the normal I/O operations are not
executed synchronously.  I.e., even if a `write' system call returns,
this does not mean the data is actually written to the media, e.g., the
disk.

   In situations where synchronization points are necessary, you can use
special functions which ensure that all operations finish before they
return.

 -- Function: int sync (void)
     A call to this function will not return as long as there is data
     which has not been written to the device.  All dirty buffers in
     the kernel will be written and so an overall consistent system can
     be achieved (if no other process in parallel writes data).

     A prototype for `sync' can be found in `unistd.h'.

     The return value is zero to indicate no error.

   Programs more often want to ensure that data written to a given file
is committed, rather than all data in the system.  For this, `sync' is
overkill.

 -- Function: int fsync (int FILDES)
     The `fsync' function can be used to make sure all data associated
     with the open file FILDES is written to the device associated with
     the descriptor.  The function call does not return unless all
     actions have finished.

     A prototype for `fsync' can be found in `unistd.h'.

     This function is a cancellation point in multi-threaded programs.
     This is a problem if the thread allocates some resources (like
     memory, file descriptors, semaphores or whatever) at the time
     `fsync' is called.  If the thread gets canceled these resources
     stay allocated until the program ends.  To avoid this, calls to
     `fsync' should be protected using cancellation handlers.

     The return value of the function is zero if no error occurred.
     Otherwise it is -1 and the global variable ERRNO is set to the
     following values:
    `EBADF'
          The descriptor FILDES is not valid.

    `EINVAL'
          No synchronization is possible since the system does not
          implement this.

   Sometimes it is not even necessary to write all data associated with
a file descriptor.  E.g., in database files which do not change in size
it is enough to write all the file content data to the device.
Meta-information, like the modification time etc., are not that
important and leaving such information uncommitted does not prevent a
successful recovering of the file in case of a problem.

 -- Function: int fdatasync (int FILDES)
     When a call to the `fdatasync' function returns, it is ensured
     that all of the file data is written to the device.  For all
     pending I/O operations, the parts guaranteeing data integrity
     finished.

     Not all systems implement the `fdatasync' operation.  On systems
     missing this functionality `fdatasync' is emulated by a call to
     `fsync' since the performed actions are a superset of those
     required by `fdatasync'.

     The prototype for `fdatasync' is in `unistd.h'.

     The return value of the function is zero if no error occurred.
     Otherwise it is -1 and the global variable ERRNO is set to the
     following values:
    `EBADF'
          The descriptor FILDES is not valid.

    `EINVAL'
          No synchronization is possible since the system does not
          implement this.

File: libc.info,  Node: Asynchronous I/O,  Next: Control Operations,  Prev: Synchronizing I/O,  Up: Low-Level I/O

13.10 Perform I/O Operations in Parallel
========================================

The POSIX.1b standard defines a new set of I/O operations which can
significantly reduce the time an application spends waiting at I/O.  The
new functions allow a program to initiate one or more I/O operations and
then immediately resume normal work while the I/O operations are
executed in parallel.  This functionality is available if the
`unistd.h' file defines the symbol `_POSIX_ASYNCHRONOUS_IO'.

   These functions are part of the library with realtime functions named
`librt'.  They are not actually part of the `libc' binary.  The
implementation of these functions can be done using support in the
kernel (if available) or using an implementation based on threads at
userlevel.  In the latter case it might be necessary to link
applications with the thread library `libpthread' in addition to
`librt'.

   All AIO operations operate on files which were opened previously.
There might be arbitrarily many operations running for one file.  The
asynchronous I/O operations are controlled using a data structure named
`struct aiocb' ("AIO control block").  It is defined in `aio.h' as
follows.

 -- Data Type: struct aiocb
     The POSIX.1b standard mandates that the `struct aiocb' structure
     contains at least the members described in the following table.
     There might be more elements which are used by the implementation,
     but depending upon these elements is not portable and is highly
     deprecated.

    `int aio_fildes'
          This element specifies the file descriptor to be used for the
          operation.  It must be a legal descriptor, otherwise the
          operation will fail.

          The device on which the file is opened must allow the seek
          operation.  I.e., it is not possible to use any of the AIO
          operations on devices like terminals where an `lseek' call
          would lead to an error.

    `off_t aio_offset'
          This element specifies the offset in the file at which the
          operation (input or output) is performed.  Since the
          operations are carried out in arbitrary order and more than
          one operation for one file descriptor can be started, one
          cannot expect a current read/write position of the file
          descriptor.

    `volatile void *aio_buf'
          This is a pointer to the buffer with the data to be written
          or the place where the read data is stored.

    `size_t aio_nbytes'
          This element specifies the length of the buffer pointed to by
          `aio_buf'.

    `int aio_reqprio'
          If the platform has defined `_POSIX_PRIORITIZED_IO' and
          `_POSIX_PRIORITY_SCHEDULING', the AIO requests are processed
          based on the current scheduling priority.  The `aio_reqprio'
          element can then be used to lower the priority of the AIO
          operation.

    `struct sigevent aio_sigevent'
          This element specifies how the calling process is notified
          once the operation terminates.  If the `sigev_notify' element
          is `SIGEV_NONE', no notification is sent.  If it is
          `SIGEV_SIGNAL', the signal determined by `sigev_signo' is
          sent.  Otherwise, `sigev_notify' must be `SIGEV_THREAD'.  In
          this case, a thread is created which starts executing the
          function pointed to by `sigev_notify_function'.

    `int aio_lio_opcode'
          This element is only used by the `lio_listio' and
          `lio_listio64' functions.  Since these functions allow an
          arbitrary number of operations to start at once, and each
          operation can be input or output (or nothing), the
          information must be stored in the control block.  The
          possible values are:

         `LIO_READ'
               Start a read operation.  Read from the file at position
               `aio_offset' and store the next `aio_nbytes' bytes in the
               buffer pointed to by `aio_buf'.

         `LIO_WRITE'
               Start a write operation.  Write `aio_nbytes' bytes
               starting at `aio_buf' into the file starting at position
               `aio_offset'.

         `LIO_NOP'
               Do nothing for this control block.  This value is useful
               sometimes when an array of `struct aiocb' values
               contains holes, i.e., some of the values must not be
               handled although the whole array is presented to the
               `lio_listio' function.

     When the sources are compiled using `_FILE_OFFSET_BITS == 64' on a
     32 bit machine, this type is in fact `struct aiocb64', since the
     LFS interface transparently replaces the `struct aiocb' definition.

   For use with the AIO functions defined in the LFS, there is a
similar type defined which replaces the types of the appropriate
members with larger types but otherwise is equivalent to `struct
aiocb'.  Particularly, all member names are the same.

 -- Data Type: struct aiocb64
    `int aio_fildes'
          This element specifies the file descriptor which is used for
          the operation.  It must be a legal descriptor since otherwise
          the operation fails for obvious reasons.

          The device on which the file is opened must allow the seek
          operation.  I.e., it is not possible to use any of the AIO
          operations on devices like terminals where an `lseek' call
          would lead to an error.

    `off64_t aio_offset'
          This element specifies at which offset in the file the
          operation (input or output) is performed.  Since the
          operation are carried in arbitrary order and more than one
          operation for one file descriptor can be started, one cannot
          expect a current read/write position of the file descriptor.

    `volatile void *aio_buf'
          This is a pointer to the buffer with the data to be written
          or the place where the read data is stored.

    `size_t aio_nbytes'
          This element specifies the length of the buffer pointed to by
          `aio_buf'.

    `int aio_reqprio'
          If for the platform `_POSIX_PRIORITIZED_IO' and
          `_POSIX_PRIORITY_SCHEDULING' are defined the AIO requests are
          processed based on the current scheduling priority.  The
          `aio_reqprio' element can then be used to lower the priority
          of the AIO operation.

    `struct sigevent aio_sigevent'
          This element specifies how the calling process is notified
          once the operation terminates.  If the `sigev_notify',
          element is `SIGEV_NONE' no notification is sent.  If it is
          `SIGEV_SIGNAL', the signal determined by `sigev_signo' is
          sent.  Otherwise, `sigev_notify' must be `SIGEV_THREAD' in
          which case a thread which starts executing the function
          pointed to by `sigev_notify_function'.

    `int aio_lio_opcode'
          This element is only used by the `lio_listio' and
          `[lio_listio64' functions.  Since these functions allow an
          arbitrary number of operations to start at once, and since
          each operation can be input or output (or nothing), the
          information must be stored in the control block.  See the
          description of `struct aiocb' for a description of the
          possible values.

     When the sources are compiled using `_FILE_OFFSET_BITS == 64' on a
     32 bit machine, this type is available under the name `struct
     aiocb64', since the LFS transparently replaces the old interface.

* Menu:

* Asynchronous Reads/Writes::    Asynchronous Read and Write Operations.
* Status of AIO Operations::     Getting the Status of AIO Operations.
* Synchronizing AIO Operations:: Getting into a consistent state.
* Cancel AIO Operations::        Cancellation of AIO Operations.
* Configuration of AIO::         How to optimize the AIO implementation.

File: libc.info,  Node: Asynchronous Reads/Writes,  Next: Status of AIO Operations,  Up: Asynchronous I/O

13.10.1 Asynchronous Read and Write Operations
----------------------------------------------

 -- Function: int aio_read (struct aiocb *AIOCBP)
     This function initiates an asynchronous read operation.  It
     immediately returns after the operation was enqueued or when an
     error was encountered.

     The first `aiocbp->aio_nbytes' bytes of the file for which
     `aiocbp->aio_fildes' is a descriptor are written to the buffer
     starting at `aiocbp->aio_buf'.  Reading starts at the absolute
     position `aiocbp->aio_offset' in the file.

     If prioritized I/O is supported by the platform the
     `aiocbp->aio_reqprio' value is used to adjust the priority before
     the request is actually enqueued.

     The calling process is notified about the termination of the read
     request according to the `aiocbp->aio_sigevent' value.

     When `aio_read' returns, the return value is zero if no error
     occurred that can be found before the process is enqueued.  If
     such an early error is found, the function returns -1 and sets
     `errno' to one of the following values:

    `EAGAIN'
          The request was not enqueued due to (temporarily) exceeded
          resource limitations.

    `ENOSYS'
          The `aio_read' function is not implemented.

    `EBADF'
          The `aiocbp->aio_fildes' descriptor is not valid.  This
          condition need not be recognized before enqueueing the
          request and so this error might also be signaled
          asynchronously.

    `EINVAL'
          The `aiocbp->aio_offset' or `aiocbp->aio_reqpiro' value is
          invalid.  This condition need not be recognized before
          enqueueing the request and so this error might also be
          signaled asynchronously.

     If `aio_read' returns zero, the current status of the request can
     be queried using `aio_error' and `aio_return' functions.  As long
     as the value returned by `aio_error' is `EINPROGRESS' the
     operation has not yet completed.  If `aio_error' returns zero, the
     operation successfully terminated, otherwise the value is to be
     interpreted as an error code.  If the function terminated, the
     result of the operation can be obtained using a call to
     `aio_return'.  The returned value is the same as an equivalent
     call to `read' would have returned.  Possible error codes returned
     by `aio_error' are:

    `EBADF'
          The `aiocbp->aio_fildes' descriptor is not valid.

    `ECANCELED'
          The operation was canceled before the operation was finished
          (*note Cancel AIO Operations::)

    `EINVAL'
          The `aiocbp->aio_offset' value is invalid.

     When the sources are compiled with `_FILE_OFFSET_BITS == 64' this
     function is in fact `aio_read64' since the LFS interface
     transparently replaces the normal implementation.

 -- Function: int aio_read64 (struct aiocb *AIOCBP)
     This function is similar to the `aio_read' function.  The only
     difference is that on 32 bit machines, the file descriptor should
     be opened in the large file mode.  Internally, `aio_read64' uses
     functionality equivalent to `lseek64' (*note File Position
     Primitive::) to position the file descriptor correctly for the
     reading, as opposed to `lseek' functionality used in `aio_read'.

     When the sources are compiled with `_FILE_OFFSET_BITS == 64', this
     function is available under the name `aio_read' and so
     transparently replaces the interface for small files on 32 bit
     machines.

   To write data asynchronously to a file, there exists an equivalent
pair of functions with a very similar interface.

 -- Function: int aio_write (struct aiocb *AIOCBP)
     This function initiates an asynchronous write operation.  The
     function call immediately returns after the operation was enqueued
     or if before this happens an error was encountered.

     The first `aiocbp->aio_nbytes' bytes from the buffer starting at
     `aiocbp->aio_buf' are written to the file for which
     `aiocbp->aio_fildes' is an descriptor, starting at the absolute
     position `aiocbp->aio_offset' in the file.

     If prioritized I/O is supported by the platform, the
     `aiocbp->aio_reqprio' value is used to adjust the priority before
     the request is actually enqueued.

     The calling process is notified about the termination of the read
     request according to the `aiocbp->aio_sigevent' value.

     When `aio_write' returns, the return value is zero if no error
     occurred that can be found before the process is enqueued.  If
     such an early error is found the function returns -1 and sets
     `errno' to one of the following values.

    `EAGAIN'
          The request was not enqueued due to (temporarily) exceeded
          resource limitations.

    `ENOSYS'
          The `aio_write' function is not implemented.

    `EBADF'
          The `aiocbp->aio_fildes' descriptor is not valid.  This
          condition may not be recognized before enqueueing the
          request, and so this error might also be signaled
          asynchronously.

    `EINVAL'
          The `aiocbp->aio_offset' or `aiocbp->aio_reqprio' value is
          invalid.  This condition may not be recognized before
          enqueueing the request and so this error might also be
          signaled asynchronously.

     In the case `aio_write' returns zero, the current status of the
     request can be queried using `aio_error' and `aio_return'
     functions.  As long as the value returned by `aio_error' is
     `EINPROGRESS' the operation has not yet completed.  If `aio_error'
     returns zero, the operation successfully terminated, otherwise the
     value is to be interpreted as an error code.  If the function
     terminated, the result of the operation can be get using a call to
     `aio_return'.  The returned value is the same as an equivalent
     call to `read' would have returned.  Possible error codes returned
     by `aio_error' are:

    `EBADF'
          The `aiocbp->aio_fildes' descriptor is not valid.

    `ECANCELED'
          The operation was canceled before the operation was finished.
          (*note Cancel AIO Operations::)

    `EINVAL'
          The `aiocbp->aio_offset' value is invalid.

     When the sources are compiled with `_FILE_OFFSET_BITS == 64', this
     function is in fact `aio_write64' since the LFS interface
     transparently replaces the normal implementation.

 -- Function: int aio_write64 (struct aiocb *AIOCBP)
     This function is similar to the `aio_write' function.  The only
     difference is that on 32 bit machines the file descriptor should
     be opened in the large file mode.  Internally `aio_write64' uses
     functionality equivalent to `lseek64' (*note File Position
     Primitive::) to position the file descriptor correctly for the
     writing, as opposed to `lseek' functionality used in `aio_write'.

     When the sources are compiled with `_FILE_OFFSET_BITS == 64', this
     function is available under the name `aio_write' and so
     transparently replaces the interface for small files on 32 bit
     machines.

   Besides these functions with the more or less traditional interface,
POSIX.1b also defines a function which can initiate more than one
operation at a time, and which can handle freely mixed read and write
operations.  It is therefore similar to a combination of `readv' and
`writev'.

 -- Function: int lio_listio (int MODE, struct aiocb *const LIST[], int
          NENT, struct sigevent *SIG)
     The `lio_listio' function can be used to enqueue an arbitrary
     number of read and write requests at one time.  The requests can
     all be meant for the same file, all for different files or every
     solution in between.

     `lio_listio' gets the NENT requests from the array pointed to by
     LIST.  The operation to be performed is determined by the
     `aio_lio_opcode' member in each element of LIST.  If this field is
     `LIO_READ' a read operation is enqueued, similar to a call of
     `aio_read' for this element of the array (except that the way the
     termination is signalled is different, as we will see below).  If
     the `aio_lio_opcode' member is `LIO_WRITE' a write operation is
     enqueued.  Otherwise the `aio_lio_opcode' must be `LIO_NOP' in
     which case this element of LIST is simply ignored.  This
     "operation" is useful in situations where one has a fixed array of
     `struct aiocb' elements from which only a few need to be handled at
     a time.  Another situation is where the `lio_listio' call was
     canceled before all requests are processed (*note Cancel AIO
     Operations::) and the remaining requests have to be reissued.

     The other members of each element of the array pointed to by
     `list' must have values suitable for the operation as described in
     the documentation for `aio_read' and `aio_write' above.

     The MODE argument determines how `lio_listio' behaves after having
     enqueued all the requests.  If MODE is `LIO_WAIT' it waits until
     all requests terminated.  Otherwise MODE must be `LIO_NOWAIT' and
     in this case the function returns immediately after having
     enqueued all the requests.  In this case the caller gets a
     notification of the termination of all requests according to the
     SIG parameter.  If SIG is `NULL' no notification is send.
     Otherwise a signal is sent or a thread is started, just as
     described in the description for `aio_read' or `aio_write'.

     If MODE is `LIO_WAIT', the return value of `lio_listio' is 0 when
     all requests completed successfully.  Otherwise the function
     return -1 and `errno' is set accordingly.  To find out which
     request or requests failed one has to use the `aio_error' function
     on all the elements of the array LIST.

     In case MODE is `LIO_NOWAIT', the function returns 0 if all
     requests were enqueued correctly.  The current state of the
     requests can be found using `aio_error' and `aio_return' as
     described above.  If `lio_listio' returns -1 in this mode, the
     global variable `errno' is set accordingly.  If a request did not
     yet terminate, a call to `aio_error' returns `EINPROGRESS'.  If
     the value is different, the request is finished and the error
     value (or 0) is returned and the result of the operation can be
     retrieved using `aio_return'.

     Possible values for `errno' are:

    `EAGAIN'
          The resources necessary to queue all the requests are not
          available at the moment.  The error status for each element
          of LIST must be checked to determine which request failed.

          Another reason could be that the system wide limit of AIO
          requests is exceeded.  This cannot be the case for the
          implementation on GNU systems since no arbitrary limits exist.

    `EINVAL'
          The MODE parameter is invalid or NENT is larger than
          `AIO_LISTIO_MAX'.

    `EIO'
          One or more of the request's I/O operations failed.  The
          error status of each request should be checked to determine
          which one failed.

    `ENOSYS'
          The `lio_listio' function is not supported.

     If the MODE parameter is `LIO_NOWAIT' and the caller cancels a
     request, the error status for this request returned by `aio_error'
     is `ECANCELED'.

     When the sources are compiled with `_FILE_OFFSET_BITS == 64', this
     function is in fact `lio_listio64' since the LFS interface
     transparently replaces the normal implementation.

 -- Function: int lio_listio64 (int MODE, struct aiocb *const LIST, int
          NENT, struct sigevent *SIG)
     This function is similar to the `lio_listio' function.  The only
     difference is that on 32 bit machines, the file descriptor should
     be opened in the large file mode.  Internally, `lio_listio64' uses
     functionality equivalent to `lseek64' (*note File Position
     Primitive::) to position the file descriptor correctly for the
     reading or writing, as opposed to `lseek' functionality used in
     `lio_listio'.

     When the sources are compiled with `_FILE_OFFSET_BITS == 64', this
     function is available under the name `lio_listio' and so
     transparently replaces the interface for small files on 32 bit
     machines.

File: libc.info,  Node: Status of AIO Operations,  Next: Synchronizing AIO Operations,  Prev: Asynchronous Reads/Writes,  Up: Asynchronous I/O

13.10.2 Getting the Status of AIO Operations
--------------------------------------------

As already described in the documentation of the functions in the last
section, it must be possible to get information about the status of an
I/O request.  When the operation is performed truly asynchronously (as
with `aio_read' and `aio_write' and with `lio_listio' when the mode is
`LIO_NOWAIT'), one sometimes needs to know whether a specific request
already terminated and if so, what the result was.  The following two
functions allow you to get this kind of information.

 -- Function: int aio_error (const struct aiocb *AIOCBP)
     This function determines the error state of the request described
     by the `struct aiocb' variable pointed to by AIOCBP.  If the
     request has not yet terminated the value returned is always
     `EINPROGRESS'.  Once the request has terminated the value
     `aio_error' returns is either 0 if the request completed
     successfully or it returns the value which would be stored in the
     `errno' variable if the request would have been done using `read',
     `write', or `fsync'.

     The function can return `ENOSYS' if it is not implemented.  It
     could also return `EINVAL' if the AIOCBP parameter does not refer
     to an asynchronous operation whose return status is not yet known.

     When the sources are compiled with `_FILE_OFFSET_BITS == 64' this
     function is in fact `aio_error64' since the LFS interface
     transparently replaces the normal implementation.

 -- Function: int aio_error64 (const struct aiocb64 *AIOCBP)
     This function is similar to `aio_error' with the only difference
     that the argument is a reference to a variable of type `struct
     aiocb64'.

     When the sources are compiled with `_FILE_OFFSET_BITS == 64' this
     function is available under the name `aio_error' and so
     transparently replaces the interface for small files on 32 bit
     machines.

 -- Function: ssize_t aio_return (const struct aiocb *AIOCBP)
     This function can be used to retrieve the return status of the
     operation carried out by the request described in the variable
     pointed to by AIOCBP.  As long as the error status of this request
     as returned by `aio_error' is `EINPROGRESS' the return of this
     function is undefined.

     Once the request is finished this function can be used exactly
     once to retrieve the return value.  Following calls might lead to
     undefined behavior.  The return value itself is the value which
     would have been returned by the `read', `write', or `fsync' call.

     The function can return `ENOSYS' if it is not implemented.  It
     could also return `EINVAL' if the AIOCBP parameter does not refer
     to an asynchronous operation whose return status is not yet known.

     When the sources are compiled with `_FILE_OFFSET_BITS == 64' this
     function is in fact `aio_return64' since the LFS interface
     transparently replaces the normal implementation.

 -- Function: int aio_return64 (const struct aiocb64 *AIOCBP)
     This function is similar to `aio_return' with the only difference
     that the argument is a reference to a variable of type `struct
     aiocb64'.

     When the sources are compiled with `_FILE_OFFSET_BITS == 64' this
     function is available under the name `aio_return' and so
     transparently replaces the interface for small files on 32 bit
     machines.

File: libc.info,  Node: Synchronizing AIO Operations,  Next: Cancel AIO Operations,  Prev: Status of AIO Operations,  Up: Asynchronous I/O

13.10.3 Getting into a Consistent State
---------------------------------------

When dealing with asynchronous operations it is sometimes necessary to
get into a consistent state.  This would mean for AIO that one wants to
know whether a certain request or a group of request were processed.
This could be done by waiting for the notification sent by the system
after the operation terminated, but this sometimes would mean wasting
resources (mainly computation time).  Instead POSIX.1b defines two
functions which will help with most kinds of consistency.

   The `aio_fsync' and `aio_fsync64' functions are only available if
the symbol `_POSIX_SYNCHRONIZED_IO' is defined in `unistd.h'.

 -- Function: int aio_fsync (int OP, struct aiocb *AIOCBP)
     Calling this function forces all I/O operations operating queued
     at the time of the function call operating on the file descriptor
     `aiocbp->aio_fildes' into the synchronized I/O completion state
     (*note Synchronizing I/O::).  The `aio_fsync' function returns
     immediately but the notification through the method described in
     `aiocbp->aio_sigevent' will happen only after all requests for this
     file descriptor have terminated and the file is synchronized.
     This also means that requests for this very same file descriptor
     which are queued after the synchronization request are not
     affected.

     If OP is `O_DSYNC' the synchronization happens as with a call to
     `fdatasync'.  Otherwise OP should be `O_SYNC' and the
     synchronization happens as with `fsync'.

     As long as the synchronization has not happened, a call to
     `aio_error' with the reference to the object pointed to by AIOCBP
     returns `EINPROGRESS'.  Once the synchronization is done
     `aio_error' return 0 if the synchronization was not successful.
     Otherwise the value returned is the value to which the `fsync' or
     `fdatasync' function would have set the `errno' variable.  In this
     case nothing can be assumed about the consistency for the data
     written to this file descriptor.

     The return value of this function is 0 if the request was
     successfully enqueued.  Otherwise the return value is -1 and
     `errno' is set to one of the following values:

    `EAGAIN'
          The request could not be enqueued due to temporary lack of
          resources.

    `EBADF'
          The file descriptor `aiocbp->aio_fildes' is not valid or not
          open for writing.

    `EINVAL'
          The implementation does not support I/O synchronization or
          the OP parameter is other than `O_DSYNC' and `O_SYNC'.

    `ENOSYS'
          This function is not implemented.

     When the sources are compiled with `_FILE_OFFSET_BITS == 64' this
     function is in fact `aio_fsync64' since the LFS interface
     transparently replaces the normal implementation.

 -- Function: int aio_fsync64 (int OP, struct aiocb64 *AIOCBP)
     This function is similar to `aio_fsync' with the only difference
     that the argument is a reference to a variable of type `struct
     aiocb64'.

     When the sources are compiled with `_FILE_OFFSET_BITS == 64' this
     function is available under the name `aio_fsync' and so
     transparently replaces the interface for small files on 32 bit
     machines.

   Another method of synchronization is to wait until one or more
requests of a specific set terminated.  This could be achieved by the
`aio_*' functions to notify the initiating process about the
termination but in some situations this is not the ideal solution.  In
a program which constantly updates clients somehow connected to the
server it is not always the best solution to go round robin since some
connections might be slow.  On the other hand letting the `aio_*'
function notify the caller might also be not the best solution since
whenever the process works on preparing data for on client it makes no
sense to be interrupted by a notification since the new client will not
be handled before the current client is served.  For situations like
this `aio_suspend' should be used.

 -- Function: int aio_suspend (const struct aiocb *const LIST[], int
          NENT, const struct timespec *TIMEOUT)
     When calling this function, the calling thread is suspended until
     at least one of the requests pointed to by the NENT elements of the
     array LIST has completed.  If any of the requests has already
     completed at the time `aio_suspend' is called, the function returns
     immediately.  Whether a request has terminated or not is
     determined by comparing the error status of the request with
     `EINPROGRESS'.  If an element of LIST is `NULL', the entry is
     simply ignored.

     If no request has finished, the calling process is suspended.  If
     TIMEOUT is `NULL', the process is not woken until a request has
     finished.  If TIMEOUT is not `NULL', the process remains suspended
     at least as long as specified in TIMEOUT.  In this case,
     `aio_suspend' returns with an error.

     The return value of the function is 0 if one or more requests from
     the LIST have terminated.  Otherwise the function returns -1 and
     `errno' is set to one of the following values:

    `EAGAIN'
          None of the requests from the LIST completed in the time
          specified by TIMEOUT.

    `EINTR'
          A signal interrupted the `aio_suspend' function.  This signal
          might also be sent by the AIO implementation while signalling
          the termination of one of the requests.

    `ENOSYS'
          The `aio_suspend' function is not implemented.

     When the sources are compiled with `_FILE_OFFSET_BITS == 64' this
     function is in fact `aio_suspend64' since the LFS interface
     transparently replaces the normal implementation.

 -- Function: int aio_suspend64 (const struct aiocb64 *const LIST[],
          int NENT, const struct timespec *TIMEOUT)
     This function is similar to `aio_suspend' with the only difference
     that the argument is a reference to a variable of type `struct
     aiocb64'.

     When the sources are compiled with `_FILE_OFFSET_BITS == 64' this
     function is available under the name `aio_suspend' and so
     transparently replaces the interface for small files on 32 bit
     machines.

File: libc.info,  Node: Cancel AIO Operations,  Next: Configuration of AIO,  Prev: Synchronizing AIO Operations,  Up: Asynchronous I/O

13.10.4 Cancellation of AIO Operations
--------------------------------------

When one or more requests are asynchronously processed, it might be
useful in some situations to cancel a selected operation, e.g., if it
becomes obvious that the written data is no longer accurate and would
have to be overwritten soon.  As an example, assume an application,
which writes data in files in a situation where new incoming data would
have to be written in a file which will be updated by an enqueued
request.  The POSIX AIO implementation provides such a function, but
this function is not capable of forcing the cancellation of the
request.  It is up to the implementation to decide whether it is
possible to cancel the operation or not.  Therefore using this function
is merely a hint.

 -- Function: int aio_cancel (int FILDES, struct aiocb *AIOCBP)
     The `aio_cancel' function can be used to cancel one or more
     outstanding requests.  If the AIOCBP parameter is `NULL', the
     function tries to cancel all of the outstanding requests which
     would process the file descriptor FILDES (i.e., whose `aio_fildes'
     member is FILDES).  If AIOCBP is not `NULL', `aio_cancel' attempts
     to cancel the specific request pointed to by AIOCBP.

     For requests which were successfully canceled, the normal
     notification about the termination of the request should take
     place.  I.e., depending on the `struct sigevent' object which
     controls this, nothing happens, a signal is sent or a thread is
     started.  If the request cannot be canceled, it terminates the
     usual way after performing the operation.

     After a request is successfully canceled, a call to `aio_error'
     with a reference to this request as the parameter will return
     `ECANCELED' and a call to `aio_return' will return -1.  If the
     request wasn't canceled and is still running the error status is
     still `EINPROGRESS'.

     The return value of the function is `AIO_CANCELED' if there were
     requests which haven't terminated and which were successfully
     canceled.  If there is one or more requests left which couldn't be
     canceled, the return value is `AIO_NOTCANCELED'.  In this case
     `aio_error' must be used to find out which of the, perhaps
     multiple, requests (in AIOCBP is `NULL') weren't successfully
     canceled.  If all requests already terminated at the time
     `aio_cancel' is called the return value is `AIO_ALLDONE'.

     If an error occurred during the execution of `aio_cancel' the
     function returns -1 and sets `errno' to one of the following
     values.

    `EBADF'
          The file descriptor FILDES is not valid.

    `ENOSYS'
          `aio_cancel' is not implemented.

     When the sources are compiled with `_FILE_OFFSET_BITS == 64', this
     function is in fact `aio_cancel64' since the LFS interface
     transparently replaces the normal implementation.

 -- Function: int aio_cancel64 (int FILDES, struct aiocb64 *AIOCBP)
     This function is similar to `aio_cancel' with the only difference
     that the argument is a reference to a variable of type `struct
     aiocb64'.

     When the sources are compiled with `_FILE_OFFSET_BITS == 64', this
     function is available under the name `aio_cancel' and so
     transparently replaces the interface for small files on 32 bit
     machines.

File: libc.info,  Node: Configuration of AIO,  Prev: Cancel AIO Operations,  Up: Asynchronous I/O

13.10.5 How to optimize the AIO implementation
----------------------------------------------

The POSIX standard does not specify how the AIO functions are
implemented.  They could be system calls, but it is also possible to
emulate them at userlevel.

   At the point of this writing, the available implementation is a
userlevel implementation which uses threads for handling the enqueued
requests.  While this implementation requires making some decisions
about limitations, hard limitations are something which is best avoided
in the GNU C library.  Therefore, the GNU C library provides a means
for tuning the AIO implementation according to the individual use.

 -- Data Type: struct aioinit
     This data type is used to pass the configuration or tunable
     parameters to the implementation.  The program has to initialize
     the members of this struct and pass it to the implementation using
     the `aio_init' function.

    `int aio_threads'
          This member specifies the maximal number of threads which may
          be used at any one time.

    `int aio_num'
          This number provides an estimate on the maximal number of
          simultaneously enqueued requests.

    `int aio_locks'
          Unused.

    `int aio_usedba'
          Unused.

    `int aio_debug'
          Unused.

    `int aio_numusers'
          Unused.

    `int aio_reserved[2]'
          Unused.

 -- Function: void aio_init (const struct aioinit *INIT)
     This function must be called before any other AIO function.
     Calling it is completely voluntary, as it is only meant to help
     the AIO implementation perform better.

     Before calling the `aio_init', function the members of a variable
     of type `struct aioinit' must be initialized.  Then a reference to
     this variable is passed as the parameter to `aio_init' which itself
     may or may not pay attention to the hints.

     The function has no return value and no error cases are defined.
     It is a extension which follows a proposal from the SGI
     implementation in Irix 6.  It is not covered by POSIX.1b or Unix98.

File: libc.info,  Node: Control Operations,  Next: Duplicating Descriptors,  Prev: Asynchronous I/O,  Up: Low-Level I/O

13.11 Control Operations on Files
=================================

This section describes how you can perform various other operations on
file descriptors, such as inquiring about or setting flags describing
the status of the file descriptor, manipulating record locks, and the
like.  All of these operations are performed by the function `fcntl'.

   The second argument to the `fcntl' function is a command that
specifies which operation to perform.  The function and macros that name
various flags that are used with it are declared in the header file
`fcntl.h'.  Many of these flags are also used by the `open' function;
see *note Opening and Closing Files::.

 -- Function: int fcntl (int FILEDES, int COMMAND, ...)
     The `fcntl' function performs the operation specified by COMMAND
     on the file descriptor FILEDES.  Some commands require additional
     arguments to be supplied.  These additional arguments and the
     return value and error conditions are given in the detailed
     descriptions of the individual commands.

     Briefly, here is a list of what the various commands are.

    `F_DUPFD'
          Duplicate the file descriptor (return another file descriptor
          pointing to the same open file).  *Note Duplicating
          Descriptors::.

    `F_GETFD'
          Get flags associated with the file descriptor.  *Note
          Descriptor Flags::.

    `F_SETFD'
          Set flags associated with the file descriptor.  *Note
          Descriptor Flags::.

    `F_GETFL'
          Get flags associated with the open file.  *Note File Status
          Flags::.

    `F_SETFL'
          Set flags associated with the open file.  *Note File Status
          Flags::.

    `F_GETLK'
          Get a file lock.  *Note File Locks::.

    `F_SETLK'
          Set or clear a file lock.  *Note File Locks::.

    `F_SETLKW'
          Like `F_SETLK', but wait for completion.  *Note File Locks::.

    `F_GETOWN'
          Get process or process group ID to receive `SIGIO' signals.
          *Note Interrupt Input::.

    `F_SETOWN'
          Set process or process group ID to receive `SIGIO' signals.
          *Note Interrupt Input::.

     This function is a cancellation point in multi-threaded programs.
     This is a problem if the thread allocates some resources (like
     memory, file descriptors, semaphores or whatever) at the time
     `fcntl' is called.  If the thread gets canceled these resources
     stay allocated until the program ends.  To avoid this calls to
     `fcntl' should be protected using cancellation handlers.

File: libc.info,  Node: Duplicating Descriptors,  Next: Descriptor Flags,  Prev: Control Operations,  Up: Low-Level I/O

13.12 Duplicating Descriptors
=============================

You can "duplicate" a file descriptor, or allocate another file
descriptor that refers to the same open file as the original.  Duplicate
descriptors share one file position and one set of file status flags
(*note File Status Flags::), but each has its own set of file descriptor
flags (*note Descriptor Flags::).

   The major use of duplicating a file descriptor is to implement
"redirection" of input or output:  that is, to change the file or pipe
that a particular file descriptor corresponds to.

   You can perform this operation using the `fcntl' function with the
`F_DUPFD' command, but there are also convenient functions `dup' and
`dup2' for duplicating descriptors.

   The `fcntl' function and flags are declared in `fcntl.h', while
prototypes for `dup' and `dup2' are in the header file `unistd.h'.

 -- Function: int dup (int OLD)
     This function copies descriptor OLD to the first available
     descriptor number (the first number not currently open).  It is
     equivalent to `fcntl (OLD, F_DUPFD, 0)'.

 -- Function: int dup2 (int OLD, int NEW)
     This function copies the descriptor OLD to descriptor number NEW.

     If OLD is an invalid descriptor, then `dup2' does nothing; it does
     not close NEW.  Otherwise, the new duplicate of OLD replaces any
     previous meaning of descriptor NEW, as if NEW were closed first.

     If OLD and NEW are different numbers, and OLD is a valid
     descriptor number, then `dup2' is equivalent to:

          close (NEW);
          fcntl (OLD, F_DUPFD, NEW)

     However, `dup2' does this atomically; there is no instant in the
     middle of calling `dup2' at which NEW is closed and not yet a
     duplicate of OLD.

 -- Macro: int F_DUPFD
     This macro is used as the COMMAND argument to `fcntl', to copy the
     file descriptor given as the first argument.

     The form of the call in this case is:

          fcntl (OLD, F_DUPFD, NEXT-FILEDES)

     The NEXT-FILEDES argument is of type `int' and specifies that the
     file descriptor returned should be the next available one greater
     than or equal to this value.

     The return value from `fcntl' with this command is normally the
     value of the new file descriptor.  A return value of -1 indicates
     an error.  The following `errno' error conditions are defined for
     this command:

    `EBADF'
          The OLD argument is invalid.

    `EINVAL'
          The NEXT-FILEDES argument is invalid.

    `EMFILE'
          There are no more file descriptors available--your program is
          already using the maximum.  In BSD and GNU, the maximum is
          controlled by a resource limit that can be changed; *note
          Limits on Resources::, for more information about the
          `RLIMIT_NOFILE' limit.

     `ENFILE' is not a possible error code for `dup2' because `dup2'
     does not create a new opening of a file; duplicate descriptors do
     not count toward the limit which `ENFILE' indicates.  `EMFILE' is
     possible because it refers to the limit on distinct descriptor
     numbers in use in one process.

   Here is an example showing how to use `dup2' to do redirection.
Typically, redirection of the standard streams (like `stdin') is done
by a shell or shell-like program before calling one of the `exec'
functions (*note Executing a File::) to execute a new program in a
child process.  When the new program is executed, it creates and
initializes the standard streams to point to the corresponding file
descriptors, before its `main' function is invoked.

   So, to redirect standard input to a file, the shell could do
something like:

     pid = fork ();
     if (pid == 0)
       {
         char *filename;
         char *program;
         int file;
         ...
         file = TEMP_FAILURE_RETRY (open (filename, O_RDONLY));
         dup2 (file, STDIN_FILENO);
         TEMP_FAILURE_RETRY (close (file));
         execv (program, NULL);
       }

   There is also a more detailed example showing how to implement
redirection in the context of a pipeline of processes in *note
Launching Jobs::.

File: libc.info,  Node: Descriptor Flags,  Next: File Status Flags,  Prev: Duplicating Descriptors,  Up: Low-Level I/O

13.13 File Descriptor Flags
===========================

"File descriptor flags" are miscellaneous attributes of a file
descriptor.  These flags are associated with particular file
descriptors, so that if you have created duplicate file descriptors
from a single opening of a file, each descriptor has its own set of
flags.

   Currently there is just one file descriptor flag: `FD_CLOEXEC',
which causes the descriptor to be closed if you use any of the
`exec...' functions (*note Executing a File::).

   The symbols in this section are defined in the header file `fcntl.h'.

 -- Macro: int F_GETFD
     This macro is used as the COMMAND argument to `fcntl', to specify
     that it should return the file descriptor flags associated with
     the FILEDES argument.

     The normal return value from `fcntl' with this command is a
     nonnegative number which can be interpreted as the bitwise OR of
     the individual flags (except that currently there is only one flag
     to use).

     In case of an error, `fcntl' returns -1.  The following `errno'
     error conditions are defined for this command:

    `EBADF'
          The FILEDES argument is invalid.

 -- Macro: int F_SETFD
     This macro is used as the COMMAND argument to `fcntl', to specify
     that it should set the file descriptor flags associated with the
     FILEDES argument.  This requires a third `int' argument to specify
     the new flags, so the form of the call is:

          fcntl (FILEDES, F_SETFD, NEW-FLAGS)

     The normal return value from `fcntl' with this command is an
     unspecified value other than -1, which indicates an error.  The
     flags and error conditions are the same as for the `F_GETFD'
     command.

   The following macro is defined for use as a file descriptor flag with
the `fcntl' function.  The value is an integer constant usable as a bit
mask value.

 -- Macro: int FD_CLOEXEC
     This flag specifies that the file descriptor should be closed when
     an `exec' function is invoked; see *note Executing a File::.  When
     a file descriptor is allocated (as with `open' or `dup'), this bit
     is initially cleared on the new file descriptor, meaning that
     descriptor will survive into the new program after `exec'.

   If you want to modify the file descriptor flags, you should get the
current flags with `F_GETFD' and modify the value.  Don't assume that
the flags listed here are the only ones that are implemented; your
program may be run years from now and more flags may exist then.  For
example, here is a function to set or clear the flag `FD_CLOEXEC'
without altering any other flags:

     /* Set the `FD_CLOEXEC' flag of DESC if VALUE is nonzero,
        or clear the flag if VALUE is 0.
        Return 0 on success, or -1 on error with `errno' set. */

     int
     set_cloexec_flag (int desc, int value)
     {
       int oldflags = fcntl (desc, F_GETFD, 0);
       /* If reading the flags failed, return error indication now. */
       if (oldflags < 0)
         return oldflags;
       /* Set just the flag we want to set. */
       if (value != 0)
         oldflags |= FD_CLOEXEC;
       else
         oldflags &= ~FD_CLOEXEC;
       /* Store modified flag word in the descriptor. */
       return fcntl (desc, F_SETFD, oldflags);
     }

File: libc.info,  Node: File Status Flags,  Next: File Locks,  Prev: Descriptor Flags,  Up: Low-Level I/O

13.14 File Status Flags
=======================

"File status flags" are used to specify attributes of the opening of a
file.  Unlike the file descriptor flags discussed in *note Descriptor
Flags::, the file status flags are shared by duplicated file descriptors
resulting from a single opening of the file.  The file status flags are
specified with the FLAGS argument to `open'; *note Opening and Closing
Files::.

   File status flags fall into three categories, which are described in
the following sections.

   * *note Access Modes::, specify what type of access is allowed to the
     file: reading, writing, or both.  They are set by `open' and are
     returned by `fcntl', but cannot be changed.

   * *note Open-time Flags::, control details of what `open' will do.
     These flags are not preserved after the `open' call.

   * *note Operating Modes::, affect how operations such as `read' and
     `write' are done.  They are set by `open', and can be fetched or
     changed with `fcntl'.

   The symbols in this section are defined in the header file `fcntl.h'.

* Menu:

* Access Modes::                Whether the descriptor can read or write.
* Open-time Flags::             Details of `open'.
* Operating Modes::             Special modes to control I/O operations.
* Getting File Status Flags::   Fetching and changing these flags.

File: libc.info,  Node: Access Modes,  Next: Open-time Flags,  Up: File Status Flags

13.14.1 File Access Modes
-------------------------

The file access modes allow a file descriptor to be used for reading,
writing, or both.  (In the GNU system, they can also allow none of
these, and allow execution of the file as a program.)  The access modes
are chosen when the file is opened, and never change.

 -- Macro: int O_RDONLY
     Open the file for read access.

 -- Macro: int O_WRONLY
     Open the file for write access.

 -- Macro: int O_RDWR
     Open the file for both reading and writing.

   In the GNU system (and not in other systems), `O_RDONLY' and
`O_WRONLY' are independent bits that can be bitwise-ORed together, and
it is valid for either bit to be set or clear.  This means that
`O_RDWR' is the same as `O_RDONLY|O_WRONLY'.  A file access mode of
zero is permissible; it allows no operations that do input or output to
the file, but does allow other operations such as `fchmod'.  On the GNU
system, since "read-only" or "write-only" is a misnomer, `fcntl.h'
defines additional names for the file access modes.  These names are
preferred when writing GNU-specific code.  But most programs will want
to be portable to other POSIX.1 systems and should use the POSIX.1
names above instead.

 -- Macro: int O_READ
     Open the file for reading.  Same as `O_RDONLY'; only defined on
     GNU.

 -- Macro: int O_WRITE
     Open the file for writing.  Same as `O_WRONLY'; only defined on
     GNU.

 -- Macro: int O_EXEC
     Open the file for executing.  Only defined on GNU.

   To determine the file access mode with `fcntl', you must extract the
access mode bits from the retrieved file status flags.  In the GNU
system, you can just test the `O_READ' and `O_WRITE' bits in the flags
word.  But in other POSIX.1 systems, reading and writing access modes
are not stored as distinct bit flags.  The portable way to extract the
file access mode bits is with `O_ACCMODE'.

 -- Macro: int O_ACCMODE
     This macro stands for a mask that can be bitwise-ANDed with the
     file status flag value to produce a value representing the file
     access mode.  The mode will be `O_RDONLY', `O_WRONLY', or `O_RDWR'.
     (In the GNU system it could also be zero, and it never includes the
     `O_EXEC' bit.)

File: libc.info,  Node: Open-time Flags,  Next: Operating Modes,  Prev: Access Modes,  Up: File Status Flags

13.14.2 Open-time Flags
-----------------------

The open-time flags specify options affecting how `open' will behave.
These options are not preserved once the file is open.  The exception to
this is `O_NONBLOCK', which is also an I/O operating mode and so it
_is_ saved.  *Note Opening and Closing Files::, for how to call `open'.

   There are two sorts of options specified by open-time flags.

   * "File name translation flags" affect how `open' looks up the file
     name to locate the file, and whether the file can be created.

   * "Open-time action flags" specify extra operations that `open' will
     perform on the file once it is open.

   Here are the file name translation flags.

 -- Macro: int O_CREAT
     If set, the file will be created if it doesn't already exist.

 -- Macro: int O_EXCL
     If both `O_CREAT' and `O_EXCL' are set, then `open' fails if the
     specified file already exists.  This is guaranteed to never
     clobber an existing file.

 -- Macro: int O_NONBLOCK
     This prevents `open' from blocking for a "long time" to open the
     file.  This is only meaningful for some kinds of files, usually
     devices such as serial ports; when it is not meaningful, it is
     harmless and ignored.  Often opening a port to a modem blocks
     until the modem reports carrier detection; if `O_NONBLOCK' is
     specified, `open' will return immediately without a carrier.

     Note that the `O_NONBLOCK' flag is overloaded as both an I/O
     operating mode and a file name translation flag.  This means that
     specifying `O_NONBLOCK' in `open' also sets nonblocking I/O mode;
     *note Operating Modes::.  To open the file without blocking but do
     normal I/O that blocks, you must call `open' with `O_NONBLOCK' set
     and then call `fcntl' to turn the bit off.

 -- Macro: int O_NOCTTY
     If the named file is a terminal device, don't make it the
     controlling terminal for the process.  *Note Job Control::, for
     information about what it means to be the controlling terminal.

     In the GNU system and 4.4 BSD, opening a file never makes it the
     controlling terminal and `O_NOCTTY' is zero.  However, other
     systems may use a nonzero value for `O_NOCTTY' and set the
     controlling terminal when you open a file that is a terminal
     device; so to be portable, use `O_NOCTTY' when it is important to
     avoid this.

   The following three file name translation flags exist only in the
GNU system.

 -- Macro: int O_IGNORE_CTTY
     Do not recognize the named file as the controlling terminal, even
     if it refers to the process's existing controlling terminal
     device.  Operations on the new file descriptor will never induce
     job control signals.  *Note Job Control::.

 -- Macro: int O_NOLINK
     If the named file is a symbolic link, open the link itself instead
     of the file it refers to.  (`fstat' on the new file descriptor will
     return the information returned by `lstat' on the link's name.)

 -- Macro: int O_NOTRANS
     If the named file is specially translated, do not invoke the
     translator.  Open the bare file the translator itself sees.

   The open-time action flags tell `open' to do additional operations
which are not really related to opening the file.  The reason to do them
as part of `open' instead of in separate calls is that `open' can do
them atomically.

 -- Macro: int O_TRUNC
     Truncate the file to zero length.  This option is only useful for
     regular files, not special files such as directories or FIFOs.
     POSIX.1 requires that you open the file for writing to use
     `O_TRUNC'.  In BSD and GNU you must have permission to write the
     file to truncate it, but you need not open for write access.

     This is the only open-time action flag specified by POSIX.1.
     There is no good reason for truncation to be done by `open',
     instead of by calling `ftruncate' afterwards.  The `O_TRUNC' flag
     existed in Unix before `ftruncate' was invented, and is retained
     for backward compatibility.

   The remaining operating modes are BSD extensions.  They exist only
on some systems.  On other systems, these macros are not defined.

 -- Macro: int O_SHLOCK
     Acquire a shared lock on the file, as with `flock'.  *Note File
     Locks::.

     If `O_CREAT' is specified, the locking is done atomically when
     creating the file.  You are guaranteed that no other process will
     get the lock on the new file first.

 -- Macro: int O_EXLOCK
     Acquire an exclusive lock on the file, as with `flock'.  *Note
     File Locks::.  This is atomic like `O_SHLOCK'.

File: libc.info,  Node: Operating Modes,  Next: Getting File Status Flags,  Prev: Open-time Flags,  Up: File Status Flags

13.14.3 I/O Operating Modes
---------------------------

The operating modes affect how input and output operations using a file
descriptor work.  These flags are set by `open' and can be fetched and
changed with `fcntl'.

 -- Macro: int O_APPEND
     The bit that enables append mode for the file.  If set, then all
     `write' operations write the data at the end of the file, extending
     it, regardless of the current file position.  This is the only
     reliable way to append to a file.  In append mode, you are
     guaranteed that the data you write will always go to the current
     end of the file, regardless of other processes writing to the
     file.  Conversely, if you simply set the file position to the end
     of file and write, then another process can extend the file after
     you set the file position but before you write, resulting in your
     data appearing someplace before the real end of file.

 -- Macro: int O_NONBLOCK
     The bit that enables nonblocking mode for the file.  If this bit
     is set, `read' requests on the file can return immediately with a
     failure status if there is no input immediately available, instead
     of blocking.  Likewise, `write' requests can also return
     immediately with a failure status if the output can't