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File: gdbm.info,  Node: Top,  Next: Copying,  Prev: (dir),  Up: (dir)

   GNU `dbm' is a library of functions implementing a hashed database
on a disk file.  This manual documents GNU `dbm' Version 1.8 (`gdbm').
The software was written by Philip A. Nelson. This document was
originally written by Pierre Gaumond from texts written by Phil.

* Menu:

Introduction:

* Copying::                    Your rights.
* Intro::                      Introduction to GNU dbm.
* List::                       List of functions.

Functions:

* Open::                       Opening the database.
* Close::                      Closing the database.
* Store::                      Inserting and replacing records in the database.
* Fetch::                      Searching records in the database.
* Delete::                     Removing records from the database.
* Sequential::                 Sequential access to records.
* Reorganization::             Database reorganization.
* Sync::                       Insure all writes to disk have competed.
* Errors::                     Convert internal error codes into English.
* Options::                    Setting internal options.
* Locking::                    File locking.

Other topics:

* Variables::                  Two useful variables.
* Compatibility::              Compatibility with UNIX dbm and ndbm.
* Conversion::                 Converting dbm files to gdbm format.
* Bugs::                       Problems and bugs.

File: gdbm.info,  Node: Copying,  Next: Intro,  Prev: Top,  Up: Top

Copying Conditions.
*******************

   This library is "free"; this means that everyone is free to use it
and free to redistribute it on a free basis.  GNU `dbm' (`gdbm') is not
in the public domain; it is copyrighted and there are restrictions on
its distribution, but these restrictions are designed to permit
everything that a good cooperating citizen would want to do.  What is
not allowed is to try to prevent others from further sharing any
version of `gdbm' that they might get from you.

   Specifically, we want to make sure that you have the right to give
away copies `gdbm', that you receive source code or else can get it if
you want it, that you can change these functions or use pieces of them
in new free programs, and that you know you can do these things.

   To make sure that everyone has such rights, we have to forbid you to
deprive anyone else of these rights.  For example, if you distribute
copies `gdbm', you must give the recipients all the rights that you
have.  You must make sure that they, too, receive or can get the source
code.  And you must tell them their rights.

   Also, for our own protection, we must make certain that everyone
finds out that there is no warranty for anything in the `gdbm'
distribution.  If these functions are modified by someone else and
passed on, we want their recipients to know that what they have is not
what we distributed, so that any problems introduced by others will not
reflect on our reputation.

   `gdbm' is currently distributed under the terms of the GNU General
Public License, Version 2.  (*NOT* under the GNU General Library Public
License.)  A copy the GNU General Public License is included with the
distribution of `gdbm'.

File: gdbm.info,  Node: Intro,  Next: List,  Prev: Copying,  Up: Top

Introduction to GNU `dbm'.
**************************

   GNU `dbm' (`gdbm')is a library of database functions that use
extendible hashing and works similar to the standard UNIX `dbm'
functions.  These routines are provided to a programmer needing to
create and manipulate a hashed database. (`gdbm' is *NOT* a complete
database package for an end user.)

   The basic use of `gdbm' is to store key/data pairs in a data file.
Each key must be unique and each key is paired with only one data item.
The keys can not be directly accessed in sorted order.  The basic unit
of data in `gdbm' is the structure:

       typedef struct {
                  char *dptr;
                  int  dsize;
               } datum;

   This structure allows for arbitrary sized keys and data items.

   The key/data pairs are stored in a `gdbm' disk file, called a `gdbm'
database.  An application must open a `gdbm' database to be able
manipulate the keys and data contained in the database.  `gdbm' allows
an application to have multiple databases open at the same time.  When
an application opens a `gdbm' database, it is designated as a `reader'
or a `writer'.  A `gdbm' database opened by at most one writer at a
time.  However, many readers may open the database open simultaneously.
Readers and writers can not open the `gdbm' database at the same time.

File: gdbm.info,  Node: List,  Next: Open,  Prev: Intro,  Up: Top

List of functions.
******************

   The following is a quick list of the functions contained in the
`gdbm' library. The include file `gdbm.h', that can be included by the
user, contains a definition of these functions.

     #include <gdbm.h>

     GDBM_FILE gdbm_open(name, block_size, flags, mode, fatal_func);
     void gdbm_close(dbf);
     int gdbm_store(dbf, key, content, flag);
     datum gdbm_fetch(dbf, key);
     int gdbm_delete(dbf, key);
     datum gdbm_firstkey(dbf);
     datum gdbm_nextkey(dbf, key);
     int gdbm_reorganize(dbf);
     void gdbm_sync(dbf);
     int gdbm_exists(dbf, key);
     char *gdbm_strerror(errno);
     int gdbm_setopt(dbf, option, value, size);
     int gdbm_fdesc(dbf);

   The `gdbm.h' include file is often in the `/usr/local/include'
directory. (The actual location of `gdbm.h' depends on your local
installation of `gdbm'.)

File: gdbm.info,  Node: Open,  Next: Close,  Prev: List,  Up: Top

Opening the database.
*********************

   Initialize `gdbm' system. If the file has a size of zero bytes, a
file initialization procedure is performed, setting up the initial
structure in the file.

   The procedure for opening a `gdbm' file is:

     GDBM_FILE dbf;

     dbf = gdbm_open(name, block_size, flags, mode, fatal_func);

   The parameters are:

char *name
     The name of the file (the complete name, `gdbm' does not append any
     characters to this name).

int block_size
     It is used during initialization to determine the size of various
     constructs. It is the size of a single transfer from disk to
     memory. This parameter is ignored if the file has been previously
     initialized. The minimum size is 512.  If the value is less than
     512, the file system blocksize is used, otherwise the value of
     `block_size' is used.

int flags
     If `flags' is set to GDBM_READER, the user wants to just read the
     database and any call to `gdbm_store' or `gdbm_delete' will fail.
     Many readers can access the database at the same time. If `flags'
     is set to GDBM_WRITER, the user wants both read and write access
     to the database and requires exclusive access. If `flags' is set
     to GDBM_WRCREAT, the user wants both read and write access to the
     database and if the database does not exist, create a new one. If
     `flags' is set to GDBM_NEWDB, the user want a new database
     created, regardless of whether one existed, and wants read and
     write access to the new database.  The following may also be
     logically or'd into the database flags: GDBM_SYNC, which causes
     all database operations to be synchronized to the disk, and
     GDBM_NOLOCK, which prevents the library from performing any
     locking on the database file.  The option GDBM_FAST is now
     obsolete, since `gdbm' defaults to no-sync mode.  Any error
     detected will cause a return value of NULL and an appropriate
     value will be in `gdbm_errno' (see Variables). If no errors occur,
     a pointer to the `gdbm' file descriptor will be returned.

int mode
     File mode (see chmod(2) and open(2) if the file is created).

void (*fatal_func) ()
     A function for `gdbm' to call if it detects a fatal error. The only
     parameter of this function is a string. If the value of NULL is
     provided, `gdbm' will use a default function.

   The return value, `dbf', is the pointer needed by all other
functions to access that `gdbm' file. If the return is the NULL pointer,
`gdbm_open' was not successful. The errors can be found in `gdbm_errno'
for `gdbm' errors and in `errno' for file system errors (for error
codes, see `gdbm.h').

   In all of the following calls, the parameter `dbf' refers to the
pointer returned from `gdbm_open'.

File: gdbm.info,  Node: Close,  Next: Store,  Prev: Open,  Up: Top

Closing the database.
*********************

   It is important that every file opened is also closed. This is
needed to update the reader/writer count on the file. This is done by:

     gdbm_close(dbf);

   The parameter is:

GDBM_FILE dbf
     The pointer returned by `gdbm_open'.

   Closes the `gdbm' file and frees all memory associated with the file
`dbf'.

File: gdbm.info,  Node: Store,  Next: Fetch,  Prev: Close,  Up: Top

Inserting and replacing records in the database.
************************************************

   The function `gdbm_store' inserts or replaces records in the
database.

     ret = gdbm_store(dbf, key, content, flag);

   The parameters are:

GDBM_FILE dbf
     The pointer returned by `gdbm_open'.

datum key
     The `key' data.

datum content
     The data to be associated with the key.

int flag
     Defines the action to take when the key is already in the
     database. The value GDBM_REPLACE (defined in `gdbm.h') asks that
     the old data be replaced by the new `content'. The value
     GDBM_INSERT asks that an error be returned and no action taken if
     the `key' already exists.

   The values returned in `ret' are:

-1
     The item was not stored in the database because the caller was not
     an official writer or either `key' or `content' have a NULL dptr
     field.  Both `key' and `content' must have the dptr field be a
     non-NULL value.  Since a NULL dptr field is used by other
     functions to indicate an error, a NULL field cannot be valid data.

+1
     The item was not stored because the argument `flag' was
     GDBM_INSERT and the `key' was already in the database.

0
     No error. `content' is keyed by `key'. The file on disk is updated
     to reflect the structure of the new database before returning from
     this function.

   If you store data for a `key' that is already in the data base,
`gdbm' replaces the old data with the new data if called with
GDBM_REPLACE. You do not get two data items for the same `key' and you
do not get an error from `gdbm_store'.

   The size in `gdbm' is not restricted like `dbm' or `ndbm'. Your data
can be as large as you want.

File: gdbm.info,  Node: Fetch,  Next: Delete,  Prev: Store,  Up: Top

Searching for records in the database.
**************************************

   Looks up a given `key' and returns the information associated with
that key. The pointer in the structure that is  returned is a pointer
to dynamically allocated memory block. To search for some data:

     content = gdbm_fetch(dbf, key);

   The parameters are:

GDBM_FILE dbf
     The pointer returned by `gdbm_open'.

datum key
     The `key' data.

   The datum returned in `content' is a pointer to the data found. If
the dptr is NULL, no data was found. If dptr is not NULL, then it points
to data allocated by malloc. `gdbm' does not automatically free this
data.  The user must free this storage when done using it. This
eliminates the need to copy the result to save it for later use (you
just save the pointer).

   You may also search for a particular key without retrieving it,
using:

     ret = gdbm_exists(dbf, key);

   The parameters are:

GDBM_FILE dbf
     The pointer returned by `gdbm_open'.

datum key
     The `key' data.

   Unlike `gdbm_fetch', this routine does not allocate any memory, and
simply returns true or false, depending on whether the `key' exists, or
not.

File: gdbm.info,  Node: Delete,  Next: Sequential,  Prev: Fetch,  Up: Top

Removing records from the database.
***********************************

   To remove some data from the database:

     ret = gdbm_delete(dbf, key);

   The parameters are:

GDBM_FILE dbf
     The pointer returned by `gdbm_open'.

datum key
     The `key' data.

   The ret value is -1 if the item is not present or the requester is a
reader.  The ret value is 0 if there was a successful delete.

   `gdbm_delete' removes the keyed item and the `key' from the database
`dbf'. The file on disk is updated to reflect the structure of the new
database before returning from this function.

File: gdbm.info,  Node: Sequential,  Next: Reorganization,  Prev: Delete,  Up: Top

Sequential access to records.
*****************************

   The next two functions allow for accessing all items in the
database. This access is not `key' sequential, but it is guaranteed to
visit every `key' in the database once. The order has to do with the
hash values.  `gdbm_firstkey' starts the visit of all keys in the
database.  `gdbm_nextkey' finds and reads the next entry in the hash
structure for `dbf'.

     key = gdbm_firstkey(dbf);

     nextkey = gdbm_nextkey(dbf, key);

   The parameters are:

GDBM_FILE dbf
     The pointer returned by `gdbm_open'.

datum `key'

datum nextkey
     The `key' data.

   The return values are both datum. If `key'.dptr or nextkey.dptr is
NULL, there is no first `key' or next `key'. Again notice that dptr
points to data allocated by malloc and `gdbm' will not free it for you.

   These functions were intended to visit the database in read-only
algorithms, for instance, to validate the database or similar
operations.

   File `visiting' is based on a `hash table'. `gdbm_delete'
re-arranges the hash table to make sure that any collisions in the
table do not leave some item `un-findable'. The original key order is
NOT guaranteed to remain unchanged in ALL instances. It is possible
that some key will not be visited if a loop like the following is
executed:

        key = gdbm_firstkey ( dbf );
        while ( key.dptr ) {
           nextkey = gdbm_nextkey ( dbf, key );
           if ( some condition ) {
              gdbm_delete ( dbf, key );
              free ( key.dptr );
           }
           key = nextkey;
        }

File: gdbm.info,  Node: Reorganization,  Next: Sync,  Prev: Sequential,  Up: Top

Database reorganization.
************************

   The following function should be used very seldom.

     ret = gdbm_reorganize(dbf);

   The parameter is:

GDBM_FILE dbf
     The pointer returned by `gdbm_open'.

   If you have had a lot of deletions and would like to shrink the space
used by the `gdbm' file, this function will reorganize the database.
`gdbm' will not shorten the length of a `gdbm' file (deleted file space
will be reused) except by using this reorganization.

   This reorganization requires creating a new file and inserting all
the elements in the old file `dbf' into the new file. The new file is
then renamed to the same name as the old file and `dbf' is updated to
contain all the correct information about the new file. If an error is
detected, the return value is negative. The value zero is returned
after a successful reorganization.

File: gdbm.info,  Node: Sync,  Next: Errors,  Prev: Reorganization,  Up: Top

Database Synchronization
************************

   Unless your database was opened with the GDBM_SYNC flag, `gdbm' does
not wait for writes to the disk to complete before continuing.  This
allows faster writing of databases at the risk of having a corrupted
database if the application terminates in an abnormal fashion.  The
following function allows the programmer to make sure the disk version
of the database has been completely updated with all changes to the
current time.

     gdbm_sync(dbf);

   The parameter is:

GDBM_FILE dbf
     The pointer returned by `gdbm_open'.

   This would usually be called after a complete set of changes have
been made to the database and before some long waiting time.
`gdbm_close' automatically calls the equivalent of `gdbm_sync' so no
call is needed if the database is to be closed immediately after the
set of changes have been made.

File: gdbm.info,  Node: Errors,  Next: Options,  Prev: Sync,  Up: Top

Error strings.
**************

   To convert a `gdbm' error code into English text, use this routine:

     ret = gdbm_strerror(errno)

   The parameter is:

gdbm_error errno
     The `gdbm' error code, usually `gdbm_errno'.

   The appropiate phrase for reading by humans is returned.

File: gdbm.info,  Node: Options,  Next: Locking,  Prev: Errors,  Up: Top

Seting options.
***************

   `Gdbm' supports the ability to set certain options on an already
open database.

     ret = gdbm_setopt(dbf, option, value, size);

   The parameters are:

GDBM_FILE dbf
     The pointer returned by `gdbm_open'.

int option
     The option to be set.

int *value
     A pointer to the value to which `option' will be set.

int size
     The length of the data pointed to by `value'.

   The valid options are:

   GDBM_CACHESIZE - Set the size of the internal bucket cache.  This
option may only be set once on each GDBM_FILE descriptor, and   is set
automatically to 100 upon the first access to the database.

   GDBM_FASTMODE - Set fast mode to either on or off.  This allows
fast mode to be toggled on an already open and active database.
value (see below) should be set to either TRUE or FALSE.    *This
option is now obsolete.*

   GDBM_SYNCMODE - Turn on or off file system synchronization
operations.  This   setting defaults to off; value (see below) should
be set to either TRUE or   FALSE.

   GDBM_CENTFREE - Set central free block pool to either on or off.
The default is off, which is how previous versions of `Gdbm'   handled
free blocks.  If set, this option causes all subsequent free   blocks
to be placed in the *global* pool, allowing (in theory)   more file
space to be reused more quickly.  value (see below) should   be set to
either TRUE or FALSE.    *NOTICE: This feature is still under study.*

   GDBM_COALESCEBLKS - Set free block merging to either on or off.
The default is off, which is how previous versions of `Gdbm'   handled
free blocks.  If set, this option causes adjacent free blocks   to be
merged.  This can become a CPU expensive process with time, though,
especially if used in conjunction with GDBM_CENTFREE.  value (see below)
 should be set to either TRUE or FALSE.    *NOTICE: This feature is
still under study.*

   The return value will be -1 upon failure, or 0 upon success.  The
global variable `gdbm_errno' will be set upon failure.

   For instance, to set a database to use a cache of 10, after opening
it with `gdbm_open', but prior to accessing it in any way, the following
code could be used:

     int value = 10;
     ret = gdbm_setopt(dbf, GDBM_CACHESIZE, &value, sizeof(int));

File: gdbm.info,  Node: Locking,  Next: Variables,  Prev: Options,  Up: Top

File Locking.
*************

   With locking disabled (if `gdbm_open' was called with GDBM_NOLOCK),
the user may want to perform their own file locking on the database file
in order to prevent multiple writers operating on the same file
simultaneously.

   In order to support this, the `gdbm_fdesc' routine is provided.

     ret = gdbm_fdesc(dbf);

   The single valid parameter is:

GDBM_FILE dbf
     The pointer returned by `gdbm_open'.

File: gdbm.info,  Node: Variables,  Next: Compatibility,  Prev: Locking,  Up: Top

Two useful variables.
*********************

   The following two variables are variables that may need to be used:

gdbm_error gdbm_errno
     The variable that contains more information about `gdbm' errors
     (`gdbm.h' has the definitions of the error values).

const char * gdbm_version
     The string containing the version information.

File: gdbm.info,  Node: Compatibility,  Next: Conversion,  Prev: Variables,  Up: Top

Compatibility with standard `dbm' and `ndbm'.
*********************************************

   GNU `dbm' files are not `sparse'. You can copy them with the UNIX
`cp' command and they will not expand in the copying process.

   There is a compatibility mode for use with programs that already use
UNIX `dbm' and UNIX `ndbm'.

   GNU `dbm' has compatibility functions for `dbm'. For `dbm'
compatibility functions, you need the include file `dbm.h'.

   In this compatibility mode, no `gdbm' file pointer is required by
the user, and Only one file may be opened at a time. All users in
compatibility mode are assumed to be writers. If the `gdbm' file is a
read only, it will fail as a writer, but will also try to open it as a
reader.  All returned pointers in datum structures point to data that
`gdbm' WILL free. They should be treated as static pointers (as
standard UNIX `dbm' does). The compatibility function names are the
same as the UNIX `dbm' function names. Their definitions follow:

     int dbminit(name);
     int store(key, content);
     datum fetch(key);
     int delete(key);
     datum firstkey();
     datum nextkey(key);
     int dbmclose();

   Standard UNIX `dbm' and GNU `dbm' do not have the same data format
in the file. You cannot access a standard UNIX `dbm' file with GNU
`dbm'!  If you want to use an old database with GNU `dbm', you must use
the `conv2gdbm' program.

   Also, GNU `dbm' has compatibility functions for `ndbm'. For `ndbm'
compatibility functions, you need the include file `ndbm.h'.

   Again, just like `ndbm', any returned datum can be assumed to be
static storage. You do not have to free that memory, the `ndbm'
compatibility functions will do it for you.

   The functions are:

     DBM *dbm_open(name, flags, mode);
     void dbm_close(file);
     datum dbm_fetch(file, key);
     int dbm_store(file, key, `content', flags);
     int dbm_delete(file, key);
     datum dbm_firstkey(file);
     datum dbm_nextkey(file);
     int dbm_error(file);
     int dbm_clearerr(file);
     int dbm_dirfno(file);
     int dbm_pagfno(file);
     int dbm_rdonly(file);

   If you want to compile an old C program that used UNIX `dbm' or
`ndbm' and want to use `gdbm' files, execute the following `cc' command:

     cc ... -L /usr/local/lib -lgdbm

File: gdbm.info,  Node: Conversion,  Next: Bugs,  Prev: Compatibility,  Up: Top

Converting `dbm' files to `gdbm' format.
****************************************

   The program `conv2gdbm' has been provided to help you convert from
`dbm' databases to `gdbm'. The usage is:

     conv2gdbm [-q] [-b block_size] dbm_file [gdbm_file]

   The options are:

-q
     Causes `conv2gdbm' to work quietly.

block_size
     Is the same as in `gdbm_open'.

dbm_file
     Is the name of the `dbm' file without the `.pag' or `.dir'
     extensions.

gdbm_file
     Is the complete file name. If not included, the `gdbm' file name
     is the same as the `dbm' file name without any extensions. That is
     `conv2gdbm' `dbmfile' converts the files `dbmfile.pag' and
     `dbmfile.dir' into a `gdbm' file called `dbmfile'.

File: gdbm.info,  Node: Bugs,  Prev: Conversion,  Up: Top

Problems and bugs.
******************

   If you have problems with GNU `dbm' or think you've found a bug,
please report it. Before reporting a bug, make sure you've actually
found a real bug. Carefully reread the documentation and see if it
really says you can do what you're trying to do. If it's not clear
whether you should be able to do something or not, report that too; it's
a bug in the documentation!

   Before reporting a bug or trying to fix it yourself, try to isolate
it to the smallest possible input file that reproduces the problem. Then
send us the input file and the exact results `gdbm' gave you. Also say
what you expected to occur; this will help us decide whether the
problem was really in the documentation.

   Once you've got a precise problem, send e-mail to:

     Internet: `bug-gnu-utilsATprep.edu'.
     UUCP: `mit-eddie!prep.ai.mit.edu!bug-gnu-utils'.

   Please include the version number of GNU `dbm' you are using. You
can get this information by printing the variable `gdbm_version' (see
Variables).

   Non-bug suggestions are always welcome as well. If you have questions
about things that are unclear in the documentation or are just obscure
features, please report them too.

   You may contact the author by:
         e-mail:  philATcs.edu
        us-mail:  Philip A. Nelson
                  Computer Science Department
                  Western Washington University
                  Bellingham, WA 98226

   You may contact the current maintainer by:
         e-mail:  downsjATdownsj.com