Man Pages

perliol(1) - phpMan perliol(1) - phpMan

Command: man perldoc info search(apropos)  


PERLIOL(1)             Perl Programmers Reference Guide             PERLIOL(1)



NAME
       perliol - C API for Perl's implementation of IO in Layers.

SYNOPSIS
           /* Defining a layer ... */
           #include <perliol.h>

DESCRIPTION
       This document describes the behavior and implementation of the PerlIO abstraction described in perlapio when
       "USE_PERLIO" is defined (and "USE_SFIO" is not).

       History and Background

       The PerlIO abstraction was introduced in perl5.003_02 but languished as just an abstraction until perl5.7.0.
       However during that time a number of perl extensions switched to using it, so the API is mostly fixed to main-
       tain (source) compatibility.

       The aim of the implementation is to provide the PerlIO API in a flexible and platform neutral manner. It is
       also a trial of an "Object Oriented C, with vtables" approach which may be applied to perl6.

       Basic Structure

       PerlIO is a stack of layers.

       The low levels of the stack work with the low-level operating system calls (file descriptors in C) getting
       bytes in and out, the higher layers of the stack buffer, filter, and otherwise manipulate the I/O, and return
       characters (or bytes) to Perl.  Terms above and below are used to refer to the relative positioning of the
       stack layers.

       A layer contains a "vtable", the table of I/O operations (at C level a table of function pointers), and status
       flags.  The functions in the vtable implement operations like "open", "read", and "write".

       When I/O, for example "read", is requested, the request goes from Perl first down the stack using "read" func-
       tions of each layer, then at the bottom the input is requested from the operating system services, then the
       result is returned up the stack, finally being interpreted as Perl data.

       The requests do not necessarily go always all the way down to the operating system: that's where PerlIO buffer-
       ing comes into play.

       When you do an open() and specify extra PerlIO layers to be deployed, the layers you specify are "pushed" on
       top of the already existing default stack.  One way to see it is that "operating system is on the left" and
       "Perl is on the right".

       What exact layers are in this default stack depends on a lot of things: your operating system, Perl version,
       Perl compile time configuration, and Perl runtime configuration.  See PerlIO, "PERLIO" in perlrun, and open for
       more information.

       binmode() operates similarly to open(): by default the specified layers are pushed on top of the existing
       stack.

       However, note that even as the specified layers are "pushed on top" for open() and binmode(), this doesn't mean
       that the effects are limited to the "top": PerlIO layers can be very 'active' and inspect and affect layers
       also deeper in the stack.  As an example there is a layer called "raw" which repeatedly "pops" layers until it
       reaches the first layer that has declared itself capable of handling binary data.  The "pushed" layers are pro-
       cessed in left-to-right order.

       sysopen() operates (unsurprisingly) at a lower level in the stack than open().  For example in UNIX or UNIX-
       like systems sysopen() operates directly at the level of file descriptors: in the terms of PerlIO layers, it
       uses only the "unix" layer, which is a rather thin wrapper on top of the UNIX file descriptors.

       Layers vs Disciplines

       Initial discussion of the ability to modify IO streams behaviour used the term "discipline" for the entities
       which were added. This came (I believe) from the use of the term in "sfio", which in turn borrowed it from
       "line disciplines" on Unix terminals. However, this document (and the C code) uses the term "layer".

       This is, I hope, a natural term given the implementation, and should avoid connotations that are inherent in
       earlier uses of "discipline" for things which are rather different.

       Data Structures

       The basic data structure is a PerlIOl:

               typedef struct _PerlIO PerlIOl;
               typedef struct _PerlIO_funcs PerlIO_funcs;
               typedef PerlIOl *PerlIO;

               struct _PerlIO
               {
                PerlIOl *      next;       /* Lower layer */
                PerlIO_funcs * tab;        /* Functions for this layer */
                IV             flags;      /* Various flags for state */
               };

       A "PerlIOl *" is a pointer to the struct, and the application level "PerlIO *" is a pointer to a "PerlIOl *" -
       i.e. a pointer to a pointer to the struct. This allows the application level "PerlIO *" to remain constant
       while the actual "PerlIOl *" underneath changes. (Compare perl's "SV *" which remains constant while its
       "sv_any" field changes as the scalar's type changes.) An IO stream is then in general represented as a pointer
       to this linked-list of "layers".

       It should be noted that because of the double indirection in a "PerlIO *", a "&(perlio->next)" "is" a "PerlIO
       *", and so to some degree at least one layer can use the "standard" API on the next layer down.

       A "layer" is composed of two parts:

       1.  The functions and attributes of the "layer class".

       2.  The per-instance data for a particular handle.

       Functions and Attributes

       The functions and attributes are accessed via the "tab" (for table) member of "PerlIOl". The functions (methods
       of the layer "class") are fixed, and are defined by the "PerlIO_funcs" type. They are broadly the same as the
       public "PerlIO_xxxxx" functions:

         struct _PerlIO_funcs
         {
          Size_t               fsize;
          char *               name;
          Size_t               size;
          IV           kind;
          IV           (*Pushed)(pTHX_ PerlIO *f,const char *mode,SV *arg, PerlIO_funcs *tab);
          IV           (*Popped)(pTHX_ PerlIO *f);
          PerlIO *     (*Open)(pTHX_ PerlIO_funcs *tab,
                               AV *layers, IV n,
                               const char *mode,
                               int fd, int imode, int perm,
                               PerlIO *old,
                               int narg, SV **args);
          IV           (*Binmode)(pTHX_ PerlIO *f);
          SV *         (*Getarg)(pTHX_ PerlIO *f, CLONE_PARAMS *param, int flags)
          IV           (*Fileno)(pTHX_ PerlIO *f);
          PerlIO *     (*Dup)(pTHX_ PerlIO *f, PerlIO *o, CLONE_PARAMS *param, int flags)
          /* Unix-like functions - cf sfio line disciplines */
          SSize_t      (*Read)(pTHX_ PerlIO *f, void *vbuf, Size_t count);
          SSize_t      (*Unread)(pTHX_ PerlIO *f, const void *vbuf, Size_t count);
          SSize_t      (*Write)(pTHX_ PerlIO *f, const void *vbuf, Size_t count);
          IV           (*Seek)(pTHX_ PerlIO *f, Off_t offset, int whence);
          Off_t        (*Tell)(pTHX_ PerlIO *f);
          IV           (*Close)(pTHX_ PerlIO *f);
          /* Stdio-like buffered IO functions */
          IV           (*Flush)(pTHX_ PerlIO *f);
          IV           (*Fill)(pTHX_ PerlIO *f);
          IV           (*Eof)(pTHX_ PerlIO *f);
          IV           (*Error)(pTHX_ PerlIO *f);
          void         (*Clearerr)(pTHX_ PerlIO *f);
          void         (*Setlinebuf)(pTHX_ PerlIO *f);
          /* Perl's snooping functions */
          STDCHAR *    (*Get_base)(pTHX_ PerlIO *f);
          Size_t       (*Get_bufsiz)(pTHX_ PerlIO *f);
          STDCHAR *    (*Get_ptr)(pTHX_ PerlIO *f);
          SSize_t      (*Get_cnt)(pTHX_ PerlIO *f);
          void         (*Set_ptrcnt)(pTHX_ PerlIO *f,STDCHAR *ptr,SSize_t cnt);
         };

       The first few members of the struct give a function table size for compatibility check "name" for the layer,
       the  size to "malloc" for the per-instance data, and some flags which are attributes of the class as whole
       (such as whether it is a buffering layer), then follow the functions which fall into four basic groups:

       1.  Opening and setup functions

       2.  Basic IO operations

       3.  Stdio class buffering options.

       4.  Functions to support Perl's traditional "fast" access to the buffer.

       A layer does not have to implement all the functions, but the whole table has to be present. Unimplemented
       slots can be NULL (which will result in an error when called) or can be filled in with stubs to "inherit"
       behaviour from a "base class". This "inheritance" is fixed for all instances of the layer, but as the layer
       chooses which stubs to populate the table, limited "multiple inheritance" is possible.

       Per-instance Data

       The per-instance data are held in memory beyond the basic PerlIOl struct, by making a PerlIOl the first member
       of the layer's struct thus:

               typedef struct
               {
                struct _PerlIO base;       /* Base "class" info */
                STDCHAR *      buf;        /* Start of buffer */
                STDCHAR *      end;        /* End of valid part of buffer */
                STDCHAR *      ptr;        /* Current position in buffer */
                Off_t          posn;       /* Offset of buf into the file */
                Size_t         bufsiz;     /* Real size of buffer */
                IV             oneword;    /* Emergency buffer */
               } PerlIOBuf;

       In this way (as for perl's scalars) a pointer to a PerlIOBuf can be treated as a pointer to a PerlIOl.

       Layers in action.

                       table           perlio          unix
                   |           |
                   +-----------+    +----------+    +--------+
          PerlIO ->|           |--->|  next    |--->|  NULL  |
                   +-----------+    +----------+    +--------+
                   |           |    |  buffer  |    |   fd   |
                   +-----------+    |          |    +--------+
                   |           |    +----------+

       The above attempts to show how the layer scheme works in a simple case.  The application's "PerlIO *" points to
       an entry in the table(s) representing open (allocated) handles. For example the first three slots in the table
       correspond to "stdin","stdout" and "stderr". The table in turn points to the current "top" layer for the handle
       - in this case an instance of the generic buffering layer "perlio". That layer in turn points to the next layer
       down - in this case the lowlevel "unix" layer.

       The above is roughly equivalent to a "stdio" buffered stream, but with much more flexibility:

       ?   If Unix level "read"/"write"/"lseek" is not appropriate for (say) sockets then the "unix" layer can be
           replaced (at open time or even dynamically) with a "socket" layer.

       ?   Different handles can have different buffering schemes. The "top" layer could be the "mmap" layer if read-
           ing disk files was quicker using "mmap" than "read". An "unbuffered" stream can be implemented simply by
           not having a buffer layer.

       ?   Extra layers can be inserted to process the data as it flows through.  This was the driving need for
           including the scheme in perl 5.7.0+ - we needed a mechanism to allow data to be translated between perl's
           internal encoding (conceptually at least Unicode as UTF-8), and the "native" format used by the system.
           This is provided by the ":encoding(xxxx)" layer which typically sits above the buffering layer.

       ?   A layer can be added that does "\n" to CRLF translation. This layer can be used on any platform, not just
           those that normally do such things.

       Per-instance flag bits

       The generic flag bits are a hybrid of "O_XXXXX" style flags deduced from the mode string passed to "Per-
       lIO_open()", and state bits for typical buffer layers.

       PERLIO_F_EOF
           End of file.

       PERLIO_F_CANWRITE
           Writes are permitted, i.e. opened as "w" or "r+" or "a", etc.

       PERLIO_F_CANREAD
           Reads are permitted i.e. opened "r" or "w+" (or even "a+" - ick).

       PERLIO_F_ERROR
           An error has occurred (for "PerlIO_error()").

       PERLIO_F_TRUNCATE
           Truncate file suggested by open mode.

       PERLIO_F_APPEND
           All writes should be appends.

       PERLIO_F_CRLF
           Layer is performing Win32-like "\n" mapped to CR,LF for output and CR,LF mapped to "\n" for input. Normally
           the provided "crlf" layer is the only layer that need bother about this. "PerlIO_binmode()" will mess with
           this flag rather than add/remove layers if the "PERLIO_K_CANCRLF" bit is set for the layers class.

       PERLIO_F_UTF8
           Data written to this layer should be UTF-8 encoded; data provided by this layer should be considered UTF-8
           encoded. Can be set on any layer by ":utf8" dummy layer. Also set on ":encoding" layer.

       PERLIO_F_UNBUF
           Layer is unbuffered - i.e. write to next layer down should occur for each write to this layer.

       PERLIO_F_WRBUF
           The buffer for this layer currently holds data written to it but not sent to next layer.

       PERLIO_F_RDBUF
           The buffer for this layer currently holds unconsumed data read from layer below.

       PERLIO_F_LINEBUF
           Layer is line buffered. Write data should be passed to next layer down whenever a "\n" is seen. Any data
           beyond the "\n" should then be processed.

       PERLIO_F_TEMP
           File has been "unlink()"ed, or should be deleted on "close()".

       PERLIO_F_OPEN
           Handle is open.

       PERLIO_F_FASTGETS
           This instance of this layer supports the "fast "gets"" interface.  Normally set based on "PERLIO_K_FAST-
           GETS" for the class and by the existence of the function(s) in the table. However a class that normally
           provides that interface may need to avoid it on a particular instance. The "pending" layer needs to do this
           when it is pushed above a layer which does not support the interface.  (Perl's "sv_gets()" does not expect
           the streams fast "gets" behaviour to change during one "get".)

       Methods in Detail


       fsize
                   Size_t fsize;

           Size of the function table. This is compared against the value PerlIO code "knows" as a compatibility
           check. Future versions may be able to tolerate layers compiled against an old version of the headers.

       name
                   char * name;

           The name of the layer whose open() method Perl should invoke on open().  For example if the layer is called
           APR, you will call:

             open $fh, ">:APR", ...

           and Perl knows that it has to invoke the PerlIOAPR_open() method implemented by the APR layer.

       size
                   Size_t size;

           The size of the per-instance data structure, e.g.:

             sizeof(PerlIOAPR)

           If this field is zero then "PerlIO_pushed" does not malloc anything and assumes layer's Pushed function
           will do any required layer stack manipulation - used to avoid malloc/free overhead for dummy layers.  If
           the field is non-zero it must be at least the size of "PerlIOl", "PerlIO_pushed" will allocate memory for
           the layer's data structures and link new layer onto the stream's stack. (If the layer's Pushed method
           returns an error indication the layer is popped again.)

       kind
                   IV kind;

           * PERLIO_K_BUFFERED
               The layer is buffered.

           * PERLIO_K_RAW
               The layer is acceptable to have in a binmode(FH) stack - i.e. it does not (or will configure itself not
               to) transform bytes passing through it.

           * PERLIO_K_CANCRLF
               Layer can translate between "\n" and CRLF line ends.

           * PERLIO_K_FASTGETS
               Layer allows buffer snooping.

           * PERLIO_K_MULTIARG
               Used when the layer's open() accepts more arguments than usual. The extra arguments should come not
               before the "MODE" argument. When this flag is used it's up to the layer to validate the args.

       Pushed
                   IV      (*Pushed)(pTHX_ PerlIO *f,const char *mode, SV *arg);

           The only absolutely mandatory method. Called when the layer is pushed onto the stack.  The "mode" argument
           may be NULL if this occurs post-open. The "arg" will be non-"NULL" if an argument string was passed. In
           most cases this should call "PerlIOBase_pushed()" to convert "mode" into the appropriate "PERLIO_F_XXXXX"
           flags in addition to any actions the layer itself takes.  If a layer is not expecting an argument it need
           neither save the one passed to it, nor provide "Getarg()" (it could perhaps "Perl_warn" that the argument
           was un-expected).

           Returns 0 on success. On failure returns -1 and should set errno.

       Popped
                   IV      (*Popped)(pTHX_ PerlIO *f);

           Called when the layer is popped from the stack. A layer will normally be popped after "Close()" is called.
           But a layer can be popped without being closed if the program is dynamically managing layers on the stream.
           In such cases "Popped()" should free any resources (buffers, translation tables, ...) not held directly in
           the layer's struct.  It should also "Unread()" any unconsumed data that has been read and buffered from the
           layer below back to that layer, so that it can be re-provided to what ever is now above.

           Returns 0 on success and failure.  If "Popped()" returns true then perlio.c assumes that either the layer
           has popped itself, or the layer is super special and needs to be retained for other reasons.  In most cases
           it should return false.

       Open
                   PerlIO *        (*Open)(...);

           The "Open()" method has lots of arguments because it combines the functions of perl's "open", "Per-
           lIO_open", perl's "sysopen", "PerlIO_fdopen" and "PerlIO_reopen".  The full prototype is as follows:

            PerlIO *       (*Open)(pTHX_ PerlIO_funcs *tab,
                                   AV *layers, IV n,
                                   const char *mode,
                                   int fd, int imode, int perm,
                                   PerlIO *old,
                                   int narg, SV **args);

           Open should (perhaps indirectly) call "PerlIO_allocate()" to allocate a slot in the table and associate it
           with the layers information for the opened file, by calling "PerlIO_push".  The layers AV is an array of
           all the layers destined for the "PerlIO *", and any arguments passed to them, n is the index into that
           array of the layer being called. The macro "PerlIOArg" will return a (possibly "NULL") SV * for the argu-
           ment passed to the layer.

           The mode string is an ""fopen()"-like" string which would match the regular expression
           "/^[I#]?[rwa]\+?[bt]?$/".

           The 'I' prefix is used during creation of "stdin".."stderr" via special "PerlIO_fdopen" calls; the '#' pre-
           fix means that this is "sysopen" and that imode and perm should be passed to "PerlLIO_open3"; 'r' means
           read, 'w' means write and 'a' means append. The '+' suffix means that both reading and writing/appending
           are permitted.  The 'b' suffix means file should be binary, and 't' means it is text. (Almost all layers
           should do the IO in binary mode, and ignore the b/t bits. The ":crlf" layer should be pushed to handle the
           distinction.)

           If old is not "NULL" then this is a "PerlIO_reopen". Perl itself does not use this (yet?) and semantics are
           a little vague.

           If fd not negative then it is the numeric file descriptor fd, which will be open in a manner compatible
           with the supplied mode string, the call is thus equivalent to "PerlIO_fdopen". In this case nargs will be
           zero.

           If nargs is greater than zero then it gives the number of arguments passed to "open", otherwise it will be
           1 if for example "PerlIO_open" was called.  In simple cases SvPV_nolen(*args) is the pathname to open.

           Having said all that translation-only layers do not need to provide "Open()" at all, but rather leave the
           opening to a lower level layer and wait to be "pushed".  If a layer does provide "Open()" it should nor-
           mally call the "Open()" method of next layer down (if any) and then push itself on top if that succeeds.

           If "PerlIO_push" was performed and open has failed, it must "PerlIO_pop" itself, since if it's not, the
           layer won't be removed and may cause bad problems.

           Returns "NULL" on failure.

       Binmode
                   IV        (*Binmode)(pTHX_ PerlIO *f);

           Optional. Used when ":raw" layer is pushed (explicitly or as a result of binmode(FH)). If not present layer
           will be popped. If present should configure layer as binary (or pop itself) and return 0.  If it returns -1
           for error "binmode" will fail with layer still on the stack.

       Getarg
                   SV *      (*Getarg)(pTHX_ PerlIO *f,
                                       CLONE_PARAMS *param, int flags);

           Optional. If present should return an SV * representing the string argument passed to the layer when it was
           pushed. e.g. ":encoding(ascii)" would return an SvPV with value "ascii". (param and flags arguments can be
           ignored in most cases)

           "Dup" uses "Getarg" to retrieve the argument originally passed to "Pushed", so you must implement this
           function if your layer has an extra argument to "Pushed" and will ever be "Dup"ed.

       Fileno
                   IV        (*Fileno)(pTHX_ PerlIO *f);

           Returns the Unix/Posix numeric file descriptor for the handle. Normally "PerlIOBase_fileno()" (which just
           asks next layer down) will suffice for this.

           Returns -1 on error, which is considered to include the case where the layer cannot provide such a file
           descriptor.

       Dup
                   PerlIO * (*Dup)(pTHX_ PerlIO *f, PerlIO *o,
                                   CLONE_PARAMS *param, int flags);

           XXX: Needs more docs.

           Used as part of the "clone" process when a thread is spawned (in which case param will be non-NULL) and
           when a stream is being duplicated via '&' in the "open".

           Similar to "Open", returns PerlIO* on success, "NULL" on failure.

       Read
                   SSize_t (*Read)(pTHX_ PerlIO *f, void *vbuf, Size_t count);

           Basic read operation.

           Typically will call "Fill" and manipulate pointers (possibly via the API).  "PerlIOBuf_read()" may be suit-
           able for derived classes which provide "fast gets" methods.

           Returns actual bytes read, or -1 on an error.

       Unread
                   SSize_t (*Unread)(pTHX_ PerlIO *f,
                                     const void *vbuf, Size_t count);

           A superset of stdio's "ungetc()". Should arrange for future reads to see the bytes in "vbuf". If there is
           no obviously better implementation then "PerlIOBase_unread()" provides the function by pushing a "fake"
           "pending" layer above the calling layer.

           Returns the number of unread chars.

       Write
                   SSize_t (*Write)(PerlIO *f, const void *vbuf, Size_t count);

           Basic write operation.

           Returns bytes written or -1 on an error.

       Seek
                   IV      (*Seek)(pTHX_ PerlIO *f, Off_t offset, int whence);

           Position the file pointer. Should normally call its own "Flush" method and then the "Seek" method of next
           layer down.

           Returns 0 on success, -1 on failure.

       Tell
                   Off_t   (*Tell)(pTHX_ PerlIO *f);

           Return the file pointer. May be based on layers cached concept of position to avoid overhead.

           Returns -1 on failure to get the file pointer.

       Close
                   IV      (*Close)(pTHX_ PerlIO *f);

           Close the stream. Should normally call "PerlIOBase_close()" to flush itself and close layers below, and
           then deallocate any data structures (buffers, translation tables, ...) not  held directly in the data
           structure.

           Returns 0 on success, -1 on failure.

       Flush
                   IV      (*Flush)(pTHX_ PerlIO *f);

           Should make stream's state consistent with layers below. That is, any buffered write data should be writ-
           ten, and file position of lower layers adjusted for data read from below but not actually consumed.
           (Should perhaps "Unread()" such data to the lower layer.)

           Returns 0 on success, -1 on failure.

       Fill
                   IV      (*Fill)(pTHX_ PerlIO *f);

           The buffer for this layer should be filled (for read) from layer below.  When you "subclass" PerlIOBuf
           layer, you want to use its _read method and to supply your own fill method, which fills the PerlIOBuf's
           buffer.

           Returns 0 on success, -1 on failure.

       Eof
                   IV      (*Eof)(pTHX_ PerlIO *f);

           Return end-of-file indicator. "PerlIOBase_eof()" is normally sufficient.

           Returns 0 on end-of-file, 1 if not end-of-file, -1 on error.

       Error
                   IV      (*Error)(pTHX_ PerlIO *f);

           Return error indicator. "PerlIOBase_error()" is normally sufficient.

           Returns 1 if there is an error (usually when "PERLIO_F_ERROR" is set, 0 otherwise.

       Clearerr
                   void    (*Clearerr)(pTHX_ PerlIO *f);

           Clear end-of-file and error indicators. Should call "PerlIOBase_clearerr()" to set the "PERLIO_F_XXXXX"
           flags, which may suffice.

       Setlinebuf
                   void    (*Setlinebuf)(pTHX_ PerlIO *f);

           Mark the stream as line buffered. "PerlIOBase_setlinebuf()" sets the PERLIO_F_LINEBUF flag and is normally
           sufficient.

       Get_base
                   STDCHAR *       (*Get_base)(pTHX_ PerlIO *f);

           Allocate (if not already done so) the read buffer for this layer and return pointer to it. Return NULL on
           failure.

       Get_bufsiz
                   Size_t  (*Get_bufsiz)(pTHX_ PerlIO *f);

           Return the number of bytes that last "Fill()" put in the buffer.

       Get_ptr
                   STDCHAR *       (*Get_ptr)(pTHX_ PerlIO *f);

           Return the current read pointer relative to this layer's buffer.

       Get_cnt
                   SSize_t (*Get_cnt)(pTHX_ PerlIO *f);

           Return the number of bytes left to be read in the current buffer.

       Set_ptrcnt
                   void    (*Set_ptrcnt)(pTHX_ PerlIO *f,
                                         STDCHAR *ptr, SSize_t cnt);

           Adjust the read pointer and count of bytes to match "ptr" and/or "cnt".  The application (or layer above)
           must ensure they are consistent.  (Checking is allowed by the paranoid.)

       Utilities

       To ask for the next layer down use PerlIONext(PerlIO *f).

       To check that a PerlIO* is valid use PerlIOValid(PerlIO *f).  (All this does is really just to check that the
       pointer is non-NULL and that the pointer behind that is non-NULL.)

       PerlIOBase(PerlIO *f) returns the "Base" pointer, or in other words, the "PerlIOl*" pointer.

       PerlIOSelf(PerlIO* f, type) return the PerlIOBase cast to a type.

       Perl_PerlIO_or_Base(PerlIO* f, callback, base, failure, args) either calls the callback from the functions of
       the layer f (just by the name of the IO function, like "Read") with the args, or if there is no such callback,
       calls the base version of the callback with the same args, or if the f is invalid, set errno to EBADF and
       return failure.

       Perl_PerlIO_or_fail(PerlIO* f, callback, failure, args) either calls the callback of the functions of the layer
       f with the args, or if there is no such callback, set errno to EINVAL.  Or if the f is invalid, set errno to
       EBADF and return failure.

       Perl_PerlIO_or_Base_void(PerlIO* f, callback, base, args) either calls the callback of the functions of the
       layer f with the args, or if there is no such callback, calls the base version of the callback with the same
       args, or if the f is invalid, set errno to EBADF.

       Perl_PerlIO_or_fail_void(PerlIO* f, callback, args) either calls the callback of the functions of the layer f
       with the args, or if there is no such callback, set errno to EINVAL.  Or if the f is invalid, set errno to
       EBADF.

       Implementing PerlIO Layers

       If you find the implementation document unclear or not sufficient, look at the existing PerlIO layer implemen-
       tations, which include:

       * C implementations
           The perlio.c and perliol.h in the Perl core implement the "unix", "perlio", "stdio", "crlf", "utf8",
           "byte", "raw", "pending" layers, and also the "mmap" and "win32" layers if applicable.  (The "win32" is
           currently unfinished and unused, to see what is used instead in Win32, see "Querying the layers of filehan-
           dles" in PerlIO .)

           PerlIO::encoding, PerlIO::scalar, PerlIO::via in the Perl core.

           PerlIO::gzip and APR::PerlIO (mod_perl 2.0) on CPAN.

       * Perl implementations
           PerlIO::via::QuotedPrint in the Perl core and PerlIO::via::* on CPAN.

       If you are creating a PerlIO layer, you may want to be lazy, in other words, implement only the methods that
       interest you.  The other methods you can either replace with the "blank" methods

           PerlIOBase_noop_ok
           PerlIOBase_noop_fail

       (which do nothing, and return zero and -1, respectively) or for certain methods you may assume a default
       behaviour by using a NULL method.  The Open method looks for help in the 'parent' layer.  The following table
       summarizes the behaviour:

           method      behaviour with NULL

           Clearerr    PerlIOBase_clearerr
           Close       PerlIOBase_close
           Dup         PerlIOBase_dup
           Eof         PerlIOBase_eof
           Error       PerlIOBase_error
           Fileno      PerlIOBase_fileno
           Fill        FAILURE
           Flush       SUCCESS
           Getarg      SUCCESS
           Get_base    FAILURE
           Get_bufsiz  FAILURE
           Get_cnt     FAILURE
           Get_ptr     FAILURE
           Open        INHERITED
           Popped      SUCCESS
           Pushed      SUCCESS
           Read        PerlIOBase_read
           Seek        FAILURE
           Set_cnt     FAILURE
           Set_ptrcnt  FAILURE
           Setlinebuf  PerlIOBase_setlinebuf
           Tell        FAILURE
           Unread      PerlIOBase_unread
           Write       FAILURE

        FAILURE        Set errno (to EINVAL in UNIXish, to LIB$_INVARG in VMS) and
                       return -1 (for numeric return values) or NULL (for pointers)
        INHERITED      Inherited from the layer below
        SUCCESS        Return 0 (for numeric return values) or a pointer

       Core Layers

       The file "perlio.c" provides the following layers:

       "unix"
           A basic non-buffered layer which calls Unix/POSIX "read()", "write()", "lseek()", "close()". No buffering.
           Even on platforms that distinguish between O_TEXT and O_BINARY this layer is always O_BINARY.

       "perlio"
           A very complete generic buffering layer which provides the whole of PerlIO API. It is also intended to be
           used as a "base class" for other layers. (For example its "Read()" method is implemented in terms of the
           "Get_cnt()"/"Get_ptr()"/"Set_ptrcnt()" methods).

           "perlio" over "unix" provides a complete replacement for stdio as seen via PerlIO API. This is the default
           for USE_PERLIO when system's stdio does not permit perl's "fast gets" access, and which do not distinguish
           between "O_TEXT" and "O_BINARY".

       "stdio"
           A layer which provides the PerlIO API via the layer scheme, but implements it by calling system's stdio.
           This is (currently) the default if system's stdio provides sufficient access to allow perl's "fast gets"
           access and which do not distinguish between "O_TEXT" and "O_BINARY".

       "crlf"
           A layer derived using "perlio" as a base class. It provides Win32-like "\n" to CR,LF translation. Can
           either be applied above "perlio" or serve as the buffer layer itself. "crlf" over "unix" is the default if
           system distinguishes between "O_TEXT" and "O_BINARY" opens. (At some point "unix" will be replaced by a
           "native" Win32 IO layer on that platform, as Win32's read/write layer has various drawbacks.) The "crlf"
           layer is a reasonable model for a layer which transforms data in some way.

       "mmap"
           If Configure detects "mmap()" functions this layer is provided (with "perlio" as a "base") which does
           "read" operations by mmap()ing the file. Performance improvement is marginal on modern systems, so it is
           mainly there as a proof of concept. It is likely to be unbundled from the core at some point. The "mmap"
           layer is a reasonable model for a minimalist "derived" layer.

       "pending"
           An "internal" derivative of "perlio" which can be used to provide Unread() function for layers which have
           no buffer or cannot be bothered.  (Basically this layer's "Fill()" pops itself off the stack and so resumes
           reading from layer below.)

       "raw"
           A dummy layer which never exists on the layer stack. Instead when "pushed" it actually pops the stack
           removing itself, it then calls Binmode function table entry on all the layers in the stack - normally this
           (via PerlIOBase_binmode) removes any layers which do not have "PERLIO_K_RAW" bit set. Layers can modify
           that behaviour by defining their own Binmode entry.

       "utf8"
           Another dummy layer. When pushed it pops itself and sets the "PERLIO_F_UTF8" flag on the layer which was
           (and now is once more) the top of the stack.

       In addition perlio.c also provides a number of "PerlIOBase_xxxx()" functions which are intended to be used in
       the table slots of classes which do not need to do anything special for a particular method.

       Extension Layers

       Layers can made available by extension modules. When an unknown layer is encountered the PerlIO code will per-
       form the equivalent of :

          use PerlIO 'layer';

       Where layer is the unknown layer. PerlIO.pm will then attempt to:

          require PerlIO::layer;

       If after that process the layer is still not defined then the "open" will fail.

       The following extension layers are bundled with perl:

       ":encoding"
              use Encoding;

           makes this layer available, although PerlIO.pm "knows" where to find it.  It is an example of a layer which
           takes an argument as it is called thus:

              open( $fh, "<:encoding(iso-8859-7)", $pathname );

       ":scalar"
           Provides support for reading data from and writing data to a scalar.

              open( $fh, "+<:scalar", \$scalar );

           When a handle is so opened, then reads get bytes from the string value of $scalar, and writes change the
           value. In both cases the position in $scalar starts as zero but can be altered via "seek", and determined
           via "tell".

           Please note that this layer is implied when calling open() thus:

              open( $fh, "+<", \$scalar );

       ":via"
           Provided to allow layers to be implemented as Perl code.  For instance:

              use PerlIO::via::StripHTML;
              open( my $fh, "<:via(StripHTML)", "index.html" );

           See PerlIO::via for details.

TODO
       Things that need to be done to improve this document.

       ?   Explain how to make a valid fh without going through open()(i.e. apply a layer). For example if the file is
           not opened through perl, but we want to get back a fh, like it was opened by Perl.

           How PerlIO_apply_layera fits in, where its docs, was it made public?

           Currently the example could be something like this:

             PerlIO *foo_to_PerlIO(pTHX_ char *mode, ...)
             {
                 char *mode; /* "w", "r", etc */
                 const char *layers = ":APR"; /* the layer name */
                 PerlIO *f = PerlIO_allocate(aTHX);
                 if (!f) {
                     return NULL;
                 }

                 PerlIO_apply_layers(aTHX_ f, mode, layers);

                 if (f) {
                     PerlIOAPR *st = PerlIOSelf(f, PerlIOAPR);
                     /* fill in the st struct, as in _open() */
                     st->file = file;
                     PerlIOBase(f)->flags |= PERLIO_F_OPEN;

                     return f;
                 }
                 return NULL;
             }

       ?   fix/add the documentation in places marked as XXX.

       ?   The handling of errors by the layer is not specified. e.g. when $!  should be set explicitly, when the
           error handling should be just delegated to the top layer.

           Probably give some hints on using SETERRNO() or pointers to where they can be found.

       ?   I think it would help to give some concrete examples to make it easier to understand the API. Of course I
           agree that the API has to be concise, but since there is no second document that is more of a guide, I
           think that it'd make it easier to start with the doc which is an API, but has examples in it in places
           where things are unclear, to a person who is not a PerlIO guru (yet).



perl v5.8.8                       2006-01-07                        PERLIOL(1)