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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
       maintain (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 Perl 6.

   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" functions 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 buffering 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 processed 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,
			       PerlIO_list_t *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 low-
       level "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 reading 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 "PerlIO_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_FASTGETS" 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", "PerlIO_open", perl's "sysopen",
	   "PerlIO_fdopen" and "PerlIO_reopen".	 The full prototype is as
	   follows:

	    PerlIO *	   (*Open)(pTHX_ PerlIO_funcs *tab,
				   PerlIO_list_t *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 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 argument 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 '#' prefix 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 normally 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 suitable 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 written, 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 implementations, 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 filehandles" 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 perform 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 v5.10.1			  2009-02-12			    PERLIOL(1)
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