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PERLAPIO(1)			 Perl Programmers Reference Guide		      PERLAPIO(1)

NAME
       perlapio - perl's IO abstraction interface.

SYNOPSIS
	   #define PERLIO_NOT_STDIO 0	 /* For co-existence with stdio only */
	   #include <perlio.h>		 /* Usually via #include <perl.h> */

	   PerlIO *PerlIO_stdin(void);
	   PerlIO *PerlIO_stdout(void);
	   PerlIO *PerlIO_stderr(void);

	   PerlIO *PerlIO_open(const char *path,const char *mode);
	   PerlIO *PerlIO_fdopen(int fd, const char *mode);
	   PerlIO *PerlIO_reopen(const char *path, const char *mode, PerlIO *old);  /* deprecated */
	   int	   PerlIO_close(PerlIO *f);

	   int	   PerlIO_stdoutf(const char *fmt,...)
	   int	   PerlIO_puts(PerlIO *f,const char *string);
	   int	   PerlIO_putc(PerlIO *f,int ch);
	   int	   PerlIO_write(PerlIO *f,const void *buf,size_t numbytes);
	   int	   PerlIO_printf(PerlIO *f, const char *fmt,...);
	   int	   PerlIO_vprintf(PerlIO *f, const char *fmt, va_list args);
	   int	   PerlIO_flush(PerlIO *f);

	   int	   PerlIO_eof(PerlIO *f);
	   int	   PerlIO_error(PerlIO *f);
	   void    PerlIO_clearerr(PerlIO *f);

	   int	   PerlIO_getc(PerlIO *d);
	   int	   PerlIO_ungetc(PerlIO *f,int ch);
	   int	   PerlIO_read(PerlIO *f, void *buf, size_t numbytes);

	   int	   PerlIO_fileno(PerlIO *f);

	   void    PerlIO_setlinebuf(PerlIO *f);

	   Off_t   PerlIO_tell(PerlIO *f);
	   int	   PerlIO_seek(PerlIO *f, Off_t offset, int whence);
	   void    PerlIO_rewind(PerlIO *f);

	   int	   PerlIO_getpos(PerlIO *f, SV *save);	      /* prototype changed */
	   int	   PerlIO_setpos(PerlIO *f, SV *saved);       /* prototype changed */

	   int	   PerlIO_fast_gets(PerlIO *f);
	   int	   PerlIO_has_cntptr(PerlIO *f);
	   int	   PerlIO_get_cnt(PerlIO *f);
	   char   *PerlIO_get_ptr(PerlIO *f);
	   void    PerlIO_set_ptrcnt(PerlIO *f, char *ptr, int count);

	   int	   PerlIO_canset_cnt(PerlIO *f);	      /* deprecated */
	   void    PerlIO_set_cnt(PerlIO *f, int count);      /* deprecated */

	   int	   PerlIO_has_base(PerlIO *f);
	   char   *PerlIO_get_base(PerlIO *f);
	   int	   PerlIO_get_bufsiz(PerlIO *f);

	   PerlIO *PerlIO_importFILE(FILE *stdio, const char *mode);
	   FILE   *PerlIO_exportFILE(PerlIO *f, int flags);
	   FILE   *PerlIO_findFILE(PerlIO *f);
	   void    PerlIO_releaseFILE(PerlIO *f,FILE *stdio);

	   int	   PerlIO_apply_layers(PerlIO *f, const char *mode, const char *layers);
	   int	   PerlIO_binmode(PerlIO *f, int ptype, int imode, const char *layers);
	   void    PerlIO_debug(const char *fmt,...)

DESCRIPTION
       Perl's source code, and extensions that want maximum portability, should use the above
       functions instead of those defined in ANSI C's stdio.h.	The perl headers (in particular
       "perlio.h") will "#define" them to the I/O mechanism selected at Configure time.

       The functions are modeled on those in stdio.h, but parameter order has been "tidied up a
       little".

       "PerlIO *" takes the place of FILE *. Like FILE * it should be treated as opaque (it is
       probably safe to assume it is a pointer to something).

       There are currently three implementations:

       1. USE_STDIO
	   All above are #define'd to stdio functions or are trivial wrapper functions which call
	   stdio. In this case only PerlIO * is a FILE *.  This has been the default
	   implementation since the abstraction was introduced in perl5.003_02.

       2. USE_SFIO
	   A "legacy" implementation in terms of the "sfio" library. Used for some specialist
	   applications on Unix machines ("sfio" is not widely ported away from Unix).	Most of
	   above are #define'd to the sfio functions. PerlIO * is in this case Sfio_t *.

       3. USE_PERLIO
	   Introduced just after perl5.7.0, this is a re-implementation of the above abstraction
	   which allows perl more control over how IO is done as it decouples IO from the way the
	   operating system and C library choose to do things. For USE_PERLIO PerlIO * has an
	   extra layer of indirection - it is a pointer-to-a-pointer.  This allows the PerlIO *
	   to remain with a known value while swapping the implementation around underneath at
	   run time. In this case all the above are true (but very simple) functions which call
	   the underlying implementation.

	   This is the only implementation for which "PerlIO_apply_layers()" does anything
	   "interesting".

	   The USE_PERLIO implementation is described in perliol.

       Because "perlio.h" is a thin layer (for efficiency) the semantics of these functions are
       somewhat dependent on the underlying implementation.  Where these variations are
       understood they are noted below.

       Unless otherwise noted, functions return 0 on success, or a negative value (usually "EOF"
       which is usually -1) and set "errno" on error.

       PerlIO_stdin(), PerlIO_stdout(), PerlIO_stderr()
	   Use these rather than "stdin", "stdout", "stderr". They are written to look like
	   "function calls" rather than variables because this makes it easier to make them
	   function calls if platform cannot export data to loaded modules, or if (say) different
	   "threads" might have different values.

       PerlIO_open(path, mode), PerlIO_fdopen(fd,mode)
	   These correspond to fopen()/fdopen() and the arguments are the same.  Return "NULL"
	   and set "errno" if there is an error.  There may be an implementation limit on the
	   number of open handles, which may be lower than the limit on the number of open files
	   - "errno" may not be set when "NULL" is returned if this limit is exceeded.

       PerlIO_reopen(path,mode,f)
	   While this currently exists in all three implementations perl itself does not use it.
	   As perl does not use it, it is not well tested.

	   Perl prefers to "dup" the new low-level descriptor to the descriptor used by the
	   existing PerlIO. This may become the behaviour of this function in the future.

       PerlIO_printf(f,fmt,...), PerlIO_vprintf(f,fmt,a)
	   These are fprintf()/vfprintf() equivalents.

       PerlIO_stdoutf(fmt,...)
	   This is printf() equivalent. printf is #defined to this function, so it is (currently)
	   legal to use "printf(fmt,...)" in perl sources.

       PerlIO_read(f,buf,count), PerlIO_write(f,buf,count)
	   These correspond functionally to fread() and fwrite() but the arguments and return
	   values are different.  The PerlIO_read() and PerlIO_write() signatures have been
	   modeled on the more sane low level read() and write() functions instead: The "file"
	   argument is passed first, there is only one "count", and the return value can
	   distinguish between error and "EOF".

	   Returns a byte count if successful (which may be zero or positive), returns negative
	   value and sets "errno" on error.  Depending on implementation "errno" may be "EINTR"
	   if operation was interrupted by a signal.

       PerlIO_close(f)
	   Depending on implementation "errno" may be "EINTR" if operation was interrupted by a
	   signal.

       PerlIO_puts(f,s), PerlIO_putc(f,c)
	   These correspond to fputs() and fputc().  Note that arguments have been revised to
	   have "file" first.

       PerlIO_ungetc(f,c)
	   This corresponds to ungetc().  Note that arguments have been revised to have "file"
	   first.  Arranges that next read operation will return the byte c.  Despite the implied
	   "character" in the name only values in the range 0..0xFF are defined. Returns the byte
	   c on success or -1 ("EOF") on error.  The number of bytes that can be "pushed back"
	   may vary, only 1 character is certain, and then only if it is the last character that
	   was read from the handle.

       PerlIO_getc(f)
	   This corresponds to getc().	Despite the c in the name only byte range 0..0xFF is
	   supported.  Returns the character read or -1 ("EOF") on error.

       PerlIO_eof(f)
	   This corresponds to feof().	Returns a true/false indication of whether the handle is
	   at end of file.  For terminal devices this may or may not be "sticky" depending on the
	   implementation.  The flag is cleared by PerlIO_seek(), or PerlIO_rewind().

       PerlIO_error(f)
	   This corresponds to ferror().  Returns a true/false indication of whether there has
	   been an IO error on the handle.

       PerlIO_fileno(f)
	   This corresponds to fileno(), note that on some platforms, the meaning of "fileno" may
	   not match Unix. Returns -1 if the handle has no open descriptor associated with it.

       PerlIO_clearerr(f)
	   This corresponds to clearerr(), i.e., clears 'error' and (usually) 'eof' flags for the
	   "stream". Does not return a value.

       PerlIO_flush(f)
	   This corresponds to fflush().  Sends any buffered write data to the underlying file.
	   If called with "NULL" this may flush all open streams (or core dump with some
	   USE_STDIO implementations).	Calling on a handle open for read only, or on which last
	   operation was a read of some kind may lead to undefined behaviour on some USE_STDIO
	   implementations.  The USE_PERLIO (layers) implementation tries to behave better: it
	   flushes all open streams when passed "NULL", and attempts to retain data on read
	   streams either in the buffer or by seeking the handle to the current logical position.

       PerlIO_seek(f,offset,whence)
	   This corresponds to fseek().  Sends buffered write data to the underlying file, or
	   discards any buffered read data, then positions the file descriptor as specified by
	   offset and whence (sic).  This is the correct thing to do when switching between read
	   and write on the same handle (see issues with PerlIO_flush() above).  Offset is of
	   type "Off_t" which is a perl Configure value which may not be same as stdio's "off_t".

       PerlIO_tell(f)
	   This corresponds to ftell().  Returns the current file position, or (Off_t) -1 on
	   error.  May just return value system "knows" without making a system call or checking
	   the underlying file descriptor (so use on shared file descriptors is not safe without
	   a PerlIO_seek()). Return value is of type "Off_t" which is a perl Configure value
	   which may not be same as stdio's "off_t".

       PerlIO_getpos(f,p), PerlIO_setpos(f,p)
	   These correspond (loosely) to fgetpos() and fsetpos(). Rather than stdio's Fpos_t they
	   expect a "Perl Scalar Value" to be passed. What is stored there should be considered
	   opaque. The layout of the data may vary from handle to handle.  When not using stdio
	   or if platform does not have the stdio calls then they are implemented in terms of
	   PerlIO_tell() and PerlIO_seek().

       PerlIO_rewind(f)
	   This corresponds to rewind(). It is usually defined as being

	       PerlIO_seek(f,(Off_t)0L, SEEK_SET);
	       PerlIO_clearerr(f);

       PerlIO_tmpfile()
	   This corresponds to tmpfile(), i.e., returns an anonymous PerlIO or NULL on error.
	   The system will attempt to automatically delete the file when closed.  On Unix the
	   file is usually "unlink"-ed just after it is created so it does not matter how it gets
	   closed. On other systems the file may only be deleted if closed via PerlIO_close()
	   and/or the program exits via "exit".  Depending on the implementation there may be
	   "race conditions" which allow other processes access to the file, though in general it
	   will be safer in this regard than ad. hoc. schemes.

       PerlIO_setlinebuf(f)
	   This corresponds to setlinebuf().  Does not return a value. What constitutes a "line"
	   is implementation dependent but usually means that writing "\n" flushes the buffer.
	   What happens with things like "this\nthat" is uncertain.  (Perl core uses it only when
	   "dumping"; it has nothing to do with $| auto-flush.)

   Co-existence with stdio
       There is outline support for co-existence of PerlIO with stdio.	Obviously if PerlIO is
       implemented in terms of stdio there is no problem. However in other cases then mechanisms
       must exist to create a FILE * which can be passed to library code which is going to use
       stdio calls.

       The first step is to add this line:

	  #define PERLIO_NOT_STDIO 0

       before including any perl header files. (This will probably become the default at some
       point).	That prevents "perlio.h" from attempting to #define stdio functions onto PerlIO
       functions.

       XS code is probably better using "typemap" if it expects FILE * arguments.  The standard
       typemap will be adjusted to comprehend any changes in this area.

       PerlIO_importFILE(f,mode)
	   Used to get a PerlIO * from a FILE *.

	   The mode argument should be a string as would be passed to fopen/PerlIO_open.  If it
	   is NULL then - for legacy support - the code will (depending upon the platform and the
	   implementation) either attempt to empirically determine the mode in which f is open,
	   or use "r+" to indicate a read/write stream.

	   Once called the FILE * should ONLY be closed by calling "PerlIO_close()" on the
	   returned PerlIO *.

	   The PerlIO is set to textmode. Use PerlIO_binmode if this is not the desired mode.

	   This is not the reverse of PerlIO_exportFILE().

       PerlIO_exportFILE(f,mode)
	   Given a PerlIO * create a 'native' FILE * suitable for passing to code expecting to be
	   compiled and linked with ANSI C stdio.h.  The mode argument should be a string as
	   would be passed to fopen/PerlIO_open.  If it is NULL then - for legacy support - the
	   FILE * is opened in same mode as the PerlIO *.

	   The fact that such a FILE * has been 'exported' is recorded, (normally by pushing a
	   new :stdio "layer" onto the PerlIO *), which may affect future PerlIO operations on
	   the original PerlIO *.  You should not call "fclose()" on the file unless you call
	   "PerlIO_releaseFILE()" to disassociate it from the PerlIO *.  (Do not use
	   PerlIO_importFILE() for doing the disassociation.)

	   Calling this function repeatedly will create a FILE * on each call (and will push an
	   :stdio layer each time as well).

       PerlIO_releaseFILE(p,f)
	   Calling PerlIO_releaseFILE informs PerlIO that all use of FILE * is complete. It is
	   removed from the list of 'exported' FILE *s, and the associated PerlIO * should revert
	   to its original behaviour.

	   Use this to disassociate a file from a PerlIO * that was associated using
	   PerlIO_exportFILE().

       PerlIO_findFILE(f)
	   Returns a native FILE * used by a stdio layer. If there is none, it will create one
	   with PerlIO_exportFILE. In either case the FILE * should be considered as belonging to
	   PerlIO subsystem and should only be closed by calling "PerlIO_close()".

   "Fast gets" Functions
       In addition to standard-like API defined so far above there is an "implementation"
       interface which allows perl to get at internals of PerlIO.  The following calls correspond
       to the various FILE_xxx macros determined by Configure - or their equivalent in other
       implementations. This section is really of interest to only those concerned with detailed
       perl-core behaviour, implementing a PerlIO mapping or writing code which can make use of
       the "read ahead" that has been done by the IO system in the same way perl does. Note that
       any code that uses these interfaces must be prepared to do things the traditional way if a
       handle does not support them.

       PerlIO_fast_gets(f)
	   Returns true if implementation has all the interfaces required to allow perl's
	   "sv_gets" to "bypass" normal IO mechanism.  This can vary from handle to handle.

	     PerlIO_fast_gets(f) = PerlIO_has_cntptr(f) && \
				   PerlIO_canset_cnt(f) && \
				   'Can set pointer into buffer'

       PerlIO_has_cntptr(f)
	   Implementation can return pointer to current position in the "buffer" and a count of
	   bytes available in the buffer.  Do not use this - use PerlIO_fast_gets.

       PerlIO_get_cnt(f)
	   Return count of readable bytes in the buffer. Zero or negative return means no more
	   bytes available.

       PerlIO_get_ptr(f)
	   Return pointer to next readable byte in buffer, accessing via the pointer
	   (dereferencing) is only safe if PerlIO_get_cnt() has returned a positive value.  Only
	   positive offsets up to value returned by PerlIO_get_cnt() are allowed.

       PerlIO_set_ptrcnt(f,p,c)
	   Set pointer into buffer, and a count of bytes still in the buffer. Should be used only
	   to set pointer to within range implied by previous calls to "PerlIO_get_ptr" and
	   "PerlIO_get_cnt". The two values must be consistent with each other (implementation
	   may only use one or the other or may require both).

       PerlIO_canset_cnt(f)
	   Implementation can adjust its idea of number of bytes in the buffer.  Do not use this
	   - use PerlIO_fast_gets.

       PerlIO_set_cnt(f,c)
	   Obscure - set count of bytes in the buffer. Deprecated.  Only usable if
	   PerlIO_canset_cnt() returns true.  Currently used in only doio.c to force count less
	   than -1 to -1.  Perhaps should be PerlIO_set_empty or similar.  This call may actually
	   do nothing if "count" is deduced from pointer and a "limit".  Do not use this - use
	   PerlIO_set_ptrcnt().

       PerlIO_has_base(f)
	   Returns true if implementation has a buffer, and can return pointer to whole buffer
	   and its size. Used by perl for -T / -B tests.  Other uses would be very obscure...

       PerlIO_get_base(f)
	   Return start of buffer. Access only positive offsets in the buffer up to the value
	   returned by PerlIO_get_bufsiz().

       PerlIO_get_bufsiz(f)
	   Return the total number of bytes in the buffer, this is neither the number that can be
	   read, nor the amount of memory allocated to the buffer. Rather it is what the
	   operating system and/or implementation happened to "read()" (or whatever) last time IO
	   was requested.

   Other Functions
       PerlIO_apply_layers(f,mode,layers)
	   The new interface to the USE_PERLIO implementation. The layers ":crlf" and ":raw" are
	   only ones allowed for other implementations and those are silently ignored. (As of
	   perl5.8 ":raw" is deprecated.)  Use PerlIO_binmode() below for the portable case.

       PerlIO_binmode(f,ptype,imode,layers)
	   The hook used by perl's "binmode" operator.	ptype is perl's character for the kind of
	   IO:

	   '<' read
	   '>' write
	   '+' read/write

	   imode is "O_BINARY" or "O_TEXT".

	   layers is a string of layers to apply, only ":crlf" makes sense in the non USE_PERLIO
	   case. (As of perl5.8 ":raw" is deprecated in favour of passing NULL.)

	   Portable cases are:

	       PerlIO_binmode(f,ptype,O_BINARY,NULL);
	   and
	       PerlIO_binmode(f,ptype,O_TEXT,":crlf");

	   On Unix these calls probably have no effect whatsoever.  Elsewhere they alter "\n" to
	   CR,LF translation and possibly cause a special text "end of file" indicator to be
	   written or honoured on read. The effect of making the call after doing any IO to the
	   handle depends on the implementation. (It may be ignored, affect any data which is
	   already buffered as well, or only apply to subsequent data.)

       PerlIO_debug(fmt,...)
	   PerlIO_debug is a printf()-like function which can be used for debugging.  No return
	   value. Its main use is inside PerlIO where using real printf, warn() etc. would
	   recursively call PerlIO and be a problem.

	   PerlIO_debug writes to the file named by $ENV{'PERLIO_DEBUG'} typical use might be

	     Bourne shells (sh, ksh, bash, zsh, ash, ...):
	      PERLIO_DEBUG=/dev/tty ./perl somescript some args

	     Csh/Tcsh:
	      setenv PERLIO_DEBUG /dev/tty
	      ./perl somescript some args

	     If you have the "env" utility:
	      env PERLIO_DEBUG=/dev/tty ./perl somescript some args

	     Win32:
	      set PERLIO_DEBUG=CON
	      perl somescript some args

	   If $ENV{'PERLIO_DEBUG'} is not set PerlIO_debug() is a no-op.

perl v5.16.3				    2013-03-04				      PERLAPIO(1)
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