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database/perl/lib/pods/perlapio.pod

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+=head1 NAME
+
+perlapio - perl's IO abstraction interface.
+
+=head1 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, /* deprecated */
+          const char *mode, PerlIO *old);
+  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);
+  SSize_t 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);
+  SSize_t 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);
+  SSize_t PerlIO_get_cnt(PerlIO *f);
+  char   *PerlIO_get_ptr(PerlIO *f);
+  void    PerlIO_set_ptrcnt(PerlIO *f, char *ptr, SSize_t 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);
+  SSize_t PerlIO_get_bufsiz(PerlIO *f);
+
+  PerlIO *PerlIO_importFILE(FILE *stdio, const char *mode);
+  FILE   *PerlIO_exportFILE(PerlIO *f, const char *mode);
+  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,...);
+
+=for apidoc Amh|int     |PerlIO_apply_layers|PerlIO *f|const char *mode|const char *layers
+=for apidoc Amh|int     |PerlIO_binmode|PerlIO *f|int ptype|int imode|const char *layers
+=for apidoc ATmh|int    |PerlIO_canset_cnt|PerlIO *f
+=for apidoc Amh|void    |PerlIO_debug|const char *fmt|...
+=for apidoc ATmh|FILE  *|PerlIO_exportFILE|PerlIO *f|const char *mode
+=for apidoc ATmh|int    |PerlIO_fast_gets|PerlIO *f
+=for apidoc ATmh|PerlIO*|PerlIO_fdopen|int fd|const char *mode
+=for apidoc ATmh|FILE  *|PerlIO_findFILE|PerlIO *f
+=for apidoc ATmh|int    |PerlIO_getc|PerlIO *d
+=for apidoc ATmh|int    |PerlIO_getpos|PerlIO *f|SV *save
+=for apidoc ATmh|int    |PerlIO_has_base|PerlIO *f
+=for apidoc ATmh|int    |PerlIO_has_cntptr|PerlIO *f
+=for apidoc ATmh|PerlIO*|PerlIO_importFILE|FILE *stdio|const char *mode
+=for apidoc ATmh|PerlIO*|PerlIO_open|const char *path|const char *mode
+=for apidoc Amh|int     |PerlIO_printf|PerlIO *f|const char *fmt|...
+=for apidoc ATmh|int    |PerlIO_putc|PerlIO *f|int ch
+=for apidoc ATmh|int    |PerlIO_puts|PerlIO *f|const char *string
+=for apidoc ATmh|void   |PerlIO_releaseFILE|PerlIO *f|FILE *stdio
+=for apidoc Amh|PerlIO *|PerlIO_reopen|const char *path|const char *mode|PerlIO *old
+=for apidoc ATmh|void   |PerlIO_rewind|PerlIO *f
+=for apidoc ATmh|int    |PerlIO_setpos|PerlIO *f|SV *saved
+=for apidoc Amh|int     |PerlIO_stdoutf|const char *fmt|...
+=for apidoc ATmh|int    |PerlIO_ungetc|PerlIO *f|int ch
+=for apidoc ATmh|int    |PerlIO_vprintf|PerlIO *f|const char *fmt|va_list args
+
+=for apidoc PerlIO_stdin
+=for apidoc PerlIO_stdout
+=for apidoc PerlIO_stderr
+=for apidoc PerlIO_close
+=for apidoc PerlIO_write
+=for apidoc PerlIO_flush
+=for apidoc PerlIO_eof
+=for apidoc PerlIO_error
+=for apidoc PerlIO_clearerr
+=for apidoc PerlIO_read
+=for apidoc PerlIO_fileno
+=for apidoc PerlIO_setlinebuf
+=for apidoc PerlIO_tell
+=for apidoc PerlIO_seek
+=for apidoc PerlIO_get_cnt
+=for apidoc PerlIO_get_ptr
+=for apidoc PerlIO_set_ptrcnt
+=for apidoc PerlIO_set_cnt
+=for apidoc PerlIO_get_base
+=for apidoc PerlIO_get_bufsiz
+
+=head1 DESCRIPTION
+
+Perl's source code, and extensions that want maximum portability,
+should use the above functions instead of those defined in ANSI C's
+I<stdio.h>.  The perl headers (in particular "perlio.h") will
+C<#define> them to the I/O mechanism selected at Configure time.
+
+The functions are modeled on those in I<stdio.h>, but parameter order
+has been "tidied up a little".
+
+C<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 two implementations:
+
+=over 4
+
+=item 1. USE_STDIO
+
+All above are #define'd to stdio functions or are trivial wrapper
+functions which call stdio. In this case I<only> PerlIO * is a FILE *.
+This has been the default implementation since the abstraction was
+introduced in perl5.003_02.
+
+=item 2. 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 I<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 C<PerlIO_apply_layers()>
+does anything "interesting".
+
+The USE_PERLIO implementation is described in L<perliol>.
+
+=back
+
+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 C<EOF> which is usually -1) and set C<errno> on error.
+
+=over 4
+
+=item B<PerlIO_stdin()>, B<PerlIO_stdout()>, B<PerlIO_stderr()>
+
+Use these rather than C<stdin>, C<stdout>, C<stderr>. They are written
+to look like "function calls" rather than variables because this makes
+it easier to I<make them> function calls if platform cannot export data
+to loaded modules, or if (say) different "threads" might have different
+values.
+
+=item B<PerlIO_open(path, mode)>, B<PerlIO_fdopen(fd,mode)>
+
+These correspond to fopen()/fdopen() and the arguments are the same.
+Return C<NULL> and set C<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 - C<errno> may not be set
+when C<NULL> is returned if this limit is exceeded.
+
+=item B<PerlIO_reopen(path,mode,f)>
+
+While this currently exists in both implementations, perl itself
+does not use it. I<As perl does not use it, it is not well tested.>
+
+Perl prefers to C<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.
+
+=item B<PerlIO_printf(f,fmt,...)>, B<PerlIO_vprintf(f,fmt,a)>
+
+These are fprintf()/vfprintf() equivalents.
+
+=item B<PerlIO_stdoutf(fmt,...)>
+
+This is printf() equivalent. printf is #defined to this function,
+so it is (currently) legal to use C<printf(fmt,...)> in perl sources.
+
+=item B<PerlIO_read(f,buf,count)>, B<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 C<EOF>.
+
+Returns a byte count if successful (which may be zero or
+positive), returns negative value and sets C<errno> on error.
+Depending on implementation C<errno> may be C<EINTR> if operation was
+interrupted by a signal.
+
+=item B<PerlIO_close(f)>
+
+Depending on implementation C<errno> may be C<EINTR> if operation was
+interrupted by a signal.
+
+=item B<PerlIO_puts(f,s)>, B<PerlIO_putc(f,c)>
+
+These correspond to fputs() and fputc().
+Note that arguments have been revised to have "file" first.
+
+=item B<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 B<c>.  Despite the implied "character" in the name only
+values in the range 0..0xFF are defined. Returns the byte B<c> on
+success or -1 (C<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.
+
+=item B<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 (C<EOF>) on error.
+
+=item B<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().
+
+=item B<PerlIO_error(f)>
+
+This corresponds to ferror().  Returns a true/false indication of
+whether there has been an IO error on the handle.
+
+=item B<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.
+
+=item B<PerlIO_clearerr(f)>
+
+This corresponds to clearerr(), i.e., clears 'error' and (usually)
+'eof' flags for the "stream". Does not return a value.
+
+=item B<PerlIO_flush(f)>
+
+This corresponds to fflush().  Sends any buffered write data to the
+underlying file.  If called with C<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 C<NULL>, and
+attempts to retain data on read streams either in the buffer or by
+seeking the handle to the current logical position.
+
+=item B<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 B<offset> and B<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 C<Off_t> which is a perl Configure value which may not be same
+as stdio's C<off_t>.
+
+=item B<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 C<Off_t> which is a perl
+Configure value which may not be same as stdio's C<off_t>.
+
+=item B<PerlIO_getpos(f,p)>, B<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().
+
+=item B<PerlIO_rewind(f)>
+
+This corresponds to rewind(). It is usually defined as being
+
+    PerlIO_seek(f,(Off_t)0L, SEEK_SET);
+    PerlIO_clearerr(f);
+
+=item B<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 C<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 C<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.
+
+=item B<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 I<only> when "dumping";
+it has nothing to do with $| auto-flush.)
+
+=back
+
+=head2 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
+
+I<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.
+
+=over 4
+
+=item B<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 I<f> is open, or
+use "r+" to indicate a read/write stream.
+
+Once called the FILE * should I<ONLY> be closed by calling
+C<PerlIO_close()> on the returned PerlIO *.
+
+The PerlIO is set to textmode. Use PerlIO_binmode if this is
+not the desired mode.
+
+This is B<not> the reverse of PerlIO_exportFILE().
+
+=item B<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 I<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 C<fclose()> on the file unless you call C<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).
+
+=item B<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().
+
+=item B<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 C<PerlIO_close()>.
+
+
+=back
+
+=head2 "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.
+
+=over 4
+
+=item B<PerlIO_fast_gets(f)>
+
+Returns true if implementation has all the interfaces required to
+allow perl's C<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'
+
+=item B<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.
+
+=item B<PerlIO_get_cnt(f)>
+
+Return count of readable bytes in the buffer. Zero or negative return
+means no more bytes available.
+
+=item B<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.
+
+=item B<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 C<PerlIO_get_ptr> and C<PerlIO_get_cnt>. The two
+values I<must> be consistent with each other (implementation may only
+use one or the other or may require both).
+
+=item B<PerlIO_canset_cnt(f)>
+
+Implementation can adjust its idea of number of bytes in the buffer.
+Do not use this - use PerlIO_fast_gets.
+
+=item B<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().
+
+=item B<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 B<-T> / B<-B> tests.
+Other uses would be very obscure...
+
+=item B<PerlIO_get_base(f)>
+
+Return I<start> of buffer. Access only positive offsets in the buffer
+up to the value returned by PerlIO_get_bufsiz().
+
+=item B<PerlIO_get_bufsiz(f)>
+
+Return the I<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 C<read()> (or whatever) last time IO was requested.
+
+=back
+
+=head2 Other Functions
+
+=over 4
+
+=item 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.
+
+=item PerlIO_binmode(f,ptype,imode,layers)
+
+The hook used by perl's C<binmode> operator.
+B<ptype> is perl's character for the kind of IO:
+
+=over 8
+
+=item 'E<lt>' read
+
+=item 'E<gt>' write
+
+=item '+' read/write
+
+=back
+
+B<imode> is C<O_BINARY> or C<O_TEXT>.
+
+B<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.)
+
+=item 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'} or defaults
+to stderr if the environment variable is not defined. Typical
+use might be
+
+  Bourne shells (sh, ksh, bash, zsh, ash, ...):
+   PERLIO_DEBUG=/tmp/perliodebug.log ./perl -Di somescript some args
+
+  Csh/Tcsh:
+   setenv PERLIO_DEBUG /tmp/perliodebug.log
+   ./perl -Di somescript some args
+
+  If you have the "env" utility:
+   env PERLIO_DEBUG=/tmp/perliodebug.log ./perl -Di somescript args
+
+  Win32:
+   set PERLIO_DEBUG=perliodebug.log
+   perl -Di somescript some args
+
+On a Perl built without C<-DDEBUGGING>, or when the C<-Di> command-line switch
+is not specified, or under taint, PerlIO_debug() is a no-op.
+
+=back