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, 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,...)

=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 three 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_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 *.

=item 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 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 all three 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)>