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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, int flags);
    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 arround 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 thing layer (for efficiency) the semantics of these functions are
somewhat dependent on the 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 limnit 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 to fread() and fwrite(). Note that arguments
are different, there is only one "count" and order has
"file" first. Returns a byte count if successful (which may be zero),
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).
Calling on a handle open for read only, or on which last operation was a read of some kind
may lead to undefined behaviour.

=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 desciptor 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 fisrt 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,flags)>

Used to get a PerlIO * from a FILE *.
May need additional arguments, interface under review.

The flags argument was meant to be used for read vs write vs read/write
information. In hindsight it would have been better to make it a char *mode
as in fopen/freopen.

=item B<PerlIO_exportFILE(f,flags)>

Given a PerlIO * return a 'native' FILE * suitable for
passing to code expecting to be compiled and linked with
ANSI C I<stdio.h>.

The fact that such a FILE * has been 'exported' is recorded,
and may affect future PerlIO operations on the original
PerlIO *.

=item B<PerlIO_findFILE(f)>

Returns previously 'exported' FILE * (if any).
Place holder until interface is fully defined.

=item B<PerlIO_releaseFILE(p,f)>

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

=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. 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 charcter 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 ":raw" or :"crlf" make
sense in the non USE_PERLIO case.

Portable cases are:

    PerlIO_binmode(f,ptype,O_BINARY,":raw");
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'} typical use
might be

  Bourne shells:
   PERLIO_DEBUG=/dev/tty ./perl somescript some args

  Csh:
   setenv 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.

=back