#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);

       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

       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.

           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.

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

       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.

           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.

           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.

           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.

           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().

           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.

           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".

           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().

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

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

           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.


       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.

           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().

           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).

           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().

       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.

           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'

           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.

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

           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.

           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).

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

           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().

           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...

   Other Functions
           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.

           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:


           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 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

              setenv PERLIO_DEBUG /dev/tty
              ./perl somescript some args

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

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