=head1 NAME
-perlio - perl's IO abstraction interface.
+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 *,const char *);
- int PerlIO_close(PerlIO *);
-
- int PerlIO_stdoutf(const char *,...)
- int PerlIO_puts(PerlIO *,const char *);
- int PerlIO_putc(PerlIO *,int);
- int PerlIO_write(PerlIO *,const void *,size_t);
- int PerlIO_printf(PerlIO *, const char *,...);
- int PerlIO_vprintf(PerlIO *, const char *, va_list);
- int PerlIO_flush(PerlIO *);
-
- int PerlIO_eof(PerlIO *);
- int PerlIO_error(PerlIO *);
- void PerlIO_clearerr(PerlIO *);
-
- int PerlIO_getc(PerlIO *);
- int PerlIO_ungetc(PerlIO *,int);
- int PerlIO_read(PerlIO *,void *,size_t);
-
- int PerlIO_fileno(PerlIO *);
- PerlIO *PerlIO_fdopen(int, const char *);
- PerlIO *PerlIO_importFILE(FILE *);
- FILE *PerlIO_exportFILE(PerlIO *);
- FILE *PerlIO_findFILE(PerlIO *);
- void PerlIO_releaseFILE(PerlIO *,FILE *);
-
- void PerlIO_setlinebuf(PerlIO *);
-
- long PerlIO_tell(PerlIO *);
- int PerlIO_seek(PerlIO *,off_t,int);
- int PerlIO_getpos(PerlIO *,Fpos_t *)
- int PerlIO_setpos(PerlIO *,Fpos_t *)
- void PerlIO_rewind(PerlIO *);
-
- int PerlIO_has_base(PerlIO *);
- int PerlIO_has_cntptr(PerlIO *);
- int PerlIO_fast_gets(PerlIO *);
- int PerlIO_canset_cnt(PerlIO *);
-
- char *PerlIO_get_ptr(PerlIO *);
- int PerlIO_get_cnt(PerlIO *);
- void PerlIO_set_cnt(PerlIO *,int);
- void PerlIO_set_ptrcnt(PerlIO *,char *,int);
- char *PerlIO_get_base(PerlIO *);
- int PerlIO_get_bufsiz(PerlIO *);
+
+ 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 should use the above functions instead of those
-defined in ANSI C's I<stdio.h>, I<perlio.h> will the C<#define> them to
-the I/O mechanism selected at Configure time.
+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 B<PerlIO *>
+=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.
-This takes the place of FILE *. Unlike FILE * it should be treated as
-opaque (it is probably safe to assume it is a pointer to something).
+=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
+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() arguments are the same.
+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 is fprintf()/vfprintf equivalents.
+These are fprintf()/vfprintf() equivalents.
=item B<PerlIO_stdoutf(fmt,...)>
=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.
+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)>
-=item B<PerlIO_puts(s,f)>, B<PerlIO_putc(c,f)>
+Depending on implementation C<errno> may be C<EINTR> if operation was
+interrupted by a signal.
-These correspond to fputs() and fputc().
+=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(c,f)>
+=item B<PerlIO_ungetc(f,c)>
-This corresponds to ungetc().
-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 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().
+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().
+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.
+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 'eof' and 'error'
-flags for the "stream".
+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().
+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().
-
-=item B<PerlIO_seek(f,o,w)>
-
-This corresponds to fseek().
+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 to fgetpos() and fsetpos(). If platform does not
-have the stdio calls then they are implemented in terms of PerlIO_tell()
-and PerlIO_seek().
+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(). Note may be redefined
-in terms of PerlIO_seek() at some point.
+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 which will automatically be deleted when closed.
+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.
-=back
+=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 if perlio is implemented on top of (say) sfio
-then mechanisms must exist to create a FILE * which can be passed
-to library code which is going to use stdio calls.
+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,flags)>
+=item B<PerlIO_importFILE(f,mode)>
Used to get a PerlIO * from a FILE *.
-May need additional arguments, interface under review.
-=item B<PerlIO_exportFILE(f,flags)>
+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.
-Given an PerlIO * return a 'native' FILE * suitable for
-passing to code expecting to be compiled and linked with
-ANSI C I<stdio.h>.
+Once called the FILE * should I<ONLY> be closed by calling
+C<PerlIO_close()> on the returned PerlIO *.
-The fact that such a FILE * has been 'exported' is recorded,
-and may affect future PerlIO operations on the original
-PerlIO *.
+The PerlIO is set to textmode. Use PerlIO_binmode if this is
+not the desired mode.
-=item B<PerlIO_findFILE(f)>
+This is B<not> the reverse of PerlIO_exportFILE().
+
+=item B<PerlIO_exportFILE(f,mode)>
-Returns previously 'exported' FILE * (if any).
-Place holder until interface is fully defined.
+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 list of 'exported'
-FILE *s, and associated PerlIO * should revert to original
-behaviour.
+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.
-=item B<PerlIO_setlinebuf(f)>
+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()>.
-This corresponds to setlinebuf(). Use is deprecated pending
-further discussion. (Perl core uses it I<only> when "dumping"
-is has nothing to do with $| auto-flush.)
=back
-In addition to user API 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. This section is really of interest to only those
-concerned with detailed perl-core behaviour or implementing a
-PerlIO mapping.
+=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_has_cntptr(f)>
+=item B<PerlIO_fast_gets(f)>
-Implementation can return pointer to current position in the "buffer" and
-a count of bytes available in the buffer.
+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.
-=item B<PerlIO_get_ptr(f)>
-
-Return pointer to next readable byte in buffer.
+ PerlIO_fast_gets(f) = PerlIO_has_cntptr(f) && \
+ PerlIO_canset_cnt(f) && \
+ `Can set pointer into buffer'
-=item B<PerlIO_get_cnt(f)>
-Return count of readable bytes in the buffer.
+=item B<PerlIO_has_cntptr(f)>
-=item B<PerlIO_canset_cnt(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.
-Implementation can adjust its idea of number of
-bytes in the buffer.
+=item B<PerlIO_get_cnt(f)>
-=item B<PerlIO_fast_gets(f)>
+Return count of readable bytes in the buffer. Zero or negative return
+means no more bytes available.
-Implementation has all the interfaces required to
-allow perl's fast code to handle <FILE> mechanism.
+=item B<PerlIO_get_ptr(f)>
- PerlIO_fast_gets(f) = PerlIO_has_cntptr(f) && \
- PerlIO_canset_cnt(f) && \
- `Can set pointer into buffer'
+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>.
+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.
-Currently used in only doio.c to force count < -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".
+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)>
-Implementation has a buffer, and can return pointer
+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.
+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 I<total size> of buffer.
+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.)
-=back
+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'} 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.
+
+=back
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