PerlIO documentation. Proofing and sub-editing requested.

p4raw-id: //depot/perlio@8581

2 files changed, 823 insertions, 99 deletions

diff --git a/pod/perlapio.pod b/pod/perlapio.pod
index 90475a9543..88a509cfd0 100644
--- a/pod/perlapio.pod
+++ b/pod/perlapio.pod
@@ -4,71 +4,121 @@ perlapio - perl's IO abstraction interface.
 
 =head1 SYNOPSIS
 
+    #define PERLIO_NOT_STDIO 0    /* For co-existance 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 *, int flags);
-    FILE   *PerlIO_exportFILE(PerlIO *, int flags);
-    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, 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 should use the above functions instead of those
-defined in ANSI C's I<stdio.h>.  The perl headers will 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
 
-This 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).
+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 dependant on the the underlying implementation. Where these variations are
+understood they are noted below.
+
+Unless otherwise noted functions returns 0 on sucess, 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()>
 
@@ -81,6 +131,19 @@ values.
 =item B<PerlIO_open(path, mode)>, B<PerlIO_fdopen(fd,mode)>
 
 These correspond to fopen()/fdopen() arguments are the same.
+Returns C<NULL> and sets C<errno> if there is an error.
+There may be implementation limit on number of open handles, which may
+be lower than the limit on the number of open files - C<errno> may
+not be 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)>
 
@@ -95,10 +158,16 @@ so it is (currently) legal to use C<printf(fmt,...)> in perl sources.
 
 These correspond to fread() and fwrite(). Note that arguments
 are different, there is only one "count" and order has
-"file" first.
+"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().
@@ -108,56 +177,103 @@ 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 charactr 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.
+the meaning of "fileno" may not match Unix. Returns -1 if the handle has not
+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().
+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_tell(f)>
+=item B<PerlIO_seek(f,offset,whence)>
 
-This corresponds to ftell().
+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_seek(f,o,w)>
+=item B<PerlIO_tell(f)>
 
-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 descriptior (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. They 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.
+PerlIO  or NULL on error.
+The system will attempt to automatically be delete then 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
+dependant 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
 
@@ -165,9 +281,19 @@ PerlIO which will automatically be deleted when closed.
 
 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.
+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 and changes in this area.
 
 =over 4
 
@@ -176,6 +302,10 @@ to library code which is going to use stdio calls.
 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 an PerlIO * return a 'native' FILE * suitable for
@@ -198,77 +328,158 @@ of FILE * is complete. It is removed from list of 'exported'
 FILE *s, and associated PerlIO * should revert to original
 behaviour.
 
-=item B<PerlIO_setlinebuf(f)>
-
-This corresponds to setlinebuf(). Use is deprecated pending
-further discussion. (Perl core uses it I<only> when "dumping";
-it has nothing to do with $| auto-flush.)
-
 =back
 
-In addition to user API above there is an "implementation" interface
+=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. This section is really of interest to only those
-concerned with detailed perl-core behaviour or implementing a
-PerlIO mapping.
+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)>
-
-Implementation can return pointer to current position in the "buffer" and
-a count of bytes available in the buffer.
+=item B<PerlIO_fast_gets(f)>
 
-=item B<PerlIO_get_ptr(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.
 
-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>.
+to C<PerlIO_get_ptr> and C<PerlIO_get_cnt>. The two values I<must> be consistent
+with each other (implementartion 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 < -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. 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 '<' read
+
+=item '>' 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 haver no effect what so ever.
+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
diff --git a/pod/perliol.pod b/pod/perliol.pod
new file mode 100644
index 0000000000..6169d43949
--- /dev/null
+++ b/pod/perliol.pod
@@ -0,0 +1,513 @@
+
+=head1 NAME
+
+perliol - C API for Perl's implementation of IO in Layers.
+
+=head1 SYNOPSIS
+
+    /* Defining a layer ... */
+    #include <perliol.h>
+
+
+=head1 DESCRIPTION
+
+This document describes the behavior and implementation of the PerlIO abstraction
+described in L<perlapio> when C<USE_PERLIO> is defined (and C<USE_SFIO> is not).
+
+=head2 History and Background
+
+The PerlIO abstraction was introduced in perl5.003_02 but languished as just
+an abstraction until perl5.7.0. However during that time a number of perl extenstions
+switch to using it, so the API is mostly fixed to maintain (source) compatibility.
+
+The aim of the implementation is to provide the PerlIO API in a flexible and
+platform neutral manner. It is also a trial of an "Object Oriented C, with vtables"
+approach which may be applied to perl6.
+
+=head2 Layers vs Disciplines
+
+Initial discussion of the ability to modify IO streams behaviour used the term
+"discipline" for the entities which were added. This came (I believe) from the use
+of the term in "sfio", which in turn borowed it from "line disciplines" on Unix
+terminals. However, this document (and the C code) uses the term "layer".
+This is I hope a natural term given the implementation, and should avoid conotations
+that are inherent in earlier uses of "discipline" for things which are rather different.
+
+=head2 Data Structures
+
+The basic data structure is a PerlIOl:
+
+	typedef struct _PerlIO PerlIOl;
+	typedef struct _PerlIO_funcs PerlIO_funcs;
+	typedef PerlIOl *PerlIO;
+
+	struct _PerlIO
+	{
+	 PerlIOl *	next;       /* Lower layer */
+	 PerlIO_funcs *	tab;        /* Functions for this layer */
+	 IV		flags;      /* Various flags for state */
+	};
+
+A PerlIOl * is a pointer to to the struct, and the I<application> level PerlIO *
+is a pointer to a PerlIOl * - i.e. a pointer to a pointer to the struct.
+This allows the application level PerlIO * to remain constant while the actual
+PerlIOl * underneath changes. (Compare perl's SV * which remains constant
+while its sv_any field changes as the scalar's type changes.)
+An IO stream is then in general represented as a pointer to this linked-list
+of "layers".
+
+It should be noted that because of the double indirection in a PerlIO *,
+a &(perlio->next) "is" a PerlIO *, and so to some degree at least
+one layer can use the "standard" API on the next layer down.
+
+A "layer" is composed of two parts:
+
+=over 4
+
+=item 1. The functions and attributes of the "layer class".
+
+=item 2. The per-instance data for a particular handle.
+
+=back
+
+=head2 Functions and Attributes
+
+The functions and attributes are accessed via the "tab" (for table) member of
+PerlIOl. The functions (methods of the layer "class") are fixed, and are defined by the
+PerlIO_funcs type. They are broadly the same as the public PerlIO_xxxxx functions:
+
+	struct _PerlIO_funcs
+	{
+	 char *		name;
+	 Size_t		size;
+	 IV		kind;
+	 IV		(*Fileno)(PerlIO *f);
+	 PerlIO *	(*Fdopen)(PerlIO_funcs *tab, int fd, const char *mode);
+	 PerlIO *	(*Open)(PerlIO_funcs *tab, const char *path, const char *mode);
+	 int		(*Reopen)(const char *path, const char *mode, PerlIO *f);
+	 IV		(*Pushed)(PerlIO *f,const char *mode,const char *arg,STRLEN len);
+	 IV		(*Popped)(PerlIO *f);
+	 /* Unix-like functions - cf sfio line disciplines */
+	 SSize_t	(*Read)(PerlIO *f, void *vbuf, Size_t count);
+	 SSize_t	(*Unread)(PerlIO *f, const void *vbuf, Size_t count);
+	 SSize_t	(*Write)(PerlIO *f, const void *vbuf, Size_t count);
+	 IV		(*Seek)(PerlIO *f, Off_t offset, int whence);
+	 Off_t		(*Tell)(PerlIO *f);
+	 IV		(*Close)(PerlIO *f);
+	 /* Stdio-like buffered IO functions */
+	 IV		(*Flush)(PerlIO *f);
+	 IV		(*Fill)(PerlIO *f);
+	 IV		(*Eof)(PerlIO *f);
+	 IV		(*Error)(PerlIO *f);
+	 void		(*Clearerr)(PerlIO *f);
+	 void		(*Setlinebuf)(PerlIO *f);
+	 /* Perl's snooping functions */
+	 STDCHAR *	(*Get_base)(PerlIO *f);
+	 Size_t		(*Get_bufsiz)(PerlIO *f);
+	 STDCHAR *	(*Get_ptr)(PerlIO *f);
+	 SSize_t	(*Get_cnt)(PerlIO *f);
+	 void		(*Set_ptrcnt)(PerlIO *f,STDCHAR *ptr,SSize_t cnt);
+	};
+
+The first few members of the struct give a "name" for the layer, the size to C<malloc>
+for the per-instance data, and some flags which are attributes of the class as whole
+(such as whether it is a buffering layer), then follow the functions which fall into
+four basic groups:
+
+=over 4
+
+=item 1. Opening and setup functions
+
+=item 2. Basic IO operations
+
+=item 3. Stdio class buffering options.
+
+=item 4. Functions to support Perl's traditional "fast" access to the buffer.
+
+=back
+
+A layer does not have to implement all the functions, but the whole table has
+to be present. Unimplemented slots can be NULL (which will will result in an error
+when called) or can be filled in with stubs to "inherit" behaviour from
+a "base class". This "inheritance" is fixed for all instances of the layer,
+but as the layer chooses which stubs to populate the table, limited
+"multiple inheritance" is possible.
+
+=head2 Per-instance Data
+
+The per-instance data are held in memory beyond the basic PerlIOl struct,
+by making a PerlIOl the first member of the layer's struct thus:
+
+	typedef struct
+	{
+	 struct _PerlIO base;       /* Base "class" info */
+	 STDCHAR *	buf;        /* Start of buffer */
+	 STDCHAR *	end;        /* End of valid part of buffer */
+	 STDCHAR *	ptr;        /* Current position in buffer */
+	 Off_t		posn;       /* Offset of buf into the file */
+	 Size_t		bufsiz;     /* Real size of buffer */
+	 IV		oneword;    /* Emergency buffer */
+	} PerlIOBuf;
+
+In this way (as for perl's scalars) a pointer to a PerlIOBuf can be treated
+as a pointer to a PerlIOl.
+
+=head2 Layers in action.
+
+                table           perlio          unix
+            |           |
+            +-----------+    +----------+    +--------+
+   PerlIO ->|           |--->|  next    |--->|  NULL  |
+            +-----------+    +----------+    +--------+
+            |           |    |  buffer  |    |   fd   |
+            +-----------+    |          |    +--------+
+            |           |    +----------+
+
+
+The above attempts to show how the layer scheme works in a simple case.
+The applications PerlIO * points to an entry in the table(s) representing open
+(allocated) handles. For example the first three slots in the table correspond
+to C<stdin>,C<stdout> and C<stderr>. The table in turn points to the current
+"top" layer for the handle - in this case an instance of the generic buffering
+layer "perlio". That layer in turn points to the next layer down - in this
+case the lowlevel "unix" layer.
+
+The above is roughly equivalent to a "stdio" buffered stream, but with much more
+flexibility:
+
+=over 4
+
+=item *
+
+If Unix level read/write/lseek is not appropriate for (say) sockets then
+the "unix" layer can be replaced (at open time or even dynamically) with a
+"socket" layer.
+
+=item *
+
+Different handles can have different buffering schemes. The "top" layer
+could be the "mmap" layer if reading disk files was quicker using C<mmap>
+than C<read>. An "unbuffered" stream can be implemented simply by
+not having a buffer layer.
+
+=item *
+
+Extra layers can be inserted to process the data as it flows through.
+This was the driving need for including the scheme in perkl5.70+ - we needed a mechanism
+to allow data to be translated bewteen perl's internal encoding (conceptually
+at least Unicode as UTF-8), and the "native" format used by the system.
+This is provided by the ":encoding(xxxx)" layer which typically sits above
+the buffering layer.
+
+=item *
+
+A layer can be added that does "\n" to CRLF translation. This layer can be used
+on any platform, not just those that normally do such things.
+
+=back
+
+=head2 Per-instance flag bits
+
+The generic flag bits are a hybrid of O_XXXXX style flags deduced from
+the mode string passed to PerlIO_open() and state bits for typical buffer
+layers.
+
+=over4
+
+=item PERLIO_F_EOF
+
+End of file.
+
+=item PERLIO_F_CANWRITE
+
+Writes are permited i.e. opened as "w" or "r+" or "a". etc.
+
+=item  PERLIO_F_CANREAD
+
+Reads are permited i.e. opened "r" or "w+" (or even "a+" - ick).
+
+=item PERLIO_F_ERROR
+
+An error has occured (for PerlIO_error())
+
+=item PERLIO_F_TRUNCATE
+
+Truncate file suggested by open mode.
+
+=item PERLIO_F_APPEND
+
+All writes should be appends.
+
+=item PERLIO_F_CRLF
+
+Layer is performing Win32-like "\n" => CR,LF for output and CR,LF => "\n" for
+input. Normally the provided "crlf" layer is only layer than need bother about
+this. PerlIO_binmode() will mess with this flag rather than add/remove layers
+if the PERLIO_K_CANCRLF bit is set for the layers class.
+
+=item PERLIO_F_UTF8
+
+Data for this written to this layer should be UTF-8 encoded, data provided
+by this layer should be considered UTF-8 encoded. Can be set on any layer
+by ":utf8" dummy layer. Also set on ":encoding" layer.
+
+=item PERLIO_F_UNBUF
+
+Layer is unbuffered - i.e. write to next layer down should occur for
+each write to this layer.
+
+=item PERLIO_F_WRBUF
+
+The buffer for this layer currently holds data written to it but not sent
+to next layer.
+
+=item PERLIO_F_RDBUF
+
+The buffer for this layer currently holds unconsumed data read from
+layer below.
+
+=item PERLIO_F_LINEBUF
+
+Layer is line buffered. Write data should be passed to next layer down whenever a
+"\n" is seen. Any data beyond the "\n" should then be processed.
+
+=item PERLIO_F_TEMP
+
+File has been unlink()ed, or should be deleted on close().
+
+=item PERLIO_F_OPEN
+
+Handle is open.
+
+=item PERLIO_F_FASTGETS
+
+This instance of this layer supports the "fast gets" interface.
+Normally set based on PERLIO_K_FASTGETS for the class and by the
+existance of the function(s) in the table. However a class that
+normally provides that interface may need to avoid it on a
+particular instance. The "pending" layer needs to do this when
+it is pushed above an layer which does not support the interface.
+(Perls sv_gets() does not expect the steams fast gets behaviour
+to change during one "get".)
+
+=back
+
+=head2 Methods in Detail
+
+=over 4
+
+=item IV	(*Fileno)(PerlIO *f);
+
+Returns the Unix/Posix numeric file decriptor for the handle.
+Normally PerlIOBase_fileno() (which just asks next layer down) will suffice for this.
+
+=item  PerlIO *	(*Fdopen)(PerlIO_funcs *tab, int fd, const char *mode);
+
+Should (perhaps indirectly) call PerlIO_allocate() to allocate a slot
+in the table and associate it with the given numeric file descriptor,
+which will be open in an manner compatible with the supplied mode string.
+
+=item  PerlIO *	(*Open)(PerlIO_funcs *tab, const char *path, const char *mode);
+
+Should attempt to open the given path and if that succeeds then (perhaps indirectly)
+call PerlIO_allocate() to allocate a slot in the table and associate it with the
+layers information for the opened file.
+
+=item  int		(*Reopen)(const char *path, const char *mode, PerlIO *f);
+
+Re-open the supplied PerlIO * to connect it to C<path> in C<mode>. Returns as success flag.
+Perl does not use this and L<perlapio> marks it as subject to change.
+
+=item  IV		(*Pushed)(PerlIO *f,const char *mode,const char *arg,STRLEN len);
+
+Called when the layer is pushed onto the stack. The C<mode> argument may be NULL if this
+occurs post-open. The C<arg> and C<len> will be present if an argument string was
+passed. In most cases this should call PerlIOBase_pushed() to conver C<mode> into
+the appropriate PERLIO_F_XXXXX flags in addition to any actions the layer itself takes.
+
+=item  IV		(*Popped)(PerlIO *f);
+
+Called when the layer is popped from the stack. A layer will normally be popped after
+Close() is called. But a layer can be popped without being closed if the program
+is dynamically managing layers on the stream. In such cases Popped() should free
+any resources (buffers, translation tables, ...) not held directly in the layer's
+struct.
+
+=item  SSize_t	(*Read)(PerlIO *f, void *vbuf, Size_t count);
+
+Basic read operation. Returns actual bytes read, or -1 on an error.
+Typically will call Fill and manipulate pointers (possibly via the API).
+PerlIOBuf_read() may be suitable for derived classes which provide "fast gets" methods.
+
+=item  SSize_t	(*Unread)(PerlIO *f, const void *vbuf, Size_t count);
+
+A superset of stdio's ungetc(). Should arrange for future reads to see the bytes in C<vbuf>.
+If there is no obviously better implementation then PerlIOBase_unread() provides
+the function by pushing a "fake" "pending" layer above the calling layer.
+
+=item  SSize_t	(*Write)(PerlIO *f, const void *vbuf, Size_t count);
+
+Basic write operation. Returns bytes written or -1 on an error.
+
+=item  IV		(*Seek)(PerlIO *f, Off_t offset, int whence);
+
+Position the file pointer. Should normally call its own Flush method and
+then the Seek method of next layer down.
+
+=item  Off_t		(*Tell)(PerlIO *f);
+
+Return the file pointer. May be based on layers cached concept of position to
+avoid overhead.
+
+=item  IV		(*Close)(PerlIO *f);
+
+Close the stream. Should normally call PerlIOBase_close() to flush itself
+and Close layers below and then deallocate any data structures (buffers, translation
+tables, ...) not  held directly in the data structure.
+
+=item  IV		(*Flush)(PerlIO *f);
+
+Should make streams state consistent with layers below. That is any
+buffered write data should be written, and file position of lower layer
+adjusted for data read fron below but not actually consumed.
+
+=item  IV		(*Fill)(PerlIO *f);
+
+The buffer for this layer should be filled (for read) from layer below.
+
+=item  IV		(*Eof)(PerlIO *f);
+
+Return end-of-file indicator. PerlIOBase_eof() is normally sufficient.
+
+=item  IV		(*Error)(PerlIO *f);
+
+Return error indicator. PerlIOBase_error() is normally sufficient.
+
+=item  void		(*Clearerr)(PerlIO *f);
+
+Clear end-of-file and error indicators. Should call PerlIOBase_clearerr()
+to set the PERLIO_F_XXXXX flags, which may suffice.
+
+=item  void		(*Setlinebuf)(PerlIO *f);
+
+Mark the stream as line buffered.
+
+=item  STDCHAR *	(*Get_base)(PerlIO *f);
+
+Allocate (if not already done so) the read buffer for this layer and
+return pointer to it.
+
+=item  Size_t		(*Get_bufsiz)(PerlIO *f);
+
+Return the number of bytes that last Fill() put in the buffer.
+
+=item  STDCHAR *	(*Get_ptr)(PerlIO *f);
+
+Return the current read pointer relative to this layers buffer.
+
+=item  SSize_t	(*Get_cnt)(PerlIO *f);
+
+Return the number of bytes left to be read in the current buffer.
+
+=item  void		(*Set_ptrcnt)(PerlIO *f,STDCHAR *ptr,SSize_t cnt);
+
+Adjust the read pointer and count of bytes to match C<ptr> and/or C<cnt>.
+The application (or layer above) must ensure they are consistent.
+(Checking is allowed by the paranoid.)
+
+=back
+
+
+=head2 Core Layers
+
+The file C<perlio.c> provides the following layers:
+
+=over 4
+
+=item "unix"
+
+A basic non-buffered layer which calls Unix/POSIX read(), write(), lseek(), close().
+No buffering. Even on platforms that distinguish between O_TEXT and O_BINARY
+this layer is always O_BINARY.
+
+=item "perlio"
+
+A very complete generic buffering layer which provides the whole of PerlIO API.
+It is also intended to be used as a "base class" for other layers. (For example
+its Read() method is implemented in terms of the Get_cnt()/Get_ptr()/Set_ptrcnt()
+methods).
+
+"perlio" over "unix" provides a complete replacement for stdio as seen via PerlIO API.
+This is the default for USE_PERLIO when system's stdio does not permit perl's
+"fast gets" access, and which do not distinguish between O_TEXT and O_BINARY.
+
+=item "stdio"
+
+A layer which provides the PerlIO API via the layer scheme, but implements it by calling
+system's stdio. This is (currently) the default if system's stdio provides sufficient
+access to allow perl's "fast gets" access and which do not distinguish between O_TEXT and
+O_BINARY.
+
+=item "crlf"
+
+A layer derived using "perlio" as a base class. It provides Win32-like "\n" to CR,LF
+translation. Can either be applied above "perlio" or serve as the buffer layer itself.
+"crlf" over "unix" is the default if system distinguishes between O_TEXT and O_BINARY
+opens. (At some point "unix" will be replaced by a "native" Win32 IO layer on that
+platform, as Win32's read/write layer has various drawbacks.)
+The "crlf" layer is a reasonable model for a layer which transforms data in some way.
+
+=item "mmap"
+
+If Configure detects C<mmap()> functions this layer is provided (with "perlio" as a
+"base") which does "read" operations by mmap()ing the file. Performance improvement
+is marginal on modern systems, so it is mainly there as a proof of concept.
+It is likely to be unbundled from the core at some point.
+The "mmap" layer is a reasonable model for a minimalist "derived" layer.
+
+=item "pending"
+
+An "internal" derivative of "perlio" which can be used to provide Unread() function
+for layers which have no buffer or cannot be bothered.
+(Basically this layer's Fill() pops itself off the stack and so resumes reading
+from layer below.)
+
+=item "raw"
+
+A dummy layer which never exists on the layer stack. Instead when "pushed" it
+actually pops the stack!, removing itself, and any other layers until it reaches
+a layer with the class PERLIO_K_RAW bit set.
+
+=item "utf8"
+
+Another dummy layer. When pushed it pops itself and sets the PERLIO_F_UTF8 flag
+on the layer which was (and now is once more) the top of the stack.
+
+=back
+
+In addition C<perlio.c> also provides a number of PerlIOBase_xxxx() functions
+which are intended to be used in the table slots of classes which do not need
+to do anything special for a particular method.
+
+=head2 Extension Layers
+
+Layers can made available by extension modules.
+
+=over 4
+
+=item "encoding"
+
+   use Encoding;
+
+makes this layer available. It is an example of a layer which takes an argument.
+as it is called as:
+
+   open($fh,"<:encoding(iso-8859-7)",$pathname)
+
+=back
+
+
+=cut
+
+
+