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=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