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[p5sagit/p5-mst-13.2.git] / pod / perliol.pod

=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 extentions switched 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 borrowed 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
connotations 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 C<PerlIOl *> is a pointer to to the struct, and the I<application> level
C<PerlIO *> is a pointer to a C<PerlIOl *> - i.e. a pointer to a pointer to
the struct. This allows the application level C<PerlIO *> to remain
constant while the actual C<PerlIOl *> underneath changes. (Compare perl's
C<SV *> which remains constant while its C<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 C<PerlIO *>,
a C<< &(perlio-E<gt>next) >> "is" a C<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 C<PerlIOl>. The functions (methods of the layer "class") are
fixed, and are defined by the C<PerlIO_funcs> type. They are broadly the
same as the public C<PerlIO_xxxxx> functions:

  struct _PerlIO_funcs
  {
   char *		name;
   Size_t		size;
   IV		kind;
   IV		(*Pushed)(PerlIO *f,const char *mode,SV *arg);
   IV		(*Popped)(PerlIO *f);
   PerlIO *	(*Open)(pTHX_ PerlIO_funcs *tab,
  			AV *layers, IV n,
  			const char *mode,
  			int fd, int imode, int perm,
  			PerlIO *old,
  			int narg, SV **args);
   SV *		(*Getarg)(PerlIO *f);
   IV		(*Fileno)(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 application's C<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 C<read>/C<write>/C<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 perl 5.7.0+ - 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 C<O_XXXXX> style flags deduced from
the mode string passed to C<PerlIO_open()>, and state bits for typical buffer
layers.

=over 4

=item PERLIO_F_EOF

End of file.

=item PERLIO_F_CANWRITE

Writes are permitted, i.e. opened as "w" or "r+" or "a", etc.

=item  PERLIO_F_CANREAD

Reads are permitted i.e. opened "r" or "w+" (or even "a+" - ick).

=item PERLIO_F_ERROR

An error has occured (for C<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" mapped to CR,LF for output and CR,LF
mapped to "\n" for input. Normally the provided "crlf" layer is the only
layer that need bother about this. C<PerlIO_binmode()> will mess with this
flag rather than add/remove layers if the C<PERLIO_K_CANCRLF> bit is set
for the layers class.

=item PERLIO_F_UTF8

Data 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 C<unlink()>ed, or should be deleted on C<close()>.

=item PERLIO_F_OPEN

Handle is open.

=item PERLIO_F_FASTGETS

This instance of this layer supports the "fast C<gets>" interface.
Normally set based on C<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.
(Perl's C<sv_gets()> does not expect the streams fast C<gets> behaviour
to change during one "get".)

=back

=head2 Methods in Detail

=over 4

=item  IV		(*Pushed)(PerlIO *f,const char *mode, SV *arg);

The only absoultely mandatory method. Called when the layer is pushed onto the stack.
The C<mode> argument may  be NULL if this occurs post-open. The C<arg> will be non-C<NULL>
if an argument string was passed. In most cases this should call
C<PerlIOBase_pushed()> to convert C<mode> into the appropriate
C<PERLIO_F_XXXXX> flags in addition to any actions the layer itself takes.
If a layer is not expecting an argument it need neither save the one passed to it, nor
provide C<Getarg()> (it could perhaps C<Perl_warn> that the argument was un-expected).

=item  IV		(*Popped)(PerlIO *f);

Called when the layer is popped from the stack. A layer will normally be
popped after C<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 C<Popped()> should free any resources (buffers, translation
tables, ...) not held directly in the layer's struct.
It should also C<Unread()> any unconsumed data that has been read and buffered
from the layer below back to that layer, so that it can be re-provided to what
ever is now above.

=item  PerlIO *	(*Open)(...);

The C<Open()> method has lots of arguments because it combines the functions
of perl's C<open>, C<PerlIO_open>, perl's C<sysopen>, C<PerlIO_fdopen> and C<PerlIO_reopen>.
The full prototype is as follows:

 PerlIO *	(*Open)(pTHX_ PerlIO_funcs *tab,
			AV *layers, IV n,
			const char *mode,
			int fd, int imode, int perm,
			PerlIO *old,
			int narg, SV **args);

Open should (perhaps indirectly) call C<PerlIO_allocate()> to allocate a slot in the table and
associate it with the layers information for the opened file, by calling C<PerlIO_push>.
The I<layers> AV is an array of all the layers destined for the C<PerlIO *>,
and any arguments passed to them,  I<n> is the index into that array of the
layer being called. The macro C<PerlIOArg> will return a (possibly C<NULL>) SV *
for the argument passed to the layer.

The I<mode> string is an "C<fopen()>-like" string which would match the regular
expression C</^[I#]?[rwa]\+?[bt]?$/>.

The C<'I'> prefix is used during creation of C<stdin>..C<stderr> via special
C<PerlIO_fdopen> calls; the C<'#'> prefix means that this is C<sysopen> and that I<imode> and
I<perm> should be passed to C<PerlLIO_open3>; C<'r'> means B<r>ead, C<'w'> means B<w>rite
and C<'a'> means B<a>ppend. The C<'+'> suffix means that both reading and writing/appending
are permited. The C<'b'> suffix means file should be binary, and C<'t'> means it
is text. (Binary/Text should be ignored by almost all layers and binary IO done,
with PerlIO. The C<:crlf> layer should be pushed to handle the distinction.)

If I<old> is not C<NULL> then this is a C<PerlIO_reopen>. Perl iteself does not use
this (yet?) and semantics are a little vague.

If I<fd> not negative then it is the numeric file descriptor I<fd>, which will
be open in an manner compatible with the supplied mode string, the call is
thus equivalent to C<PerlIO_fdopen>. In this case I<nargs> will be zero.

If I<nargs> is greater than zero then it gives the number of arguments passed
to C<open>, otherwise it will be 1 if for example C<PerlIO_open> was called.
In simple cases SvPV(*args) is the pathname to open.

Having said all that translation-only layers do not need to provide C<Open()> at all,
but rather leave the opening to a lower level layer and wait to be "pushed".
If a layer does provide C<Open()> it should normaly call the C<Open()> method
of next layer down (if any) and then push itself on top if that succeeds.

=item SV *		(*Getarg)(PerlIO *f);

Optional. If present should return an SV * representing the string argument
passed to the layer when it was pushed. e.g. ":encoding(ascii)" would
return an SvPV with value "ascii".

=item IV	(*Fileno)(PerlIO *f);

Returns the Unix/Posix numeric file decriptor for the handle. Normally
C<PerlIOBase_fileno()> (which just asks next layer down) will suffice
for this.

=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).
C<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 C<ungetc()>. Should arrange for future reads to
see the bytes in C<vbuf>. If there is no obviously better implementation
then C<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 C<Flush> method and
then the C<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 C<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 stream's state consistent with layers below. That is, any
buffered write data should be written, and file position of lower layers
adjusted for data read fron below but not actually consumed.
(Should perhaps C<Unread()> such data to the lower layer.)

=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. C<PerlIOBase_eof()> is normally sufficient.

=item  IV		(*Error)(PerlIO *f);

Return error indicator. C<PerlIOBase_error()> is normally sufficient.

=item  void		(*Clearerr)(PerlIO *f);

Clear end-of-file and error indicators. Should call C<PerlIOBase_clearerr()>
to set the C<PERLIO_F_XXXXX> flags, which may suffice.

=item  void		(*Setlinebuf)(PerlIO *f);

Mark the stream as line buffered. C<PerlIOBase_setlinebuf()> sets the
PERLIO_F_LINEBUF flag and is normally sufficient.

=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 C<Fill()> put in the buffer.

=item  STDCHAR *	(*Get_ptr)(PerlIO *f);

Return the current read pointer relative to this layer's 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 C<read()>, C<write()>,
C<lseek()>, C<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 C<Read()> method is implemented in terms of the
C<Get_cnt()>/C<Get_ptr()>/C<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 C<O_TEXT> and C<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 C<O_TEXT> and C<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 C<O_TEXT> and C<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 C<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 C<PERLIO_K_RAW> bit set.

=item "utf8"

Another dummy layer. When pushed it pops itself and sets the
C<PERLIO_F_UTF8> flag on the layer which was (and now is once more) the top
of the stack.

=back

In addition F<perlio.c> also provides a number of C<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. When an unknown layer is encountered
the PerlIO code will perform the equivalent of :

   use PerlIO 'layer';

Where I<layer> is the unknown layer. F<PerlIO.pm> will then attempt to :

   require PerlIO::layer;

If after that process the layer is still not defined then the C<open> will fail.

The following extension layers are bundled with perl:

=over 4

=item ":encoding"

   use Encoding;

makes this layer available, although F<PerlIO.pm> "knows" where to find it.
It is an example of a layer which takes an argument as it is called thus:

   open($fh,"<:encoding(iso-8859-7)",$pathname)

=item ":Scalar"

Provides support for

   open($fh,"...",\$scalar)

When a handle is so opened, then reads get bytes from the string value of I<$scalar>,
and writes change the value. In both cases the position in I<$scalar> starts as zero
but can be altered via C<seek>, and determined via C<tell>.

=item ":Object" or ":Perl"

May be provided to allow layers to be implemented as perl code - implementation
is being investigated.

=back

=cut