perl/lib/PerlIO.pm

package PerlIO;

our $VERSION = '1.03';

# Map layer name to package that defines it
our %alias;

sub import
{
 my $class = shift;
 while (@_)
  {
   my $layer = shift;
   if (exists $alias{$layer})
    {
     $layer = $alias{$layer}
    }
   else
    {
     $layer = "${class}::$layer";
    }
   eval "require $layer";
   warn $@ if $@;
  }
}

sub F_UTF8 () { 0x8000 }

1;
__END__

=head1 NAME

PerlIO - On demand loader for PerlIO layers and root of PerlIO::* name space

=head1 SYNOPSIS

  open($fh,"<:crlf", "my.txt"); # portably open a text file for reading

  open($fh,"<","his.jpg");      # portably open a binary file for reading
  binmode($fh);

  Shell:
    PERLIO=perlio perl ....

=head1 DESCRIPTION

When an undefined layer 'foo' is encountered in an C<open> or
C<binmode> layer specification then C code performs the equivalent of:

  use PerlIO 'foo';

The perl code in PerlIO.pm then attempts to locate a layer by doing

  require PerlIO::foo;

Otherwise the C<PerlIO> package is a place holder for additional
PerlIO related functions.

The following layers are currently defined:

=over 4

=item :unix

Lowest level layer which provides basic PerlIO operations in terms of
UNIX/POSIX numeric file descriptor calls
(open(), read(), write(), lseek(), close()).

=item :stdio

Layer which calls C<fread>, C<fwrite> and C<fseek>/C<ftell> etc.  Note
that as this is "real" stdio it will ignore any layers beneath it and
got straight to the operating system via the C library as usual.

=item :perlio

A from scratch implementation of buffering for PerlIO. Provides fast
access to the buffer for C<sv_gets> which implements perl's readline/E<lt>E<gt>
and in general attempts to minimize data copying.

C<:perlio> will insert a C<:unix> layer below itself to do low level IO.

=item :crlf

A layer that implements DOS/Windows like CRLF line endings.  On read
converts pairs of CR,LF to a single "\n" newline character.  On write
converts each "\n" to a CR,LF pair.  Note that this layer likes to be
one of its kind: it silently ignores attempts to be pushed into the
layer stack more than once.

It currently does I<not> mimic MS-DOS as far as treating of Control-Z
as being an end-of-file marker.

(Gory details follow) To be more exact what happens is this: after
pushing itself to the stack, the C<:crlf> layer checks all the layers
below itself to find the first layer that is capable of being a CRLF
layer but is not yet enabled to be a CRLF layer.  If it finds such a
layer, it enables the CRLFness of that other deeper layer, and then
pops itself off the stack.  If not, fine, use the one we just pushed.

The end result is that a C<:crlf> means "please enable the first CRLF
layer you can find, and if you can't find one, here would be a good
spot to place a new one."

Based on the C<:perlio> layer.

=item :mmap

A layer which implements "reading" of files by using C<mmap()> to
make (whole) file appear in the process's address space, and then
using that as PerlIO's "buffer". This I<may> be faster in certain
circumstances for large files, and may result in less physical memory
use when multiple processes are reading the same file.

Files which are not C<mmap()>-able revert to behaving like the C<:perlio>
layer. Writes also behave like C<:perlio> layer as C<mmap()> for write
needs extra house-keeping (to extend the file) which negates any advantage.

The C<:mmap> layer will not exist if platform does not support C<mmap()>.

=item :utf8

Declares that the stream accepts perl's internal encoding of
characters.  (Which really is UTF-8 on ASCII machines, but is
UTF-EBCDIC on EBCDIC machines.)  This allows any character perl can
represent to be read from or written to the stream. The UTF-X encoding
is chosen to render simple text parts (i.e.  non-accented letters,
digits and common punctuation) human readable in the encoded file.

Here is how to write your native data out using UTF-8 (or UTF-EBCDIC)
and then read it back in.

        open(F, ">:utf8", "data.utf");
        print F $out;
        close(F);

        open(F, "<:utf8", "data.utf");
        $in = <F>;
        close(F);

=item :bytes

This is the inverse of C<:utf8> layer. It turns off the flag
on the layer below so that data read from it is considered to
be "octets" i.e. characters in range 0..255 only. Likewise
on output perl will warn if a "wide" character is written
to a such a stream.

=item :raw

The C<:raw> layer is I<defined> as being identical to calling
C<binmode($fh)> - the stream is made suitable for passing binary data
i.e. each byte is passed as-is. The stream will still be
buffered.

In Perl 5.6 and some books the C<:raw> layer (previously sometimes also
referred to as a "discipline") is documented as the inverse of the
C<:crlf> layer. That is no longer the case - other layers which would
alter binary nature of the stream are also disabled.  If you want UNIX
line endings on a platform that normally does CRLF translation, but still
want UTF-8 or encoding defaults the appropriate thing to do is to add
C<:perlio> to PERLIO environment variable.

The implementation of C<:raw> is as a pseudo-layer which when "pushed"
pops itself and then any layers which do not declare themselves as suitable
for binary data. (Undoing :utf8 and :crlf are implemented by clearing
flags rather than popping layers but that is an implementation detail.)

As a consequence of the fact that C<:raw> normally pops layers
it usually only makes sense to have it as the only or first element in
a layer specification.  When used as the first element it provides
a known base on which to build e.g.

    open($fh,":raw:utf8",...)

will construct a "binary" stream, but then enable UTF-8 translation.

=item :pop

A pseudo layer that removes the top-most layer. Gives perl code
a way to manipulate the layer stack. Should be considered
as experimental. Note that C<:pop> only works on real layers
and will not undo the effects of pseudo layers like C<:utf8>.
An example of a possible use might be:

    open($fh,...)
    ...
    binmode($fh,":encoding(...)");  # next chunk is encoded
    ...
    binmode($fh,":pop");            # back to un-encocded

A more elegant (and safer) interface is needed.

=item :win32

On Win32 platforms this I<experimental> layer uses native "handle" IO
rather than unix-like numeric file descriptor layer. Known to be
buggy as of perl 5.8.2.

=back

=head2 Custom Layers

It is possible to write custom layers in addition to the above builtin
ones, both in C/XS and Perl.  Two such layers (and one example written
in Perl using the latter) come with the Perl distribution.

=over 4

=item :encoding

Use C<:encoding(ENCODING)> either in open() or binmode() to install
a layer that does transparently character set and encoding transformations,
for example from Shift-JIS to Unicode.  Note that under C<stdio>
an C<:encoding> also enables C<:utf8>.  See L<PerlIO::encoding>
for more information.

=item :via

Use C<:via(MODULE)> either in open() or binmode() to install a layer
that does whatever transformation (for example compression /
decompression, encryption / decryption) to the filehandle.
See L<PerlIO::via> for more information.

=back

=head2 Alternatives to raw

To get a binary stream an alternate method is to use:

    open($fh,"whatever")
    binmode($fh);

this has advantage of being backward compatible with how such things have
had to be coded on some platforms for years.

To get an un-buffered stream specify an unbuffered layer (e.g. C<:unix>)
in the open call:

    open($fh,"<:unix",$path)

=head2 Defaults and how to override them

If the platform is MS-DOS like and normally does CRLF to "\n"
translation for text files then the default layers are :

  unix crlf

(The low level "unix" layer may be replaced by a platform specific low
level layer.)

Otherwise if C<Configure> found out how to do "fast" IO using system's
stdio, then the default layers are:

  unix stdio

Otherwise the default layers are

  unix perlio

These defaults may change once perlio has been better tested and tuned.

The default can be overridden by setting the environment variable
PERLIO to a space separated list of layers (C<unix> or platform low
level layer is always pushed first).

This can be used to see the effect of/bugs in the various layers e.g.

  cd .../perl/t
  PERLIO=stdio  ./perl harness
  PERLIO=perlio ./perl harness

For the various value of PERLIO see L<perlrun/PERLIO>.

=head2 Querying the layers of filehandles

The following returns the B<names> of the PerlIO layers on a filehandle.

   my @layers = PerlIO::get_layers($fh); # Or FH, *FH, "FH".

The layers are returned in the order an open() or binmode() call would
use them.  Note that the "default stack" depends on the operating
system and on the Perl version, and both the compile-time and
runtime configurations of Perl.

The following table summarizes the default layers on UNIX-like and
DOS-like platforms and depending on the setting of the C<$ENV{PERLIO}>:

 PERLIO     UNIX-like                   DOS-like
 
 unset / "" unix perlio / stdio [1]     unix crlf
 stdio      unix perlio / stdio [1]     stdio
 perlio     unix perlio                 unix perlio
 mmap       unix mmap                   unix mmap

 # [1] "stdio" if Configure found out how to do "fast stdio" (depends
 # on the stdio implementation) and in Perl 5.8, otherwise "unix perlio"

By default the layers from the input side of the filehandle is
returned, to get the output side use the optional C<output> argument:

   my @layers = PerlIO::get_layers($fh, output => 1);

(Usually the layers are identical on either side of a filehandle but
for example with sockets there may be differences, or if you have
been using the C<open> pragma.)

There is no set_layers(), nor does get_layers() return a tied array
mirroring the stack, or anything fancy like that.  This is not
accidental or unintentional.  The PerlIO layer stack is a bit more
complicated than just a stack (see for example the behaviour of C<:raw>).
You are supposed to use open() and binmode() to manipulate the stack.

B<Implementation details follow, please close your eyes.>

The arguments to layers are by default returned in parenthesis after
the name of the layer, and certain layers (like C<utf8>) are not real
layers but instead flags on real layers: to get all of these returned
separately use the optional C<details> argument:

   my @layer_and_args_and_flags = PerlIO::get_layers($fh, details => 1);

The result will be up to be three times the number of layers:
the first element will be a name, the second element the arguments
(unspecified arguments will be C<undef>), the third element the flags,
the fourth element a name again, and so forth.

B<You may open your eyes now.>

=head1 AUTHOR

Nick Ing-Simmons E<lt>nick@ing-simmons.netE<gt>

=head1 SEE ALSO

L<perlfunc/"binmode">, L<perlfunc/"open">, L<perlunicode>, L<perliol>,
L<Encode>

=cut

Revision:	1.1
Committed:	Thu Jun 30 14:27:08 2005 UTC (21 years, 3 months ago) by root
Branch:	MAIN
CVS Tags:	PERL-5-8-7, HEAD
Branch point for:	PERL
Log Message:	* empty log message *
#	User	Rev	Content
1	root	1.1	package PerlIO;
2
3			our $VERSION = '1.03';
4
5			# Map layer name to package that defines it
6			our %alias;
7
8			sub import
9			{
10			my $class = shift;
11			while (@_)
12			{
13			my $layer = shift;
14			if (exists $alias{$layer})
15			{
16			$layer = $alias{$layer}
17			}
18			else
19			{
20			$layer = "${class}::$layer";
21			}
22			eval "require $layer";
23			warn $@ if $@;
24			}
25			}
26
27			sub F_UTF8 () { 0x8000 }
28
29			1;
30			__END__
31
32			=head1 NAME
33
34			PerlIO - On demand loader for PerlIO layers and root of PerlIO::* name space
35
36			=head1 SYNOPSIS
37
38			open($fh,"<:crlf", "my.txt"); # portably open a text file for reading
39
40			open($fh,"<","his.jpg"); # portably open a binary file for reading
41			binmode($fh);
42
43			Shell:
44			PERLIO=perlio perl ....
45
46			=head1 DESCRIPTION
47
48			When an undefined layer 'foo' is encountered in an C<open> or
49			C<binmode> layer specification then C code performs the equivalent of:
50
51			use PerlIO 'foo';
52
53			The perl code in PerlIO.pm then attempts to locate a layer by doing
54
55			require PerlIO::foo;
56
57			Otherwise the C<PerlIO> package is a place holder for additional
58			PerlIO related functions.
59
60			The following layers are currently defined:
61
62			=over 4
63
64			=item :unix
65
66			Lowest level layer which provides basic PerlIO operations in terms of
67			UNIX/POSIX numeric file descriptor calls
68			(open(), read(), write(), lseek(), close()).
69
70			=item :stdio
71
72			Layer which calls C<fread>, C<fwrite> and C<fseek>/C<ftell> etc. Note
73			that as this is "real" stdio it will ignore any layers beneath it and
74			got straight to the operating system via the C library as usual.
75
76			=item :perlio
77
78			A from scratch implementation of buffering for PerlIO. Provides fast
79			access to the buffer for C<sv_gets> which implements perl's readline/E<lt>E<gt>
80			and in general attempts to minimize data copying.
81
82			C<:perlio> will insert a C<:unix> layer below itself to do low level IO.
83
84			=item :crlf
85
86			A layer that implements DOS/Windows like CRLF line endings. On read
87			converts pairs of CR,LF to a single "\n" newline character. On write
88			converts each "\n" to a CR,LF pair. Note that this layer likes to be
89			one of its kind: it silently ignores attempts to be pushed into the
90			layer stack more than once.
91
92			It currently does I<not> mimic MS-DOS as far as treating of Control-Z
93			as being an end-of-file marker.
94
95			(Gory details follow) To be more exact what happens is this: after
96			pushing itself to the stack, the C<:crlf> layer checks all the layers
97			below itself to find the first layer that is capable of being a CRLF
98			layer but is not yet enabled to be a CRLF layer. If it finds such a
99			layer, it enables the CRLFness of that other deeper layer, and then
100			pops itself off the stack. If not, fine, use the one we just pushed.
101
102			The end result is that a C<:crlf> means "please enable the first CRLF
103			layer you can find, and if you can't find one, here would be a good
104			spot to place a new one."
105
106			Based on the C<:perlio> layer.
107
108			=item :mmap
109
110			A layer which implements "reading" of files by using C<mmap()> to
111			make (whole) file appear in the process's address space, and then
112			using that as PerlIO's "buffer". This I<may> be faster in certain
113			circumstances for large files, and may result in less physical memory
114			use when multiple processes are reading the same file.
115
116			Files which are not C<mmap()>-able revert to behaving like the C<:perlio>
117			layer. Writes also behave like C<:perlio> layer as C<mmap()> for write
118			needs extra house-keeping (to extend the file) which negates any advantage.
119
120			The C<:mmap> layer will not exist if platform does not support C<mmap()>.
121
122			=item :utf8
123
124			Declares that the stream accepts perl's internal encoding of
125			characters. (Which really is UTF-8 on ASCII machines, but is
126			UTF-EBCDIC on EBCDIC machines.) This allows any character perl can
127			represent to be read from or written to the stream. The UTF-X encoding
128			is chosen to render simple text parts (i.e. non-accented letters,
129			digits and common punctuation) human readable in the encoded file.
130
131			Here is how to write your native data out using UTF-8 (or UTF-EBCDIC)
132			and then read it back in.
133
134			open(F, ">:utf8", "data.utf");
135			print F $out;
136			close(F);
137
138			open(F, "<:utf8", "data.utf");
139			$in = <F>;
140			close(F);
141
142			=item :bytes
143
144			This is the inverse of C<:utf8> layer. It turns off the flag
145			on the layer below so that data read from it is considered to
146			be "octets" i.e. characters in range 0..255 only. Likewise
147			on output perl will warn if a "wide" character is written
148			to a such a stream.
149
150			=item :raw
151
152			The C<:raw> layer is I<defined> as being identical to calling
153			C<binmode($fh)> - the stream is made suitable for passing binary data
154			i.e. each byte is passed as-is. The stream will still be
155			buffered.
156
157			In Perl 5.6 and some books the C<:raw> layer (previously sometimes also
158			referred to as a "discipline") is documented as the inverse of the
159			C<:crlf> layer. That is no longer the case - other layers which would
160			alter binary nature of the stream are also disabled. If you want UNIX
161			line endings on a platform that normally does CRLF translation, but still
162			want UTF-8 or encoding defaults the appropriate thing to do is to add
163			C<:perlio> to PERLIO environment variable.
164
165			The implementation of C<:raw> is as a pseudo-layer which when "pushed"
166			pops itself and then any layers which do not declare themselves as suitable
167			for binary data. (Undoing :utf8 and :crlf are implemented by clearing
168			flags rather than popping layers but that is an implementation detail.)
169
170			As a consequence of the fact that C<:raw> normally pops layers
171			it usually only makes sense to have it as the only or first element in
172			a layer specification. When used as the first element it provides
173			a known base on which to build e.g.
174
175			open($fh,":raw:utf8",...)
176
177			will construct a "binary" stream, but then enable UTF-8 translation.
178
179			=item :pop
180
181			A pseudo layer that removes the top-most layer. Gives perl code
182			a way to manipulate the layer stack. Should be considered
183			as experimental. Note that C<:pop> only works on real layers
184			and will not undo the effects of pseudo layers like C<:utf8>.
185			An example of a possible use might be:
186
187			open($fh,...)
188			...
189			binmode($fh,":encoding(...)"); # next chunk is encoded
190			...
191			binmode($fh,":pop"); # back to un-encocded
192
193			A more elegant (and safer) interface is needed.
194
195			=item :win32
196
197			On Win32 platforms this I<experimental> layer uses native "handle" IO
198			rather than unix-like numeric file descriptor layer. Known to be
199			buggy as of perl 5.8.2.
200
201			=back
202
203			=head2 Custom Layers
204
205			It is possible to write custom layers in addition to the above builtin
206			ones, both in C/XS and Perl. Two such layers (and one example written
207			in Perl using the latter) come with the Perl distribution.
208
209			=over 4
210
211			=item :encoding
212
213			Use C<:encoding(ENCODING)> either in open() or binmode() to install
214			a layer that does transparently character set and encoding transformations,
215			for example from Shift-JIS to Unicode. Note that under C<stdio>
216			an C<:encoding> also enables C<:utf8>. See L<PerlIO::encoding>
217			for more information.
218
219			=item :via
220
221			Use C<:via(MODULE)> either in open() or binmode() to install a layer
222			that does whatever transformation (for example compression /
223			decompression, encryption / decryption) to the filehandle.
224			See L<PerlIO::via> for more information.
225
226			=back
227
228			=head2 Alternatives to raw
229
230			To get a binary stream an alternate method is to use:
231
232			open($fh,"whatever")
233			binmode($fh);
234
235			this has advantage of being backward compatible with how such things have
236			had to be coded on some platforms for years.
237
238			To get an un-buffered stream specify an unbuffered layer (e.g. C<:unix>)
239			in the open call:
240
241			open($fh,"<:unix",$path)
242
243			=head2 Defaults and how to override them
244
245			If the platform is MS-DOS like and normally does CRLF to "\n"
246			translation for text files then the default layers are :
247
248			unix crlf
249
250			(The low level "unix" layer may be replaced by a platform specific low
251			level layer.)
252
253			Otherwise if C<Configure> found out how to do "fast" IO using system's
254			stdio, then the default layers are:
255
256			unix stdio
257
258			Otherwise the default layers are
259
260			unix perlio
261
262			These defaults may change once perlio has been better tested and tuned.
263
264			The default can be overridden by setting the environment variable
265			PERLIO to a space separated list of layers (C<unix> or platform low
266			level layer is always pushed first).
267
268			This can be used to see the effect of/bugs in the various layers e.g.
269
270			cd .../perl/t
271			PERLIO=stdio ./perl harness
272			PERLIO=perlio ./perl harness
273
274			For the various value of PERLIO see L<perlrun/PERLIO>.
275
276			=head2 Querying the layers of filehandles
277
278			The following returns the B<names> of the PerlIO layers on a filehandle.
279
280			my @layers = PerlIO::get_layers($fh); # Or FH, *FH, "FH".
281
282			The layers are returned in the order an open() or binmode() call would
283			use them. Note that the "default stack" depends on the operating
284			system and on the Perl version, and both the compile-time and
285			runtime configurations of Perl.
286
287			The following table summarizes the default layers on UNIX-like and
288			DOS-like platforms and depending on the setting of the C<$ENV{PERLIO}>:
289
290			PERLIO UNIX-like DOS-like
291
292			unset / "" unix perlio / stdio [1] unix crlf
293			stdio unix perlio / stdio [1] stdio
294			perlio unix perlio unix perlio
295			mmap unix mmap unix mmap
296
297			# [1] "stdio" if Configure found out how to do "fast stdio" (depends
298			# on the stdio implementation) and in Perl 5.8, otherwise "unix perlio"
299
300			By default the layers from the input side of the filehandle is
301			returned, to get the output side use the optional C<output> argument:
302
303			my @layers = PerlIO::get_layers($fh, output => 1);
304
305			(Usually the layers are identical on either side of a filehandle but
306			for example with sockets there may be differences, or if you have
307			been using the C<open> pragma.)
308
309			There is no set_layers(), nor does get_layers() return a tied array
310			mirroring the stack, or anything fancy like that. This is not
311			accidental or unintentional. The PerlIO layer stack is a bit more
312			complicated than just a stack (see for example the behaviour of C<:raw>).
313			You are supposed to use open() and binmode() to manipulate the stack.
314
315			B<Implementation details follow, please close your eyes.>
316
317			The arguments to layers are by default returned in parenthesis after
318			the name of the layer, and certain layers (like C<utf8>) are not real
319			layers but instead flags on real layers: to get all of these returned
320			separately use the optional C<details> argument:
321
322			my @layer_and_args_and_flags = PerlIO::get_layers($fh, details => 1);
323
324			The result will be up to be three times the number of layers:
325			the first element will be a name, the second element the arguments
326			(unspecified arguments will be C<undef>), the third element the flags,
327			the fourth element a name again, and so forth.
328
329			B<You may open your eyes now.>
330
331			=head1 AUTHOR
332
333			Nick Ing-Simmons E<lt>nick@ing-simmons.netE<gt>
334
335			=head1 SEE ALSO
336
337			L<perlfunc/"binmode">, L<perlfunc/"open">, L<perlunicode>, L<perliol>,
338			L<Encode>
339
340			=cut
341