5 # Map layer name to package that defines it
14 if (exists $alias{$layer})
16 $layer = $alias{$layer}
20 $layer = "${class}::$layer";
22 eval "require $layer";
27 sub F_UTF8 () { 0x8000 }
34 PerlIO - On demand loader for PerlIO layers and root of PerlIO::* name space
38 open($fh,"<:crlf", "my.txt"); # portably open a text file for reading
40 open($fh,"<","his.jpg"); # portably open a binary file for reading
44 PERLIO=perlio perl ....
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:
53 The perl code in PerlIO.pm then attempts to locate a layer by doing
57 Otherwise the C<PerlIO> package is a place holder for additional
58 PerlIO related functions.
60 The following layers are currently defined:
66 Low level layer which calls C<read>, C<write> and C<lseek> etc.
70 Layer which calls C<fread>, C<fwrite> and C<fseek>/C<ftell> etc. Note
71 that as this is "real" stdio it will ignore any layers beneath it and
72 got straight to the operating system via the C library as usual.
76 This is a re-implementation of "stdio-like" buffering written as a
77 PerlIO "layer". As such it will call whatever layer is below it for
82 A layer which does CRLF to "\n" translation distinguishing "text" and
83 "binary" files in the manner of MS-DOS and similar operating systems.
84 (It currently does I<not> mimic MS-DOS as far as treating of Control-Z
85 as being an end-of-file marker.)
89 Declares that the stream accepts perl's internal encoding of
90 characters. (Which really is UTF-8 on ASCII machines, but is
91 UTF-EBCDIC on EBCDIC machines.) This allows any character perl can
92 represent to be read from or written to the stream. The UTF-X encoding
93 is chosen to render simple text parts (i.e. non-accented letters,
94 digits and common punctuation) human readable in the encoded file.
96 Here is how to write your native data out using UTF-8 (or UTF-EBCDIC)
97 and then read it back in.
99 open(F, ">:utf8", "data.utf");
103 open(F, "<:utf8", "data.utf");
109 This is the inverse of C<:utf8> layer. It turns off the flag
110 on the layer below so that data read from it is considered to
111 be "octets" i.e. characters in range 0..255 only. Likewise
112 on output perl will warn if a "wide" character is written
117 The C<:raw> layer is I<defined> as being identical to calling
118 C<binmode($fh)> - the stream is made suitable for passing binary data
119 i.e. each byte is passed as-is. The stream will still be
120 buffered. Unlike in the earlier versions of Perl C<:raw> is I<not>
121 just the inverse of C<:crlf> - other layers which would affect the
122 binary nature of the stream are also removed or disabled.
124 The implementation of C<:raw> is as a pseudo-layer which when "pushed"
125 pops itself and then any layers which do not declare themselves as suitable
126 for binary data. (Undoing :utf8 and :crlf are implemented by clearing
127 flags rather than popping layers but that is an implementation detail.)
129 As a consequence of the fact that C<:raw> normally pops layers
130 it usually only makes sense to have it as the only or first element in
131 a layer specification. When used as the first element it provides
132 a known base on which to build e.g.
134 open($fh,":raw:utf8",...)
136 will construct a "binary" stream, but then enable UTF-8 translation.
140 A pseudo layer that removes the top-most layer. Gives perl code
141 a way to manipulate the layer stack. Should be considered
142 as experimental. Note that C<:pop> only works on real layers
143 and will not undo the effects of pseudo layers like C<:utf8>.
144 An example of a possible use might be:
148 binmode($fh,":encoding(...)"); # next chunk is encoded
150 binmode($fh,":pop"); # back to un-encocded
152 A more elegant (and safer) interface is needed.
158 It is possible to write custom layers in addition to the above builtin
159 ones, both in C/XS and Perl. Two such layers (and one example written
160 in Perl using the latter) come with the Perl distribution.
166 Use C<:encoding(ENCODING)> either in open() or binmode() to install
167 a layer that does transparently character set and encoding transformations,
168 for example from Shift-JIS to Unicode. Note that under C<stdio>
169 an C<:encoding> also enables C<:utf8>. See L<PerlIO::encoding>
170 for more information.
174 Use C<:via(MODULE)> either in open() or binmode() to install a layer
175 that does whatever transformation (for example compression /
176 decompression, encryption / decryption) to the filehandle.
177 See L<PerlIO::via> for more information.
181 =head2 Alternatives to raw
183 To get a binary stream an alternate method is to use:
188 this has advantage of being backward compatible with how such things have
189 had to be coded on some platforms for years.
191 To get an un-buffered stream specify an unbuffered layer (e.g. C<:unix>)
194 open($fh,"<:unix",$path)
196 =head2 Defaults and how to override them
198 If the platform is MS-DOS like and normally does CRLF to "\n"
199 translation for text files then the default layers are :
203 (The low level "unix" layer may be replaced by a platform specific low
206 Otherwise if C<Configure> found out how to do "fast" IO using system's
207 stdio, then the default layers are:
211 Otherwise the default layers are
215 These defaults may change once perlio has been better tested and tuned.
217 The default can be overridden by setting the environment variable
218 PERLIO to a space separated list of layers (C<unix> or platform low
219 level layer is always pushed first).
221 This can be used to see the effect of/bugs in the various layers e.g.
224 PERLIO=stdio ./perl harness
225 PERLIO=perlio ./perl harness
227 For the various value of PERLIO see L<perlrun/PERLIO>.
229 =head2 Querying the layers of filehandles
231 The following returns the B<names> of the PerlIO layers on a filehandle.
233 my @layers = PerlIO::get_layers($fh); # Or FH, *FH, "FH".
235 The layers are returned in the order an open() or binmode() call would
236 use them. Note that the "default stack" depends on the operating
237 system and on the Perl version, and both the compile-time and
238 runtime configurations of Perl.
240 The following table summarizes the default layers on UNIX-like and
241 DOS-like platforms and depending on the setting of the C<$ENV{PERLIO}>:
243 PERLIO UNIX-like DOS-like
245 unset / "" unix perlio / stdio [1] unix crlf
246 stdio unix perlio / stdio [1] stdio
247 perlio unix perlio unix perlio
248 mmap unix mmap unix mmap
250 # [1] "stdio" if Configure found out how to do "fast stdio" (depends
251 # on the stdio implementation) and in Perl 5.8, otherwise "unix perlio"
253 By default the layers from the input side of the filehandle is
254 returned, to get the output side use the optional C<output> argument:
256 my @layers = PerlIO::get_layers($fh, output => 1);
258 (Usually the layers are identical on either side of a filehandle but
259 for example with sockets there may be differences, or if you have
260 been using the C<open> pragma.)
262 There is no set_layers(), nor does get_layers() return a tied array
263 mirroring the stack, or anything fancy like that. This is not
264 accidental or unintentional. The PerlIO layer stack is a bit more
265 complicated than just a stack (see for example the behaviour of C<:raw>).
266 You are supposed to use open() and binmode() to manipulate the stack.
268 B<Implementation details follow, please close your eyes.>
270 The arguments to layers are by default returned in parenthesis after
271 the name of the layer, and certain layers (like C<utf8>) are not real
272 layers but instead flags on real layers: to get all of these returned
273 separately use the optional C<separate> argument:
275 my @layer_and_args_and_flags = PerlIO::get_layers($fh, details => 1);
277 The result will be up to be three times the number of layers:
278 the first element will be a name, the second element the arguments
279 (unspecified arguments will be C<undef>), the third element the flags,
280 the fourth element a name again, and so forth.
282 B<You may open your eyes now.>
286 Nick Ing-Simmons E<lt>nick@ing-simmons.netE<gt>
290 L<perlfunc/"binmode">, L<perlfunc/"open">, L<perlunicode>, L<perliol>,