1PerlIO(3pm)            Perl Programmers Reference Guide            PerlIO(3pm)
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NAME

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

10         open($fh,"<:crlf", "my.txt"); # support platform-native and CRLF text files
11
12         open($fh,"<","his.jpg");      # portably open a binary file for reading
13         binmode($fh);
14
15         Shell:
16           PERLIO=perlio perl ....
17

DESCRIPTION

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

AUTHOR

292       Nick Ing-Simmons <nick@ing-simmons.net>
293

SEE ALSO

295       "binmode" in perlfunc, "open" in perlfunc, perlunicode, perliol, Encode
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299perl v5.10.1                      2009-07-03                       PerlIO(3pm)
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