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($fh, "<:crlf", "my.txt"); # support platform-native and
11                                        # CRLF text files
12
13         open($fh, "<", "his.jpg"); # portably open a binary file for reading
14         binmode($fh);
15
16         Shell:
17           PERLIO=perlio perl ....
18

DESCRIPTION

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

AUTHOR

278       Nick Ing-Simmons <nick@ing-simmons.net>
279

SEE ALSO

281       "binmode" in perlfunc, "open" in perlfunc, perlunicode, perliol, Encode
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285perl v5.28.2                      2018-11-01                       PerlIO(3pm)
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