1PerlIO(3pm) Perl Programmers Reference Guide PerlIO(3pm)
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6 PerlIO - On demand loader for PerlIO layers and root of PerlIO::* name
7 space
8
10 open($fh,"<:crlf", "my.txt"); # support platform-native and CRLF text files
11 open($fh,"<","his.jpg"); # portably open a binary file for reading
13 binmode($fh);
14 Shell:
16 PERLIO=perlio perl ....
17
19 When an undefined layer 'foo' is encountered in an "open" or "binmode"
20 layer specification then C code performs the equivalent of:
21 use PerlIO 'foo';
23 The perl code in PerlIO.pm then attempts to locate a layer by doing
25 require PerlIO::foo;
27 Otherwise the "PerlIO" package is a place holder for additional PerlIO
29 related functions.
30 The following layers are currently defined:
32 :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 :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 got straight to the operating system via the C library as
42 usual.
43 :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 ":perlio" will insert a ":unix" layer below itself to do low level
50 IO.
51 :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 likes to be one of its kind: it silently ignores attempts to be
57 pushed into the layer stack more than once.
58 It currently does not mimic MS-DOS as far as treating of Control-Z
60 as being an end-of-file marker.
61 (Gory details follow) To be more exact what happens is this: after
63 pushing itself to the stack, the ":crlf" layer checks all the lay‐
64 ers below itself to find the first layer that is capable of being a
65 CRLF layer but is not yet enabled to be a CRLF layer. If it finds
66 such a layer, it enables the CRLFness of that other deeper layer,
67 and then pops itself off the stack. If not, fine, use the one we
68 just pushed.
69 The end result is that a ":crlf" means "please enable the first
71 CRLF layer you can find, and if you can't find one, here would be a
72 good spot to place a new one."
73 Based on the ":perlio" layer.
75 :mmap
77 A layer which implements "reading" of files by using "mmap()" to
78 make (whole) file appear in the process's address space, and then
79 using that as PerlIO's "buffer". This may be faster in certain cir‐
80 cumstances for large files, and may result in less physical memory
81 use when multiple processes are reading the same file.
82 Files which are not "mmap()"-able revert to behaving like the
84 ":perlio" layer. Writes also behave like ":perlio" layer as
85 "mmap()" for write needs extra house-keeping (to extend the file)
86 which negates any advantage.
87 The ":mmap" layer will not exist if platform does not support
89 "mmap()".
90 :utf8
92 Declares that the stream accepts perl's internal encoding of char‐
93 acters. (Which really is UTF-8 on ASCII machines, but is UTF-
94 EBCDIC on EBCDIC machines.) This allows any character perl can
95 represent to be read from or written to the stream. The UTF-X
96 encoding is chosen to render simple text parts (i.e. non-accented
97 letters, digits and common punctuation) human readable in the
98 encoded file.
99 Here is how to write your native data out using UTF-8 (or
101 UTF-EBCDIC) and then read it back in.
102 open(F, ">:utf8", "data.utf");
104 print F $out;
105 close(F);
106 open(F, "<:utf8", "data.utf");
108 $in = <F>;
109 close(F);
110 :bytes
112 This is the inverse of ":utf8" layer. It turns off the flag on the
113 layer below so that data read from it is considered to be "octets"
114 i.e. characters in range 0..255 only. Likewise on output perl will
115 warn if a "wide" character is written to a such a stream.
116 :raw
118 The ":raw" layer is defined as being identical to calling "bin‐
119 mode($fh)" - the stream is made suitable for passing binary data
120 i.e. each byte is passed as-is. The stream will still be buffered.
121 In Perl 5.6 and some books the ":raw" layer (previously sometimes
123 also referred to as a "discipline") is documented as the inverse of
124 the ":crlf" layer. That is no longer the case - other layers which
125 would alter binary nature of the stream are also disabled. If you
126 want UNIX line endings on a platform that normally does CRLF trans‐
127 lation, but still want UTF-8 or encoding defaults the appropriate
128 thing to do is to add ":perlio" to PERLIO environment variable.
129 The implementation of ":raw" is as a pseudo-layer which when
131 "pushed" pops itself and then any layers which do not declare them‐
132 selves as suitable for binary data. (Undoing :utf8 and :crlf are
133 implemented by clearing flags rather than popping layers but that
134 is an implementation detail.)
135 As a consequence of the fact that ":raw" normally pops layers it
137 usually only makes sense to have it as the only or first element in
138 a layer specification. When used as the first element it provides
139 a known base on which to build e.g.
140 open($fh,":raw:utf8",...)
142 will construct a "binary" stream, but then enable UTF-8 transla‐
144 tion.
145 :pop
147 A pseudo layer that removes the top-most layer. Gives perl code a
148 way to manipulate the layer stack. Should be considered as experi‐
149 mental. Note that ":pop" only works on real layers and will not
150 undo the effects of pseudo layers like ":utf8". An example of a
151 possible use might be:
152 open($fh,...)
154 ...
155 binmode($fh,":encoding(...)"); # next chunk is encoded
156 ...
157 binmode($fh,":pop"); # back to un-encoded
158 A more elegant (and safer) interface is needed.
160 :win32
162 On Win32 platforms this experimental layer uses native "handle" IO
163 rather than unix-like numeric file descriptor layer. Known to be
164 buggy as of perl 5.8.2.
165 Custom Layers
167 It is possible to write custom layers in addition to the above builtin
169 ones, both in C/XS and Perl. Two such layers (and one example written
170 in Perl using the latter) come with the Perl distribution.
171 :encoding
173 Use ":encoding(ENCODING)" either in open() or binmode() to install
174 a layer that does transparently character set and encoding trans‐
175 formations, for example from Shift-JIS to Unicode. Note that under
176 "stdio" an ":encoding" also enables ":utf8". See PerlIO::encoding
177 for more information.
178 :via
180 Use ":via(MODULE)" either in open() or binmode() to install a layer
181 that does whatever transformation (for example compression / decom‐
182 pression, encryption / decryption) to the filehandle. See Per‐
183 lIO::via for more information.
184 Alternatives to raw
186 To get a binary stream an alternate method is to use:
188 open($fh,"whatever")
190 binmode($fh);
191 this has advantage of being backward compatible with how such things
193 have had to be coded on some platforms for years.
194 To get an un-buffered stream specify an unbuffered layer (e.g. ":unix")
196 in the open call:
197 open($fh,"<:unix",$path)
199 Defaults and how to override them
201 If the platform is MS-DOS like and normally does CRLF to "\n" transla‐
203 tion for text files then the default layers are :
204 unix crlf
206 (The low level "unix" layer may be replaced by a platform specific low
208 level layer.)
209 Otherwise if "Configure" found out how to do "fast" IO using system's
211 stdio, then the default layers are:
212 unix stdio
214 Otherwise the default layers are
216 unix perlio
218 These defaults may change once perlio has been better tested and tuned.
220 The default can be overridden by setting the environment variable PER‐
222 LIO to a space separated list of layers ("unix" or platform low level
223 layer is always pushed first).
224 This can be used to see the effect of/bugs in the various layers e.g.
226 cd .../perl/t
228 PERLIO=stdio ./perl harness
229 PERLIO=perlio ./perl harness
230 For the various value of PERLIO see "PERLIO" in perlrun.
232 Querying the layers of filehandles
234 The following returns the names of the PerlIO layers on a filehandle.
236 my @layers = PerlIO::get_layers($fh); # Or FH, *FH, "FH".
238 The layers are returned in the order an open() or binmode() call would
240 use them. Note that the "default stack" depends on the operating sys‐
241 tem and on the Perl version, and both the compile-time and runtime con‐
242 figurations of Perl.
243 The following table summarizes the default layers on UNIX-like and DOS-
245 like platforms and depending on the setting of the $ENV{PERLIO}:
246 PERLIO UNIX-like DOS-like
248 ------ --------- --------
249 unset / "" unix perlio / stdio [1] unix crlf
250 stdio unix perlio / stdio [1] stdio
251 perlio unix perlio unix perlio
252 mmap unix mmap unix mmap
253 # [1] "stdio" if Configure found out how to do "fast stdio" (depends
255 # on the stdio implementation) and in Perl 5.8, otherwise "unix perlio"
256 By default the layers from the input side of the filehandle is
258 returned, to get the output side use the optional "output" argument:
259 my @layers = PerlIO::get_layers($fh, output => 1);
261 (Usually the layers are identical on either side of a filehandle but
263 for example with sockets there may be differences, or if you have been
264 using the "open" pragma.)
265 There is no set_layers(), nor does get_layers() return a tied array
267 mirroring the stack, or anything fancy like that. This is not acciden‐
268 tal or unintentional. The PerlIO layer stack is a bit more complicated
269 than just a stack (see for example the behaviour of ":raw"). You are
270 supposed to use open() and binmode() to manipulate the stack.
271 Implementation details follow, please close your eyes.
273 The arguments to layers are by default returned in parenthesis after
275 the name of the layer, and certain layers (like "utf8") are not real
276 layers but instead flags on real layers: to get all of these returned
277 separately use the optional "details" argument:
278 my @layer_and_args_and_flags = PerlIO::get_layers($fh, details => 1);
280 The result will be up to be three times the number of layers: the first
282 element will be a name, the second element the arguments (unspecified
283 arguments will be "undef"), the third element the flags, the fourth
284 element a name again, and so forth.
285 You may open your eyes now.
287
289 Nick Ing-Simmons <nick@ing-simmons.net>
290
292 "binmode" in perlfunc, "open" in perlfunc, perlunicode, perliol, Encode
293perl v5.8.8 2001-09-21 PerlIO(3pm)