1PerlIO(3pm) Perl Programmers Reference Guide PerlIO(3pm)
2
6 PerlIO - On demand loader for PerlIO layers and root of PerlIO::* name
7 space
8
10 # support platform-native and CRLF text files
11 open(my $fh, "<:crlf", "my.txt") or die "open failed: $!";
12 # append UTF-8 encoded text
14 open(my $fh, ">>:encoding(UTF-8)", "some.log")
15 or die "open failed: $!";
16 # portably open a binary file for reading
18 open(my $fh, "<", "his.jpg") or die "open failed: $!";
19 binmode($fh) or die "binmode failed: $!";
20 Shell:
22 PERLIO=:perlio perl ....
23
25 When an undefined layer 'foo' is encountered in an "open" or "binmode"
26 layer specification then C code performs the equivalent of:
27 use PerlIO 'foo';
29 The Perl code in PerlIO.pm then attempts to locate a layer by doing
31 require PerlIO::foo;
33 Otherwise the "PerlIO" package is a place holder for additional PerlIO
35 related functions.
36 Layers
38 Generally speaking, PerlIO layers (previously sometimes referred to as
39 "disciplines") are an ordered stack applied to a filehandle (specified
40 as a space- or colon-separated list, conventionally written with a
41 leading colon). Each layer performs some operation on any input or
42 output, except when bypassed such as with "sysread" or "syswrite".
43 Read operations go through the stack in the order they are set (left to
44 right), and write operations in the reverse order.
45 There are also layers which actually just set flags on lower layers, or
47 layers that modify the current stack but don't persist on the stack
48 themselves; these are referred to as pseudo-layers.
49 When opening a handle, it will be opened with any layers specified
51 explicitly in the open() call (or the platform defaults, if specified
52 as a colon with no following layers).
53 If layers are not explicitly specified, the handle will be opened with
55 the layers specified by the ${^OPEN} variable (usually set by using the
56 open pragma for a lexical scope, or the "-C" command-line switch or
57 "PERL_UNICODE" environment variable for the main program scope).
58 If layers are not specified in the open() call or "${^OPEN}" variable,
60 the handle will be opened with the default layer stack configured for
61 that architecture; see "Defaults and how to override them".
62 Some layers will automatically insert required lower level layers if
64 not present; for example ":perlio" will insert ":unix" below itself for
65 low level IO, and ":encoding" will insert the platform defaults for
66 buffered IO.
67 The "binmode" function can be called on an opened handle to push
69 additional layers onto the stack, which may also modify the existing
70 layers. "binmode" called with no layers will remove or unset any
71 existing layers which transform the byte stream, making the handle
72 suitable for binary data.
73 The following layers are currently defined:
75 :unix
77 Lowest level layer which provides basic PerlIO operations in terms
78 of UNIX/POSIX numeric file descriptor calls (open(), read(),
79 write(), lseek(), close()). It is used even on non-Unix
80 architectures, and most other layers operate on top of it.
81 :stdio
83 Layer which calls "fread", "fwrite" and "fseek"/"ftell" etc. Note
84 that as this is "real" stdio it will ignore any layers beneath it
85 and go straight to the operating system via the C library as usual.
86 This layer implements both low level IO and buffering, but is
87 rarely used on modern architectures.
88 :perlio
90 A from scratch implementation of buffering for PerlIO. Provides
91 fast access to the buffer for "sv_gets" which implements Perl's
92 readline/<> and in general attempts to minimize data copying.
93 ":perlio" will insert a ":unix" layer below itself to do low level
95 IO.
96 :crlf
98 A layer that implements DOS/Windows like CRLF line endings. On
99 read converts pairs of CR,LF to a single "\n" newline character.
100 On write converts each "\n" to a CR,LF pair. Note that this layer
101 will silently refuse to be pushed on top of itself.
102 It currently does not mimic MS-DOS as far as treating of Control-Z
104 as being an end-of-file marker.
105 On DOS/Windows like architectures where this layer is part of the
107 defaults, it also acts like the ":perlio" layer, and removing the
108 CRLF translation (such as with ":raw") will only unset the CRLF
109 translation flag. Since Perl 5.14, you can also apply another
110 ":crlf" layer later, such as when the CRLF translation must occur
111 after an encoding layer. On other architectures, it is a mundane
112 CRLF translation layer and can be added and removed normally.
113 # translate CRLF after encoding on Perl 5.14 or newer
115 binmode $fh, ":raw:encoding(UTF-16LE):crlf"
116 or die "binmode failed: $!";
117 :utf8
119 Pseudo-layer that declares that the stream accepts Perl's internal
120 upgraded encoding of characters, which is approximately UTF-8 on
121 ASCII machines, but UTF-EBCDIC on EBCDIC machines. This allows any
122 character Perl can represent to be read from or written to the
123 stream.
124 This layer (which actually sets a flag on the preceding layer, and
126 is implicitly set by any ":encoding" layer) does not translate or
127 validate byte sequences. It instead indicates that the byte stream
128 will have been arranged by other layers to be provided in Perl's
129 internal upgraded encoding, which Perl code (and correctly written
130 XS code) will interpret as decoded Unicode characters.
131 CAUTION: Do not use this layer to translate from UTF-8 bytes, as
133 invalid UTF-8 or binary data will result in malformed Perl strings.
134 It is unlikely to produce invalid UTF-8 when used for output,
135 though it will instead produce UTF-EBCDIC on EBCDIC systems. The
136 ":encoding(UTF-8)" layer (hyphen is significant) is preferred as it
137 will ensure translation between valid UTF-8 bytes and valid Unicode
138 characters.
139 :bytes
141 This is the inverse of the ":utf8" pseudo-layer. It turns off the
142 flag on the layer below so that data read from it is considered to
143 be Perl's internal downgraded encoding, thus interpreted as the
144 native single-byte encoding of Latin-1 or EBCDIC. Likewise on
145 output Perl will warn if a "wide" character (a codepoint not in the
146 range 0..255) is written to a such a stream.
147 This is very dangerous to push on a handle using an ":encoding"
149 layer, as such a layer assumes to be working with Perl's internal
150 upgraded encoding, so you will likely get a mangled result.
151 Instead use ":raw" or ":pop" to remove encoding layers.
152 :raw
154 The ":raw" pseudo-layer is defined as being identical to calling
155 "binmode($fh)" - the stream is made suitable for passing binary
156 data, i.e. each byte is passed as-is. The stream will still be
157 buffered (but this was not always true before Perl 5.14).
158 In Perl 5.6 and some books the ":raw" layer is documented as the
160 inverse of the ":crlf" layer. That is no longer the case - other
161 layers which would alter the binary nature of the stream are also
162 disabled. If you want UNIX line endings on a platform that
163 normally does CRLF translation, but still want UTF-8 or encoding
164 defaults, the appropriate thing to do is to add ":perlio" to the
165 PERLIO environment variable, or open the handle explicitly with
166 that layer, to replace the platform default of ":crlf".
167 The implementation of ":raw" is as a pseudo-layer which when
169 "pushed" pops itself and then any layers which would modify the
170 binary data stream. (Undoing ":utf8" and ":crlf" may be
171 implemented by clearing flags rather than popping layers but that
172 is an implementation detail.)
173 As a consequence of the fact that ":raw" normally pops layers, it
175 usually only makes sense to have it as the only or first element in
176 a layer specification. When used as the first element it provides
177 a known base on which to build e.g.
178 open(my $fh,">:raw:encoding(UTF-8)",...)
180 or die "open failed: $!";
181 will construct a "binary" stream regardless of the platform
183 defaults, but then enable UTF-8 translation.
184 :pop
186 A pseudo-layer that removes the top-most layer. Gives Perl code a
187 way to manipulate the layer stack. Note that ":pop" only works on
188 real layers and will not undo the effects of pseudo-layers or flags
189 like ":utf8". An example of a possible use might be:
190 open(my $fh,...) or die "open failed: $!";
192 ...
193 binmode($fh,":encoding(...)") or die "binmode failed: $!";
194 # next chunk is encoded
195 ...
196 binmode($fh,":pop") or die "binmode failed: $!";
197 # back to un-encoded
198 A more elegant (and safer) interface is needed.
200 Custom Layers
202 It is possible to write custom layers in addition to the above builtin
203 ones, both in C/XS and Perl, as a module named "PerlIO::<layer name>".
204 Some custom layers come with the Perl distribution.
205 :encoding
207 Use ":encoding(ENCODING)" to transparently do character set and
208 encoding transformations, for example from Shift-JIS to Unicode.
209 Note that an ":encoding" also enables ":utf8". See
210 PerlIO::encoding for more information.
211 :mmap
213 A layer which implements "reading" of files by using "mmap()" to
214 make a (whole) file appear in the process's address space, and then
215 using that as PerlIO's "buffer". This may be faster in certain
216 circumstances for large files, and may result in less physical
217 memory use when multiple processes are reading the same file.
218 Files which are not "mmap()"-able revert to behaving like the
220 ":perlio" layer. Writes also behave like the ":perlio" layer, as
221 "mmap()" for write needs extra house-keeping (to extend the file)
222 which negates any advantage.
223 The ":mmap" layer will not exist if the platform does not support
225 "mmap()". See PerlIO::mmap for more information.
226 :via
228 ":via(MODULE)" allows a transformation to be applied by an
229 arbitrary Perl module, for example compression / decompression,
230 encryption / decryption. See PerlIO::via for more information.
231 :scalar
233 A layer implementing "in memory" files using scalar variables,
234 automatically used in place of the platform defaults for IO when
235 opening such a handle. As such, the scalar is expected to act like
236 a file, only containing or storing bytes. See PerlIO::scalar for
237 more information.
238 Alternatives to raw
240 To get a binary stream an alternate method is to use:
241 open(my $fh,"<","whatever") or die "open failed: $!";
243 binmode($fh) or die "binmode failed: $!";
244 This has the advantage of being backward compatible with older versions
246 of Perl that did not use PerlIO or where ":raw" was buggy (as it was
247 before Perl 5.14).
248 To get an unbuffered stream specify an unbuffered layer (e.g. ":unix")
250 in the open call:
251 open(my $fh,"<:unix",$path) or die "open failed: $!";
253 Defaults and how to override them
255 If the platform is MS-DOS like and normally does CRLF to "\n"
256 translation for text files then the default layers are:
257 :unix:crlf
259 Otherwise if "Configure" found out how to do "fast" IO using the
261 system's stdio (not common on modern architectures), then the default
262 layers are:
263 :stdio
265 Otherwise the default layers are
267 :unix:perlio
269 Note that the "default stack" depends on the operating system and on
271 the Perl version, and both the compile-time and runtime configurations
272 of Perl. The default can be overridden by setting the environment
273 variable PERLIO to a space or colon separated list of layers, however
274 this cannot be used to set layers that require loading modules like
275 ":encoding".
276 This can be used to see the effect of/bugs in the various layers e.g.
278 cd .../perl/t
280 PERLIO=:stdio ./perl harness
281 PERLIO=:perlio ./perl harness
282 For the various values of PERLIO see "PERLIO" in perlrun.
284 The following table summarizes the default layers on UNIX-like and DOS-
286 like platforms and depending on the setting of $ENV{PERLIO}:
287 PERLIO UNIX-like DOS-like
289 ------ --------- --------
290 unset / "" :unix:perlio / :stdio [1] :unix:crlf
291 :stdio :stdio :stdio
292 :perlio :unix:perlio :unix:perlio
293 # [1] ":stdio" if Configure found out how to do "fast stdio" (depends
295 # on the stdio implementation) and in Perl 5.8, else ":unix:perlio"
296 Querying the layers of filehandles
298 The following returns the names of the PerlIO layers on a filehandle.
299 my @layers = PerlIO::get_layers($fh); # Or FH, *FH, "FH".
301 The layers are returned in the order an open() or binmode() call would
303 use them, and without colons.
304 By default the layers from the input side of the filehandle are
306 returned; to get the output side, use the optional "output" argument:
307 my @layers = PerlIO::get_layers($fh, output => 1);
309 (Usually the layers are identical on either side of a filehandle but
311 for example with sockets there may be differences.)
312 There is no set_layers(), nor does get_layers() return a tied array
314 mirroring the stack, or anything fancy like that. This is not
315 accidental or unintentional. The PerlIO layer stack is a bit more
316 complicated than just a stack (see for example the behaviour of
317 ":raw"). You are supposed to use open() and binmode() to manipulate
318 the stack.
319 Implementation details follow, please close your eyes.
321 The arguments to layers are by default returned in parentheses after
323 the name of the layer, and certain layers (like ":utf8") are not real
324 layers but instead flags on real layers; to get all of these returned
325 separately, use the optional "details" argument:
326 my @layer_and_args_and_flags = PerlIO::get_layers($fh, details => 1);
328 The result will be up to be three times the number of layers: the first
330 element will be a name, the second element the arguments (unspecified
331 arguments will be "undef"), the third element the flags, the fourth
332 element a name again, and so forth.
333 You may open your eyes now.
335
337 Nick Ing-Simmons <nick@ing-simmons.net>
338
340 "binmode" in perlfunc, "open" in perlfunc, perlunicode, perliol, Encode
341perl v5.36.0 2022-08-30 PerlIO(3pm)