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| 128 | .rm #[ #] #H #V #F C |
| 129 | .\" ======================================================================== |
| 130 | .\" |
| 131 | .IX Title "PerlIO 3" |
| 132 | .TH PerlIO 3 "2001-09-21" "perl v5.8.8" "Perl Programmers Reference Guide" |
| 133 | .SH "NAME" |
| 134 | PerlIO \- On demand loader for PerlIO layers and root of PerlIO::* name space |
| 135 | .SH "SYNOPSIS" |
| 136 | .IX Header "SYNOPSIS" |
| 137 | .Vb 1 |
| 138 | \& open($fh,"<:crlf", "my.txt"); # support platform-native and CRLF text files |
| 139 | .Ve |
| 140 | .PP |
| 141 | .Vb 2 |
| 142 | \& open($fh,"<","his.jpg"); # portably open a binary file for reading |
| 143 | \& binmode($fh); |
| 144 | .Ve |
| 145 | .PP |
| 146 | .Vb 2 |
| 147 | \& Shell: |
| 148 | \& PERLIO=perlio perl .... |
| 149 | .Ve |
| 150 | .SH "DESCRIPTION" |
| 151 | .IX Header "DESCRIPTION" |
| 152 | When an undefined layer 'foo' is encountered in an \f(CW\*(C`open\*(C'\fR or |
| 153 | \&\f(CW\*(C`binmode\*(C'\fR layer specification then C code performs the equivalent of: |
| 154 | .PP |
| 155 | .Vb 1 |
| 156 | \& use PerlIO 'foo'; |
| 157 | .Ve |
| 158 | .PP |
| 159 | The perl code in PerlIO.pm then attempts to locate a layer by doing |
| 160 | .PP |
| 161 | .Vb 1 |
| 162 | \& require PerlIO::foo; |
| 163 | .Ve |
| 164 | .PP |
| 165 | Otherwise the \f(CW\*(C`PerlIO\*(C'\fR package is a place holder for additional |
| 166 | PerlIO related functions. |
| 167 | .PP |
| 168 | The following layers are currently defined: |
| 169 | .IP ":unix" 4 |
| 170 | .IX Item ":unix" |
| 171 | Lowest level layer which provides basic PerlIO operations in terms of |
| 172 | \&\s-1UNIX/POSIX\s0 numeric file descriptor calls |
| 173 | (\fIopen()\fR, \fIread()\fR, \fIwrite()\fR, \fIlseek()\fR, \fIclose()\fR). |
| 174 | .IP ":stdio" 4 |
| 175 | .IX Item ":stdio" |
| 176 | Layer which calls \f(CW\*(C`fread\*(C'\fR, \f(CW\*(C`fwrite\*(C'\fR and \f(CW\*(C`fseek\*(C'\fR/\f(CW\*(C`ftell\*(C'\fR etc. Note |
| 177 | that as this is \*(L"real\*(R" stdio it will ignore any layers beneath it and |
| 178 | got straight to the operating system via the C library as usual. |
| 179 | .IP ":perlio" 4 |
| 180 | .IX Item ":perlio" |
| 181 | A from scratch implementation of buffering for PerlIO. Provides fast |
| 182 | access to the buffer for \f(CW\*(C`sv_gets\*(C'\fR which implements perl's readline/<> |
| 183 | and in general attempts to minimize data copying. |
| 184 | .Sp |
| 185 | \&\f(CW\*(C`:perlio\*(C'\fR will insert a \f(CW\*(C`:unix\*(C'\fR layer below itself to do low level \s-1IO\s0. |
| 186 | .IP ":crlf" 4 |
| 187 | .IX Item ":crlf" |
| 188 | A layer that implements DOS/Windows like \s-1CRLF\s0 line endings. On read |
| 189 | converts pairs of \s-1CR\s0,LF to a single \*(L"\en\*(R" newline character. On write |
| 190 | converts each \*(L"\en\*(R" to a \s-1CR\s0,LF pair. Note that this layer likes to be |
| 191 | one of its kind: it silently ignores attempts to be pushed into the |
| 192 | layer stack more than once. |
| 193 | .Sp |
| 194 | It currently does \fInot\fR mimic MS-DOS as far as treating of Control-Z |
| 195 | as being an end-of-file marker. |
| 196 | .Sp |
| 197 | (Gory details follow) To be more exact what happens is this: after |
| 198 | pushing itself to the stack, the \f(CW\*(C`:crlf\*(C'\fR layer checks all the layers |
| 199 | below itself to find the first layer that is capable of being a \s-1CRLF\s0 |
| 200 | layer but is not yet enabled to be a \s-1CRLF\s0 layer. If it finds such a |
| 201 | layer, it enables the CRLFness of that other deeper layer, and then |
| 202 | pops itself off the stack. If not, fine, use the one we just pushed. |
| 203 | .Sp |
| 204 | The end result is that a \f(CW\*(C`:crlf\*(C'\fR means \*(L"please enable the first \s-1CRLF\s0 |
| 205 | layer you can find, and if you can't find one, here would be a good |
| 206 | spot to place a new one.\*(R" |
| 207 | .Sp |
| 208 | Based on the \f(CW\*(C`:perlio\*(C'\fR layer. |
| 209 | .IP ":mmap" 4 |
| 210 | .IX Item ":mmap" |
| 211 | A layer which implements \*(L"reading\*(R" of files by using \f(CW\*(C`mmap()\*(C'\fR to |
| 212 | make (whole) file appear in the process's address space, and then |
| 213 | using that as PerlIO's \*(L"buffer\*(R". This \fImay\fR be faster in certain |
| 214 | circumstances for large files, and may result in less physical memory |
| 215 | use when multiple processes are reading the same file. |
| 216 | .Sp |
| 217 | Files which are not \f(CW\*(C`mmap()\*(C'\fR\-able revert to behaving like the \f(CW\*(C`:perlio\*(C'\fR |
| 218 | layer. Writes also behave like \f(CW\*(C`:perlio\*(C'\fR layer as \f(CW\*(C`mmap()\*(C'\fR for write |
| 219 | needs extra house-keeping (to extend the file) which negates any advantage. |
| 220 | .Sp |
| 221 | The \f(CW\*(C`:mmap\*(C'\fR layer will not exist if platform does not support \f(CW\*(C`mmap()\*(C'\fR. |
| 222 | .IP ":utf8" 4 |
| 223 | .IX Item ":utf8" |
| 224 | Declares that the stream accepts perl's internal encoding of |
| 225 | characters. (Which really is \s-1UTF\-8\s0 on \s-1ASCII\s0 machines, but is |
| 226 | UTF-EBCDIC on \s-1EBCDIC\s0 machines.) This allows any character perl can |
| 227 | represent to be read from or written to the stream. The UTF-X encoding |
| 228 | is chosen to render simple text parts (i.e. non-accented letters, |
| 229 | digits and common punctuation) human readable in the encoded file. |
| 230 | .Sp |
| 231 | Here is how to write your native data out using \s-1UTF\-8\s0 (or \s-1UTF\-EBCDIC\s0) |
| 232 | and then read it back in. |
| 233 | .Sp |
| 234 | .Vb 3 |
| 235 | \& open(F, ">:utf8", "data.utf"); |
| 236 | \& print F $out; |
| 237 | \& close(F); |
| 238 | .Ve |
| 239 | .Sp |
| 240 | .Vb 3 |
| 241 | \& open(F, "<:utf8", "data.utf"); |
| 242 | \& $in = <F>; |
| 243 | \& close(F); |
| 244 | .Ve |
| 245 | .IP ":bytes" 4 |
| 246 | .IX Item ":bytes" |
| 247 | This is the inverse of \f(CW\*(C`:utf8\*(C'\fR layer. It turns off the flag |
| 248 | on the layer below so that data read from it is considered to |
| 249 | be \*(L"octets\*(R" i.e. characters in range 0..255 only. Likewise |
| 250 | on output perl will warn if a \*(L"wide\*(R" character is written |
| 251 | to a such a stream. |
| 252 | .IP ":raw" 4 |
| 253 | .IX Item ":raw" |
| 254 | The \f(CW\*(C`:raw\*(C'\fR layer is \fIdefined\fR as being identical to calling |
| 255 | \&\f(CW\*(C`binmode($fh)\*(C'\fR \- the stream is made suitable for passing binary data |
| 256 | i.e. each byte is passed as\-is. The stream will still be |
| 257 | buffered. |
| 258 | .Sp |
| 259 | In Perl 5.6 and some books the \f(CW\*(C`:raw\*(C'\fR layer (previously sometimes also |
| 260 | referred to as a \*(L"discipline\*(R") is documented as the inverse of the |
| 261 | \&\f(CW\*(C`:crlf\*(C'\fR layer. That is no longer the case \- other layers which would |
| 262 | alter binary nature of the stream are also disabled. If you want \s-1UNIX\s0 |
| 263 | line endings on a platform that normally does \s-1CRLF\s0 translation, but still |
| 264 | want \s-1UTF\-8\s0 or encoding defaults the appropriate thing to do is to add |
| 265 | \&\f(CW\*(C`:perlio\*(C'\fR to \s-1PERLIO\s0 environment variable. |
| 266 | .Sp |
| 267 | The implementation of \f(CW\*(C`:raw\*(C'\fR is as a pseudo-layer which when \*(L"pushed\*(R" |
| 268 | pops itself and then any layers which do not declare themselves as suitable |
| 269 | for binary data. (Undoing :utf8 and :crlf are implemented by clearing |
| 270 | flags rather than popping layers but that is an implementation detail.) |
| 271 | .Sp |
| 272 | As a consequence of the fact that \f(CW\*(C`:raw\*(C'\fR normally pops layers |
| 273 | it usually only makes sense to have it as the only or first element in |
| 274 | a layer specification. When used as the first element it provides |
| 275 | a known base on which to build e.g. |
| 276 | .Sp |
| 277 | .Vb 1 |
| 278 | \& open($fh,":raw:utf8",...) |
| 279 | .Ve |
| 280 | .Sp |
| 281 | will construct a \*(L"binary\*(R" stream, but then enable \s-1UTF\-8\s0 translation. |
| 282 | .IP ":pop" 4 |
| 283 | .IX Item ":pop" |
| 284 | A pseudo layer that removes the top-most layer. Gives perl code |
| 285 | a way to manipulate the layer stack. Should be considered |
| 286 | as experimental. Note that \f(CW\*(C`:pop\*(C'\fR only works on real layers |
| 287 | and will not undo the effects of pseudo layers like \f(CW\*(C`:utf8\*(C'\fR. |
| 288 | An example of a possible use might be: |
| 289 | .Sp |
| 290 | .Vb 5 |
| 291 | \& open($fh,...) |
| 292 | \& ... |
| 293 | \& binmode($fh,":encoding(...)"); # next chunk is encoded |
| 294 | \& ... |
| 295 | \& binmode($fh,":pop"); # back to un-encoded |
| 296 | .Ve |
| 297 | .Sp |
| 298 | A more elegant (and safer) interface is needed. |
| 299 | .IP ":win32" 4 |
| 300 | .IX Item ":win32" |
| 301 | On Win32 platforms this \fIexperimental\fR layer uses native \*(L"handle\*(R" \s-1IO\s0 |
| 302 | rather than unix-like numeric file descriptor layer. Known to be |
| 303 | buggy as of perl 5.8.2. |
| 304 | .Sh "Custom Layers" |
| 305 | .IX Subsection "Custom Layers" |
| 306 | It is possible to write custom layers in addition to the above builtin |
| 307 | ones, both in C/XS and Perl. Two such layers (and one example written |
| 308 | in Perl using the latter) come with the Perl distribution. |
| 309 | .IP ":encoding" 4 |
| 310 | .IX Item ":encoding" |
| 311 | Use \f(CW\*(C`:encoding(ENCODING)\*(C'\fR either in \fIopen()\fR or \fIbinmode()\fR to install |
| 312 | a layer that does transparently character set and encoding transformations, |
| 313 | for example from Shift-JIS to Unicode. Note that under \f(CW\*(C`stdio\*(C'\fR |
| 314 | an \f(CW\*(C`:encoding\*(C'\fR also enables \f(CW\*(C`:utf8\*(C'\fR. See PerlIO::encoding |
| 315 | for more information. |
| 316 | .IP ":via" 4 |
| 317 | .IX Item ":via" |
| 318 | Use \f(CW\*(C`:via(MODULE)\*(C'\fR either in \fIopen()\fR or \fIbinmode()\fR to install a layer |
| 319 | that does whatever transformation (for example compression / |
| 320 | decompression, encryption / decryption) to the filehandle. |
| 321 | See PerlIO::via for more information. |
| 322 | .Sh "Alternatives to raw" |
| 323 | .IX Subsection "Alternatives to raw" |
| 324 | To get a binary stream an alternate method is to use: |
| 325 | .PP |
| 326 | .Vb 2 |
| 327 | \& open($fh,"whatever") |
| 328 | \& binmode($fh); |
| 329 | .Ve |
| 330 | .PP |
| 331 | this has advantage of being backward compatible with how such things have |
| 332 | had to be coded on some platforms for years. |
| 333 | .PP |
| 334 | To get an un-buffered stream specify an unbuffered layer (e.g. \f(CW\*(C`:unix\*(C'\fR) |
| 335 | in the open call: |
| 336 | .PP |
| 337 | .Vb 1 |
| 338 | \& open($fh,"<:unix",$path) |
| 339 | .Ve |
| 340 | .Sh "Defaults and how to override them" |
| 341 | .IX Subsection "Defaults and how to override them" |
| 342 | If the platform is MS-DOS like and normally does \s-1CRLF\s0 to \*(L"\en\*(R" |
| 343 | translation for text files then the default layers are : |
| 344 | .PP |
| 345 | .Vb 1 |
| 346 | \& unix crlf |
| 347 | .Ve |
| 348 | .PP |
| 349 | (The low level \*(L"unix\*(R" layer may be replaced by a platform specific low |
| 350 | level layer.) |
| 351 | .PP |
| 352 | Otherwise if \f(CW\*(C`Configure\*(C'\fR found out how to do \*(L"fast\*(R" \s-1IO\s0 using system's |
| 353 | stdio, then the default layers are: |
| 354 | .PP |
| 355 | .Vb 1 |
| 356 | \& unix stdio |
| 357 | .Ve |
| 358 | .PP |
| 359 | Otherwise the default layers are |
| 360 | .PP |
| 361 | .Vb 1 |
| 362 | \& unix perlio |
| 363 | .Ve |
| 364 | .PP |
| 365 | These defaults may change once perlio has been better tested and tuned. |
| 366 | .PP |
| 367 | The default can be overridden by setting the environment variable |
| 368 | \&\s-1PERLIO\s0 to a space separated list of layers (\f(CW\*(C`unix\*(C'\fR or platform low |
| 369 | level layer is always pushed first). |
| 370 | .PP |
| 371 | This can be used to see the effect of/bugs in the various layers e.g. |
| 372 | .PP |
| 373 | .Vb 3 |
| 374 | \& cd .../perl/t |
| 375 | \& PERLIO=stdio ./perl harness |
| 376 | \& PERLIO=perlio ./perl harness |
| 377 | .Ve |
| 378 | .PP |
| 379 | For the various value of \s-1PERLIO\s0 see \*(L"\s-1PERLIO\s0\*(R" in perlrun. |
| 380 | .Sh "Querying the layers of filehandles" |
| 381 | .IX Subsection "Querying the layers of filehandles" |
| 382 | The following returns the \fBnames\fR of the PerlIO layers on a filehandle. |
| 383 | .PP |
| 384 | .Vb 1 |
| 385 | \& my @layers = PerlIO::get_layers($fh); # Or FH, *FH, "FH". |
| 386 | .Ve |
| 387 | .PP |
| 388 | The layers are returned in the order an \fIopen()\fR or \fIbinmode()\fR call would |
| 389 | use them. Note that the \*(L"default stack\*(R" depends on the operating |
| 390 | system and on the Perl version, and both the compile-time and |
| 391 | runtime configurations of Perl. |
| 392 | .PP |
| 393 | The following table summarizes the default layers on UNIX-like and |
| 394 | DOS-like platforms and depending on the setting of the \f(CW$ENV{PERLIO}\fR: |
| 395 | .PP |
| 396 | .Vb 6 |
| 397 | \& PERLIO UNIX-like DOS-like |
| 398 | \& ------ --------- -------- |
| 399 | \& unset / "" unix perlio / stdio [1] unix crlf |
| 400 | \& stdio unix perlio / stdio [1] stdio |
| 401 | \& perlio unix perlio unix perlio |
| 402 | \& mmap unix mmap unix mmap |
| 403 | .Ve |
| 404 | .PP |
| 405 | .Vb 2 |
| 406 | \& # [1] "stdio" if Configure found out how to do "fast stdio" (depends |
| 407 | \& # on the stdio implementation) and in Perl 5.8, otherwise "unix perlio" |
| 408 | .Ve |
| 409 | .PP |
| 410 | By default the layers from the input side of the filehandle is |
| 411 | returned, to get the output side use the optional \f(CW\*(C`output\*(C'\fR argument: |
| 412 | .PP |
| 413 | .Vb 1 |
| 414 | \& my @layers = PerlIO::get_layers($fh, output => 1); |
| 415 | .Ve |
| 416 | .PP |
| 417 | (Usually the layers are identical on either side of a filehandle but |
| 418 | for example with sockets there may be differences, or if you have |
| 419 | been using the \f(CW\*(C`open\*(C'\fR pragma.) |
| 420 | .PP |
| 421 | There is no \fIset_layers()\fR, nor does \fIget_layers()\fR return a tied array |
| 422 | mirroring the stack, or anything fancy like that. This is not |
| 423 | accidental or unintentional. The PerlIO layer stack is a bit more |
| 424 | complicated than just a stack (see for example the behaviour of \f(CW\*(C`:raw\*(C'\fR). |
| 425 | You are supposed to use \fIopen()\fR and \fIbinmode()\fR to manipulate the stack. |
| 426 | .PP |
| 427 | \&\fBImplementation details follow, please close your eyes.\fR |
| 428 | .PP |
| 429 | The arguments to layers are by default returned in parenthesis after |
| 430 | the name of the layer, and certain layers (like \f(CW\*(C`utf8\*(C'\fR) are not real |
| 431 | layers but instead flags on real layers: to get all of these returned |
| 432 | separately use the optional \f(CW\*(C`details\*(C'\fR argument: |
| 433 | .PP |
| 434 | .Vb 1 |
| 435 | \& my @layer_and_args_and_flags = PerlIO::get_layers($fh, details => 1); |
| 436 | .Ve |
| 437 | .PP |
| 438 | The result will be up to be three times the number of layers: |
| 439 | the first element will be a name, the second element the arguments |
| 440 | (unspecified arguments will be \f(CW\*(C`undef\*(C'\fR), the third element the flags, |
| 441 | the fourth element a name again, and so forth. |
| 442 | .PP |
| 443 | \&\fBYou may open your eyes now.\fR |
| 444 | .SH "AUTHOR" |
| 445 | .IX Header "AUTHOR" |
| 446 | Nick Ing-Simmons <nick@ing\-simmons.net> |
| 447 | .SH "SEE ALSO" |
| 448 | .IX Header "SEE ALSO" |
| 449 | \&\*(L"binmode\*(R" in perlfunc, \*(L"open\*(R" in perlfunc, perlunicode, perliol, |
| 450 | Encode |