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129 | .\" ======================================================================== | |
130 | .\" | |
131 | .IX Title "PerlIO 3" | |
132 | .TH PerlIO 3 "2002-06-01" "perl v5.8.0" "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($fh,"<:crlf", "my.txt"); # portably open a text file for reading | |
139 | .Ve | |
140 | .PP | |
141 | .Vb 2 | |
142 | \& open($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 | Low level layer which calls \f(CW\*(C`read\*(C'\fR, \f(CW\*(C`write\*(C'\fR and \f(CW\*(C`lseek\*(C'\fR etc. | |
172 | .IP "stdio" 4 | |
173 | .IX Item "stdio" | |
174 | 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 | |
175 | that as this is \*(L"real\*(R" stdio it will ignore any layers beneath it and | |
176 | got straight to the operating system via the C library as usual. | |
177 | .IP "perlio" 4 | |
178 | .IX Item "perlio" | |
179 | This is a re-implementation of \*(L"stdio\-like\*(R" buffering written as a | |
180 | PerlIO \*(L"layer\*(R". As such it will call whatever layer is below it for | |
181 | its operations. | |
182 | .IP "crlf" 4 | |
183 | .IX Item "crlf" | |
184 | A layer which does \s-1CRLF\s0 to \*(L"\en\*(R" translation distinguishing \*(L"text\*(R" and | |
185 | \&\*(L"binary\*(R" files in the manner of MS-DOS and similar operating systems. | |
186 | (It currently does \fInot\fR mimic MS-DOS as far as treating of Control-Z | |
187 | as being an end-of-file marker.) | |
188 | .IP "utf8" 4 | |
189 | .IX Item "utf8" | |
190 | Declares that the stream accepts perl's internal encoding of | |
191 | characters. (Which really is \s-1UTF\-8\s0 on \s-1ASCII\s0 machines, but is | |
192 | UTF-EBCDIC on \s-1EBCDIC\s0 machines.) This allows any character perl can | |
193 | represent to be read from or written to the stream. The UTF-X encoding | |
194 | is chosen to render simple text parts (i.e. non-accented letters, | |
195 | digits and common punctuation) human readable in the encoded file. | |
196 | .Sp | |
197 | Here is how to write your native data out using \s-1UTF\-8\s0 (or \s-1UTF\-EBCDIC\s0) | |
198 | and then read it back in. | |
199 | .Sp | |
200 | .Vb 3 | |
201 | \& open(F, ">:utf8", "data.utf"); | |
202 | \& print F $out; | |
203 | \& close(F); | |
204 | .Ve | |
205 | .Sp | |
206 | .Vb 3 | |
207 | \& open(F, "<:utf8", "data.utf"); | |
208 | \& $in = <F>; | |
209 | \& close(F); | |
210 | .Ve | |
211 | .IP "bytes" 4 | |
212 | .IX Item "bytes" | |
213 | This is the inverse of \f(CW\*(C`:utf8\*(C'\fR layer. It turns off the flag | |
214 | on the layer below so that data read from it is considered to | |
215 | be \*(L"octets\*(R" i.e. characters in range 0..255 only. Likewise | |
216 | on output perl will warn if a \*(L"wide\*(R" character is written | |
217 | to a such a stream. | |
218 | .IP "raw" 4 | |
219 | .IX Item "raw" | |
220 | The \f(CW\*(C`:raw\*(C'\fR layer is \fIdefined\fR as being identical to calling | |
221 | \&\f(CW\*(C`binmode($fh)\*(C'\fR \- the stream is made suitable for passing binary | |
222 | data i.e. each byte is passed as\-is. The stream will still be | |
223 | buffered. Unlike earlier versions of perl \f(CW\*(C`:raw\*(C'\fR is \fInot\fR just the | |
224 | inverse of \f(CW\*(C`:crlf\*(C'\fR \- other layers which would affect the binary nature of | |
225 | the stream are also removed or disabled. | |
226 | .Sp | |
227 | The implementation of \f(CW\*(C`:raw\*(C'\fR is as a pseudo-layer which when \*(L"pushed\*(R" | |
228 | pops itself and then any layers which do not declare themselves as suitable | |
229 | for binary data. (Undoing :utf8 and :crlf are implemented by clearing | |
230 | flags rather than poping layers but that is an implementation detail.) | |
231 | .Sp | |
232 | As a consequence of the fact that \f(CW\*(C`:raw\*(C'\fR normally pops layers | |
233 | it usually only makes sense to have it as the only or first element in a | |
234 | layer specification. When used as the first element it provides | |
235 | a known base on which to build e.g. | |
236 | .Sp | |
237 | .Vb 1 | |
238 | \& open($fh,":raw:utf8",...) | |
239 | .Ve | |
240 | .Sp | |
241 | will construct a \*(L"binary\*(R" stream, but then enable \s-1UTF\-8\s0 translation. | |
242 | .IP "pop" 4 | |
243 | .IX Item "pop" | |
244 | A pseudo layer that removes the top-most layer. Gives perl code | |
245 | a way to manipulate the layer stack. Should be considered | |
246 | as experimental. Note that \f(CW\*(C`:pop\*(C'\fR only works on real layers | |
247 | and will not undo the effects of pseudo layers like \f(CW\*(C`:utf8\*(C'\fR. | |
248 | An example of a possible use might be: | |
249 | .Sp | |
250 | .Vb 5 | |
251 | \& open($fh,...) | |
252 | \& ... | |
253 | \& binmode($fh,":encoding(...)"); # next chunk is encoded | |
254 | \& ... | |
255 | \& binmode($fh,":pop"); # back to un-encocded | |
256 | .Ve | |
257 | .Sp | |
258 | A more elegant (and safer) interface is needed. | |
259 | .Sh "Alternatives to raw" | |
260 | .IX Subsection "Alternatives to raw" | |
261 | To get a binary stream an alternate method is to use: | |
262 | .PP | |
263 | .Vb 2 | |
264 | \& open($fh,"whatever") | |
265 | \& binmode($fh); | |
266 | .Ve | |
267 | .PP | |
268 | this has advantage of being backward compatible with how such things have | |
269 | had to be coded on some platforms for years. | |
270 | .PP | |
271 | To get an un-buffered stream specify an unbuffered layer (e.g. \f(CW\*(C`:unix\*(C'\fR) | |
272 | in the open call: | |
273 | .PP | |
274 | .Vb 1 | |
275 | \& open($fh,"<:unix",$path) | |
276 | .Ve | |
277 | .Sh "Defaults and how to override them" | |
278 | .IX Subsection "Defaults and how to override them" | |
279 | If the platform is MS-DOS like and normally does \s-1CRLF\s0 to \*(L"\en\*(R" | |
280 | translation for text files then the default layers are : | |
281 | .PP | |
282 | .Vb 1 | |
283 | \& unix crlf | |
284 | .Ve | |
285 | .PP | |
286 | (The low level \*(L"unix\*(R" layer may be replaced by a platform specific low | |
287 | level layer.) | |
288 | .PP | |
289 | Otherwise if \f(CW\*(C`Configure\*(C'\fR found out how to do \*(L"fast\*(R" \s-1IO\s0 using system's | |
290 | stdio, then the default layers are : | |
291 | .PP | |
292 | .Vb 1 | |
293 | \& unix stdio | |
294 | .Ve | |
295 | .PP | |
296 | Otherwise the default layers are | |
297 | .PP | |
298 | .Vb 1 | |
299 | \& unix perlio | |
300 | .Ve | |
301 | .PP | |
302 | These defaults may change once perlio has been better tested and tuned. | |
303 | .PP | |
304 | The default can be overridden by setting the environment variable | |
305 | \&\s-1PERLIO\s0 to a space separated list of layers (unix or platform low level | |
306 | layer is always pushed first). | |
307 | .PP | |
308 | This can be used to see the effect of/bugs in the various layers e.g. | |
309 | .PP | |
310 | .Vb 3 | |
311 | \& cd .../perl/t | |
312 | \& PERLIO=stdio ./perl harness | |
313 | \& PERLIO=perlio ./perl harness | |
314 | .Ve | |
315 | .SH "AUTHOR" | |
316 | .IX Header "AUTHOR" | |
317 | Nick Ing-Simmons <nick@ing\-simmons.net> | |
318 | .SH "SEE ALSO" | |
319 | .IX Header "SEE ALSO" | |
320 | \&\*(L"binmode\*(R" in perlfunc, \*(L"open\*(R" in perlfunc, perlunicode, Encode |