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1 | |
2 | =head1 NAME |
3 | |
4 | perliol - C API for Perl's implementation of IO in Layers. |
5 | |
6 | =head1 SYNOPSIS |
7 | |
8 | /* Defining a layer ... */ |
9 | #include <perliol.h> |
10 | |
11 | |
12 | =head1 DESCRIPTION |
13 | |
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14 | This document describes the behavior and implementation of the PerlIO |
15 | abstraction described in L<perlapio> when C<USE_PERLIO> is defined (and |
16 | C<USE_SFIO> is not). |
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17 | |
18 | =head2 History and Background |
19 | |
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20 | The PerlIO abstraction was introduced in perl5.003_02 but languished as |
21 | just an abstraction until perl5.7.0. However during that time a number |
22 | of perl extentions switched to using it, so the API is mostly fixed to |
23 | maintain (source) compatibility. |
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24 | |
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25 | The aim of the implementation is to provide the PerlIO API in a flexible |
26 | and platform neutral manner. It is also a trial of an "Object Oriented |
27 | C, with vtables" approach which may be applied to perl6. |
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28 | |
29 | =head2 Layers vs Disciplines |
30 | |
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31 | Initial discussion of the ability to modify IO streams behaviour used |
32 | the term "discipline" for the entities which were added. This came (I |
33 | believe) from the use of the term in "sfio", which in turn borrowed it |
34 | from "line disciplines" on Unix terminals. However, this document (and |
35 | the C code) uses the term "layer". |
36 | |
37 | This is, I hope, a natural term given the implementation, and should avoid |
38 | connotations that are inherent in earlier uses of "discipline" for things |
39 | which are rather different. |
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40 | |
41 | =head2 Data Structures |
42 | |
43 | The basic data structure is a PerlIOl: |
44 | |
45 | typedef struct _PerlIO PerlIOl; |
46 | typedef struct _PerlIO_funcs PerlIO_funcs; |
47 | typedef PerlIOl *PerlIO; |
48 | |
49 | struct _PerlIO |
50 | { |
51 | PerlIOl * next; /* Lower layer */ |
52 | PerlIO_funcs * tab; /* Functions for this layer */ |
53 | IV flags; /* Various flags for state */ |
54 | }; |
55 | |
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56 | A C<PerlIOl *> is a pointer to to the struct, and the I<application> level |
57 | C<PerlIO *> is a pointer to a C<PerlIOl *> - i.e. a pointer to a pointer to |
58 | the struct. This allows the application level C<PerlIO *> to remain |
59 | constant while the actual C<PerlIOl *> underneath changes. (Compare perl's |
60 | C<SV *> which remains constant while its C<sv_any> field changes as the |
61 | scalar's type changes.) An IO stream is then in general represented as a |
62 | pointer to this linked-list of "layers". |
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63 | |
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64 | It should be noted that because of the double indirection in a C<PerlIO *>, |
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65 | a C<< &(perlio-E<gt>next) >> "is" a C<PerlIO *>, and so to some degree |
66 | at least one layer can use the "standard" API on the next layer down. |
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67 | |
68 | A "layer" is composed of two parts: |
69 | |
70 | =over 4 |
71 | |
72 | =item 1. The functions and attributes of the "layer class". |
73 | |
74 | =item 2. The per-instance data for a particular handle. |
75 | |
76 | =back |
77 | |
78 | =head2 Functions and Attributes |
79 | |
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80 | The functions and attributes are accessed via the "tab" (for table) |
81 | member of C<PerlIOl>. The functions (methods of the layer "class") are |
82 | fixed, and are defined by the C<PerlIO_funcs> type. They are broadly the |
83 | same as the public C<PerlIO_xxxxx> functions: |
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84 | |
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85 | struct _PerlIO_funcs |
86 | { |
87 | char * name; |
88 | Size_t size; |
89 | IV kind; |
90 | IV (*Pushed)(PerlIO *f,const char *mode,SV *arg); |
91 | IV (*Popped)(PerlIO *f); |
92 | PerlIO * (*Open)(pTHX_ PerlIO_funcs *tab, |
93 | AV *layers, IV n, |
94 | const char *mode, |
95 | int fd, int imode, int perm, |
96 | PerlIO *old, |
97 | int narg, SV **args); |
98 | SV * (*Getarg)(PerlIO *f); |
99 | IV (*Fileno)(PerlIO *f); |
100 | /* Unix-like functions - cf sfio line disciplines */ |
101 | SSize_t (*Read)(PerlIO *f, void *vbuf, Size_t count); |
102 | SSize_t (*Unread)(PerlIO *f, const void *vbuf, Size_t count); |
103 | SSize_t (*Write)(PerlIO *f, const void *vbuf, Size_t count); |
104 | IV (*Seek)(PerlIO *f, Off_t offset, int whence); |
105 | Off_t (*Tell)(PerlIO *f); |
106 | IV (*Close)(PerlIO *f); |
107 | /* Stdio-like buffered IO functions */ |
108 | IV (*Flush)(PerlIO *f); |
109 | IV (*Fill)(PerlIO *f); |
110 | IV (*Eof)(PerlIO *f); |
111 | IV (*Error)(PerlIO *f); |
112 | void (*Clearerr)(PerlIO *f); |
113 | void (*Setlinebuf)(PerlIO *f); |
114 | /* Perl's snooping functions */ |
115 | STDCHAR * (*Get_base)(PerlIO *f); |
116 | Size_t (*Get_bufsiz)(PerlIO *f); |
117 | STDCHAR * (*Get_ptr)(PerlIO *f); |
118 | SSize_t (*Get_cnt)(PerlIO *f); |
119 | void (*Set_ptrcnt)(PerlIO *f,STDCHAR *ptr,SSize_t cnt); |
120 | }; |
121 | |
122 | |
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123 | |
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124 | The first few members of the struct give a "name" for the layer, the |
125 | size to C<malloc> for the per-instance data, and some flags which are |
126 | attributes of the class as whole (such as whether it is a buffering |
127 | layer), then follow the functions which fall into four basic groups: |
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128 | |
129 | =over 4 |
130 | |
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131 | =item 1. |
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132 | |
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133 | Opening and setup functions |
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134 | |
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135 | =item 2. |
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136 | |
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137 | Basic IO operations |
138 | |
139 | =item 3. |
140 | |
141 | Stdio class buffering options. |
142 | |
143 | =item 4. |
144 | |
145 | Functions to support Perl's traditional "fast" access to the buffer. |
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146 | |
147 | =back |
148 | |
149 | A layer does not have to implement all the functions, but the whole table has |
150 | to be present. Unimplemented slots can be NULL (which will will result in an error |
151 | when called) or can be filled in with stubs to "inherit" behaviour from |
152 | a "base class". This "inheritance" is fixed for all instances of the layer, |
153 | but as the layer chooses which stubs to populate the table, limited |
154 | "multiple inheritance" is possible. |
155 | |
156 | =head2 Per-instance Data |
157 | |
158 | The per-instance data are held in memory beyond the basic PerlIOl struct, |
159 | by making a PerlIOl the first member of the layer's struct thus: |
160 | |
161 | typedef struct |
162 | { |
163 | struct _PerlIO base; /* Base "class" info */ |
164 | STDCHAR * buf; /* Start of buffer */ |
165 | STDCHAR * end; /* End of valid part of buffer */ |
166 | STDCHAR * ptr; /* Current position in buffer */ |
167 | Off_t posn; /* Offset of buf into the file */ |
168 | Size_t bufsiz; /* Real size of buffer */ |
169 | IV oneword; /* Emergency buffer */ |
170 | } PerlIOBuf; |
171 | |
172 | In this way (as for perl's scalars) a pointer to a PerlIOBuf can be treated |
173 | as a pointer to a PerlIOl. |
174 | |
175 | =head2 Layers in action. |
176 | |
177 | table perlio unix |
178 | | | |
179 | +-----------+ +----------+ +--------+ |
180 | PerlIO ->| |--->| next |--->| NULL | |
181 | +-----------+ +----------+ +--------+ |
182 | | | | buffer | | fd | |
183 | +-----------+ | | +--------+ |
184 | | | +----------+ |
185 | |
186 | |
187 | The above attempts to show how the layer scheme works in a simple case. |
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188 | The application's C<PerlIO *> points to an entry in the table(s) |
189 | representing open (allocated) handles. For example the first three slots |
190 | in the table correspond to C<stdin>,C<stdout> and C<stderr>. The table |
191 | in turn points to the current "top" layer for the handle - in this case |
192 | an instance of the generic buffering layer "perlio". That layer in turn |
193 | points to the next layer down - in this case the lowlevel "unix" layer. |
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194 | |
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195 | The above is roughly equivalent to a "stdio" buffered stream, but with |
196 | much more flexibility: |
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197 | |
198 | =over 4 |
199 | |
200 | =item * |
201 | |
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202 | If Unix level C<read>/C<write>/C<lseek> is not appropriate for (say) |
203 | sockets then the "unix" layer can be replaced (at open time or even |
204 | dynamically) with a "socket" layer. |
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205 | |
206 | =item * |
207 | |
208 | Different handles can have different buffering schemes. The "top" layer |
209 | could be the "mmap" layer if reading disk files was quicker using C<mmap> |
210 | than C<read>. An "unbuffered" stream can be implemented simply by |
211 | not having a buffer layer. |
212 | |
213 | =item * |
214 | |
215 | Extra layers can be inserted to process the data as it flows through. |
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216 | This was the driving need for including the scheme in perl 5.7.0+ - we |
217 | needed a mechanism to allow data to be translated bewteen perl's |
218 | internal encoding (conceptually at least Unicode as UTF-8), and the |
219 | "native" format used by the system. This is provided by the |
220 | ":encoding(xxxx)" layer which typically sits above the buffering layer. |
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221 | |
222 | =item * |
223 | |
224 | A layer can be added that does "\n" to CRLF translation. This layer can be used |
225 | on any platform, not just those that normally do such things. |
226 | |
227 | =back |
228 | |
229 | =head2 Per-instance flag bits |
230 | |
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231 | The generic flag bits are a hybrid of C<O_XXXXX> style flags deduced from |
232 | the mode string passed to C<PerlIO_open()>, and state bits for typical buffer |
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233 | layers. |
234 | |
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235 | =over 4 |
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236 | |
237 | =item PERLIO_F_EOF |
238 | |
239 | End of file. |
240 | |
241 | =item PERLIO_F_CANWRITE |
242 | |
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243 | Writes are permitted, i.e. opened as "w" or "r+" or "a", etc. |
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244 | |
245 | =item PERLIO_F_CANREAD |
246 | |
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247 | Reads are permitted i.e. opened "r" or "w+" (or even "a+" - ick). |
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248 | |
249 | =item PERLIO_F_ERROR |
250 | |
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251 | An error has occured (for C<PerlIO_error()>) |
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252 | |
253 | =item PERLIO_F_TRUNCATE |
254 | |
255 | Truncate file suggested by open mode. |
256 | |
257 | =item PERLIO_F_APPEND |
258 | |
259 | All writes should be appends. |
260 | |
261 | =item PERLIO_F_CRLF |
262 | |
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263 | Layer is performing Win32-like "\n" mapped to CR,LF for output and CR,LF |
264 | mapped to "\n" for input. Normally the provided "crlf" layer is the only |
265 | layer that need bother about this. C<PerlIO_binmode()> will mess with this |
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266 | flag rather than add/remove layers if the C<PERLIO_K_CANCRLF> bit is set |
267 | for the layers class. |
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268 | |
269 | =item PERLIO_F_UTF8 |
270 | |
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271 | Data written to this layer should be UTF-8 encoded; data provided |
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272 | by this layer should be considered UTF-8 encoded. Can be set on any layer |
273 | by ":utf8" dummy layer. Also set on ":encoding" layer. |
274 | |
275 | =item PERLIO_F_UNBUF |
276 | |
277 | Layer is unbuffered - i.e. write to next layer down should occur for |
278 | each write to this layer. |
279 | |
280 | =item PERLIO_F_WRBUF |
281 | |
282 | The buffer for this layer currently holds data written to it but not sent |
283 | to next layer. |
284 | |
285 | =item PERLIO_F_RDBUF |
286 | |
287 | The buffer for this layer currently holds unconsumed data read from |
288 | layer below. |
289 | |
290 | =item PERLIO_F_LINEBUF |
291 | |
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292 | Layer is line buffered. Write data should be passed to next layer down |
293 | whenever a "\n" is seen. Any data beyond the "\n" should then be |
294 | processed. |
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295 | |
296 | =item PERLIO_F_TEMP |
297 | |
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298 | File has been C<unlink()>ed, or should be deleted on C<close()>. |
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299 | |
300 | =item PERLIO_F_OPEN |
301 | |
302 | Handle is open. |
303 | |
304 | =item PERLIO_F_FASTGETS |
305 | |
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306 | This instance of this layer supports the "fast C<gets>" interface. |
307 | Normally set based on C<PERLIO_K_FASTGETS> for the class and by the |
308 | existance of the function(s) in the table. However a class that |
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309 | normally provides that interface may need to avoid it on a |
310 | particular instance. The "pending" layer needs to do this when |
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311 | it is pushed above an layer which does not support the interface. |
312 | (Perl's C<sv_gets()> does not expect the streams fast C<gets> behaviour |
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313 | to change during one "get".) |
314 | |
315 | =back |
316 | |
317 | =head2 Methods in Detail |
318 | |
319 | =over 4 |
320 | |
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321 | =item IV (*Pushed)(PerlIO *f,const char *mode, SV *arg); |
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322 | |
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323 | The only absoultely mandatory method. Called when the layer is pushed onto the stack. |
324 | The C<mode> argument may be NULL if this occurs post-open. The C<arg> will be non-C<NULL> |
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325 | if an argument string was passed. In most cases this should call |
326 | C<PerlIOBase_pushed()> to convert C<mode> into the appropriate |
327 | C<PERLIO_F_XXXXX> flags in addition to any actions the layer itself takes. |
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328 | If a layer is not expecting an argument it need neither save the one passed to it, nor |
329 | provide C<Getarg()> (it could perhaps C<Perl_warn> that the argument was un-expected). |
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330 | |
331 | =item IV (*Popped)(PerlIO *f); |
332 | |
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333 | Called when the layer is popped from the stack. A layer will normally be |
334 | popped after C<Close()> is called. But a layer can be popped without being |
335 | closed if the program is dynamically managing layers on the stream. In |
336 | such cases C<Popped()> should free any resources (buffers, translation |
337 | tables, ...) not held directly in the layer's struct. |
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338 | It should also C<Unread()> any unconsumed data that has been read and buffered |
339 | from the layer below back to that layer, so that it can be re-provided to what |
340 | ever is now above. |
341 | |
342 | =item PerlIO * (*Open)(...); |
343 | |
344 | The C<Open()> method has lots of arguments because it combines the functions |
345 | of perl's C<open>, C<PerlIO_open>, perl's C<sysopen>, C<PerlIO_fdopen> and C<PerlIO_reopen>. |
346 | The full prototype is as follows: |
347 | |
348 | PerlIO * (*Open)(pTHX_ PerlIO_funcs *tab, |
349 | AV *layers, IV n, |
350 | const char *mode, |
351 | int fd, int imode, int perm, |
352 | PerlIO *old, |
353 | int narg, SV **args); |
354 | |
355 | Open should (perhaps indirectly) call C<PerlIO_allocate()> to allocate a slot in the table and |
356 | associate it with the layers information for the opened file, by calling C<PerlIO_push>. |
357 | The I<layers> AV is an array of all the layers destined for the C<PerlIO *>, |
358 | and any arguments passed to them, I<n> is the index into that array of the |
359 | layer being called. The macro C<PerlIOArg> will return a (possibly C<NULL>) SV * |
360 | for the argument passed to the layer. |
361 | |
362 | The I<mode> string is an "C<fopen()>-like" string which would match the regular |
363 | expression C</^[I#]?[rwa]\+?[bt]?$/>. |
364 | |
365 | The C<'I'> prefix is used during creation of C<stdin>..C<stderr> via special |
366 | C<PerlIO_fdopen> calls; the C<'#'> prefix means that this is C<sysopen> and that I<imode> and |
367 | I<perm> should be passed to C<PerlLIO_open3>; C<'r'> means B<r>ead, C<'w'> means B<w>rite |
368 | and C<'a'> means B<a>ppend. The C<'+'> suffix means that both reading and writing/appending |
369 | are permited. The C<'b'> suffix means file should be binary, and C<'t'> means it |
370 | is text. (Binary/Text should be ignored by almost all layers and binary IO done, |
371 | with PerlIO. The C<:crlf> layer should be pushed to handle the distinction.) |
372 | |
373 | If I<old> is not C<NULL> then this is a C<PerlIO_reopen>. Perl iteself does not use |
374 | this (yet?) and semantics are a little vague. |
375 | |
376 | If I<fd> not negative then it is the numeric file descriptor I<fd>, which will |
377 | be open in an manner compatible with the supplied mode string, the call is |
378 | thus equivalent to C<PerlIO_fdopen>. In this case I<nargs> will be zero. |
379 | |
380 | If I<nargs> is greater than zero then it gives the number of arguments passed |
381 | to C<open>, otherwise it will be 1 if for example C<PerlIO_open> was called. |
382 | In simple cases SvPV(*args) is the pathname to open. |
383 | |
384 | Having said all that translation-only layers do not need to provide C<Open()> at all, |
385 | but rather leave the opening to a lower level layer and wait to be "pushed". |
386 | If a layer does provide C<Open()> it should normaly call the C<Open()> method |
387 | of next layer down (if any) and then push itself on top if that succeeds. |
388 | |
389 | =item SV * (*Getarg)(PerlIO *f); |
390 | |
391 | Optional. If present should return an SV * representing the string argument |
392 | passed to the layer when it was pushed. e.g. ":encoding(ascii)" would |
393 | return an SvPV with value "ascii". |
394 | |
395 | =item IV (*Fileno)(PerlIO *f); |
396 | |
397 | Returns the Unix/Posix numeric file decriptor for the handle. Normally |
398 | C<PerlIOBase_fileno()> (which just asks next layer down) will suffice |
399 | for this. |
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400 | |
401 | =item SSize_t (*Read)(PerlIO *f, void *vbuf, Size_t count); |
402 | |
403 | Basic read operation. Returns actual bytes read, or -1 on an error. |
404 | Typically will call Fill and manipulate pointers (possibly via the API). |
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405 | C<PerlIOBuf_read()> may be suitable for derived classes which provide |
406 | "fast gets" methods. |
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407 | |
408 | =item SSize_t (*Unread)(PerlIO *f, const void *vbuf, Size_t count); |
409 | |
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410 | A superset of stdio's C<ungetc()>. Should arrange for future reads to |
411 | see the bytes in C<vbuf>. If there is no obviously better implementation |
412 | then C<PerlIOBase_unread()> provides the function by pushing a "fake" |
413 | "pending" layer above the calling layer. |
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414 | |
415 | =item SSize_t (*Write)(PerlIO *f, const void *vbuf, Size_t count); |
416 | |
417 | Basic write operation. Returns bytes written or -1 on an error. |
418 | |
419 | =item IV (*Seek)(PerlIO *f, Off_t offset, int whence); |
420 | |
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421 | Position the file pointer. Should normally call its own C<Flush> method and |
422 | then the C<Seek> method of next layer down. |
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423 | |
424 | =item Off_t (*Tell)(PerlIO *f); |
425 | |
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426 | Return the file pointer. May be based on layers cached concept of |
427 | position to avoid overhead. |
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428 | |
429 | =item IV (*Close)(PerlIO *f); |
430 | |
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431 | Close the stream. Should normally call C<PerlIOBase_close()> to flush |
432 | itself and close layers below, and then deallocate any data structures |
433 | (buffers, translation tables, ...) not held directly in the data |
434 | structure. |
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435 | |
436 | =item IV (*Flush)(PerlIO *f); |
437 | |
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438 | Should make stream's state consistent with layers below. That is, any |
439 | buffered write data should be written, and file position of lower layers |
440 | adjusted for data read fron below but not actually consumed. |
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441 | (Should perhaps C<Unread()> such data to the lower layer.) |
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442 | |
443 | =item IV (*Fill)(PerlIO *f); |
444 | |
445 | The buffer for this layer should be filled (for read) from layer below. |
446 | |
447 | =item IV (*Eof)(PerlIO *f); |
448 | |
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449 | Return end-of-file indicator. C<PerlIOBase_eof()> is normally sufficient. |
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450 | |
451 | =item IV (*Error)(PerlIO *f); |
452 | |
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453 | Return error indicator. C<PerlIOBase_error()> is normally sufficient. |
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454 | |
455 | =item void (*Clearerr)(PerlIO *f); |
456 | |
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457 | Clear end-of-file and error indicators. Should call C<PerlIOBase_clearerr()> |
458 | to set the C<PERLIO_F_XXXXX> flags, which may suffice. |
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459 | |
460 | =item void (*Setlinebuf)(PerlIO *f); |
461 | |
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462 | Mark the stream as line buffered. C<PerlIOBase_setlinebuf()> sets the |
463 | PERLIO_F_LINEBUF flag and is normally sufficient. |
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464 | |
465 | =item STDCHAR * (*Get_base)(PerlIO *f); |
466 | |
467 | Allocate (if not already done so) the read buffer for this layer and |
468 | return pointer to it. |
469 | |
470 | =item Size_t (*Get_bufsiz)(PerlIO *f); |
471 | |
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472 | Return the number of bytes that last C<Fill()> put in the buffer. |
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473 | |
474 | =item STDCHAR * (*Get_ptr)(PerlIO *f); |
475 | |
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476 | Return the current read pointer relative to this layer's buffer. |
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477 | |
478 | =item SSize_t (*Get_cnt)(PerlIO *f); |
479 | |
480 | Return the number of bytes left to be read in the current buffer. |
481 | |
482 | =item void (*Set_ptrcnt)(PerlIO *f,STDCHAR *ptr,SSize_t cnt); |
483 | |
484 | Adjust the read pointer and count of bytes to match C<ptr> and/or C<cnt>. |
485 | The application (or layer above) must ensure they are consistent. |
486 | (Checking is allowed by the paranoid.) |
487 | |
488 | =back |
489 | |
490 | |
491 | =head2 Core Layers |
492 | |
493 | The file C<perlio.c> provides the following layers: |
494 | |
495 | =over 4 |
496 | |
497 | =item "unix" |
498 | |
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499 | A basic non-buffered layer which calls Unix/POSIX C<read()>, C<write()>, |
500 | C<lseek()>, C<close()>. No buffering. Even on platforms that distinguish |
501 | between O_TEXT and O_BINARY this layer is always O_BINARY. |
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502 | |
503 | =item "perlio" |
504 | |
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505 | A very complete generic buffering layer which provides the whole of |
506 | PerlIO API. It is also intended to be used as a "base class" for other |
507 | layers. (For example its C<Read()> method is implemented in terms of the |
508 | C<Get_cnt()>/C<Get_ptr()>/C<Set_ptrcnt()> methods). |
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509 | |
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510 | "perlio" over "unix" provides a complete replacement for stdio as seen |
511 | via PerlIO API. This is the default for USE_PERLIO when system's stdio |
512 | does not permit perl's "fast gets" access, and which do not distinguish |
513 | between C<O_TEXT> and C<O_BINARY>. |
50b80e25 |
514 | |
515 | =item "stdio" |
516 | |
9d799145 |
517 | A layer which provides the PerlIO API via the layer scheme, but |
518 | implements it by calling system's stdio. This is (currently) the default |
519 | if system's stdio provides sufficient access to allow perl's "fast gets" |
520 | access and which do not distinguish between C<O_TEXT> and C<O_BINARY>. |
50b80e25 |
521 | |
522 | =item "crlf" |
523 | |
9d799145 |
524 | A layer derived using "perlio" as a base class. It provides Win32-like |
525 | "\n" to CR,LF translation. Can either be applied above "perlio" or serve |
526 | as the buffer layer itself. "crlf" over "unix" is the default if system |
527 | distinguishes between C<O_TEXT> and C<O_BINARY> opens. (At some point |
528 | "unix" will be replaced by a "native" Win32 IO layer on that platform, |
529 | as Win32's read/write layer has various drawbacks.) The "crlf" layer is |
530 | a reasonable model for a layer which transforms data in some way. |
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531 | |
532 | =item "mmap" |
533 | |
9d799145 |
534 | If Configure detects C<mmap()> functions this layer is provided (with |
535 | "perlio" as a "base") which does "read" operations by mmap()ing the |
536 | file. Performance improvement is marginal on modern systems, so it is |
537 | mainly there as a proof of concept. It is likely to be unbundled from |
538 | the core at some point. The "mmap" layer is a reasonable model for a |
539 | minimalist "derived" layer. |
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540 | |
541 | =item "pending" |
542 | |
9d799145 |
543 | An "internal" derivative of "perlio" which can be used to provide |
544 | Unread() function for layers which have no buffer or cannot be bothered. |
545 | (Basically this layer's C<Fill()> pops itself off the stack and so resumes |
546 | reading from layer below.) |
50b80e25 |
547 | |
548 | =item "raw" |
549 | |
9d799145 |
550 | A dummy layer which never exists on the layer stack. Instead when |
551 | "pushed" it actually pops the stack(!), removing itself, and any other |
552 | layers until it reaches a layer with the class C<PERLIO_K_RAW> bit set. |
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553 | |
554 | =item "utf8" |
555 | |
9d799145 |
556 | Another dummy layer. When pushed it pops itself and sets the |
557 | C<PERLIO_F_UTF8> flag on the layer which was (and now is once more) the top |
558 | of the stack. |
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559 | |
560 | =back |
561 | |
9d799145 |
562 | In addition F<perlio.c> also provides a number of C<PerlIOBase_xxxx()> |
563 | functions which are intended to be used in the table slots of classes |
564 | which do not need to do anything special for a particular method. |
50b80e25 |
565 | |
566 | =head2 Extension Layers |
567 | |
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568 | Layers can made available by extension modules. When an unknown layer is encountered |
569 | the PerlIO code will perform the equivalent of : |
570 | |
571 | use PerlIO 'layer'; |
572 | |
573 | Where I<layer> is the unknown layer. F<PerlIO.pm> will then attempt to : |
574 | |
575 | require PerlIO::layer; |
576 | |
577 | If after that process the layer is still not defined then the C<open> will fail. |
578 | |
579 | The following extension layers are bundled with perl: |
50b80e25 |
580 | |
581 | =over 4 |
582 | |
b76cc8ba |
583 | =item ":encoding" |
50b80e25 |
584 | |
585 | use Encoding; |
586 | |
b76cc8ba |
587 | makes this layer available, although F<PerlIO.pm> "knows" where to find it. |
588 | It is an example of a layer which takes an argument as it is called thus: |
50b80e25 |
589 | |
590 | open($fh,"<:encoding(iso-8859-7)",$pathname) |
591 | |
b76cc8ba |
592 | =item ":Scalar" |
593 | |
594 | Provides support for |
595 | |
596 | open($fh,"...",\$scalar) |
50b80e25 |
597 | |
b76cc8ba |
598 | When a handle is so opened, then reads get bytes from the string value of I<$scalar>, |
599 | and writes change the value. In both cases the position in I<$scalar> starts as zero |
600 | but can be altered via C<seek>, and determined via C<tell>. |
601 | |
602 | =item ":Object" or ":Perl" |
603 | |
604 | May be provided to allow layers to be implemented as perl code - implementation |
605 | is being investigated. |
606 | |
607 | =back |
50b80e25 |
608 | |
609 | =cut |
610 | |
611 | |
612 | |