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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 |
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22 | of perl extensions switched to using it, so the API is mostly fixed to |
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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 | |
1d11c889 |
37 | This is, I hope, a natural term given the implementation, and should |
38 | avoid connotations that are inherent in earlier uses of "discipline" |
39 | for things 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 the struct, and the I<application> |
57 | level C<PerlIO *> is a pointer to a C<PerlIOl *> - i.e. a pointer |
58 | to a pointer to the struct. This allows the application level C<PerlIO *> |
59 | to remain constant while the actual C<PerlIOl *> underneath |
60 | changes. (Compare perl's C<SV *> which remains constant while its |
61 | C<sv_any> field changes as the scalar's type changes.) An IO stream is |
62 | then in general represented as a pointer to this linked-list of |
63 | "layers". |
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64 | |
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65 | It should be noted that because of the double indirection in a C<PerlIO *>, |
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66 | a C<< &(perlio-E<gt>next) >> "is" a C<PerlIO *>, and so to some degree |
67 | at least one layer can use the "standard" API on the next layer down. |
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68 | |
69 | A "layer" is composed of two parts: |
70 | |
71 | =over 4 |
72 | |
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73 | =item 1. |
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74 | |
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75 | The functions and attributes of the "layer class". |
76 | |
77 | =item 2. |
78 | |
79 | The per-instance data for a particular handle. |
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80 | |
81 | =back |
82 | |
83 | =head2 Functions and Attributes |
84 | |
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85 | The functions and attributes are accessed via the "tab" (for table) |
86 | member of C<PerlIOl>. The functions (methods of the layer "class") are |
87 | fixed, and are defined by the C<PerlIO_funcs> type. They are broadly the |
88 | same as the public C<PerlIO_xxxxx> functions: |
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89 | |
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90 | struct _PerlIO_funcs |
91 | { |
92 | char * name; |
93 | Size_t size; |
94 | IV kind; |
95 | IV (*Pushed)(PerlIO *f,const char *mode,SV *arg); |
96 | IV (*Popped)(PerlIO *f); |
97 | PerlIO * (*Open)(pTHX_ PerlIO_funcs *tab, |
98 | AV *layers, IV n, |
99 | const char *mode, |
100 | int fd, int imode, int perm, |
101 | PerlIO *old, |
102 | int narg, SV **args); |
103 | SV * (*Getarg)(PerlIO *f); |
104 | IV (*Fileno)(PerlIO *f); |
105 | /* Unix-like functions - cf sfio line disciplines */ |
106 | SSize_t (*Read)(PerlIO *f, void *vbuf, Size_t count); |
107 | SSize_t (*Unread)(PerlIO *f, const void *vbuf, Size_t count); |
108 | SSize_t (*Write)(PerlIO *f, const void *vbuf, Size_t count); |
109 | IV (*Seek)(PerlIO *f, Off_t offset, int whence); |
110 | Off_t (*Tell)(PerlIO *f); |
111 | IV (*Close)(PerlIO *f); |
112 | /* Stdio-like buffered IO functions */ |
113 | IV (*Flush)(PerlIO *f); |
114 | IV (*Fill)(PerlIO *f); |
115 | IV (*Eof)(PerlIO *f); |
116 | IV (*Error)(PerlIO *f); |
117 | void (*Clearerr)(PerlIO *f); |
118 | void (*Setlinebuf)(PerlIO *f); |
119 | /* Perl's snooping functions */ |
120 | STDCHAR * (*Get_base)(PerlIO *f); |
121 | Size_t (*Get_bufsiz)(PerlIO *f); |
122 | STDCHAR * (*Get_ptr)(PerlIO *f); |
123 | SSize_t (*Get_cnt)(PerlIO *f); |
124 | void (*Set_ptrcnt)(PerlIO *f,STDCHAR *ptr,SSize_t cnt); |
125 | }; |
126 | |
127 | |
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128 | |
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129 | The first few members of the struct give a "name" for the layer, the |
130 | size to C<malloc> for the per-instance data, and some flags which are |
131 | attributes of the class as whole (such as whether it is a buffering |
132 | layer), then follow the functions which fall into four basic groups: |
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133 | |
134 | =over 4 |
135 | |
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136 | =item 1. |
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137 | |
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138 | Opening and setup functions |
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139 | |
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140 | =item 2. |
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141 | |
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142 | Basic IO operations |
143 | |
144 | =item 3. |
145 | |
146 | Stdio class buffering options. |
147 | |
148 | =item 4. |
149 | |
150 | Functions to support Perl's traditional "fast" access to the buffer. |
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151 | |
152 | =back |
153 | |
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154 | A layer does not have to implement all the functions, but the whole |
155 | table has to be present. Unimplemented slots can be NULL (which will |
156 | result in an error when called) or can be filled in with stubs to |
157 | "inherit" behaviour from a "base class". This "inheritance" is fixed |
158 | for all instances of the layer, but as the layer chooses which stubs |
159 | to populate the table, limited "multiple inheritance" is possible. |
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160 | |
161 | =head2 Per-instance Data |
162 | |
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163 | The per-instance data are held in memory beyond the basic PerlIOl |
164 | struct, by making a PerlIOl the first member of the layer's struct |
165 | thus: |
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166 | |
167 | typedef struct |
168 | { |
169 | struct _PerlIO base; /* Base "class" info */ |
170 | STDCHAR * buf; /* Start of buffer */ |
171 | STDCHAR * end; /* End of valid part of buffer */ |
172 | STDCHAR * ptr; /* Current position in buffer */ |
173 | Off_t posn; /* Offset of buf into the file */ |
174 | Size_t bufsiz; /* Real size of buffer */ |
175 | IV oneword; /* Emergency buffer */ |
176 | } PerlIOBuf; |
177 | |
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178 | In this way (as for perl's scalars) a pointer to a PerlIOBuf can be |
179 | treated as a pointer to a PerlIOl. |
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180 | |
181 | =head2 Layers in action. |
182 | |
183 | table perlio unix |
184 | | | |
185 | +-----------+ +----------+ +--------+ |
186 | PerlIO ->| |--->| next |--->| NULL | |
187 | +-----------+ +----------+ +--------+ |
188 | | | | buffer | | fd | |
189 | +-----------+ | | +--------+ |
190 | | | +----------+ |
191 | |
192 | |
193 | The above attempts to show how the layer scheme works in a simple case. |
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194 | The application's C<PerlIO *> points to an entry in the table(s) |
195 | representing open (allocated) handles. For example the first three slots |
196 | in the table correspond to C<stdin>,C<stdout> and C<stderr>. The table |
197 | in turn points to the current "top" layer for the handle - in this case |
198 | an instance of the generic buffering layer "perlio". That layer in turn |
199 | points to the next layer down - in this case the lowlevel "unix" layer. |
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200 | |
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201 | The above is roughly equivalent to a "stdio" buffered stream, but with |
202 | much more flexibility: |
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203 | |
204 | =over 4 |
205 | |
206 | =item * |
207 | |
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208 | If Unix level C<read>/C<write>/C<lseek> is not appropriate for (say) |
209 | sockets then the "unix" layer can be replaced (at open time or even |
210 | dynamically) with a "socket" layer. |
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211 | |
212 | =item * |
213 | |
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214 | Different handles can have different buffering schemes. The "top" |
215 | layer could be the "mmap" layer if reading disk files was quicker |
216 | using C<mmap> than C<read>. An "unbuffered" stream can be implemented |
217 | simply by not having a buffer layer. |
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218 | |
219 | =item * |
220 | |
221 | Extra layers can be inserted to process the data as it flows through. |
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222 | This was the driving need for including the scheme in perl 5.7.0+ - we |
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223 | needed a mechanism to allow data to be translated between perl's |
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224 | internal encoding (conceptually at least Unicode as UTF-8), and the |
225 | "native" format used by the system. This is provided by the |
226 | ":encoding(xxxx)" layer which typically sits above the buffering layer. |
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227 | |
228 | =item * |
229 | |
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230 | A layer can be added that does "\n" to CRLF translation. This layer |
231 | can be used on any platform, not just those that normally do such |
232 | things. |
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233 | |
234 | =back |
235 | |
236 | =head2 Per-instance flag bits |
237 | |
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238 | The generic flag bits are a hybrid of C<O_XXXXX> style flags deduced |
239 | from the mode string passed to C<PerlIO_open()>, and state bits for |
240 | typical buffer layers. |
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241 | |
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242 | =over 4 |
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243 | |
244 | =item PERLIO_F_EOF |
245 | |
246 | End of file. |
247 | |
248 | =item PERLIO_F_CANWRITE |
249 | |
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250 | Writes are permitted, i.e. opened as "w" or "r+" or "a", etc. |
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251 | |
252 | =item PERLIO_F_CANREAD |
253 | |
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254 | Reads are permitted i.e. opened "r" or "w+" (or even "a+" - ick). |
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255 | |
256 | =item PERLIO_F_ERROR |
257 | |
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258 | An error has occurred (for C<PerlIO_error()>) |
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259 | |
260 | =item PERLIO_F_TRUNCATE |
261 | |
262 | Truncate file suggested by open mode. |
263 | |
264 | =item PERLIO_F_APPEND |
265 | |
266 | All writes should be appends. |
267 | |
268 | =item PERLIO_F_CRLF |
269 | |
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270 | Layer is performing Win32-like "\n" mapped to CR,LF for output and CR,LF |
271 | mapped to "\n" for input. Normally the provided "crlf" layer is the only |
272 | layer that need bother about this. C<PerlIO_binmode()> will mess with this |
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273 | flag rather than add/remove layers if the C<PERLIO_K_CANCRLF> bit is set |
274 | for the layers class. |
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275 | |
276 | =item PERLIO_F_UTF8 |
277 | |
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278 | Data written to this layer should be UTF-8 encoded; data provided |
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279 | by this layer should be considered UTF-8 encoded. Can be set on any layer |
280 | by ":utf8" dummy layer. Also set on ":encoding" layer. |
281 | |
282 | =item PERLIO_F_UNBUF |
283 | |
284 | Layer is unbuffered - i.e. write to next layer down should occur for |
285 | each write to this layer. |
286 | |
287 | =item PERLIO_F_WRBUF |
288 | |
289 | The buffer for this layer currently holds data written to it but not sent |
290 | to next layer. |
291 | |
292 | =item PERLIO_F_RDBUF |
293 | |
294 | The buffer for this layer currently holds unconsumed data read from |
295 | layer below. |
296 | |
297 | =item PERLIO_F_LINEBUF |
298 | |
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299 | Layer is line buffered. Write data should be passed to next layer down |
300 | whenever a "\n" is seen. Any data beyond the "\n" should then be |
301 | processed. |
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302 | |
303 | =item PERLIO_F_TEMP |
304 | |
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305 | File has been C<unlink()>ed, or should be deleted on C<close()>. |
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306 | |
307 | =item PERLIO_F_OPEN |
308 | |
309 | Handle is open. |
310 | |
311 | =item PERLIO_F_FASTGETS |
312 | |
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313 | This instance of this layer supports the "fast C<gets>" interface. |
314 | Normally set based on C<PERLIO_K_FASTGETS> for the class and by the |
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315 | existence of the function(s) in the table. However a class that |
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316 | normally provides that interface may need to avoid it on a |
317 | particular instance. The "pending" layer needs to do this when |
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318 | it is pushed above a layer which does not support the interface. |
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319 | (Perl's C<sv_gets()> does not expect the streams fast C<gets> behaviour |
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320 | to change during one "get".) |
321 | |
322 | =back |
323 | |
324 | =head2 Methods in Detail |
325 | |
326 | =over 4 |
327 | |
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328 | =item IV (*Pushed)(PerlIO *f,const char *mode, SV *arg); |
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329 | |
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330 | The only absolutely mandatory method. Called when the layer is pushed |
331 | onto the stack. The C<mode> argument may be NULL if this occurs |
332 | post-open. The C<arg> will be non-C<NULL> if an argument string was |
333 | passed. In most cases this should call C<PerlIOBase_pushed()> to |
334 | convert C<mode> into the appropriate C<PERLIO_F_XXXXX> flags in |
335 | addition to any actions the layer itself takes. If a layer is not |
336 | expecting an argument it need neither save the one passed to it, nor |
337 | provide C<Getarg()> (it could perhaps C<Perl_warn> that the argument |
338 | was un-expected). |
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339 | |
340 | =item IV (*Popped)(PerlIO *f); |
341 | |
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342 | Called when the layer is popped from the stack. A layer will normally |
343 | be popped after C<Close()> is called. But a layer can be popped |
344 | without being closed if the program is dynamically managing layers on |
345 | the stream. In such cases C<Popped()> should free any resources |
346 | (buffers, translation tables, ...) not held directly in the layer's |
347 | struct. It should also C<Unread()> any unconsumed data that has been |
348 | read and buffered from the layer below back to that layer, so that it |
349 | can be re-provided to what ever is now above. |
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350 | |
351 | =item PerlIO * (*Open)(...); |
352 | |
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353 | The C<Open()> method has lots of arguments because it combines the |
354 | functions of perl's C<open>, C<PerlIO_open>, perl's C<sysopen>, |
355 | C<PerlIO_fdopen> and C<PerlIO_reopen>. The full prototype is as |
356 | follows: |
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357 | |
358 | PerlIO * (*Open)(pTHX_ PerlIO_funcs *tab, |
359 | AV *layers, IV n, |
360 | const char *mode, |
361 | int fd, int imode, int perm, |
362 | PerlIO *old, |
363 | int narg, SV **args); |
364 | |
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365 | Open should (perhaps indirectly) call C<PerlIO_allocate()> to allocate |
366 | a slot in the table and associate it with the layers information for |
367 | the opened file, by calling C<PerlIO_push>. The I<layers> AV is an |
368 | array of all the layers destined for the C<PerlIO *>, and any |
369 | arguments passed to them, I<n> is the index into that array of the |
370 | layer being called. The macro C<PerlIOArg> will return a (possibly |
371 | C<NULL>) SV * for the argument passed to the layer. |
372 | |
373 | The I<mode> string is an "C<fopen()>-like" string which would match |
374 | the regular expression C</^[I#]?[rwa]\+?[bt]?$/>. |
375 | |
376 | The C<'I'> prefix is used during creation of C<stdin>..C<stderr> via |
377 | special C<PerlIO_fdopen> calls; the C<'#'> prefix means that this is |
378 | C<sysopen> and that I<imode> and I<perm> should be passed to |
379 | C<PerlLIO_open3>; C<'r'> means B<r>ead, C<'w'> means B<w>rite and |
380 | C<'a'> means B<a>ppend. The C<'+'> suffix means that both reading and |
381 | writing/appending are permitted. The C<'b'> suffix means file should |
382 | be binary, and C<'t'> means it is text. (Binary/Text should be ignored |
383 | by almost all layers and binary IO done, with PerlIO. The C<:crlf> |
384 | layer should be pushed to handle the distinction.) |
385 | |
386 | If I<old> is not C<NULL> then this is a C<PerlIO_reopen>. Perl itself |
387 | does not use this (yet?) and semantics are a little vague. |
388 | |
389 | If I<fd> not negative then it is the numeric file descriptor I<fd>, |
390 | which will be open in a manner compatible with the supplied mode |
391 | string, the call is thus equivalent to C<PerlIO_fdopen>. In this case |
392 | I<nargs> will be zero. |
393 | |
394 | If I<nargs> is greater than zero then it gives the number of arguments |
395 | passed to C<open>, otherwise it will be 1 if for example |
396 | C<PerlIO_open> was called. In simple cases SvPV_nolen(*args) is the |
397 | pathname to open. |
398 | |
399 | Having said all that translation-only layers do not need to provide |
400 | C<Open()> at all, but rather leave the opening to a lower level layer |
401 | and wait to be "pushed". If a layer does provide C<Open()> it should |
402 | normally call the C<Open()> method of next layer down (if any) and |
403 | then push itself on top if that succeeds. |
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404 | |
405 | =item SV * (*Getarg)(PerlIO *f); |
406 | |
407 | Optional. If present should return an SV * representing the string argument |
408 | passed to the layer when it was pushed. e.g. ":encoding(ascii)" would |
409 | return an SvPV with value "ascii". |
410 | |
411 | =item IV (*Fileno)(PerlIO *f); |
412 | |
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413 | Returns the Unix/Posix numeric file descriptor for the handle. Normally |
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414 | C<PerlIOBase_fileno()> (which just asks next layer down) will suffice |
415 | for this. |
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416 | |
417 | =item SSize_t (*Read)(PerlIO *f, void *vbuf, Size_t count); |
418 | |
419 | Basic read operation. Returns actual bytes read, or -1 on an error. |
420 | Typically will call Fill and manipulate pointers (possibly via the API). |
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421 | C<PerlIOBuf_read()> may be suitable for derived classes which provide |
422 | "fast gets" methods. |
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423 | |
424 | =item SSize_t (*Unread)(PerlIO *f, const void *vbuf, Size_t count); |
425 | |
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426 | A superset of stdio's C<ungetc()>. Should arrange for future reads to |
427 | see the bytes in C<vbuf>. If there is no obviously better implementation |
428 | then C<PerlIOBase_unread()> provides the function by pushing a "fake" |
429 | "pending" layer above the calling layer. |
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430 | |
431 | =item SSize_t (*Write)(PerlIO *f, const void *vbuf, Size_t count); |
432 | |
433 | Basic write operation. Returns bytes written or -1 on an error. |
434 | |
435 | =item IV (*Seek)(PerlIO *f, Off_t offset, int whence); |
436 | |
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437 | Position the file pointer. Should normally call its own C<Flush> |
438 | method and then the C<Seek> method of next layer down. |
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439 | |
440 | =item Off_t (*Tell)(PerlIO *f); |
441 | |
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442 | Return the file pointer. May be based on layers cached concept of |
443 | position to avoid overhead. |
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444 | |
445 | =item IV (*Close)(PerlIO *f); |
446 | |
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447 | Close the stream. Should normally call C<PerlIOBase_close()> to flush |
448 | itself and close layers below, and then deallocate any data structures |
449 | (buffers, translation tables, ...) not held directly in the data |
450 | structure. |
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451 | |
452 | =item IV (*Flush)(PerlIO *f); |
453 | |
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454 | Should make stream's state consistent with layers below. That is, any |
455 | buffered write data should be written, and file position of lower layers |
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456 | adjusted for data read from below but not actually consumed. |
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457 | (Should perhaps C<Unread()> such data to the lower layer.) |
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458 | |
459 | =item IV (*Fill)(PerlIO *f); |
460 | |
461 | The buffer for this layer should be filled (for read) from layer below. |
462 | |
463 | =item IV (*Eof)(PerlIO *f); |
464 | |
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465 | Return end-of-file indicator. C<PerlIOBase_eof()> is normally sufficient. |
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466 | |
467 | =item IV (*Error)(PerlIO *f); |
468 | |
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469 | Return error indicator. C<PerlIOBase_error()> is normally sufficient. |
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470 | |
471 | =item void (*Clearerr)(PerlIO *f); |
472 | |
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473 | Clear end-of-file and error indicators. Should call C<PerlIOBase_clearerr()> |
474 | to set the C<PERLIO_F_XXXXX> flags, which may suffice. |
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475 | |
476 | =item void (*Setlinebuf)(PerlIO *f); |
477 | |
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478 | Mark the stream as line buffered. C<PerlIOBase_setlinebuf()> sets the |
479 | PERLIO_F_LINEBUF flag and is normally sufficient. |
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480 | |
481 | =item STDCHAR * (*Get_base)(PerlIO *f); |
482 | |
483 | Allocate (if not already done so) the read buffer for this layer and |
484 | return pointer to it. |
485 | |
486 | =item Size_t (*Get_bufsiz)(PerlIO *f); |
487 | |
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488 | Return the number of bytes that last C<Fill()> put in the buffer. |
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489 | |
490 | =item STDCHAR * (*Get_ptr)(PerlIO *f); |
491 | |
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492 | Return the current read pointer relative to this layer's buffer. |
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493 | |
494 | =item SSize_t (*Get_cnt)(PerlIO *f); |
495 | |
496 | Return the number of bytes left to be read in the current buffer. |
497 | |
498 | =item void (*Set_ptrcnt)(PerlIO *f,STDCHAR *ptr,SSize_t cnt); |
499 | |
500 | Adjust the read pointer and count of bytes to match C<ptr> and/or C<cnt>. |
501 | The application (or layer above) must ensure they are consistent. |
502 | (Checking is allowed by the paranoid.) |
503 | |
504 | =back |
505 | |
506 | |
507 | =head2 Core Layers |
508 | |
509 | The file C<perlio.c> provides the following layers: |
510 | |
511 | =over 4 |
512 | |
513 | =item "unix" |
514 | |
9d799145 |
515 | A basic non-buffered layer which calls Unix/POSIX C<read()>, C<write()>, |
516 | C<lseek()>, C<close()>. No buffering. Even on platforms that distinguish |
517 | between O_TEXT and O_BINARY this layer is always O_BINARY. |
50b80e25 |
518 | |
519 | =item "perlio" |
520 | |
9d799145 |
521 | A very complete generic buffering layer which provides the whole of |
522 | PerlIO API. It is also intended to be used as a "base class" for other |
1d11c889 |
523 | layers. (For example its C<Read()> method is implemented in terms of |
524 | the C<Get_cnt()>/C<Get_ptr()>/C<Set_ptrcnt()> methods). |
50b80e25 |
525 | |
9d799145 |
526 | "perlio" over "unix" provides a complete replacement for stdio as seen |
527 | via PerlIO API. This is the default for USE_PERLIO when system's stdio |
1d11c889 |
528 | does not permit perl's "fast gets" access, and which do not |
529 | distinguish between C<O_TEXT> and C<O_BINARY>. |
50b80e25 |
530 | |
531 | =item "stdio" |
532 | |
9d799145 |
533 | A layer which provides the PerlIO API via the layer scheme, but |
534 | implements it by calling system's stdio. This is (currently) the default |
535 | if system's stdio provides sufficient access to allow perl's "fast gets" |
536 | access and which do not distinguish between C<O_TEXT> and C<O_BINARY>. |
50b80e25 |
537 | |
538 | =item "crlf" |
539 | |
9d799145 |
540 | A layer derived using "perlio" as a base class. It provides Win32-like |
541 | "\n" to CR,LF translation. Can either be applied above "perlio" or serve |
542 | as the buffer layer itself. "crlf" over "unix" is the default if system |
543 | distinguishes between C<O_TEXT> and C<O_BINARY> opens. (At some point |
544 | "unix" will be replaced by a "native" Win32 IO layer on that platform, |
545 | as Win32's read/write layer has various drawbacks.) The "crlf" layer is |
546 | a reasonable model for a layer which transforms data in some way. |
50b80e25 |
547 | |
548 | =item "mmap" |
549 | |
9d799145 |
550 | If Configure detects C<mmap()> functions this layer is provided (with |
551 | "perlio" as a "base") which does "read" operations by mmap()ing the |
552 | file. Performance improvement is marginal on modern systems, so it is |
553 | mainly there as a proof of concept. It is likely to be unbundled from |
554 | the core at some point. The "mmap" layer is a reasonable model for a |
555 | minimalist "derived" layer. |
50b80e25 |
556 | |
557 | =item "pending" |
558 | |
9d799145 |
559 | An "internal" derivative of "perlio" which can be used to provide |
1d11c889 |
560 | Unread() function for layers which have no buffer or cannot be |
561 | bothered. (Basically this layer's C<Fill()> pops itself off the stack |
562 | and so resumes reading from layer below.) |
50b80e25 |
563 | |
564 | =item "raw" |
565 | |
9d799145 |
566 | A dummy layer which never exists on the layer stack. Instead when |
567 | "pushed" it actually pops the stack(!), removing itself, and any other |
568 | layers until it reaches a layer with the class C<PERLIO_K_RAW> bit set. |
50b80e25 |
569 | |
570 | =item "utf8" |
571 | |
9d799145 |
572 | Another dummy layer. When pushed it pops itself and sets the |
1d11c889 |
573 | C<PERLIO_F_UTF8> flag on the layer which was (and now is once more) |
574 | the top of the stack. |
50b80e25 |
575 | |
576 | =back |
577 | |
9d799145 |
578 | In addition F<perlio.c> also provides a number of C<PerlIOBase_xxxx()> |
579 | functions which are intended to be used in the table slots of classes |
580 | which do not need to do anything special for a particular method. |
50b80e25 |
581 | |
582 | =head2 Extension Layers |
583 | |
1d11c889 |
584 | Layers can made available by extension modules. When an unknown layer |
585 | is encountered the PerlIO code will perform the equivalent of : |
b76cc8ba |
586 | |
587 | use PerlIO 'layer'; |
588 | |
1d11c889 |
589 | Where I<layer> is the unknown layer. F<PerlIO.pm> will then attempt to: |
b76cc8ba |
590 | |
591 | require PerlIO::layer; |
592 | |
1d11c889 |
593 | If after that process the layer is still not defined then the C<open> |
594 | will fail. |
b76cc8ba |
595 | |
596 | The following extension layers are bundled with perl: |
50b80e25 |
597 | |
598 | =over 4 |
599 | |
b76cc8ba |
600 | =item ":encoding" |
50b80e25 |
601 | |
602 | use Encoding; |
603 | |
1d11c889 |
604 | makes this layer available, although F<PerlIO.pm> "knows" where to |
605 | find it. It is an example of a layer which takes an argument as it is |
606 | called thus: |
50b80e25 |
607 | |
608 | open($fh,"<:encoding(iso-8859-7)",$pathname) |
609 | |
b76cc8ba |
610 | =item ":Scalar" |
611 | |
612 | Provides support for |
613 | |
614 | open($fh,"...",\$scalar) |
50b80e25 |
615 | |
1d11c889 |
616 | When a handle is so opened, then reads get bytes from the string value |
617 | of I<$scalar>, and writes change the value. In both cases the position |
618 | in I<$scalar> starts as zero but can be altered via C<seek>, and |
619 | determined via C<tell>. |
b76cc8ba |
620 | |
621 | =item ":Object" or ":Perl" |
622 | |
1d11c889 |
623 | May be provided to allow layers to be implemented as perl code - |
624 | implementation is being investigated. |
b76cc8ba |
625 | |
626 | =back |
50b80e25 |
627 | |
628 | =cut |
629 | |
630 | |
631 | |