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1 | =head1 NAME |
2 | |
3 | perliol - C API for Perl's implementation of IO in Layers. |
4 | |
5 | =head1 SYNOPSIS |
6 | |
7 | /* Defining a layer ... */ |
8 | #include <perliol.h> |
9 | |
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10 | =head1 DESCRIPTION |
11 | |
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12 | This document describes the behavior and implementation of the PerlIO |
13 | abstraction described in L<perlapio> when C<USE_PERLIO> is defined (and |
14 | C<USE_SFIO> is not). |
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15 | |
16 | =head2 History and Background |
17 | |
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18 | The PerlIO abstraction was introduced in perl5.003_02 but languished as |
19 | just an abstraction until perl5.7.0. However during that time a number |
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20 | of perl extensions switched to using it, so the API is mostly fixed to |
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21 | maintain (source) compatibility. |
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22 | |
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23 | The aim of the implementation is to provide the PerlIO API in a flexible |
24 | and platform neutral manner. It is also a trial of an "Object Oriented |
25 | C, with vtables" approach which may be applied to perl6. |
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26 | |
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27 | =head2 Basic Structure |
28 | |
29 | PerlIO is as a stack of layers. |
30 | |
31 | The low levels of the stack work with the low-level operating system |
32 | calls (file descriptors in C) getting bytes in and out, the higher |
33 | layers of the stack buffer, filter, and otherwise manipulate the I/O. |
34 | Terms I<above> and I<below> are used to refer to the relative |
35 | positioning of the stack layers. |
36 | |
37 | A layer contains a "vtable", the table of I/O operations (at C level |
38 | a table of function pointers), and status flags. The functions in the |
39 | vtable implement operations like "open", "read", and "write". |
40 | |
41 | When I/O, for example "read", is requested, the request goes from Perl |
42 | first down the stack using "read" functions of each layer, then at the |
43 | bottom the input is requested from the operating system services, then |
44 | the result is returned up the stack, finally being interpreted as Perl |
45 | data. |
46 | |
47 | When you do an open() and specify extra PerlIO layers to be deployed, |
48 | the layers you specify are "pushed" on top of the already existing |
49 | default stack. What exact layers are in this default stack depends on |
50 | a lot of things: your operating system, Perl version, Perl compile |
51 | time configuration, and Perl runtime configuration. See L<PerlIO>, |
52 | L<perlrun/PERLIO>, and L<open> for more information. |
53 | |
54 | binmode() operates similarly to open(): by default the specified |
55 | layers are pushed on top of the existing stack. |
56 | |
57 | However, note that even as the specified layers are "pushed on top" |
58 | for open() and binmode(), this doesn't mean that the effects are |
59 | limited to the "top": PerlIO layers can be very 'active' and inspect |
60 | and affect layers also deeper in the stack. As an example there |
61 | is a layer called "raw" which repeatedly "pops" layers until |
62 | it reaches the first layer that has declared itself capable of |
63 | handling binary data. The "pushed" layers are processed in left-to-right |
64 | order. |
65 | |
66 | sysopen() operates (unsurprisingly) at a lower level in the stack than |
67 | open(). For example in UNIX or UNIX-like systems sysopen() operates |
68 | directly at the level of file descriptors: in the terms of PerlIO |
69 | layers, it uses only the "unix" layer, which is a rather thin wrapper |
70 | on top of the UNIX file descriptors. |
71 | |
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72 | =head2 Layers vs Disciplines |
73 | |
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74 | Initial discussion of the ability to modify IO streams behaviour used |
75 | the term "discipline" for the entities which were added. This came (I |
76 | believe) from the use of the term in "sfio", which in turn borrowed it |
77 | from "line disciplines" on Unix terminals. However, this document (and |
78 | the C code) uses the term "layer". |
79 | |
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80 | This is, I hope, a natural term given the implementation, and should |
81 | avoid connotations that are inherent in earlier uses of "discipline" |
82 | for things which are rather different. |
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83 | |
84 | =head2 Data Structures |
85 | |
86 | The basic data structure is a PerlIOl: |
87 | |
88 | typedef struct _PerlIO PerlIOl; |
89 | typedef struct _PerlIO_funcs PerlIO_funcs; |
90 | typedef PerlIOl *PerlIO; |
91 | |
92 | struct _PerlIO |
93 | { |
94 | PerlIOl * next; /* Lower layer */ |
95 | PerlIO_funcs * tab; /* Functions for this layer */ |
96 | IV flags; /* Various flags for state */ |
97 | }; |
98 | |
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99 | A C<PerlIOl *> is a pointer to the struct, and the I<application> |
100 | level C<PerlIO *> is a pointer to a C<PerlIOl *> - i.e. a pointer |
101 | to a pointer to the struct. This allows the application level C<PerlIO *> |
102 | to remain constant while the actual C<PerlIOl *> underneath |
103 | changes. (Compare perl's C<SV *> which remains constant while its |
104 | C<sv_any> field changes as the scalar's type changes.) An IO stream is |
105 | then in general represented as a pointer to this linked-list of |
106 | "layers". |
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107 | |
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108 | It should be noted that because of the double indirection in a C<PerlIO *>, |
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109 | a C<< &(perlio->next) >> "is" a C<PerlIO *>, and so to some degree |
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110 | at least one layer can use the "standard" API on the next layer down. |
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111 | |
112 | A "layer" is composed of two parts: |
113 | |
114 | =over 4 |
115 | |
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116 | =item 1. |
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117 | |
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118 | The functions and attributes of the "layer class". |
119 | |
120 | =item 2. |
121 | |
122 | The per-instance data for a particular handle. |
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123 | |
124 | =back |
125 | |
126 | =head2 Functions and Attributes |
127 | |
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128 | The functions and attributes are accessed via the "tab" (for table) |
129 | member of C<PerlIOl>. The functions (methods of the layer "class") are |
130 | fixed, and are defined by the C<PerlIO_funcs> type. They are broadly the |
131 | same as the public C<PerlIO_xxxxx> functions: |
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132 | |
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133 | struct _PerlIO_funcs |
134 | { |
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135 | Size_t fsize; |
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136 | char * name; |
137 | Size_t size; |
138 | IV kind; |
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139 | IV (*Pushed)(pTHX_ PerlIO *f,const char *mode,SV *arg, PerlIO_funcs *tab); |
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140 | IV (*Popped)(pTHX_ PerlIO *f); |
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141 | PerlIO * (*Open)(pTHX_ PerlIO_funcs *tab, |
142 | AV *layers, IV n, |
143 | const char *mode, |
144 | int fd, int imode, int perm, |
145 | PerlIO *old, |
146 | int narg, SV **args); |
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147 | IV (*Binmode)(pTHX_ PerlIO *f); |
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148 | SV * (*Getarg)(pTHX_ PerlIO *f, CLONE_PARAMS *param, int flags) |
149 | IV (*Fileno)(pTHX_ PerlIO *f); |
150 | PerlIO * (*Dup)(pTHX_ PerlIO *f, PerlIO *o, CLONE_PARAMS *param, int flags) |
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151 | /* Unix-like functions - cf sfio line disciplines */ |
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152 | SSize_t (*Read)(pTHX_ PerlIO *f, void *vbuf, Size_t count); |
153 | SSize_t (*Unread)(pTHX_ PerlIO *f, const void *vbuf, Size_t count); |
154 | SSize_t (*Write)(pTHX_ PerlIO *f, const void *vbuf, Size_t count); |
155 | IV (*Seek)(pTHX_ PerlIO *f, Off_t offset, int whence); |
156 | Off_t (*Tell)(pTHX_ PerlIO *f); |
157 | IV (*Close)(pTHX_ PerlIO *f); |
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158 | /* Stdio-like buffered IO functions */ |
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159 | IV (*Flush)(pTHX_ PerlIO *f); |
160 | IV (*Fill)(pTHX_ PerlIO *f); |
161 | IV (*Eof)(pTHX_ PerlIO *f); |
162 | IV (*Error)(pTHX_ PerlIO *f); |
163 | void (*Clearerr)(pTHX_ PerlIO *f); |
164 | void (*Setlinebuf)(pTHX_ PerlIO *f); |
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165 | /* Perl's snooping functions */ |
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166 | STDCHAR * (*Get_base)(pTHX_ PerlIO *f); |
167 | Size_t (*Get_bufsiz)(pTHX_ PerlIO *f); |
168 | STDCHAR * (*Get_ptr)(pTHX_ PerlIO *f); |
169 | SSize_t (*Get_cnt)(pTHX_ PerlIO *f); |
170 | void (*Set_ptrcnt)(pTHX_ PerlIO *f,STDCHAR *ptr,SSize_t cnt); |
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171 | }; |
172 | |
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173 | The first few members of the struct give a function table size for |
174 | compatibility check "name" for the layer, the size to C<malloc> for the per-instance data, |
175 | and some flags which are attributes of the class as whole (such as whether it is a buffering |
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176 | layer), then follow the functions which fall into four basic groups: |
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177 | |
178 | =over 4 |
179 | |
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180 | =item 1. |
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181 | |
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182 | Opening and setup functions |
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183 | |
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184 | =item 2. |
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185 | |
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186 | Basic IO operations |
187 | |
188 | =item 3. |
189 | |
190 | Stdio class buffering options. |
191 | |
192 | =item 4. |
193 | |
194 | Functions to support Perl's traditional "fast" access to the buffer. |
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195 | |
196 | =back |
197 | |
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198 | A layer does not have to implement all the functions, but the whole |
199 | table has to be present. Unimplemented slots can be NULL (which will |
200 | result in an error when called) or can be filled in with stubs to |
201 | "inherit" behaviour from a "base class". This "inheritance" is fixed |
202 | for all instances of the layer, but as the layer chooses which stubs |
203 | to populate the table, limited "multiple inheritance" is possible. |
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204 | |
205 | =head2 Per-instance Data |
206 | |
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207 | The per-instance data are held in memory beyond the basic PerlIOl |
208 | struct, by making a PerlIOl the first member of the layer's struct |
209 | thus: |
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210 | |
211 | typedef struct |
212 | { |
213 | struct _PerlIO base; /* Base "class" info */ |
214 | STDCHAR * buf; /* Start of buffer */ |
215 | STDCHAR * end; /* End of valid part of buffer */ |
216 | STDCHAR * ptr; /* Current position in buffer */ |
217 | Off_t posn; /* Offset of buf into the file */ |
218 | Size_t bufsiz; /* Real size of buffer */ |
219 | IV oneword; /* Emergency buffer */ |
220 | } PerlIOBuf; |
221 | |
1d11c889 |
222 | In this way (as for perl's scalars) a pointer to a PerlIOBuf can be |
223 | treated as a pointer to a PerlIOl. |
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224 | |
225 | =head2 Layers in action. |
226 | |
227 | table perlio unix |
228 | | | |
229 | +-----------+ +----------+ +--------+ |
230 | PerlIO ->| |--->| next |--->| NULL | |
231 | +-----------+ +----------+ +--------+ |
232 | | | | buffer | | fd | |
233 | +-----------+ | | +--------+ |
234 | | | +----------+ |
235 | |
236 | |
237 | The above attempts to show how the layer scheme works in a simple case. |
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238 | The application's C<PerlIO *> points to an entry in the table(s) |
239 | representing open (allocated) handles. For example the first three slots |
240 | in the table correspond to C<stdin>,C<stdout> and C<stderr>. The table |
241 | in turn points to the current "top" layer for the handle - in this case |
242 | an instance of the generic buffering layer "perlio". That layer in turn |
243 | points to the next layer down - in this case the lowlevel "unix" layer. |
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244 | |
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245 | The above is roughly equivalent to a "stdio" buffered stream, but with |
246 | much more flexibility: |
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247 | |
248 | =over 4 |
249 | |
250 | =item * |
251 | |
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252 | If Unix level C<read>/C<write>/C<lseek> is not appropriate for (say) |
253 | sockets then the "unix" layer can be replaced (at open time or even |
254 | dynamically) with a "socket" layer. |
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255 | |
256 | =item * |
257 | |
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258 | Different handles can have different buffering schemes. The "top" |
259 | layer could be the "mmap" layer if reading disk files was quicker |
260 | using C<mmap> than C<read>. An "unbuffered" stream can be implemented |
261 | simply by not having a buffer layer. |
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262 | |
263 | =item * |
264 | |
265 | Extra layers can be inserted to process the data as it flows through. |
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266 | This was the driving need for including the scheme in perl 5.7.0+ - we |
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267 | needed a mechanism to allow data to be translated between perl's |
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268 | internal encoding (conceptually at least Unicode as UTF-8), and the |
269 | "native" format used by the system. This is provided by the |
270 | ":encoding(xxxx)" layer which typically sits above the buffering layer. |
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271 | |
272 | =item * |
273 | |
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274 | A layer can be added that does "\n" to CRLF translation. This layer |
275 | can be used on any platform, not just those that normally do such |
276 | things. |
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277 | |
278 | =back |
279 | |
280 | =head2 Per-instance flag bits |
281 | |
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282 | The generic flag bits are a hybrid of C<O_XXXXX> style flags deduced |
283 | from the mode string passed to C<PerlIO_open()>, and state bits for |
284 | typical buffer layers. |
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285 | |
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286 | =over 4 |
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287 | |
288 | =item PERLIO_F_EOF |
289 | |
290 | End of file. |
291 | |
292 | =item PERLIO_F_CANWRITE |
293 | |
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294 | Writes are permitted, i.e. opened as "w" or "r+" or "a", etc. |
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295 | |
296 | =item PERLIO_F_CANREAD |
297 | |
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298 | Reads are permitted i.e. opened "r" or "w+" (or even "a+" - ick). |
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299 | |
300 | =item PERLIO_F_ERROR |
301 | |
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302 | An error has occurred (for C<PerlIO_error()>). |
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303 | |
304 | =item PERLIO_F_TRUNCATE |
305 | |
306 | Truncate file suggested by open mode. |
307 | |
308 | =item PERLIO_F_APPEND |
309 | |
310 | All writes should be appends. |
311 | |
312 | =item PERLIO_F_CRLF |
313 | |
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314 | Layer is performing Win32-like "\n" mapped to CR,LF for output and CR,LF |
315 | mapped to "\n" for input. Normally the provided "crlf" layer is the only |
316 | layer that need bother about this. C<PerlIO_binmode()> will mess with this |
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317 | flag rather than add/remove layers if the C<PERLIO_K_CANCRLF> bit is set |
318 | for the layers class. |
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319 | |
320 | =item PERLIO_F_UTF8 |
321 | |
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322 | Data written to this layer should be UTF-8 encoded; data provided |
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323 | by this layer should be considered UTF-8 encoded. Can be set on any layer |
324 | by ":utf8" dummy layer. Also set on ":encoding" layer. |
325 | |
326 | =item PERLIO_F_UNBUF |
327 | |
328 | Layer is unbuffered - i.e. write to next layer down should occur for |
329 | each write to this layer. |
330 | |
331 | =item PERLIO_F_WRBUF |
332 | |
333 | The buffer for this layer currently holds data written to it but not sent |
334 | to next layer. |
335 | |
336 | =item PERLIO_F_RDBUF |
337 | |
338 | The buffer for this layer currently holds unconsumed data read from |
339 | layer below. |
340 | |
341 | =item PERLIO_F_LINEBUF |
342 | |
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343 | Layer is line buffered. Write data should be passed to next layer down |
344 | whenever a "\n" is seen. Any data beyond the "\n" should then be |
345 | processed. |
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346 | |
347 | =item PERLIO_F_TEMP |
348 | |
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349 | File has been C<unlink()>ed, or should be deleted on C<close()>. |
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350 | |
351 | =item PERLIO_F_OPEN |
352 | |
353 | Handle is open. |
354 | |
355 | =item PERLIO_F_FASTGETS |
356 | |
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357 | This instance of this layer supports the "fast C<gets>" interface. |
358 | Normally set based on C<PERLIO_K_FASTGETS> for the class and by the |
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359 | existence of the function(s) in the table. However a class that |
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360 | normally provides that interface may need to avoid it on a |
361 | particular instance. The "pending" layer needs to do this when |
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362 | it is pushed above a layer which does not support the interface. |
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363 | (Perl's C<sv_gets()> does not expect the streams fast C<gets> behaviour |
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364 | to change during one "get".) |
365 | |
366 | =back |
367 | |
368 | =head2 Methods in Detail |
369 | |
370 | =over 4 |
371 | |
e2d9456f |
372 | =item fsize |
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373 | |
374 | Size_t fsize; |
375 | |
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376 | Size of the function table. This is compared against the value PerlIO |
377 | code "knows" as a compatibility check. Future versions I<may> be able |
378 | to tolerate layers compiled against an old version of the headers. |
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379 | |
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380 | =item name |
381 | |
382 | char * name; |
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383 | |
384 | The name of the layer whose open() method Perl should invoke on |
385 | open(). For example if the layer is called APR, you will call: |
386 | |
387 | open $fh, ">:APR", ... |
388 | |
389 | and Perl knows that it has to invoke the PerlIOAPR_open() method |
390 | implemented by the APR layer. |
391 | |
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392 | =item size |
393 | |
394 | Size_t size; |
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395 | |
396 | The size of the per-instance data structure, e.g.: |
397 | |
398 | sizeof(PerlIOAPR) |
399 | |
a489db4d |
400 | If this field is zero then C<PerlIO_pushed> does not malloc anything |
401 | and assumes layer's Pushed function will do any required layer stack |
402 | manipulation - used to avoid malloc/free overhead for dummy layers. |
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403 | If the field is non-zero it must be at least the size of C<PerlIOl>, |
404 | C<PerlIO_pushed> will allocate memory for the layer's data structures |
405 | and link new layer onto the stream's stack. (If the layer's Pushed |
406 | method returns an error indication the layer is popped again.) |
407 | |
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408 | =item kind |
409 | |
410 | IV kind; |
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411 | |
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412 | =over 4 |
413 | |
414 | =item * PERLIO_K_BUFFERED |
415 | |
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416 | The layer is buffered. |
417 | |
418 | =item * PERLIO_K_RAW |
419 | |
420 | The layer is acceptable to have in a binmode(FH) stack - i.e. it does not |
421 | (or will configure itself not to) transform bytes passing through it. |
422 | |
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423 | =item * PERLIO_K_CANCRLF |
424 | |
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425 | Layer can translate between "\n" and CRLF line ends. |
426 | |
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427 | =item * PERLIO_K_FASTGETS |
428 | |
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429 | Layer allows buffer snooping. |
430 | |
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431 | =item * PERLIO_K_MULTIARG |
432 | |
433 | Used when the layer's open() accepts more arguments than usual. The |
434 | extra arguments should come not before the C<MODE> argument. When this |
435 | flag is used it's up to the layer to validate the args. |
436 | |
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437 | =back |
438 | |
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439 | =item Pushed |
440 | |
441 | IV (*Pushed)(pTHX_ PerlIO *f,const char *mode, SV *arg); |
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442 | |
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443 | The only absolutely mandatory method. Called when the layer is pushed |
444 | onto the stack. The C<mode> argument may be NULL if this occurs |
445 | post-open. The C<arg> will be non-C<NULL> if an argument string was |
446 | passed. In most cases this should call C<PerlIOBase_pushed()> to |
447 | convert C<mode> into the appropriate C<PERLIO_F_XXXXX> flags in |
448 | addition to any actions the layer itself takes. If a layer is not |
449 | expecting an argument it need neither save the one passed to it, nor |
450 | provide C<Getarg()> (it could perhaps C<Perl_warn> that the argument |
451 | was un-expected). |
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452 | |
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453 | Returns 0 on success. On failure returns -1 and should set errno. |
454 | |
5cb3728c |
455 | =item Popped |
456 | |
457 | IV (*Popped)(pTHX_ PerlIO *f); |
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458 | |
1d11c889 |
459 | Called when the layer is popped from the stack. A layer will normally |
460 | be popped after C<Close()> is called. But a layer can be popped |
461 | without being closed if the program is dynamically managing layers on |
462 | the stream. In such cases C<Popped()> should free any resources |
463 | (buffers, translation tables, ...) not held directly in the layer's |
464 | struct. It should also C<Unread()> any unconsumed data that has been |
465 | read and buffered from the layer below back to that layer, so that it |
466 | can be re-provided to what ever is now above. |
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467 | |
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468 | Returns 0 on success and failure. If C<Popped()> returns I<true> then |
469 | I<perlio.c> assumes that either the layer has popped itself, or the |
470 | layer is super special and needs to be retained for other reasons. |
471 | In most cases it should return I<false>. |
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472 | |
5cb3728c |
473 | =item Open |
474 | |
475 | PerlIO * (*Open)(...); |
b76cc8ba |
476 | |
1d11c889 |
477 | The C<Open()> method has lots of arguments because it combines the |
478 | functions of perl's C<open>, C<PerlIO_open>, perl's C<sysopen>, |
479 | C<PerlIO_fdopen> and C<PerlIO_reopen>. The full prototype is as |
480 | follows: |
b76cc8ba |
481 | |
482 | PerlIO * (*Open)(pTHX_ PerlIO_funcs *tab, |
483 | AV *layers, IV n, |
484 | const char *mode, |
485 | int fd, int imode, int perm, |
486 | PerlIO *old, |
487 | int narg, SV **args); |
488 | |
1d11c889 |
489 | Open should (perhaps indirectly) call C<PerlIO_allocate()> to allocate |
490 | a slot in the table and associate it with the layers information for |
491 | the opened file, by calling C<PerlIO_push>. The I<layers> AV is an |
492 | array of all the layers destined for the C<PerlIO *>, and any |
493 | arguments passed to them, I<n> is the index into that array of the |
494 | layer being called. The macro C<PerlIOArg> will return a (possibly |
495 | C<NULL>) SV * for the argument passed to the layer. |
496 | |
497 | The I<mode> string is an "C<fopen()>-like" string which would match |
498 | the regular expression C</^[I#]?[rwa]\+?[bt]?$/>. |
499 | |
500 | The C<'I'> prefix is used during creation of C<stdin>..C<stderr> via |
501 | special C<PerlIO_fdopen> calls; the C<'#'> prefix means that this is |
502 | C<sysopen> and that I<imode> and I<perm> should be passed to |
503 | C<PerlLIO_open3>; C<'r'> means B<r>ead, C<'w'> means B<w>rite and |
504 | C<'a'> means B<a>ppend. The C<'+'> suffix means that both reading and |
a489db4d |
505 | writing/appending are permitted. The C<'b'> suffix means file should |
506 | be binary, and C<'t'> means it is text. (Almost all layers should do |
507 | the IO in binary mode, and ignore the b/t bits. The C<:crlf> layer |
508 | should be pushed to handle the distinction.) |
1d11c889 |
509 | |
510 | If I<old> is not C<NULL> then this is a C<PerlIO_reopen>. Perl itself |
511 | does not use this (yet?) and semantics are a little vague. |
512 | |
513 | If I<fd> not negative then it is the numeric file descriptor I<fd>, |
514 | which will be open in a manner compatible with the supplied mode |
515 | string, the call is thus equivalent to C<PerlIO_fdopen>. In this case |
516 | I<nargs> will be zero. |
517 | |
518 | If I<nargs> is greater than zero then it gives the number of arguments |
519 | passed to C<open>, otherwise it will be 1 if for example |
520 | C<PerlIO_open> was called. In simple cases SvPV_nolen(*args) is the |
521 | pathname to open. |
522 | |
523 | Having said all that translation-only layers do not need to provide |
524 | C<Open()> at all, but rather leave the opening to a lower level layer |
525 | and wait to be "pushed". If a layer does provide C<Open()> it should |
526 | normally call the C<Open()> method of next layer down (if any) and |
527 | then push itself on top if that succeeds. |
b76cc8ba |
528 | |
3077d0b1 |
529 | If C<PerlIO_push> was performed and open has failed, it must |
530 | C<PerlIO_pop> itself, since if it's not, the layer won't be removed |
531 | and may cause bad problems. |
532 | |
d4165bde |
533 | Returns C<NULL> on failure. |
534 | |
86e05cf2 |
535 | =item Binmode |
536 | |
537 | IV (*Binmode)(pTHX_ PerlIO *f); |
538 | |
539 | Optional. Used when C<:raw> layer is pushed (explicitly or as a result |
540 | of binmode(FH)). If not present layer will be popped. If present |
541 | should configure layer as binary (or pop itself) and return 0. |
542 | If it returns -1 for error C<binmode> will fail with layer |
543 | still on the stack. |
544 | |
5cb3728c |
545 | =item Getarg |
546 | |
547 | SV * (*Getarg)(pTHX_ PerlIO *f, |
548 | CLONE_PARAMS *param, int flags); |
b76cc8ba |
549 | |
d4165bde |
550 | Optional. If present should return an SV * representing the string |
551 | argument passed to the layer when it was |
552 | pushed. e.g. ":encoding(ascii)" would return an SvPV with value |
553 | "ascii". (I<param> and I<flags> arguments can be ignored in most |
554 | cases) |
b76cc8ba |
555 | |
5cb3728c |
556 | =item Fileno |
557 | |
558 | IV (*Fileno)(pTHX_ PerlIO *f); |
b76cc8ba |
559 | |
d1be9408 |
560 | Returns the Unix/Posix numeric file descriptor for the handle. Normally |
b76cc8ba |
561 | C<PerlIOBase_fileno()> (which just asks next layer down) will suffice |
562 | for this. |
50b80e25 |
563 | |
a489db4d |
564 | Returns -1 on error, which is considered to include the case where the |
565 | layer cannot provide such a file descriptor. |
d4165bde |
566 | |
5cb3728c |
567 | =item Dup |
568 | |
569 | PerlIO * (*Dup)(pTHX_ PerlIO *f, PerlIO *o, |
570 | CLONE_PARAMS *param, int flags); |
d4165bde |
571 | |
2dc2558e |
572 | XXX: Needs more docs. |
573 | |
a489db4d |
574 | Used as part of the "clone" process when a thread is spawned (in which |
575 | case param will be non-NULL) and when a stream is being duplicated via |
576 | '&' in the C<open>. |
d4165bde |
577 | |
578 | Similar to C<Open>, returns PerlIO* on success, C<NULL> on failure. |
579 | |
5cb3728c |
580 | =item Read |
581 | |
582 | SSize_t (*Read)(pTHX_ PerlIO *f, void *vbuf, Size_t count); |
d4165bde |
583 | |
584 | Basic read operation. |
50b80e25 |
585 | |
d4165bde |
586 | Typically will call C<Fill> and manipulate pointers (possibly via the |
587 | API). C<PerlIOBuf_read()> may be suitable for derived classes which |
588 | provide "fast gets" methods. |
50b80e25 |
589 | |
d4165bde |
590 | Returns actual bytes read, or -1 on an error. |
591 | |
5cb3728c |
592 | =item Unread |
593 | |
594 | SSize_t (*Unread)(pTHX_ PerlIO *f, |
595 | const void *vbuf, Size_t count); |
50b80e25 |
596 | |
9d799145 |
597 | A superset of stdio's C<ungetc()>. Should arrange for future reads to |
598 | see the bytes in C<vbuf>. If there is no obviously better implementation |
599 | then C<PerlIOBase_unread()> provides the function by pushing a "fake" |
600 | "pending" layer above the calling layer. |
50b80e25 |
601 | |
d4165bde |
602 | Returns the number of unread chars. |
603 | |
5cb3728c |
604 | =item Write |
605 | |
606 | SSize_t (*Write)(PerlIO *f, const void *vbuf, Size_t count); |
50b80e25 |
607 | |
d4165bde |
608 | Basic write operation. |
50b80e25 |
609 | |
d4165bde |
610 | Returns bytes written or -1 on an error. |
611 | |
5cb3728c |
612 | =item Seek |
613 | |
614 | IV (*Seek)(pTHX_ PerlIO *f, Off_t offset, int whence); |
50b80e25 |
615 | |
1d11c889 |
616 | Position the file pointer. Should normally call its own C<Flush> |
617 | method and then the C<Seek> method of next layer down. |
50b80e25 |
618 | |
d4165bde |
619 | Returns 0 on success, -1 on failure. |
620 | |
5cb3728c |
621 | =item Tell |
622 | |
623 | Off_t (*Tell)(pTHX_ PerlIO *f); |
50b80e25 |
624 | |
9d799145 |
625 | Return the file pointer. May be based on layers cached concept of |
626 | position to avoid overhead. |
50b80e25 |
627 | |
d4165bde |
628 | Returns -1 on failure to get the file pointer. |
629 | |
5cb3728c |
630 | =item Close |
631 | |
632 | IV (*Close)(pTHX_ PerlIO *f); |
50b80e25 |
633 | |
9d799145 |
634 | Close the stream. Should normally call C<PerlIOBase_close()> to flush |
635 | itself and close layers below, and then deallocate any data structures |
636 | (buffers, translation tables, ...) not held directly in the data |
637 | structure. |
50b80e25 |
638 | |
d4165bde |
639 | Returns 0 on success, -1 on failure. |
640 | |
5cb3728c |
641 | =item Flush |
642 | |
643 | IV (*Flush)(pTHX_ PerlIO *f); |
50b80e25 |
644 | |
9d799145 |
645 | Should make stream's state consistent with layers below. That is, any |
646 | buffered write data should be written, and file position of lower layers |
d1be9408 |
647 | adjusted for data read from below but not actually consumed. |
b76cc8ba |
648 | (Should perhaps C<Unread()> such data to the lower layer.) |
50b80e25 |
649 | |
d4165bde |
650 | Returns 0 on success, -1 on failure. |
651 | |
5cb3728c |
652 | =item Fill |
653 | |
654 | IV (*Fill)(pTHX_ PerlIO *f); |
d4165bde |
655 | |
656 | The buffer for this layer should be filled (for read) from layer |
657 | below. When you "subclass" PerlIOBuf layer, you want to use its |
658 | I<_read> method and to supply your own fill method, which fills the |
659 | PerlIOBuf's buffer. |
50b80e25 |
660 | |
d4165bde |
661 | Returns 0 on success, -1 on failure. |
50b80e25 |
662 | |
5cb3728c |
663 | =item Eof |
664 | |
665 | IV (*Eof)(pTHX_ PerlIO *f); |
50b80e25 |
666 | |
9d799145 |
667 | Return end-of-file indicator. C<PerlIOBase_eof()> is normally sufficient. |
50b80e25 |
668 | |
d4165bde |
669 | Returns 0 on end-of-file, 1 if not end-of-file, -1 on error. |
670 | |
5cb3728c |
671 | =item Error |
672 | |
673 | IV (*Error)(pTHX_ PerlIO *f); |
50b80e25 |
674 | |
9d799145 |
675 | Return error indicator. C<PerlIOBase_error()> is normally sufficient. |
50b80e25 |
676 | |
d4165bde |
677 | Returns 1 if there is an error (usually when C<PERLIO_F_ERROR> is set, |
678 | 0 otherwise. |
679 | |
5cb3728c |
680 | =item Clearerr |
681 | |
682 | void (*Clearerr)(pTHX_ PerlIO *f); |
50b80e25 |
683 | |
9d799145 |
684 | Clear end-of-file and error indicators. Should call C<PerlIOBase_clearerr()> |
685 | to set the C<PERLIO_F_XXXXX> flags, which may suffice. |
50b80e25 |
686 | |
5cb3728c |
687 | =item Setlinebuf |
688 | |
689 | void (*Setlinebuf)(pTHX_ PerlIO *f); |
50b80e25 |
690 | |
b76cc8ba |
691 | Mark the stream as line buffered. C<PerlIOBase_setlinebuf()> sets the |
692 | PERLIO_F_LINEBUF flag and is normally sufficient. |
50b80e25 |
693 | |
5cb3728c |
694 | =item Get_base |
695 | |
696 | STDCHAR * (*Get_base)(pTHX_ PerlIO *f); |
50b80e25 |
697 | |
698 | Allocate (if not already done so) the read buffer for this layer and |
d4165bde |
699 | return pointer to it. Return NULL on failure. |
50b80e25 |
700 | |
5cb3728c |
701 | =item Get_bufsiz |
702 | |
703 | Size_t (*Get_bufsiz)(pTHX_ PerlIO *f); |
50b80e25 |
704 | |
9d799145 |
705 | Return the number of bytes that last C<Fill()> put in the buffer. |
50b80e25 |
706 | |
5cb3728c |
707 | =item Get_ptr |
708 | |
709 | STDCHAR * (*Get_ptr)(pTHX_ PerlIO *f); |
50b80e25 |
710 | |
3039a93d |
711 | Return the current read pointer relative to this layer's buffer. |
50b80e25 |
712 | |
5cb3728c |
713 | =item Get_cnt |
714 | |
715 | SSize_t (*Get_cnt)(pTHX_ PerlIO *f); |
50b80e25 |
716 | |
717 | Return the number of bytes left to be read in the current buffer. |
718 | |
5cb3728c |
719 | =item Set_ptrcnt |
720 | |
721 | void (*Set_ptrcnt)(pTHX_ PerlIO *f, |
722 | STDCHAR *ptr, SSize_t cnt); |
50b80e25 |
723 | |
724 | Adjust the read pointer and count of bytes to match C<ptr> and/or C<cnt>. |
725 | The application (or layer above) must ensure they are consistent. |
726 | (Checking is allowed by the paranoid.) |
727 | |
728 | =back |
729 | |
210e727c |
730 | =head2 Implementing PerlIO Layers |
731 | |
2535a4f7 |
732 | If you find the implementation document unclear or not sufficient, |
733 | look at the existing perlio layer implementations, which include: |
734 | |
735 | =over |
736 | |
737 | =item * C implementations |
738 | |
eae154c7 |
739 | The F<perlio.c> and F<perliol.h> in the Perl core implement the |
740 | "unix", "perlio", "stdio", "crlf", "utf8", "byte", "raw", "pending" |
741 | layers, and also the "mmap" and "win32" layers if applicable. |
742 | (The "win32" is currently unfinished and unused, to see what is used |
743 | instead in Win32, see L<PerlIO/"Querying the layers of filehandles"> .) |
744 | |
2535a4f7 |
745 | PerlIO::encoding, PerlIO::scalar, PerlIO::via in the Perl core. |
746 | |
747 | PerlIO::gzip and APR::PerlIO (mod_perl 2.0) on CPAN. |
748 | |
749 | =item * Perl implementations |
750 | |
751 | PerlIO::via::QuotedPrint in the Perl core and PerlIO::via::* on CPAN. |
752 | |
753 | =back |
754 | |
210e727c |
755 | If you are creating a PerlIO layer, you may want to be lazy, in other |
756 | words, implement only the methods that interest you. The other methods |
757 | you can either replace with the "blank" methods |
758 | |
759 | PerlIOBase_noop_ok |
760 | PerlIOBase_noop_fail |
761 | |
762 | (which do nothing, and return zero and -1, respectively) or for |
763 | certain methods you may assume a default behaviour by using a NULL |
61bdadae |
764 | method. The Open method looks for help in the 'parent' layer. |
765 | The following table summarizes the behaviour: |
210e727c |
766 | |
767 | method behaviour with NULL |
768 | |
769 | Clearerr PerlIOBase_clearerr |
770 | Close PerlIOBase_close |
61bdadae |
771 | Dup PerlIOBase_dup |
210e727c |
772 | Eof PerlIOBase_eof |
773 | Error PerlIOBase_error |
774 | Fileno PerlIOBase_fileno |
775 | Fill FAILURE |
776 | Flush SUCCESS |
61bdadae |
777 | Getarg SUCCESS |
210e727c |
778 | Get_base FAILURE |
779 | Get_bufsiz FAILURE |
780 | Get_cnt FAILURE |
781 | Get_ptr FAILURE |
61bdadae |
782 | Open INHERITED |
783 | Popped SUCCESS |
784 | Pushed SUCCESS |
210e727c |
785 | Read PerlIOBase_read |
786 | Seek FAILURE |
787 | Set_cnt FAILURE |
788 | Set_ptrcnt FAILURE |
789 | Setlinebuf PerlIOBase_setlinebuf |
790 | Tell FAILURE |
791 | Unread PerlIOBase_unread |
792 | Write FAILURE |
50b80e25 |
793 | |
61bdadae |
794 | FAILURE Set errno (to EINVAL in UNIXish, to LIB$_INVARG in VMS) and |
795 | return -1 (for numeric return values) or NULL (for pointers) |
796 | INHERITED Inherited from the layer below |
797 | SUCCESS Return 0 (for numeric return values) or a pointer |
798 | |
50b80e25 |
799 | =head2 Core Layers |
800 | |
801 | The file C<perlio.c> provides the following layers: |
802 | |
803 | =over 4 |
804 | |
805 | =item "unix" |
806 | |
9d799145 |
807 | A basic non-buffered layer which calls Unix/POSIX C<read()>, C<write()>, |
808 | C<lseek()>, C<close()>. No buffering. Even on platforms that distinguish |
809 | between O_TEXT and O_BINARY this layer is always O_BINARY. |
50b80e25 |
810 | |
811 | =item "perlio" |
812 | |
9d799145 |
813 | A very complete generic buffering layer which provides the whole of |
814 | PerlIO API. It is also intended to be used as a "base class" for other |
1d11c889 |
815 | layers. (For example its C<Read()> method is implemented in terms of |
816 | the C<Get_cnt()>/C<Get_ptr()>/C<Set_ptrcnt()> methods). |
50b80e25 |
817 | |
9d799145 |
818 | "perlio" over "unix" provides a complete replacement for stdio as seen |
819 | via PerlIO API. This is the default for USE_PERLIO when system's stdio |
1d11c889 |
820 | does not permit perl's "fast gets" access, and which do not |
821 | distinguish between C<O_TEXT> and C<O_BINARY>. |
50b80e25 |
822 | |
823 | =item "stdio" |
824 | |
9d799145 |
825 | A layer which provides the PerlIO API via the layer scheme, but |
826 | implements it by calling system's stdio. This is (currently) the default |
827 | if system's stdio provides sufficient access to allow perl's "fast gets" |
828 | access and which do not distinguish between C<O_TEXT> and C<O_BINARY>. |
50b80e25 |
829 | |
830 | =item "crlf" |
831 | |
9d799145 |
832 | A layer derived using "perlio" as a base class. It provides Win32-like |
833 | "\n" to CR,LF translation. Can either be applied above "perlio" or serve |
834 | as the buffer layer itself. "crlf" over "unix" is the default if system |
835 | distinguishes between C<O_TEXT> and C<O_BINARY> opens. (At some point |
836 | "unix" will be replaced by a "native" Win32 IO layer on that platform, |
837 | as Win32's read/write layer has various drawbacks.) The "crlf" layer is |
838 | a reasonable model for a layer which transforms data in some way. |
50b80e25 |
839 | |
840 | =item "mmap" |
841 | |
9d799145 |
842 | If Configure detects C<mmap()> functions this layer is provided (with |
843 | "perlio" as a "base") which does "read" operations by mmap()ing the |
844 | file. Performance improvement is marginal on modern systems, so it is |
845 | mainly there as a proof of concept. It is likely to be unbundled from |
846 | the core at some point. The "mmap" layer is a reasonable model for a |
847 | minimalist "derived" layer. |
50b80e25 |
848 | |
849 | =item "pending" |
850 | |
9d799145 |
851 | An "internal" derivative of "perlio" which can be used to provide |
1d11c889 |
852 | Unread() function for layers which have no buffer or cannot be |
853 | bothered. (Basically this layer's C<Fill()> pops itself off the stack |
854 | and so resumes reading from layer below.) |
50b80e25 |
855 | |
856 | =item "raw" |
857 | |
9d799145 |
858 | A dummy layer which never exists on the layer stack. Instead when |
86e05cf2 |
859 | "pushed" it actually pops the stack removing itself, it then calls |
860 | Binmode function table entry on all the layers in the stack - normally |
861 | this (via PerlIOBase_binmode) removes any layers which do not have |
862 | C<PERLIO_K_RAW> bit set. Layers can modify that behaviour by defining |
863 | their own Binmode entry. |
50b80e25 |
864 | |
865 | =item "utf8" |
866 | |
9d799145 |
867 | Another dummy layer. When pushed it pops itself and sets the |
1d11c889 |
868 | C<PERLIO_F_UTF8> flag on the layer which was (and now is once more) |
869 | the top of the stack. |
50b80e25 |
870 | |
871 | =back |
872 | |
9d799145 |
873 | In addition F<perlio.c> also provides a number of C<PerlIOBase_xxxx()> |
874 | functions which are intended to be used in the table slots of classes |
875 | which do not need to do anything special for a particular method. |
50b80e25 |
876 | |
877 | =head2 Extension Layers |
878 | |
1d11c889 |
879 | Layers can made available by extension modules. When an unknown layer |
880 | is encountered the PerlIO code will perform the equivalent of : |
b76cc8ba |
881 | |
882 | use PerlIO 'layer'; |
883 | |
1d11c889 |
884 | Where I<layer> is the unknown layer. F<PerlIO.pm> will then attempt to: |
b76cc8ba |
885 | |
886 | require PerlIO::layer; |
887 | |
1d11c889 |
888 | If after that process the layer is still not defined then the C<open> |
889 | will fail. |
b76cc8ba |
890 | |
891 | The following extension layers are bundled with perl: |
50b80e25 |
892 | |
893 | =over 4 |
894 | |
b76cc8ba |
895 | =item ":encoding" |
50b80e25 |
896 | |
897 | use Encoding; |
898 | |
1d11c889 |
899 | makes this layer available, although F<PerlIO.pm> "knows" where to |
900 | find it. It is an example of a layer which takes an argument as it is |
901 | called thus: |
50b80e25 |
902 | |
b31b80f9 |
903 | open( $fh, "<:encoding(iso-8859-7)", $pathname ); |
50b80e25 |
904 | |
385e1f9f |
905 | =item ":scalar" |
b76cc8ba |
906 | |
b31b80f9 |
907 | Provides support for reading data from and writing data to a scalar. |
b76cc8ba |
908 | |
385e1f9f |
909 | open( $fh, "+<:scalar", \$scalar ); |
50b80e25 |
910 | |
1d11c889 |
911 | When a handle is so opened, then reads get bytes from the string value |
912 | of I<$scalar>, and writes change the value. In both cases the position |
913 | in I<$scalar> starts as zero but can be altered via C<seek>, and |
914 | determined via C<tell>. |
b76cc8ba |
915 | |
385e1f9f |
916 | Please note that this layer is implied when calling open() thus: |
917 | |
918 | open( $fh, "+<", \$scalar ); |
919 | |
920 | =item ":via" |
b76cc8ba |
921 | |
4f7853f4 |
922 | Provided to allow layers to be implemented as Perl code. For instance: |
923 | |
e934609f |
924 | use PerlIO::via::StripHTML; |
385e1f9f |
925 | open( my $fh, "<:via(StripHTML)", "index.html" ); |
4f7853f4 |
926 | |
e934609f |
927 | See L<PerlIO::via> for details. |
b76cc8ba |
928 | |
929 | =back |
50b80e25 |
930 | |
d4165bde |
931 | =head1 TODO |
932 | |
933 | Things that need to be done to improve this document. |
934 | |
935 | =over |
936 | |
937 | =item * |
938 | |
939 | Explain how to make a valid fh without going through open()(i.e. apply |
940 | a layer). For example if the file is not opened through perl, but we |
941 | want to get back a fh, like it was opened by Perl. |
942 | |
943 | How PerlIO_apply_layera fits in, where its docs, was it made public? |
944 | |
945 | Currently the example could be something like this: |
946 | |
947 | PerlIO *foo_to_PerlIO(pTHX_ char *mode, ...) |
948 | { |
949 | char *mode; /* "w", "r", etc */ |
950 | const char *layers = ":APR"; /* the layer name */ |
951 | PerlIO *f = PerlIO_allocate(aTHX); |
952 | if (!f) { |
953 | return NULL; |
954 | } |
955 | |
956 | PerlIO_apply_layers(aTHX_ f, mode, layers); |
957 | |
958 | if (f) { |
959 | PerlIOAPR *st = PerlIOSelf(f, PerlIOAPR); |
960 | /* fill in the st struct, as in _open() */ |
961 | st->file = file; |
962 | PerlIOBase(f)->flags |= PERLIO_F_OPEN; |
963 | |
964 | return f; |
965 | } |
966 | return NULL; |
967 | } |
968 | |
969 | =item * |
970 | |
971 | fix/add the documentation in places marked as XXX. |
972 | |
973 | =item * |
974 | |
975 | The handling of errors by the layer is not specified. e.g. when $! |
976 | should be set explicitly, when the error handling should be just |
977 | delegated to the top layer. |
978 | |
979 | Probably give some hints on using SETERRNO() or pointers to where they |
980 | can be found. |
981 | |
982 | =item * |
983 | |
984 | I think it would help to give some concrete examples to make it easier |
985 | to understand the API. Of course I agree that the API has to be |
986 | concise, but since there is no second document that is more of a |
987 | guide, I think that it'd make it easier to start with the doc which is |
988 | an API, but has examples in it in places where things are unclear, to |
989 | a person who is not a PerlIO guru (yet). |
990 | |
991 | =back |
992 | |
50b80e25 |
993 | =cut |