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1 | =head1 NAME |
2 | ||
3 | perlcall - Perl calling conventions from C | |
4 | ||
5 | =head1 DESCRIPTION | |
6 | ||
7 | The purpose of this document is to show you how to call Perl subroutines | |
8 | directly from C, i.e., how to write I<callbacks>. | |
9 | ||
10 | Apart from discussing the C interface provided by Perl for writing | |
11 | callbacks the document uses a series of examples to show how the | |
12 | interface actually works in practice. In addition some techniques for | |
13 | coding callbacks are covered. | |
14 | ||
15 | Examples where callbacks are necessary include | |
16 | ||
17 | =over 5 | |
18 | ||
19 | =item * An Error Handler | |
20 | ||
21 | You have created an XSUB interface to an application's C API. | |
22 | ||
23 | A fairly common feature in applications is to allow you to define a C | |
24 | function that will be called whenever something nasty occurs. What we | |
25 | would like is to be able to specify a Perl subroutine that will be | |
26 | called instead. | |
27 | ||
28 | =item * An Event Driven Program | |
29 | ||
30 | The classic example of where callbacks are used is when writing an | |
31 | event driven program like for an X windows application. In this case | |
32 | you register functions to be called whenever specific events occur, | |
33 | e.g., a mouse button is pressed, the cursor moves into a window or a | |
34 | menu item is selected. | |
35 | ||
36 | =back | |
37 | ||
38 | Although the techniques described here are applicable when embedding | |
39 | Perl in a C program, this is not the primary goal of this document. | |
40 | There are other details that must be considered and are specific to | |
41 | embedding Perl. For details on embedding Perl in C refer to | |
42 | L<perlembed>. | |
43 | ||
44 | Before you launch yourself head first into the rest of this document, | |
45 | it would be a good idea to have read the following two documents - | |
46 | L<perlxs> and L<perlguts>. | |
47 | ||
48 | =head1 THE CALL_ FUNCTIONS | |
49 | ||
50 | Although this stuff is easier to explain using examples, you first need | |
51 | be aware of a few important definitions. | |
52 | ||
53 | Perl has a number of C functions that allow you to call Perl | |
54 | subroutines. They are | |
55 | ||
56 | I32 call_sv(SV* sv, I32 flags); | |
57 | I32 call_pv(char *subname, I32 flags); | |
58 | I32 call_method(char *methname, I32 flags); | |
59 | I32 call_argv(char *subname, I32 flags, register char **argv); | |
60 | ||
61 | The key function is I<call_sv>. All the other functions are | |
62 | fairly simple wrappers which make it easier to call Perl subroutines in | |
63 | special cases. At the end of the day they will all call I<call_sv> | |
64 | to invoke the Perl subroutine. | |
65 | ||
66 | All the I<call_*> functions have a C<flags> parameter which is | |
67 | used to pass a bit mask of options to Perl. This bit mask operates | |
68 | identically for each of the functions. The settings available in the | |
69 | bit mask are discussed in L<FLAG VALUES>. | |
70 | ||
71 | Each of the functions will now be discussed in turn. | |
72 | ||
73 | =over 5 | |
74 | ||
75 | =item call_sv | |
76 | ||
77 | I<call_sv> takes two parameters, the first, C<sv>, is an SV*. | |
78 | This allows you to specify the Perl subroutine to be called either as a | |
79 | C string (which has first been converted to an SV) or a reference to a | |
80 | subroutine. The section, I<Using call_sv>, shows how you can make | |
81 | use of I<call_sv>. | |
82 | ||
83 | =item call_pv | |
84 | ||
85 | The function, I<call_pv>, is similar to I<call_sv> except it | |
86 | expects its first parameter to be a C char* which identifies the Perl | |
87 | subroutine you want to call, e.g., C<call_pv("fred", 0)>. If the | |
88 | subroutine you want to call is in another package, just include the | |
89 | package name in the string, e.g., C<"pkg::fred">. | |
90 | ||
91 | =item call_method | |
92 | ||
93 | The function I<call_method> is used to call a method from a Perl | |
94 | class. The parameter C<methname> corresponds to the name of the method | |
95 | to be called. Note that the class that the method belongs to is passed | |
96 | on the Perl stack rather than in the parameter list. This class can be | |
97 | either the name of the class (for a static method) or a reference to an | |
98 | object (for a virtual method). See L<perlobj> for more information on | |
99 | static and virtual methods and L<Using call_method> for an example | |
100 | of using I<call_method>. | |
101 | ||
102 | =item call_argv | |
103 | ||
104 | I<call_argv> calls the Perl subroutine specified by the C string | |
105 | stored in the C<subname> parameter. It also takes the usual C<flags> | |
106 | parameter. The final parameter, C<argv>, consists of a NULL terminated | |
107 | list of C strings to be passed as parameters to the Perl subroutine. | |
108 | See I<Using call_argv>. | |
109 | ||
110 | =back | |
111 | ||
112 | All the functions return an integer. This is a count of the number of | |
113 | items returned by the Perl subroutine. The actual items returned by the | |
114 | subroutine are stored on the Perl stack. | |
115 | ||
116 | As a general rule you should I<always> check the return value from | |
117 | these functions. Even if you are expecting only a particular number of | |
118 | values to be returned from the Perl subroutine, there is nothing to | |
119 | stop someone from doing something unexpected--don't say you haven't | |
120 | been warned. | |
121 | ||
122 | =head1 FLAG VALUES | |
123 | ||
124 | The C<flags> parameter in all the I<call_*> functions is a bit mask | |
125 | which can consist of any combination of the symbols defined below, | |
126 | OR'ed together. | |
127 | ||
128 | ||
129 | =head2 G_VOID | |
130 | ||
131 | Calls the Perl subroutine in a void context. | |
132 | ||
133 | This flag has 2 effects: | |
134 | ||
135 | =over 5 | |
136 | ||
137 | =item 1. | |
138 | ||
139 | It indicates to the subroutine being called that it is executing in | |
140 | a void context (if it executes I<wantarray> the result will be the | |
141 | undefined value). | |
142 | ||
143 | =item 2. | |
144 | ||
145 | It ensures that nothing is actually returned from the subroutine. | |
146 | ||
147 | =back | |
148 | ||
149 | The value returned by the I<call_*> function indicates how many | |
150 | items have been returned by the Perl subroutine - in this case it will | |
151 | be 0. | |
152 | ||
153 | ||
154 | =head2 G_SCALAR | |
155 | ||
156 | Calls the Perl subroutine in a scalar context. This is the default | |
157 | context flag setting for all the I<call_*> functions. | |
158 | ||
159 | This flag has 2 effects: | |
160 | ||
161 | =over 5 | |
162 | ||
163 | =item 1. | |
164 | ||
165 | It indicates to the subroutine being called that it is executing in a | |
166 | scalar context (if it executes I<wantarray> the result will be false). | |
167 | ||
168 | =item 2. | |
169 | ||
170 | It ensures that only a scalar is actually returned from the subroutine. | |
171 | The subroutine can, of course, ignore the I<wantarray> and return a | |
172 | list anyway. If so, then only the last element of the list will be | |
173 | returned. | |
174 | ||
175 | =back | |
176 | ||
177 | The value returned by the I<call_*> function indicates how many | |
178 | items have been returned by the Perl subroutine - in this case it will | |
179 | be either 0 or 1. | |
180 | ||
181 | If 0, then you have specified the G_DISCARD flag. | |
182 | ||
183 | If 1, then the item actually returned by the Perl subroutine will be | |
184 | stored on the Perl stack - the section I<Returning a Scalar> shows how | |
185 | to access this value on the stack. Remember that regardless of how | |
186 | many items the Perl subroutine returns, only the last one will be | |
187 | accessible from the stack - think of the case where only one value is | |
188 | returned as being a list with only one element. Any other items that | |
189 | were returned will not exist by the time control returns from the | |
190 | I<call_*> function. The section I<Returning a list in a scalar | |
191 | context> shows an example of this behavior. | |
192 | ||
193 | ||
194 | =head2 G_ARRAY | |
195 | ||
196 | Calls the Perl subroutine in a list context. | |
197 | ||
198 | As with G_SCALAR, this flag has 2 effects: | |
199 | ||
200 | =over 5 | |
201 | ||
202 | =item 1. | |
203 | ||
204 | It indicates to the subroutine being called that it is executing in a | |
205 | list context (if it executes I<wantarray> the result will be true). | |
206 | ||
207 | ||
208 | =item 2. | |
209 | ||
210 | It ensures that all items returned from the subroutine will be | |
211 | accessible when control returns from the I<call_*> function. | |
212 | ||
213 | =back | |
214 | ||
215 | The value returned by the I<call_*> function indicates how many | |
216 | items have been returned by the Perl subroutine. | |
217 | ||
218 | If 0, then you have specified the G_DISCARD flag. | |
219 | ||
220 | If not 0, then it will be a count of the number of items returned by | |
221 | the subroutine. These items will be stored on the Perl stack. The | |
222 | section I<Returning a list of values> gives an example of using the | |
223 | G_ARRAY flag and the mechanics of accessing the returned items from the | |
224 | Perl stack. | |
225 | ||
226 | =head2 G_DISCARD | |
227 | ||
228 | By default, the I<call_*> functions place the items returned from | |
229 | by the Perl subroutine on the stack. If you are not interested in | |
230 | these items, then setting this flag will make Perl get rid of them | |
231 | automatically for you. Note that it is still possible to indicate a | |
232 | context to the Perl subroutine by using either G_SCALAR or G_ARRAY. | |
233 | ||
234 | If you do not set this flag then it is I<very> important that you make | |
235 | sure that any temporaries (i.e., parameters passed to the Perl | |
236 | subroutine and values returned from the subroutine) are disposed of | |
237 | yourself. The section I<Returning a Scalar> gives details of how to | |
238 | dispose of these temporaries explicitly and the section I<Using Perl to | |
239 | dispose of temporaries> discusses the specific circumstances where you | |
240 | can ignore the problem and let Perl deal with it for you. | |
241 | ||
242 | =head2 G_NOARGS | |
243 | ||
244 | Whenever a Perl subroutine is called using one of the I<call_*> | |
245 | functions, it is assumed by default that parameters are to be passed to | |
246 | the subroutine. If you are not passing any parameters to the Perl | |
247 | subroutine, you can save a bit of time by setting this flag. It has | |
248 | the effect of not creating the C<@_> array for the Perl subroutine. | |
249 | ||
250 | Although the functionality provided by this flag may seem | |
251 | straightforward, it should be used only if there is a good reason to do | |
252 | so. The reason for being cautious is that even if you have specified | |
253 | the G_NOARGS flag, it is still possible for the Perl subroutine that | |
254 | has been called to think that you have passed it parameters. | |
255 | ||
256 | In fact, what can happen is that the Perl subroutine you have called | |
257 | can access the C<@_> array from a previous Perl subroutine. This will | |
258 | occur when the code that is executing the I<call_*> function has | |
259 | itself been called from another Perl subroutine. The code below | |
260 | illustrates this | |
261 | ||
262 | sub fred | |
263 | { print "@_\n" } | |
264 | ||
265 | sub joe | |
266 | { &fred } | |
267 | ||
268 | &joe(1,2,3); | |
269 | ||
270 | This will print | |
271 | ||
272 | 1 2 3 | |
273 | ||
274 | What has happened is that C<fred> accesses the C<@_> array which | |
275 | belongs to C<joe>. | |
276 | ||
277 | ||
278 | =head2 G_EVAL | |
279 | ||
280 | It is possible for the Perl subroutine you are calling to terminate | |
281 | abnormally, e.g., by calling I<die> explicitly or by not actually | |
282 | existing. By default, when either of these events occurs, the | |
283 | process will terminate immediately. If you want to trap this | |
284 | type of event, specify the G_EVAL flag. It will put an I<eval { }> | |
285 | around the subroutine call. | |
286 | ||
287 | Whenever control returns from the I<call_*> function you need to | |
288 | check the C<$@> variable as you would in a normal Perl script. | |
289 | ||
290 | The value returned from the I<call_*> function is dependent on | |
291 | what other flags have been specified and whether an error has | |
292 | occurred. Here are all the different cases that can occur: | |
293 | ||
294 | =over 5 | |
295 | ||
296 | =item * | |
297 | ||
298 | If the I<call_*> function returns normally, then the value | |
299 | returned is as specified in the previous sections. | |
300 | ||
301 | =item * | |
302 | ||
303 | If G_DISCARD is specified, the return value will always be 0. | |
304 | ||
305 | =item * | |
306 | ||
307 | If G_ARRAY is specified I<and> an error has occurred, the return value | |
308 | will always be 0. | |
309 | ||
310 | =item * | |
311 | ||
312 | If G_SCALAR is specified I<and> an error has occurred, the return value | |
313 | will be 1 and the value on the top of the stack will be I<undef>. This | |
314 | means that if you have already detected the error by checking C<$@> and | |
315 | you want the program to continue, you must remember to pop the I<undef> | |
316 | from the stack. | |
317 | ||
318 | =back | |
319 | ||
320 | See I<Using G_EVAL> for details on using G_EVAL. | |
321 | ||
322 | =head2 G_KEEPERR | |
323 | ||
324 | You may have noticed that using the G_EVAL flag described above will | |
325 | B<always> clear the C<$@> variable and set it to a string describing | |
326 | the error iff there was an error in the called code. This unqualified | |
327 | resetting of C<$@> can be problematic in the reliable identification of | |
328 | errors using the C<eval {}> mechanism, because the possibility exists | |
329 | that perl will call other code (end of block processing code, for | |
330 | example) between the time the error causes C<$@> to be set within | |
331 | C<eval {}>, and the subsequent statement which checks for the value of | |
332 | C<$@> gets executed in the user's script. | |
333 | ||
334 | This scenario will mostly be applicable to code that is meant to be | |
335 | called from within destructors, asynchronous callbacks, signal | |
336 | handlers, C<__DIE__> or C<__WARN__> hooks, and C<tie> functions. In | |
337 | such situations, you will not want to clear C<$@> at all, but simply to | |
338 | append any new errors to any existing value of C<$@>. | |
339 | ||
340 | The G_KEEPERR flag is meant to be used in conjunction with G_EVAL in | |
341 | I<call_*> functions that are used to implement such code. This flag | |
342 | has no effect when G_EVAL is not used. | |
343 | ||
344 | When G_KEEPERR is used, any errors in the called code will be prefixed | |
345 | with the string "\t(in cleanup)", and appended to the current value | |
346 | of C<$@>. an error will not be appended if that same error string is | |
347 | already at the end of C<$@>. | |
348 | ||
349 | In addition, a warning is generated using the appended string. This can be | |
350 | disabled using C<no warnings 'misc'>. | |
351 | ||
352 | The G_KEEPERR flag was introduced in Perl version 5.002. | |
353 | ||
354 | See I<Using G_KEEPERR> for an example of a situation that warrants the | |
355 | use of this flag. | |
356 | ||
357 | =head2 Determining the Context | |
358 | ||
359 | As mentioned above, you can determine the context of the currently | |
360 | executing subroutine in Perl with I<wantarray>. The equivalent test | |
361 | can be made in C by using the C<GIMME_V> macro, which returns | |
362 | C<G_ARRAY> if you have been called in a list context, C<G_SCALAR> if | |
363 | in a scalar context, or C<G_VOID> if in a void context (i.e. the | |
364 | return value will not be used). An older version of this macro is | |
365 | called C<GIMME>; in a void context it returns C<G_SCALAR> instead of | |
366 | C<G_VOID>. An example of using the C<GIMME_V> macro is shown in | |
367 | section I<Using GIMME_V>. | |
368 | ||
369 | =head1 EXAMPLES | |
370 | ||
371 | Enough of the definition talk, let's have a few examples. | |
372 | ||
373 | Perl provides many macros to assist in accessing the Perl stack. | |
374 | Wherever possible, these macros should always be used when interfacing | |
375 | to Perl internals. We hope this should make the code less vulnerable | |
376 | to any changes made to Perl in the future. | |
377 | ||
378 | Another point worth noting is that in the first series of examples I | |
379 | have made use of only the I<call_pv> function. This has been done | |
380 | to keep the code simpler and ease you into the topic. Wherever | |
381 | possible, if the choice is between using I<call_pv> and | |
382 | I<call_sv>, you should always try to use I<call_sv>. See | |
383 | I<Using call_sv> for details. | |
384 | ||
385 | =head2 No Parameters, Nothing returned | |
386 | ||
387 | This first trivial example will call a Perl subroutine, I<PrintUID>, to | |
388 | print out the UID of the process. | |
389 | ||
390 | sub PrintUID | |
391 | { | |
392 | print "UID is $<\n"; | |
393 | } | |
394 | ||
395 | and here is a C function to call it | |
396 | ||
397 | static void | |
398 | call_PrintUID() | |
399 | { | |
400 | dSP; | |
401 | ||
402 | PUSHMARK(SP); | |
403 | call_pv("PrintUID", G_DISCARD|G_NOARGS); | |
404 | } | |
405 | ||
406 | Simple, eh. | |
407 | ||
408 | A few points to note about this example. | |
409 | ||
410 | =over 5 | |
411 | ||
412 | =item 1. | |
413 | ||
414 | Ignore C<dSP> and C<PUSHMARK(SP)> for now. They will be discussed in | |
415 | the next example. | |
416 | ||
417 | =item 2. | |
418 | ||
419 | We aren't passing any parameters to I<PrintUID> so G_NOARGS can be | |
420 | specified. | |
421 | ||
422 | =item 3. | |
423 | ||
424 | We aren't interested in anything returned from I<PrintUID>, so | |
425 | G_DISCARD is specified. Even if I<PrintUID> was changed to | |
426 | return some value(s), having specified G_DISCARD will mean that they | |
427 | will be wiped by the time control returns from I<call_pv>. | |
428 | ||
429 | =item 4. | |
430 | ||
431 | As I<call_pv> is being used, the Perl subroutine is specified as a | |
432 | C string. In this case the subroutine name has been 'hard-wired' into the | |
433 | code. | |
434 | ||
435 | =item 5. | |
436 | ||
437 | Because we specified G_DISCARD, it is not necessary to check the value | |
438 | returned from I<call_pv>. It will always be 0. | |
439 | ||
440 | =back | |
441 | ||
442 | =head2 Passing Parameters | |
443 | ||
444 | Now let's make a slightly more complex example. This time we want to | |
445 | call a Perl subroutine, C<LeftString>, which will take 2 parameters--a | |
446 | string ($s) and an integer ($n). The subroutine will simply | |
447 | print the first $n characters of the string. | |
448 | ||
449 | So the Perl subroutine would look like this | |
450 | ||
451 | sub LeftString | |
452 | { | |
453 | my($s, $n) = @_; | |
454 | print substr($s, 0, $n), "\n"; | |
455 | } | |
456 | ||
457 | The C function required to call I<LeftString> would look like this. | |
458 | ||
459 | static void | |
460 | call_LeftString(a, b) | |
461 | char * a; | |
462 | int b; | |
463 | { | |
464 | dSP; | |
465 | ||
466 | ENTER; | |
467 | SAVETMPS; | |
468 | ||
469 | PUSHMARK(SP); | |
470 | XPUSHs(sv_2mortal(newSVpv(a, 0))); | |
471 | XPUSHs(sv_2mortal(newSViv(b))); | |
472 | PUTBACK; | |
473 | ||
474 | call_pv("LeftString", G_DISCARD); | |
475 | ||
476 | FREETMPS; | |
477 | LEAVE; | |
478 | } | |
479 | ||
480 | Here are a few notes on the C function I<call_LeftString>. | |
481 | ||
482 | =over 5 | |
483 | ||
484 | =item 1. | |
485 | ||
486 | Parameters are passed to the Perl subroutine using the Perl stack. | |
487 | This is the purpose of the code beginning with the line C<dSP> and | |
488 | ending with the line C<PUTBACK>. The C<dSP> declares a local copy | |
489 | of the stack pointer. This local copy should B<always> be accessed | |
490 | as C<SP>. | |
491 | ||
492 | =item 2. | |
493 | ||
494 | If you are going to put something onto the Perl stack, you need to know | |
495 | where to put it. This is the purpose of the macro C<dSP>--it declares | |
496 | and initializes a I<local> copy of the Perl stack pointer. | |
497 | ||
498 | All the other macros which will be used in this example require you to | |
499 | have used this macro. | |
500 | ||
501 | The exception to this rule is if you are calling a Perl subroutine | |
502 | directly from an XSUB function. In this case it is not necessary to | |
503 | use the C<dSP> macro explicitly--it will be declared for you | |
504 | automatically. | |
505 | ||
506 | =item 3. | |
507 | ||
508 | Any parameters to be pushed onto the stack should be bracketed by the | |
509 | C<PUSHMARK> and C<PUTBACK> macros. The purpose of these two macros, in | |
510 | this context, is to count the number of parameters you are | |
511 | pushing automatically. Then whenever Perl is creating the C<@_> array for the | |
512 | subroutine, it knows how big to make it. | |
513 | ||
514 | The C<PUSHMARK> macro tells Perl to make a mental note of the current | |
515 | stack pointer. Even if you aren't passing any parameters (like the | |
516 | example shown in the section I<No Parameters, Nothing returned>) you | |
517 | must still call the C<PUSHMARK> macro before you can call any of the | |
518 | I<call_*> functions--Perl still needs to know that there are no | |
519 | parameters. | |
520 | ||
521 | The C<PUTBACK> macro sets the global copy of the stack pointer to be | |
522 | the same as our local copy. If we didn't do this I<call_pv> | |
523 | wouldn't know where the two parameters we pushed were--remember that | |
524 | up to now all the stack pointer manipulation we have done is with our | |
525 | local copy, I<not> the global copy. | |
526 | ||
527 | =item 4. | |
528 | ||
529 | Next, we come to XPUSHs. This is where the parameters actually get | |
530 | pushed onto the stack. In this case we are pushing a string and an | |
531 | integer. | |
532 | ||
533 | See L<perlguts/"XSUBs and the Argument Stack"> for details | |
534 | on how the XPUSH macros work. | |
535 | ||
536 | =item 5. | |
537 | ||
538 | Because we created temporary values (by means of sv_2mortal() calls) | |
539 | we will have to tidy up the Perl stack and dispose of mortal SVs. | |
540 | ||
541 | This is the purpose of | |
542 | ||
543 | ENTER; | |
544 | SAVETMPS; | |
545 | ||
546 | at the start of the function, and | |
547 | ||
548 | FREETMPS; | |
549 | LEAVE; | |
550 | ||
551 | at the end. The C<ENTER>/C<SAVETMPS> pair creates a boundary for any | |
552 | temporaries we create. This means that the temporaries we get rid of | |
553 | will be limited to those which were created after these calls. | |
554 | ||
555 | The C<FREETMPS>/C<LEAVE> pair will get rid of any values returned by | |
556 | the Perl subroutine (see next example), plus it will also dump the | |
557 | mortal SVs we have created. Having C<ENTER>/C<SAVETMPS> at the | |
558 | beginning of the code makes sure that no other mortals are destroyed. | |
559 | ||
560 | Think of these macros as working a bit like using C<{> and C<}> in Perl | |
561 | to limit the scope of local variables. | |
562 | ||
563 | See the section I<Using Perl to dispose of temporaries> for details of | |
564 | an alternative to using these macros. | |
565 | ||
566 | =item 6. | |
567 | ||
568 | Finally, I<LeftString> can now be called via the I<call_pv> function. | |
569 | The only flag specified this time is G_DISCARD. Because we are passing | |
570 | 2 parameters to the Perl subroutine this time, we have not specified | |
571 | G_NOARGS. | |
572 | ||
573 | =back | |
574 | ||
575 | =head2 Returning a Scalar | |
576 | ||
577 | Now for an example of dealing with the items returned from a Perl | |
578 | subroutine. | |
579 | ||
580 | Here is a Perl subroutine, I<Adder>, that takes 2 integer parameters | |
581 | and simply returns their sum. | |
582 | ||
583 | sub Adder | |
584 | { | |
585 | my($a, $b) = @_; | |
586 | $a + $b; | |
587 | } | |
588 | ||
589 | Because we are now concerned with the return value from I<Adder>, the C | |
590 | function required to call it is now a bit more complex. | |
591 | ||
592 | static void | |
593 | call_Adder(a, b) | |
594 | int a; | |
595 | int b; | |
596 | { | |
597 | dSP; | |
598 | int count; | |
599 | ||
600 | ENTER; | |
601 | SAVETMPS; | |
602 | ||
603 | PUSHMARK(SP); | |
604 | XPUSHs(sv_2mortal(newSViv(a))); | |
605 | XPUSHs(sv_2mortal(newSViv(b))); | |
606 | PUTBACK; | |
607 | ||
608 | count = call_pv("Adder", G_SCALAR); | |
609 | ||
610 | SPAGAIN; | |
611 | ||
612 | if (count != 1) | |
613 | croak("Big trouble\n"); | |
614 | ||
615 | printf ("The sum of %d and %d is %d\n", a, b, POPi); | |
616 | ||
617 | PUTBACK; | |
618 | FREETMPS; | |
619 | LEAVE; | |
620 | } | |
621 | ||
622 | Points to note this time are | |
623 | ||
624 | =over 5 | |
625 | ||
626 | =item 1. | |
627 | ||
628 | The only flag specified this time was G_SCALAR. That means the C<@_> | |
629 | array will be created and that the value returned by I<Adder> will | |
630 | still exist after the call to I<call_pv>. | |
631 | ||
632 | =item 2. | |
633 | ||
634 | The purpose of the macro C<SPAGAIN> is to refresh the local copy of the | |
635 | stack pointer. This is necessary because it is possible that the memory | |
636 | allocated to the Perl stack has been reallocated whilst in the | |
637 | I<call_pv> call. | |
638 | ||
639 | If you are making use of the Perl stack pointer in your code you must | |
640 | always refresh the local copy using SPAGAIN whenever you make use | |
641 | of the I<call_*> functions or any other Perl internal function. | |
642 | ||
643 | =item 3. | |
644 | ||
645 | Although only a single value was expected to be returned from I<Adder>, | |
646 | it is still good practice to check the return code from I<call_pv> | |
647 | anyway. | |
648 | ||
649 | Expecting a single value is not quite the same as knowing that there | |
650 | will be one. If someone modified I<Adder> to return a list and we | |
651 | didn't check for that possibility and take appropriate action the Perl | |
652 | stack would end up in an inconsistent state. That is something you | |
653 | I<really> don't want to happen ever. | |
654 | ||
655 | =item 4. | |
656 | ||
657 | The C<POPi> macro is used here to pop the return value from the stack. | |
658 | In this case we wanted an integer, so C<POPi> was used. | |
659 | ||
660 | ||
661 | Here is the complete list of POP macros available, along with the types | |
662 | they return. | |
663 | ||
664 | POPs SV | |
665 | POPp pointer | |
666 | POPn double | |
667 | POPi integer | |
668 | POPl long | |
669 | ||
670 | =item 5. | |
671 | ||
672 | The final C<PUTBACK> is used to leave the Perl stack in a consistent | |
673 | state before exiting the function. This is necessary because when we | |
674 | popped the return value from the stack with C<POPi> it updated only our | |
675 | local copy of the stack pointer. Remember, C<PUTBACK> sets the global | |
676 | stack pointer to be the same as our local copy. | |
677 | ||
678 | =back | |
679 | ||
680 | ||
681 | =head2 Returning a list of values | |
682 | ||
683 | Now, let's extend the previous example to return both the sum of the | |
684 | parameters and the difference. | |
685 | ||
686 | Here is the Perl subroutine | |
687 | ||
688 | sub AddSubtract | |
689 | { | |
690 | my($a, $b) = @_; | |
691 | ($a+$b, $a-$b); | |
692 | } | |
693 | ||
694 | and this is the C function | |
695 | ||
696 | static void | |
697 | call_AddSubtract(a, b) | |
698 | int a; | |
699 | int b; | |
700 | { | |
701 | dSP; | |
702 | int count; | |
703 | ||
704 | ENTER; | |
705 | SAVETMPS; | |
706 | ||
707 | PUSHMARK(SP); | |
708 | XPUSHs(sv_2mortal(newSViv(a))); | |
709 | XPUSHs(sv_2mortal(newSViv(b))); | |
710 | PUTBACK; | |
711 | ||
712 | count = call_pv("AddSubtract", G_ARRAY); | |
713 | ||
714 | SPAGAIN; | |
715 | ||
716 | if (count != 2) | |
717 | croak("Big trouble\n"); | |
718 | ||
719 | printf ("%d - %d = %d\n", a, b, POPi); | |
720 | printf ("%d + %d = %d\n", a, b, POPi); | |
721 | ||
722 | PUTBACK; | |
723 | FREETMPS; | |
724 | LEAVE; | |
725 | } | |
726 | ||
727 | If I<call_AddSubtract> is called like this | |
728 | ||
729 | call_AddSubtract(7, 4); | |
730 | ||
731 | then here is the output | |
732 | ||
733 | 7 - 4 = 3 | |
734 | 7 + 4 = 11 | |
735 | ||
736 | Notes | |
737 | ||
738 | =over 5 | |
739 | ||
740 | =item 1. | |
741 | ||
742 | We wanted list context, so G_ARRAY was used. | |
743 | ||
744 | =item 2. | |
745 | ||
746 | Not surprisingly C<POPi> is used twice this time because we were | |
747 | retrieving 2 values from the stack. The important thing to note is that | |
748 | when using the C<POP*> macros they come off the stack in I<reverse> | |
749 | order. | |
750 | ||
751 | =back | |
752 | ||
753 | =head2 Returning a list in a scalar context | |
754 | ||
755 | Say the Perl subroutine in the previous section was called in a scalar | |
756 | context, like this | |
757 | ||
758 | static void | |
759 | call_AddSubScalar(a, b) | |
760 | int a; | |
761 | int b; | |
762 | { | |
763 | dSP; | |
764 | int count; | |
765 | int i; | |
766 | ||
767 | ENTER; | |
768 | SAVETMPS; | |
769 | ||
770 | PUSHMARK(SP); | |
771 | XPUSHs(sv_2mortal(newSViv(a))); | |
772 | XPUSHs(sv_2mortal(newSViv(b))); | |
773 | PUTBACK; | |
774 | ||
775 | count = call_pv("AddSubtract", G_SCALAR); | |
776 | ||
777 | SPAGAIN; | |
778 | ||
779 | printf ("Items Returned = %d\n", count); | |
780 | ||
781 | for (i = 1; i <= count; ++i) | |
782 | printf ("Value %d = %d\n", i, POPi); | |
783 | ||
784 | PUTBACK; | |
785 | FREETMPS; | |
786 | LEAVE; | |
787 | } | |
788 | ||
789 | The other modification made is that I<call_AddSubScalar> will print the | |
790 | number of items returned from the Perl subroutine and their value (for | |
791 | simplicity it assumes that they are integer). So if | |
792 | I<call_AddSubScalar> is called | |
793 | ||
794 | call_AddSubScalar(7, 4); | |
795 | ||
796 | then the output will be | |
797 | ||
798 | Items Returned = 1 | |
799 | Value 1 = 3 | |
800 | ||
801 | In this case the main point to note is that only the last item in the | |
802 | list is returned from the subroutine, I<AddSubtract> actually made it back to | |
803 | I<call_AddSubScalar>. | |
804 | ||
805 | ||
806 | =head2 Returning Data from Perl via the parameter list | |
807 | ||
808 | It is also possible to return values directly via the parameter list - | |
809 | whether it is actually desirable to do it is another matter entirely. | |
810 | ||
811 | The Perl subroutine, I<Inc>, below takes 2 parameters and increments | |
812 | each directly. | |
813 | ||
814 | sub Inc | |
815 | { | |
816 | ++ $_[0]; | |
817 | ++ $_[1]; | |
818 | } | |
819 | ||
820 | and here is a C function to call it. | |
821 | ||
822 | static void | |
823 | call_Inc(a, b) | |
824 | int a; | |
825 | int b; | |
826 | { | |
827 | dSP; | |
828 | int count; | |
829 | SV * sva; | |
830 | SV * svb; | |
831 | ||
832 | ENTER; | |
833 | SAVETMPS; | |
834 | ||
835 | sva = sv_2mortal(newSViv(a)); | |
836 | svb = sv_2mortal(newSViv(b)); | |
837 | ||
838 | PUSHMARK(SP); | |
839 | XPUSHs(sva); | |
840 | XPUSHs(svb); | |
841 | PUTBACK; | |
842 | ||
843 | count = call_pv("Inc", G_DISCARD); | |
844 | ||
845 | if (count != 0) | |
846 | croak ("call_Inc: expected 0 values from 'Inc', got %d\n", | |
847 | count); | |
848 | ||
849 | printf ("%d + 1 = %d\n", a, SvIV(sva)); | |
850 | printf ("%d + 1 = %d\n", b, SvIV(svb)); | |
851 | ||
852 | FREETMPS; | |
853 | LEAVE; | |
854 | } | |
855 | ||
856 | To be able to access the two parameters that were pushed onto the stack | |
857 | after they return from I<call_pv> it is necessary to make a note | |
858 | of their addresses--thus the two variables C<sva> and C<svb>. | |
859 | ||
860 | The reason this is necessary is that the area of the Perl stack which | |
861 | held them will very likely have been overwritten by something else by | |
862 | the time control returns from I<call_pv>. | |
863 | ||
864 | ||
865 | ||
866 | ||
867 | =head2 Using G_EVAL | |
868 | ||
869 | Now an example using G_EVAL. Below is a Perl subroutine which computes | |
870 | the difference of its 2 parameters. If this would result in a negative | |
871 | result, the subroutine calls I<die>. | |
872 | ||
873 | sub Subtract | |
874 | { | |
875 | my ($a, $b) = @_; | |
876 | ||
877 | die "death can be fatal\n" if $a < $b; | |
878 | ||
879 | $a - $b; | |
880 | } | |
881 | ||
882 | and some C to call it | |
883 | ||
884 | static void | |
885 | call_Subtract(a, b) | |
886 | int a; | |
887 | int b; | |
888 | { | |
889 | dSP; | |
890 | int count; | |
891 | ||
892 | ENTER; | |
893 | SAVETMPS; | |
894 | ||
895 | PUSHMARK(SP); | |
896 | XPUSHs(sv_2mortal(newSViv(a))); | |
897 | XPUSHs(sv_2mortal(newSViv(b))); | |
898 | PUTBACK; | |
899 | ||
900 | count = call_pv("Subtract", G_EVAL|G_SCALAR); | |
901 | ||
902 | SPAGAIN; | |
903 | ||
904 | /* Check the eval first */ | |
905 | if (SvTRUE(ERRSV)) | |
906 | { | |
907 | STRLEN n_a; | |
908 | printf ("Uh oh - %s\n", SvPV(ERRSV, n_a)); | |
909 | POPs; | |
910 | } | |
911 | else | |
912 | { | |
913 | if (count != 1) | |
914 | croak("call_Subtract: wanted 1 value from 'Subtract', got %d\n", | |
915 | count); | |
916 | ||
917 | printf ("%d - %d = %d\n", a, b, POPi); | |
918 | } | |
919 | ||
920 | PUTBACK; | |
921 | FREETMPS; | |
922 | LEAVE; | |
923 | } | |
924 | ||
925 | If I<call_Subtract> is called thus | |
926 | ||
927 | call_Subtract(4, 5) | |
928 | ||
929 | the following will be printed | |
930 | ||
931 | Uh oh - death can be fatal | |
932 | ||
933 | Notes | |
934 | ||
935 | =over 5 | |
936 | ||
937 | =item 1. | |
938 | ||
939 | We want to be able to catch the I<die> so we have used the G_EVAL | |
940 | flag. Not specifying this flag would mean that the program would | |
941 | terminate immediately at the I<die> statement in the subroutine | |
942 | I<Subtract>. | |
943 | ||
944 | =item 2. | |
945 | ||
946 | The code | |
947 | ||
948 | if (SvTRUE(ERRSV)) | |
949 | { | |
950 | STRLEN n_a; | |
951 | printf ("Uh oh - %s\n", SvPV(ERRSV, n_a)); | |
952 | POPs; | |
953 | } | |
954 | ||
955 | is the direct equivalent of this bit of Perl | |
956 | ||
957 | print "Uh oh - $@\n" if $@; | |
958 | ||
959 | C<PL_errgv> is a perl global of type C<GV *> that points to the | |
960 | symbol table entry containing the error. C<ERRSV> therefore | |
961 | refers to the C equivalent of C<$@>. | |
962 | ||
963 | =item 3. | |
964 | ||
965 | Note that the stack is popped using C<POPs> in the block where | |
966 | C<SvTRUE(ERRSV)> is true. This is necessary because whenever a | |
967 | I<call_*> function invoked with G_EVAL|G_SCALAR returns an error, | |
968 | the top of the stack holds the value I<undef>. Because we want the | |
969 | program to continue after detecting this error, it is essential that | |
970 | the stack is tidied up by removing the I<undef>. | |
971 | ||
972 | =back | |
973 | ||
974 | ||
975 | =head2 Using G_KEEPERR | |
976 | ||
977 | Consider this rather facetious example, where we have used an XS | |
978 | version of the call_Subtract example above inside a destructor: | |
979 | ||
980 | package Foo; | |
981 | sub new { bless {}, $_[0] } | |
982 | sub Subtract { | |
983 | my($a,$b) = @_; | |
984 | die "death can be fatal" if $a < $b; | |
985 | $a - $b; | |
986 | } | |
987 | sub DESTROY { call_Subtract(5, 4); } | |
988 | sub foo { die "foo dies"; } | |
989 | ||
990 | package main; | |
991 | eval { Foo->new->foo }; | |
992 | print "Saw: $@" if $@; # should be, but isn't | |
993 | ||
994 | This example will fail to recognize that an error occurred inside the | |
995 | C<eval {}>. Here's why: the call_Subtract code got executed while perl | |
996 | was cleaning up temporaries when exiting the eval block, and because | |
997 | call_Subtract is implemented with I<call_pv> using the G_EVAL | |
998 | flag, it promptly reset C<$@>. This results in the failure of the | |
999 | outermost test for C<$@>, and thereby the failure of the error trap. | |
1000 | ||
1001 | Appending the G_KEEPERR flag, so that the I<call_pv> call in | |
1002 | call_Subtract reads: | |
1003 | ||
1004 | count = call_pv("Subtract", G_EVAL|G_SCALAR|G_KEEPERR); | |
1005 | ||
1006 | will preserve the error and restore reliable error handling. | |
1007 | ||
1008 | =head2 Using call_sv | |
1009 | ||
1010 | In all the previous examples I have 'hard-wired' the name of the Perl | |
1011 | subroutine to be called from C. Most of the time though, it is more | |
1012 | convenient to be able to specify the name of the Perl subroutine from | |
1013 | within the Perl script. | |
1014 | ||
1015 | Consider the Perl code below | |
1016 | ||
1017 | sub fred | |
1018 | { | |
1019 | print "Hello there\n"; | |
1020 | } | |
1021 | ||
1022 | CallSubPV("fred"); | |
1023 | ||
1024 | Here is a snippet of XSUB which defines I<CallSubPV>. | |
1025 | ||
1026 | void | |
1027 | CallSubPV(name) | |
1028 | char * name | |
1029 | CODE: | |
1030 | PUSHMARK(SP); | |
1031 | call_pv(name, G_DISCARD|G_NOARGS); | |
1032 | ||
1033 | That is fine as far as it goes. The thing is, the Perl subroutine | |
1034 | can be specified as only a string. For Perl 4 this was adequate, | |
1035 | but Perl 5 allows references to subroutines and anonymous subroutines. | |
1036 | This is where I<call_sv> is useful. | |
1037 | ||
1038 | The code below for I<CallSubSV> is identical to I<CallSubPV> except | |
1039 | that the C<name> parameter is now defined as an SV* and we use | |
1040 | I<call_sv> instead of I<call_pv>. | |
1041 | ||
1042 | void | |
1043 | CallSubSV(name) | |
1044 | SV * name | |
1045 | CODE: | |
1046 | PUSHMARK(SP); | |
1047 | call_sv(name, G_DISCARD|G_NOARGS); | |
1048 | ||
1049 | Because we are using an SV to call I<fred> the following can all be used | |
1050 | ||
1051 | CallSubSV("fred"); | |
1052 | CallSubSV(\&fred); | |
1053 | $ref = \&fred; | |
1054 | CallSubSV($ref); | |
1055 | CallSubSV( sub { print "Hello there\n" } ); | |
1056 | ||
1057 | As you can see, I<call_sv> gives you much greater flexibility in | |
1058 | how you can specify the Perl subroutine. | |
1059 | ||
1060 | You should note that if it is necessary to store the SV (C<name> in the | |
1061 | example above) which corresponds to the Perl subroutine so that it can | |
1062 | be used later in the program, it not enough just to store a copy of the | |
1063 | pointer to the SV. Say the code above had been like this | |
1064 | ||
1065 | static SV * rememberSub; | |
1066 | ||
1067 | void | |
1068 | SaveSub1(name) | |
1069 | SV * name | |
1070 | CODE: | |
1071 | rememberSub = name; | |
1072 | ||
1073 | void | |
1074 | CallSavedSub1() | |
1075 | CODE: | |
1076 | PUSHMARK(SP); | |
1077 | call_sv(rememberSub, G_DISCARD|G_NOARGS); | |
1078 | ||
1079 | The reason this is wrong is that by the time you come to use the | |
1080 | pointer C<rememberSub> in C<CallSavedSub1>, it may or may not still refer | |
1081 | to the Perl subroutine that was recorded in C<SaveSub1>. This is | |
1082 | particularly true for these cases | |
1083 | ||
1084 | SaveSub1(\&fred); | |
1085 | CallSavedSub1(); | |
1086 | ||
1087 | SaveSub1( sub { print "Hello there\n" } ); | |
1088 | CallSavedSub1(); | |
1089 | ||
1090 | By the time each of the C<SaveSub1> statements above have been executed, | |
1091 | the SV*s which corresponded to the parameters will no longer exist. | |
1092 | Expect an error message from Perl of the form | |
1093 | ||
1094 | Can't use an undefined value as a subroutine reference at ... | |
1095 | ||
1096 | for each of the C<CallSavedSub1> lines. | |
1097 | ||
1098 | Similarly, with this code | |
1099 | ||
1100 | $ref = \&fred; | |
1101 | SaveSub1($ref); | |
1102 | $ref = 47; | |
1103 | CallSavedSub1(); | |
1104 | ||
1105 | you can expect one of these messages (which you actually get is dependent on | |
1106 | the version of Perl you are using) | |
1107 | ||
1108 | Not a CODE reference at ... | |
1109 | Undefined subroutine &main::47 called ... | |
1110 | ||
1111 | The variable $ref may have referred to the subroutine C<fred> | |
1112 | whenever the call to C<SaveSub1> was made but by the time | |
1113 | C<CallSavedSub1> gets called it now holds the number C<47>. Because we | |
1114 | saved only a pointer to the original SV in C<SaveSub1>, any changes to | |
1115 | $ref will be tracked by the pointer C<rememberSub>. This means that | |
1116 | whenever C<CallSavedSub1> gets called, it will attempt to execute the | |
1117 | code which is referenced by the SV* C<rememberSub>. In this case | |
1118 | though, it now refers to the integer C<47>, so expect Perl to complain | |
1119 | loudly. | |
1120 | ||
1121 | A similar but more subtle problem is illustrated with this code | |
1122 | ||
1123 | $ref = \&fred; | |
1124 | SaveSub1($ref); | |
1125 | $ref = \&joe; | |
1126 | CallSavedSub1(); | |
1127 | ||
1128 | This time whenever C<CallSavedSub1> get called it will execute the Perl | |
1129 | subroutine C<joe> (assuming it exists) rather than C<fred> as was | |
1130 | originally requested in the call to C<SaveSub1>. | |
1131 | ||
1132 | To get around these problems it is necessary to take a full copy of the | |
1133 | SV. The code below shows C<SaveSub2> modified to do that | |
1134 | ||
1135 | static SV * keepSub = (SV*)NULL; | |
1136 | ||
1137 | void | |
1138 | SaveSub2(name) | |
1139 | SV * name | |
1140 | CODE: | |
1141 | /* Take a copy of the callback */ | |
1142 | if (keepSub == (SV*)NULL) | |
1143 | /* First time, so create a new SV */ | |
1144 | keepSub = newSVsv(name); | |
1145 | else | |
1146 | /* Been here before, so overwrite */ | |
1147 | SvSetSV(keepSub, name); | |
1148 | ||
1149 | void | |
1150 | CallSavedSub2() | |
1151 | CODE: | |
1152 | PUSHMARK(SP); | |
1153 | call_sv(keepSub, G_DISCARD|G_NOARGS); | |
1154 | ||
1155 | To avoid creating a new SV every time C<SaveSub2> is called, | |
1156 | the function first checks to see if it has been called before. If not, | |
1157 | then space for a new SV is allocated and the reference to the Perl | |
1158 | subroutine, C<name> is copied to the variable C<keepSub> in one | |
1159 | operation using C<newSVsv>. Thereafter, whenever C<SaveSub2> is called | |
1160 | the existing SV, C<keepSub>, is overwritten with the new value using | |
1161 | C<SvSetSV>. | |
1162 | ||
1163 | =head2 Using call_argv | |
1164 | ||
1165 | Here is a Perl subroutine which prints whatever parameters are passed | |
1166 | to it. | |
1167 | ||
1168 | sub PrintList | |
1169 | { | |
1170 | my(@list) = @_; | |
1171 | ||
1172 | foreach (@list) { print "$_\n" } | |
1173 | } | |
1174 | ||
1175 | and here is an example of I<call_argv> which will call | |
1176 | I<PrintList>. | |
1177 | ||
1178 | static char * words[] = {"alpha", "beta", "gamma", "delta", NULL}; | |
1179 | ||
1180 | static void | |
1181 | call_PrintList() | |
1182 | { | |
1183 | dSP; | |
1184 | ||
1185 | call_argv("PrintList", G_DISCARD, words); | |
1186 | } | |
1187 | ||
1188 | Note that it is not necessary to call C<PUSHMARK> in this instance. | |
1189 | This is because I<call_argv> will do it for you. | |
1190 | ||
1191 | =head2 Using call_method | |
1192 | ||
1193 | Consider the following Perl code | |
1194 | ||
1195 | { | |
1196 | package Mine; | |
1197 | ||
1198 | sub new | |
1199 | { | |
1200 | my($type) = shift; | |
1201 | bless [@_] | |
1202 | } | |
1203 | ||
1204 | sub Display | |
1205 | { | |
1206 | my ($self, $index) = @_; | |
1207 | print "$index: $$self[$index]\n"; | |
1208 | } | |
1209 | ||
1210 | sub PrintID | |
1211 | { | |
1212 | my($class) = @_; | |
1213 | print "This is Class $class version 1.0\n"; | |
1214 | } | |
1215 | } | |
1216 | ||
1217 | It implements just a very simple class to manage an array. Apart from | |
1218 | the constructor, C<new>, it declares methods, one static and one | |
1219 | virtual. The static method, C<PrintID>, prints out simply the class | |
1220 | name and a version number. The virtual method, C<Display>, prints out a | |
1221 | single element of the array. Here is an all Perl example of using it. | |
1222 | ||
1223 | $a = new Mine ('red', 'green', 'blue'); | |
1224 | $a->Display(1); | |
1225 | PrintID Mine; | |
1226 | ||
1227 | will print | |
1228 | ||
1229 | 1: green | |
1230 | This is Class Mine version 1.0 | |
1231 | ||
1232 | Calling a Perl method from C is fairly straightforward. The following | |
1233 | things are required | |
1234 | ||
1235 | =over 5 | |
1236 | ||
1237 | =item * | |
1238 | ||
1239 | a reference to the object for a virtual method or the name of the class | |
1240 | for a static method. | |
1241 | ||
1242 | =item * | |
1243 | ||
1244 | the name of the method. | |
1245 | ||
1246 | =item * | |
1247 | ||
1248 | any other parameters specific to the method. | |
1249 | ||
1250 | =back | |
1251 | ||
1252 | Here is a simple XSUB which illustrates the mechanics of calling both | |
1253 | the C<PrintID> and C<Display> methods from C. | |
1254 | ||
1255 | void | |
1256 | call_Method(ref, method, index) | |
1257 | SV * ref | |
1258 | char * method | |
1259 | int index | |
1260 | CODE: | |
1261 | PUSHMARK(SP); | |
1262 | XPUSHs(ref); | |
1263 | XPUSHs(sv_2mortal(newSViv(index))); | |
1264 | PUTBACK; | |
1265 | ||
1266 | call_method(method, G_DISCARD); | |
1267 | ||
1268 | void | |
1269 | call_PrintID(class, method) | |
1270 | char * class | |
1271 | char * method | |
1272 | CODE: | |
1273 | PUSHMARK(SP); | |
1274 | XPUSHs(sv_2mortal(newSVpv(class, 0))); | |
1275 | PUTBACK; | |
1276 | ||
1277 | call_method(method, G_DISCARD); | |
1278 | ||
1279 | ||
1280 | So the methods C<PrintID> and C<Display> can be invoked like this | |
1281 | ||
1282 | $a = new Mine ('red', 'green', 'blue'); | |
1283 | call_Method($a, 'Display', 1); | |
1284 | call_PrintID('Mine', 'PrintID'); | |
1285 | ||
1286 | The only thing to note is that in both the static and virtual methods, | |
1287 | the method name is not passed via the stack--it is used as the first | |
1288 | parameter to I<call_method>. | |
1289 | ||
1290 | =head2 Using GIMME_V | |
1291 | ||
1292 | Here is a trivial XSUB which prints the context in which it is | |
1293 | currently executing. | |
1294 | ||
1295 | void | |
1296 | PrintContext() | |
1297 | CODE: | |
1298 | I32 gimme = GIMME_V; | |
1299 | if (gimme == G_VOID) | |
1300 | printf ("Context is Void\n"); | |
1301 | else if (gimme == G_SCALAR) | |
1302 | printf ("Context is Scalar\n"); | |
1303 | else | |
1304 | printf ("Context is Array\n"); | |
1305 | ||
1306 | and here is some Perl to test it | |
1307 | ||
1308 | PrintContext; | |
1309 | $a = PrintContext; | |
1310 | @a = PrintContext; | |
1311 | ||
1312 | The output from that will be | |
1313 | ||
1314 | Context is Void | |
1315 | Context is Scalar | |
1316 | Context is Array | |
1317 | ||
1318 | =head2 Using Perl to dispose of temporaries | |
1319 | ||
1320 | In the examples given to date, any temporaries created in the callback | |
1321 | (i.e., parameters passed on the stack to the I<call_*> function or | |
1322 | values returned via the stack) have been freed by one of these methods | |
1323 | ||
1324 | =over 5 | |
1325 | ||
1326 | =item * | |
1327 | ||
1328 | specifying the G_DISCARD flag with I<call_*>. | |
1329 | ||
1330 | =item * | |
1331 | ||
1332 | explicitly disposed of using the C<ENTER>/C<SAVETMPS> - | |
1333 | C<FREETMPS>/C<LEAVE> pairing. | |
1334 | ||
1335 | =back | |
1336 | ||
1337 | There is another method which can be used, namely letting Perl do it | |
1338 | for you automatically whenever it regains control after the callback | |
1339 | has terminated. This is done by simply not using the | |
1340 | ||
1341 | ENTER; | |
1342 | SAVETMPS; | |
1343 | ... | |
1344 | FREETMPS; | |
1345 | LEAVE; | |
1346 | ||
1347 | sequence in the callback (and not, of course, specifying the G_DISCARD | |
1348 | flag). | |
1349 | ||
1350 | If you are going to use this method you have to be aware of a possible | |
1351 | memory leak which can arise under very specific circumstances. To | |
1352 | explain these circumstances you need to know a bit about the flow of | |
1353 | control between Perl and the callback routine. | |
1354 | ||
1355 | The examples given at the start of the document (an error handler and | |
1356 | an event driven program) are typical of the two main sorts of flow | |
1357 | control that you are likely to encounter with callbacks. There is a | |
1358 | very important distinction between them, so pay attention. | |
1359 | ||
1360 | In the first example, an error handler, the flow of control could be as | |
1361 | follows. You have created an interface to an external library. | |
1362 | Control can reach the external library like this | |
1363 | ||
1364 | perl --> XSUB --> external library | |
1365 | ||
1366 | Whilst control is in the library, an error condition occurs. You have | |
1367 | previously set up a Perl callback to handle this situation, so it will | |
1368 | get executed. Once the callback has finished, control will drop back to | |
1369 | Perl again. Here is what the flow of control will be like in that | |
1370 | situation | |
1371 | ||
1372 | perl --> XSUB --> external library | |
1373 | ... | |
1374 | error occurs | |
1375 | ... | |
1376 | external library --> call_* --> perl | |
1377 | | | |
1378 | perl <-- XSUB <-- external library <-- call_* <----+ | |
1379 | ||
1380 | After processing of the error using I<call_*> is completed, | |
1381 | control reverts back to Perl more or less immediately. | |
1382 | ||
1383 | In the diagram, the further right you go the more deeply nested the | |
1384 | scope is. It is only when control is back with perl on the extreme | |
1385 | left of the diagram that you will have dropped back to the enclosing | |
1386 | scope and any temporaries you have left hanging around will be freed. | |
1387 | ||
1388 | In the second example, an event driven program, the flow of control | |
1389 | will be more like this | |
1390 | ||
1391 | perl --> XSUB --> event handler | |
1392 | ... | |
1393 | event handler --> call_* --> perl | |
1394 | | | |
1395 | event handler <-- call_* <----+ | |
1396 | ... | |
1397 | event handler --> call_* --> perl | |
1398 | | | |
1399 | event handler <-- call_* <----+ | |
1400 | ... | |
1401 | event handler --> call_* --> perl | |
1402 | | | |
1403 | event handler <-- call_* <----+ | |
1404 | ||
1405 | In this case the flow of control can consist of only the repeated | |
1406 | sequence | |
1407 | ||
1408 | event handler --> call_* --> perl | |
1409 | ||
1410 | for practically the complete duration of the program. This means that | |
1411 | control may I<never> drop back to the surrounding scope in Perl at the | |
1412 | extreme left. | |
1413 | ||
1414 | So what is the big problem? Well, if you are expecting Perl to tidy up | |
1415 | those temporaries for you, you might be in for a long wait. For Perl | |
1416 | to dispose of your temporaries, control must drop back to the | |
1417 | enclosing scope at some stage. In the event driven scenario that may | |
1418 | never happen. This means that as time goes on, your program will | |
1419 | create more and more temporaries, none of which will ever be freed. As | |
1420 | each of these temporaries consumes some memory your program will | |
1421 | eventually consume all the available memory in your system--kapow! | |
1422 | ||
1423 | So here is the bottom line--if you are sure that control will revert | |
1424 | back to the enclosing Perl scope fairly quickly after the end of your | |
1425 | callback, then it isn't absolutely necessary to dispose explicitly of | |
1426 | any temporaries you may have created. Mind you, if you are at all | |
1427 | uncertain about what to do, it doesn't do any harm to tidy up anyway. | |
1428 | ||
1429 | ||
1430 | =head2 Strategies for storing Callback Context Information | |
1431 | ||
1432 | ||
1433 | Potentially one of the trickiest problems to overcome when designing a | |
1434 | callback interface can be figuring out how to store the mapping between | |
1435 | the C callback function and the Perl equivalent. | |
1436 | ||
1437 | To help understand why this can be a real problem first consider how a | |
1438 | callback is set up in an all C environment. Typically a C API will | |
1439 | provide a function to register a callback. This will expect a pointer | |
1440 | to a function as one of its parameters. Below is a call to a | |
1441 | hypothetical function C<register_fatal> which registers the C function | |
1442 | to get called when a fatal error occurs. | |
1443 | ||
1444 | register_fatal(cb1); | |
1445 | ||
1446 | The single parameter C<cb1> is a pointer to a function, so you must | |
1447 | have defined C<cb1> in your code, say something like this | |
1448 | ||
1449 | static void | |
1450 | cb1() | |
1451 | { | |
1452 | printf ("Fatal Error\n"); | |
1453 | exit(1); | |
1454 | } | |
1455 | ||
1456 | Now change that to call a Perl subroutine instead | |
1457 | ||
1458 | static SV * callback = (SV*)NULL; | |
1459 | ||
1460 | static void | |
1461 | cb1() | |
1462 | { | |
1463 | dSP; | |
1464 | ||
1465 | PUSHMARK(SP); | |
1466 | ||
1467 | /* Call the Perl sub to process the callback */ | |
1468 | call_sv(callback, G_DISCARD); | |
1469 | } | |
1470 | ||
1471 | ||
1472 | void | |
1473 | register_fatal(fn) | |
1474 | SV * fn | |
1475 | CODE: | |
1476 | /* Remember the Perl sub */ | |
1477 | if (callback == (SV*)NULL) | |
1478 | callback = newSVsv(fn); | |
1479 | else | |
1480 | SvSetSV(callback, fn); | |
1481 | ||
1482 | /* register the callback with the external library */ | |
1483 | register_fatal(cb1); | |
1484 | ||
1485 | where the Perl equivalent of C<register_fatal> and the callback it | |
1486 | registers, C<pcb1>, might look like this | |
1487 | ||
1488 | # Register the sub pcb1 | |
1489 | register_fatal(\&pcb1); | |
1490 | ||
1491 | sub pcb1 | |
1492 | { | |
1493 | die "I'm dying...\n"; | |
1494 | } | |
1495 | ||
1496 | The mapping between the C callback and the Perl equivalent is stored in | |
1497 | the global variable C<callback>. | |
1498 | ||
1499 | This will be adequate if you ever need to have only one callback | |
1500 | registered at any time. An example could be an error handler like the | |
1501 | code sketched out above. Remember though, repeated calls to | |
1502 | C<register_fatal> will replace the previously registered callback | |
1503 | function with the new one. | |
1504 | ||
1505 | Say for example you want to interface to a library which allows asynchronous | |
1506 | file i/o. In this case you may be able to register a callback whenever | |
1507 | a read operation has completed. To be of any use we want to be able to | |
1508 | call separate Perl subroutines for each file that is opened. As it | |
1509 | stands, the error handler example above would not be adequate as it | |
1510 | allows only a single callback to be defined at any time. What we | |
1511 | require is a means of storing the mapping between the opened file and | |
1512 | the Perl subroutine we want to be called for that file. | |
1513 | ||
1514 | Say the i/o library has a function C<asynch_read> which associates a C | |
1515 | function C<ProcessRead> with a file handle C<fh>--this assumes that it | |
1516 | has also provided some routine to open the file and so obtain the file | |
1517 | handle. | |
1518 | ||
1519 | asynch_read(fh, ProcessRead) | |
1520 | ||
1521 | This may expect the C I<ProcessRead> function of this form | |
1522 | ||
1523 | void | |
1524 | ProcessRead(fh, buffer) | |
1525 | int fh; | |
1526 | char * buffer; | |
1527 | { | |
1528 | ... | |
1529 | } | |
1530 | ||
1531 | To provide a Perl interface to this library we need to be able to map | |
1532 | between the C<fh> parameter and the Perl subroutine we want called. A | |
1533 | hash is a convenient mechanism for storing this mapping. The code | |
1534 | below shows a possible implementation | |
1535 | ||
1536 | static HV * Mapping = (HV*)NULL; | |
1537 | ||
1538 | void | |
1539 | asynch_read(fh, callback) | |
1540 | int fh | |
1541 | SV * callback | |
1542 | CODE: | |
1543 | /* If the hash doesn't already exist, create it */ | |
1544 | if (Mapping == (HV*)NULL) | |
1545 | Mapping = newHV(); | |
1546 | ||
1547 | /* Save the fh -> callback mapping */ | |
1548 | hv_store(Mapping, (char*)&fh, sizeof(fh), newSVsv(callback), 0); | |
1549 | ||
1550 | /* Register with the C Library */ | |
1551 | asynch_read(fh, asynch_read_if); | |
1552 | ||
1553 | and C<asynch_read_if> could look like this | |
1554 | ||
1555 | static void | |
1556 | asynch_read_if(fh, buffer) | |
1557 | int fh; | |
1558 | char * buffer; | |
1559 | { | |
1560 | dSP; | |
1561 | SV ** sv; | |
1562 | ||
1563 | /* Get the callback associated with fh */ | |
1564 | sv = hv_fetch(Mapping, (char*)&fh , sizeof(fh), FALSE); | |
1565 | if (sv == (SV**)NULL) | |
1566 | croak("Internal error...\n"); | |
1567 | ||
1568 | PUSHMARK(SP); | |
1569 | XPUSHs(sv_2mortal(newSViv(fh))); | |
1570 | XPUSHs(sv_2mortal(newSVpv(buffer, 0))); | |
1571 | PUTBACK; | |
1572 | ||
1573 | /* Call the Perl sub */ | |
1574 | call_sv(*sv, G_DISCARD); | |
1575 | } | |
1576 | ||
1577 | For completeness, here is C<asynch_close>. This shows how to remove | |
1578 | the entry from the hash C<Mapping>. | |
1579 | ||
1580 | void | |
1581 | asynch_close(fh) | |
1582 | int fh | |
1583 | CODE: | |
1584 | /* Remove the entry from the hash */ | |
1585 | (void) hv_delete(Mapping, (char*)&fh, sizeof(fh), G_DISCARD); | |
1586 | ||
1587 | /* Now call the real asynch_close */ | |
1588 | asynch_close(fh); | |
1589 | ||
1590 | So the Perl interface would look like this | |
1591 | ||
1592 | sub callback1 | |
1593 | { | |
1594 | my($handle, $buffer) = @_; | |
1595 | } | |
1596 | ||
1597 | # Register the Perl callback | |
1598 | asynch_read($fh, \&callback1); | |
1599 | ||
1600 | asynch_close($fh); | |
1601 | ||
1602 | The mapping between the C callback and Perl is stored in the global | |
1603 | hash C<Mapping> this time. Using a hash has the distinct advantage that | |
1604 | it allows an unlimited number of callbacks to be registered. | |
1605 | ||
1606 | What if the interface provided by the C callback doesn't contain a | |
1607 | parameter which allows the file handle to Perl subroutine mapping? Say | |
1608 | in the asynchronous i/o package, the callback function gets passed only | |
1609 | the C<buffer> parameter like this | |
1610 | ||
1611 | void | |
1612 | ProcessRead(buffer) | |
1613 | char * buffer; | |
1614 | { | |
1615 | ... | |
1616 | } | |
1617 | ||
1618 | Without the file handle there is no straightforward way to map from the | |
1619 | C callback to the Perl subroutine. | |
1620 | ||
1621 | In this case a possible way around this problem is to predefine a | |
1622 | series of C functions to act as the interface to Perl, thus | |
1623 | ||
1624 | #define MAX_CB 3 | |
1625 | #define NULL_HANDLE -1 | |
1626 | typedef void (*FnMap)(); | |
1627 | ||
1628 | struct MapStruct { | |
1629 | FnMap Function; | |
1630 | SV * PerlSub; | |
1631 | int Handle; | |
1632 | }; | |
1633 | ||
1634 | static void fn1(); | |
1635 | static void fn2(); | |
1636 | static void fn3(); | |
1637 | ||
1638 | static struct MapStruct Map [MAX_CB] = | |
1639 | { | |
1640 | { fn1, NULL, NULL_HANDLE }, | |
1641 | { fn2, NULL, NULL_HANDLE }, | |
1642 | { fn3, NULL, NULL_HANDLE } | |
1643 | }; | |
1644 | ||
1645 | static void | |
1646 | Pcb(index, buffer) | |
1647 | int index; | |
1648 | char * buffer; | |
1649 | { | |
1650 | dSP; | |
1651 | ||
1652 | PUSHMARK(SP); | |
1653 | XPUSHs(sv_2mortal(newSVpv(buffer, 0))); | |
1654 | PUTBACK; | |
1655 | ||
1656 | /* Call the Perl sub */ | |
1657 | call_sv(Map[index].PerlSub, G_DISCARD); | |
1658 | } | |
1659 | ||
1660 | static void | |
1661 | fn1(buffer) | |
1662 | char * buffer; | |
1663 | { | |
1664 | Pcb(0, buffer); | |
1665 | } | |
1666 | ||
1667 | static void | |
1668 | fn2(buffer) | |
1669 | char * buffer; | |
1670 | { | |
1671 | Pcb(1, buffer); | |
1672 | } | |
1673 | ||
1674 | static void | |
1675 | fn3(buffer) | |
1676 | char * buffer; | |
1677 | { | |
1678 | Pcb(2, buffer); | |
1679 | } | |
1680 | ||
1681 | void | |
1682 | array_asynch_read(fh, callback) | |
1683 | int fh | |
1684 | SV * callback | |
1685 | CODE: | |
1686 | int index; | |
1687 | int null_index = MAX_CB; | |
1688 | ||
1689 | /* Find the same handle or an empty entry */ | |
1690 | for (index = 0; index < MAX_CB; ++index) | |
1691 | { | |
1692 | if (Map[index].Handle == fh) | |
1693 | break; | |
1694 | ||
1695 | if (Map[index].Handle == NULL_HANDLE) | |
1696 | null_index = index; | |
1697 | } | |
1698 | ||
1699 | if (index == MAX_CB && null_index == MAX_CB) | |
1700 | croak ("Too many callback functions registered\n"); | |
1701 | ||
1702 | if (index == MAX_CB) | |
1703 | index = null_index; | |
1704 | ||
1705 | /* Save the file handle */ | |
1706 | Map[index].Handle = fh; | |
1707 | ||
1708 | /* Remember the Perl sub */ | |
1709 | if (Map[index].PerlSub == (SV*)NULL) | |
1710 | Map[index].PerlSub = newSVsv(callback); | |
1711 | else | |
1712 | SvSetSV(Map[index].PerlSub, callback); | |
1713 | ||
1714 | asynch_read(fh, Map[index].Function); | |
1715 | ||
1716 | void | |
1717 | array_asynch_close(fh) | |
1718 | int fh | |
1719 | CODE: | |
1720 | int index; | |
1721 | ||
1722 | /* Find the file handle */ | |
1723 | for (index = 0; index < MAX_CB; ++ index) | |
1724 | if (Map[index].Handle == fh) | |
1725 | break; | |
1726 | ||
1727 | if (index == MAX_CB) | |
1728 | croak ("could not close fh %d\n", fh); | |
1729 | ||
1730 | Map[index].Handle = NULL_HANDLE; | |
1731 | SvREFCNT_dec(Map[index].PerlSub); | |
1732 | Map[index].PerlSub = (SV*)NULL; | |
1733 | ||
1734 | asynch_close(fh); | |
1735 | ||
1736 | In this case the functions C<fn1>, C<fn2>, and C<fn3> are used to | |
1737 | remember the Perl subroutine to be called. Each of the functions holds | |
1738 | a separate hard-wired index which is used in the function C<Pcb> to | |
1739 | access the C<Map> array and actually call the Perl subroutine. | |
1740 | ||
1741 | There are some obvious disadvantages with this technique. | |
1742 | ||
1743 | Firstly, the code is considerably more complex than with the previous | |
1744 | example. | |
1745 | ||
1746 | Secondly, there is a hard-wired limit (in this case 3) to the number of | |
1747 | callbacks that can exist simultaneously. The only way to increase the | |
1748 | limit is by modifying the code to add more functions and then | |
1749 | recompiling. None the less, as long as the number of functions is | |
1750 | chosen with some care, it is still a workable solution and in some | |
1751 | cases is the only one available. | |
1752 | ||
1753 | To summarize, here are a number of possible methods for you to consider | |
1754 | for storing the mapping between C and the Perl callback | |
1755 | ||
1756 | =over 5 | |
1757 | ||
1758 | =item 1. Ignore the problem - Allow only 1 callback | |
1759 | ||
1760 | For a lot of situations, like interfacing to an error handler, this may | |
1761 | be a perfectly adequate solution. | |
1762 | ||
1763 | =item 2. Create a sequence of callbacks - hard wired limit | |
1764 | ||
1765 | If it is impossible to tell from the parameters passed back from the C | |
1766 | callback what the context is, then you may need to create a sequence of C | |
1767 | callback interface functions, and store pointers to each in an array. | |
1768 | ||
1769 | =item 3. Use a parameter to map to the Perl callback | |
1770 | ||
1771 | A hash is an ideal mechanism to store the mapping between C and Perl. | |
1772 | ||
1773 | =back | |
1774 | ||
1775 | ||
1776 | =head2 Alternate Stack Manipulation | |
1777 | ||
1778 | ||
1779 | Although I have made use of only the C<POP*> macros to access values | |
1780 | returned from Perl subroutines, it is also possible to bypass these | |
1781 | macros and read the stack using the C<ST> macro (See L<perlxs> for a | |
1782 | full description of the C<ST> macro). | |
1783 | ||
1784 | Most of the time the C<POP*> macros should be adequate, the main | |
1785 | problem with them is that they force you to process the returned values | |
1786 | in sequence. This may not be the most suitable way to process the | |
1787 | values in some cases. What we want is to be able to access the stack in | |
1788 | a random order. The C<ST> macro as used when coding an XSUB is ideal | |
1789 | for this purpose. | |
1790 | ||
1791 | The code below is the example given in the section I<Returning a list | |
1792 | of values> recoded to use C<ST> instead of C<POP*>. | |
1793 | ||
1794 | static void | |
1795 | call_AddSubtract2(a, b) | |
1796 | int a; | |
1797 | int b; | |
1798 | { | |
1799 | dSP; | |
1800 | I32 ax; | |
1801 | int count; | |
1802 | ||
1803 | ENTER; | |
1804 | SAVETMPS; | |
1805 | ||
1806 | PUSHMARK(SP); | |
1807 | XPUSHs(sv_2mortal(newSViv(a))); | |
1808 | XPUSHs(sv_2mortal(newSViv(b))); | |
1809 | PUTBACK; | |
1810 | ||
1811 | count = call_pv("AddSubtract", G_ARRAY); | |
1812 | ||
1813 | SPAGAIN; | |
1814 | SP -= count; | |
1815 | ax = (SP - PL_stack_base) + 1; | |
1816 | ||
1817 | if (count != 2) | |
1818 | croak("Big trouble\n"); | |
1819 | ||
1820 | printf ("%d + %d = %d\n", a, b, SvIV(ST(0))); | |
1821 | printf ("%d - %d = %d\n", a, b, SvIV(ST(1))); | |
1822 | ||
1823 | PUTBACK; | |
1824 | FREETMPS; | |
1825 | LEAVE; | |
1826 | } | |
1827 | ||
1828 | Notes | |
1829 | ||
1830 | =over 5 | |
1831 | ||
1832 | =item 1. | |
1833 | ||
1834 | Notice that it was necessary to define the variable C<ax>. This is | |
1835 | because the C<ST> macro expects it to exist. If we were in an XSUB it | |
1836 | would not be necessary to define C<ax> as it is already defined for | |
1837 | you. | |
1838 | ||
1839 | =item 2. | |
1840 | ||
1841 | The code | |
1842 | ||
1843 | SPAGAIN; | |
1844 | SP -= count; | |
1845 | ax = (SP - PL_stack_base) + 1; | |
1846 | ||
1847 | sets the stack up so that we can use the C<ST> macro. | |
1848 | ||
1849 | =item 3. | |
1850 | ||
1851 | Unlike the original coding of this example, the returned | |
1852 | values are not accessed in reverse order. So C<ST(0)> refers to the | |
1853 | first value returned by the Perl subroutine and C<ST(count-1)> | |
1854 | refers to the last. | |
1855 | ||
1856 | =back | |
1857 | ||
1858 | =head2 Creating and calling an anonymous subroutine in C | |
1859 | ||
1860 | As we've already shown, C<call_sv> can be used to invoke an | |
1861 | anonymous subroutine. However, our example showed a Perl script | |
1862 | invoking an XSUB to perform this operation. Let's see how it can be | |
1863 | done inside our C code: | |
1864 | ||
1865 | ... | |
1866 | ||
1867 | SV *cvrv = eval_pv("sub { print 'You will not find me cluttering any namespace!' }", TRUE); | |
1868 | ||
1869 | ... | |
1870 | ||
1871 | call_sv(cvrv, G_VOID|G_NOARGS); | |
1872 | ||
1873 | C<eval_pv> is used to compile the anonymous subroutine, which | |
1874 | will be the return value as well (read more about C<eval_pv> in | |
1875 | L<perlapi/eval_pv>). Once this code reference is in hand, it | |
1876 | can be mixed in with all the previous examples we've shown. | |
1877 | ||
1878 | =head1 SEE ALSO | |
1879 | ||
1880 | L<perlxs>, L<perlguts>, L<perlembed> | |
1881 | ||
1882 | =head1 AUTHOR | |
1883 | ||
1884 | Paul Marquess | |
1885 | ||
1886 | Special thanks to the following people who assisted in the creation of | |
1887 | the document. | |
1888 | ||
1889 | Jeff Okamoto, Tim Bunce, Nick Gianniotis, Steve Kelem, Gurusamy Sarathy | |
1890 | and Larry Wall. | |
1891 | ||
1892 | =head1 DATE | |
1893 | ||
1894 | Version 1.3, 14th Apr 1997 |