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a0d0e21e LW |
1 | =head1 NAME |
2 | ||
3 | perlcall - Perl calling conventions from C | |
4 | ||
5 | =head1 DESCRIPTION | |
6 | ||
d1b91892 | 7 | The purpose of this document is to show you how to call Perl subroutines |
5f05dabc | 8 | directly from C, i.e., how to write I<callbacks>. |
a0d0e21e | 9 | |
d1b91892 AD |
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. | |
a0d0e21e | 14 | |
d1b91892 | 15 | Examples where callbacks are necessary include |
a0d0e21e LW |
16 | |
17 | =over 5 | |
18 | ||
d1b91892 | 19 | =item * An Error Handler |
a0d0e21e LW |
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 | |
d1b91892 AD |
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. | |
a0d0e21e | 27 | |
95e84303 | 28 | =item * An Event-Driven Program |
a0d0e21e | 29 | |
d1b91892 | 30 | The classic example of where callbacks are used is when writing an |
b0b54b5e | 31 | event driven program, such as for an X11 application. In this case |
184e9718 | 32 | you register functions to be called whenever specific events occur, |
5f05dabc | 33 | e.g., a mouse button is pressed, the cursor moves into a window or a |
d1b91892 | 34 | menu item is selected. |
a0d0e21e LW |
35 | |
36 | =back | |
37 | ||
d1b91892 AD |
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>. | |
a0d0e21e | 43 | |
d1b91892 | 44 | Before you launch yourself head first into the rest of this document, |
02f6dca1 FC |
45 | it would be a good idea to have read the following two documents--L<perlxs> |
46 | and L<perlguts>. | |
a0d0e21e | 47 | |
4929bf7b | 48 | =head1 THE CALL_ FUNCTIONS |
a0d0e21e | 49 | |
d1b91892 AD |
50 | Although this stuff is easier to explain using examples, you first need |
51 | be aware of a few important definitions. | |
a0d0e21e | 52 | |
d1b91892 AD |
53 | Perl has a number of C functions that allow you to call Perl |
54 | subroutines. They are | |
a0d0e21e | 55 | |
4358a253 SS |
56 | I32 call_sv(SV* sv, I32 flags); |
57 | I32 call_pv(char *subname, I32 flags); | |
58 | I32 call_method(char *methname, I32 flags); | |
5aaab254 | 59 | I32 call_argv(char *subname, I32 flags, char **argv); |
a0d0e21e | 60 | |
4929bf7b | 61 | The key function is I<call_sv>. All the other functions are |
d1b91892 | 62 | fairly simple wrappers which make it easier to call Perl subroutines in |
4929bf7b | 63 | special cases. At the end of the day they will all call I<call_sv> |
5f05dabc | 64 | to invoke the Perl subroutine. |
d1b91892 | 65 | |
4929bf7b | 66 | All the I<call_*> functions have a C<flags> parameter which is |
d1b91892 AD |
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 | ||
4929bf7b | 75 | =item call_sv |
d1b91892 | 76 | |
02f6dca1 | 77 | I<call_sv> takes two parameters. The first, C<sv>, is an SV*. |
d1b91892 AD |
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 | |
4929bf7b GS |
80 | subroutine. The section, I<Using call_sv>, shows how you can make |
81 | use of I<call_sv>. | |
d1b91892 | 82 | |
4929bf7b | 83 | =item call_pv |
d1b91892 | 84 | |
4929bf7b | 85 | The function, I<call_pv>, is similar to I<call_sv> except it |
d1b91892 | 86 | expects its first parameter to be a C char* which identifies the Perl |
4929bf7b | 87 | subroutine you want to call, e.g., C<call_pv("fred", 0)>. If the |
d1b91892 | 88 | subroutine you want to call is in another package, just include the |
5f05dabc | 89 | package name in the string, e.g., C<"pkg::fred">. |
d1b91892 | 90 | |
4929bf7b | 91 | =item call_method |
d1b91892 | 92 | |
4929bf7b | 93 | The function I<call_method> is used to call a method from a Perl |
d1b91892 AD |
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 | |
4929bf7b GS |
99 | static and virtual methods and L<Using call_method> for an example |
100 | of using I<call_method>. | |
d1b91892 | 101 | |
4929bf7b | 102 | =item call_argv |
d1b91892 | 103 | |
4929bf7b | 104 | I<call_argv> calls the Perl subroutine specified by the C string |
d1b91892 | 105 | stored in the C<subname> parameter. It also takes the usual C<flags> |
02f6dca1 | 106 | parameter. The final parameter, C<argv>, consists of a NULL-terminated |
d1b91892 | 107 | list of C strings to be passed as parameters to the Perl subroutine. |
4929bf7b | 108 | See I<Using call_argv>. |
d1b91892 AD |
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 | |
19799a22 | 119 | stop someone from doing something unexpected--don't say you haven't |
d1b91892 AD |
120 | been warned. |
121 | ||
122 | =head1 FLAG VALUES | |
123 | ||
0b06a753 NT |
124 | The C<flags> parameter in all the I<call_*> functions is one of G_VOID, |
125 | G_SCALAR, or G_ARRAY, which indicate the call context, OR'ed together | |
126 | with a bit mask of any combination of the other G_* symbols defined below. | |
d1b91892 | 127 | |
54310121 | 128 | =head2 G_VOID |
129 | ||
130 | Calls the Perl subroutine in a void context. | |
131 | ||
132 | This flag has 2 effects: | |
133 | ||
134 | =over 5 | |
135 | ||
136 | =item 1. | |
137 | ||
138 | It indicates to the subroutine being called that it is executing in | |
139 | a void context (if it executes I<wantarray> the result will be the | |
140 | undefined value). | |
141 | ||
142 | =item 2. | |
143 | ||
144 | It ensures that nothing is actually returned from the subroutine. | |
145 | ||
146 | =back | |
147 | ||
4929bf7b | 148 | The value returned by the I<call_*> function indicates how many |
02f6dca1 | 149 | items have been returned by the Perl subroutine--in this case it will |
54310121 | 150 | be 0. |
151 | ||
152 | ||
d1b91892 AD |
153 | =head2 G_SCALAR |
154 | ||
155 | Calls the Perl subroutine in a scalar context. This is the default | |
4929bf7b | 156 | context flag setting for all the I<call_*> functions. |
d1b91892 | 157 | |
184e9718 | 158 | This flag has 2 effects: |
d1b91892 AD |
159 | |
160 | =over 5 | |
161 | ||
162 | =item 1. | |
163 | ||
184e9718 | 164 | It indicates to the subroutine being called that it is executing in a |
d1b91892 | 165 | scalar context (if it executes I<wantarray> the result will be false). |
a0d0e21e | 166 | |
d1b91892 AD |
167 | =item 2. |
168 | ||
184e9718 | 169 | It ensures that only a scalar is actually returned from the subroutine. |
d1b91892 AD |
170 | The subroutine can, of course, ignore the I<wantarray> and return a |
171 | list anyway. If so, then only the last element of the list will be | |
172 | returned. | |
173 | ||
174 | =back | |
175 | ||
4929bf7b | 176 | The value returned by the I<call_*> function indicates how many |
d1b91892 AD |
177 | items have been returned by the Perl subroutine - in this case it will |
178 | be either 0 or 1. | |
a0d0e21e | 179 | |
d1b91892 | 180 | If 0, then you have specified the G_DISCARD flag. |
a0d0e21e | 181 | |
d1b91892 AD |
182 | If 1, then the item actually returned by the Perl subroutine will be |
183 | stored on the Perl stack - the section I<Returning a Scalar> shows how | |
184 | to access this value on the stack. Remember that regardless of how | |
185 | many items the Perl subroutine returns, only the last one will be | |
186 | accessible from the stack - think of the case where only one value is | |
187 | returned as being a list with only one element. Any other items that | |
188 | were returned will not exist by the time control returns from the | |
4929bf7b | 189 | I<call_*> function. The section I<Returning a list in a scalar |
54310121 | 190 | context> shows an example of this behavior. |
a0d0e21e | 191 | |
a0d0e21e | 192 | |
d1b91892 | 193 | =head2 G_ARRAY |
a0d0e21e | 194 | |
d1b91892 | 195 | Calls the Perl subroutine in a list context. |
a0d0e21e | 196 | |
184e9718 | 197 | As with G_SCALAR, this flag has 2 effects: |
a0d0e21e LW |
198 | |
199 | =over 5 | |
200 | ||
d1b91892 AD |
201 | =item 1. |
202 | ||
90fdbbb7 DC |
203 | It indicates to the subroutine being called that it is executing in a |
204 | list context (if it executes I<wantarray> the result will be true). | |
a0d0e21e | 205 | |
d1b91892 | 206 | =item 2. |
a0d0e21e | 207 | |
184e9718 | 208 | It ensures that all items returned from the subroutine will be |
4929bf7b | 209 | accessible when control returns from the I<call_*> function. |
a0d0e21e | 210 | |
d1b91892 | 211 | =back |
a0d0e21e | 212 | |
4929bf7b | 213 | The value returned by the I<call_*> function indicates how many |
d1b91892 | 214 | items have been returned by the Perl subroutine. |
a0d0e21e | 215 | |
184e9718 | 216 | If 0, then you have specified the G_DISCARD flag. |
a0d0e21e | 217 | |
d1b91892 AD |
218 | If not 0, then it will be a count of the number of items returned by |
219 | the subroutine. These items will be stored on the Perl stack. The | |
220 | section I<Returning a list of values> gives an example of using the | |
221 | G_ARRAY flag and the mechanics of accessing the returned items from the | |
222 | Perl stack. | |
a0d0e21e | 223 | |
d1b91892 | 224 | =head2 G_DISCARD |
a0d0e21e | 225 | |
4929bf7b | 226 | By default, the I<call_*> functions place the items returned from |
d1b91892 AD |
227 | by the Perl subroutine on the stack. If you are not interested in |
228 | these items, then setting this flag will make Perl get rid of them | |
229 | automatically for you. Note that it is still possible to indicate a | |
230 | context to the Perl subroutine by using either G_SCALAR or G_ARRAY. | |
a0d0e21e | 231 | |
d1b91892 | 232 | If you do not set this flag then it is I<very> important that you make |
5f05dabc | 233 | sure that any temporaries (i.e., parameters passed to the Perl |
d1b91892 AD |
234 | subroutine and values returned from the subroutine) are disposed of |
235 | yourself. The section I<Returning a Scalar> gives details of how to | |
5f05dabc | 236 | dispose of these temporaries explicitly and the section I<Using Perl to |
d1b91892 AD |
237 | dispose of temporaries> discusses the specific circumstances where you |
238 | can ignore the problem and let Perl deal with it for you. | |
a0d0e21e | 239 | |
d1b91892 | 240 | =head2 G_NOARGS |
a0d0e21e | 241 | |
4929bf7b | 242 | Whenever a Perl subroutine is called using one of the I<call_*> |
d1b91892 AD |
243 | functions, it is assumed by default that parameters are to be passed to |
244 | the subroutine. If you are not passing any parameters to the Perl | |
245 | subroutine, you can save a bit of time by setting this flag. It has | |
246 | the effect of not creating the C<@_> array for the Perl subroutine. | |
a0d0e21e | 247 | |
d1b91892 AD |
248 | Although the functionality provided by this flag may seem |
249 | straightforward, it should be used only if there is a good reason to do | |
02f6dca1 | 250 | so. The reason for being cautious is that, even if you have specified |
d1b91892 AD |
251 | the G_NOARGS flag, it is still possible for the Perl subroutine that |
252 | has been called to think that you have passed it parameters. | |
a0d0e21e | 253 | |
d1b91892 AD |
254 | In fact, what can happen is that the Perl subroutine you have called |
255 | can access the C<@_> array from a previous Perl subroutine. This will | |
4929bf7b | 256 | occur when the code that is executing the I<call_*> function has |
d1b91892 AD |
257 | itself been called from another Perl subroutine. The code below |
258 | illustrates this | |
a0d0e21e | 259 | |
84f709e7 JH |
260 | sub fred |
261 | { print "@_\n" } | |
a0d0e21e | 262 | |
84f709e7 JH |
263 | sub joe |
264 | { &fred } | |
a0d0e21e | 265 | |
4358a253 | 266 | &joe(1,2,3); |
a0d0e21e LW |
267 | |
268 | This will print | |
269 | ||
d1b91892 AD |
270 | 1 2 3 |
271 | ||
272 | What has happened is that C<fred> accesses the C<@_> array which | |
273 | belongs to C<joe>. | |
a0d0e21e | 274 | |
a0d0e21e | 275 | |
54310121 | 276 | =head2 G_EVAL |
a0d0e21e | 277 | |
d1b91892 | 278 | It is possible for the Perl subroutine you are calling to terminate |
5f05dabc | 279 | abnormally, e.g., by calling I<die> explicitly or by not actually |
268118b2 HS |
280 | existing. By default, when either of these events occurs, the |
281 | process will terminate immediately. If you want to trap this | |
d1b91892 AD |
282 | type of event, specify the G_EVAL flag. It will put an I<eval { }> |
283 | around the subroutine call. | |
a0d0e21e | 284 | |
4929bf7b | 285 | Whenever control returns from the I<call_*> function you need to |
d1b91892 AD |
286 | check the C<$@> variable as you would in a normal Perl script. |
287 | ||
4929bf7b | 288 | The value returned from the I<call_*> function is dependent on |
d1b91892 | 289 | what other flags have been specified and whether an error has |
184e9718 | 290 | occurred. Here are all the different cases that can occur: |
d1b91892 AD |
291 | |
292 | =over 5 | |
293 | ||
294 | =item * | |
295 | ||
4929bf7b | 296 | If the I<call_*> function returns normally, then the value |
d1b91892 AD |
297 | returned is as specified in the previous sections. |
298 | ||
299 | =item * | |
300 | ||
301 | If G_DISCARD is specified, the return value will always be 0. | |
302 | ||
303 | =item * | |
304 | ||
305 | If G_ARRAY is specified I<and> an error has occurred, the return value | |
306 | will always be 0. | |
307 | ||
308 | =item * | |
a0d0e21e | 309 | |
d1b91892 AD |
310 | If G_SCALAR is specified I<and> an error has occurred, the return value |
311 | will be 1 and the value on the top of the stack will be I<undef>. This | |
312 | means that if you have already detected the error by checking C<$@> and | |
313 | you want the program to continue, you must remember to pop the I<undef> | |
314 | from the stack. | |
a0d0e21e LW |
315 | |
316 | =back | |
317 | ||
54310121 | 318 | See I<Using G_EVAL> for details on using G_EVAL. |
d1b91892 | 319 | |
c07a80fd | 320 | =head2 G_KEEPERR |
321 | ||
7ce09284 Z |
322 | Using the G_EVAL flag described above will always set C<$@>: clearing |
323 | it if there was no error, and setting it to describe the error if there | |
324 | was an error in the called code. This is what you want if your intention | |
325 | is to handle possible errors, but sometimes you just want to trap errors | |
326 | and stop them interfering with the rest of the program. | |
327 | ||
328 | This scenario will mostly be applicable to code that is meant to be called | |
329 | from within destructors, asynchronous callbacks, and signal handlers. | |
330 | In such situations, where the code being called has little relation to the | |
331 | surrounding dynamic context, the main program needs to be insulated from | |
332 | errors in the called code, even if they can't be handled intelligently. | |
333 | It may also be useful to do this with code for C<__DIE__> or C<__WARN__> | |
334 | hooks, and C<tie> functions. | |
c07a80fd | 335 | |
336 | The G_KEEPERR flag is meant to be used in conjunction with G_EVAL in | |
be064c4a DM |
337 | I<call_*> functions that are used to implement such code, or with |
338 | C<eval_sv>. This flag has no effect on the C<call_*> functions when | |
339 | G_EVAL is not used. | |
c07a80fd | 340 | |
7ce09284 Z |
341 | When G_KEEPERR is used, any error in the called code will terminate the |
342 | call as usual, and the error will not propagate beyond the call (as usual | |
343 | for G_EVAL), but it will not go into C<$@>. Instead the error will be | |
344 | converted into a warning, prefixed with the string "\t(in cleanup)". | |
345 | This can be disabled using C<no warnings 'misc'>. If there is no error, | |
346 | C<$@> will not be cleared. | |
c07a80fd | 347 | |
be064c4a DM |
348 | Note that the G_KEEPERR flag does not propagate into inner evals; these |
349 | may still set C<$@>. | |
350 | ||
c07a80fd | 351 | The G_KEEPERR flag was introduced in Perl version 5.002. |
352 | ||
353 | See I<Using G_KEEPERR> for an example of a situation that warrants the | |
354 | use of this flag. | |
355 | ||
54310121 | 356 | =head2 Determining the Context |
d1b91892 AD |
357 | |
358 | As mentioned above, you can determine the context of the currently | |
54310121 | 359 | executing subroutine in Perl with I<wantarray>. The equivalent test |
360 | can be made in C by using the C<GIMME_V> macro, which returns | |
90fdbbb7 | 361 | C<G_ARRAY> if you have been called in a list context, C<G_SCALAR> if |
02f6dca1 | 362 | in a scalar context, or C<G_VOID> if in a void context (i.e., the |
54310121 | 363 | return value will not be used). An older version of this macro is |
364 | called C<GIMME>; in a void context it returns C<G_SCALAR> instead of | |
365 | C<G_VOID>. An example of using the C<GIMME_V> macro is shown in | |
366 | section I<Using GIMME_V>. | |
d1b91892 | 367 | |
a0d0e21e LW |
368 | =head1 EXAMPLES |
369 | ||
02f6dca1 | 370 | Enough of the definition talk! Let's have a few examples. |
a0d0e21e | 371 | |
d1b91892 AD |
372 | Perl provides many macros to assist in accessing the Perl stack. |
373 | Wherever possible, these macros should always be used when interfacing | |
5f05dabc | 374 | to Perl internals. We hope this should make the code less vulnerable |
d1b91892 | 375 | to any changes made to Perl in the future. |
a0d0e21e | 376 | |
d1b91892 | 377 | Another point worth noting is that in the first series of examples I |
4929bf7b | 378 | have made use of only the I<call_pv> function. This has been done |
d1b91892 | 379 | to keep the code simpler and ease you into the topic. Wherever |
4929bf7b GS |
380 | possible, if the choice is between using I<call_pv> and |
381 | I<call_sv>, you should always try to use I<call_sv>. See | |
382 | I<Using call_sv> for details. | |
a0d0e21e | 383 | |
02f6dca1 | 384 | =head2 No Parameters, Nothing Returned |
a0d0e21e | 385 | |
d1b91892 AD |
386 | This first trivial example will call a Perl subroutine, I<PrintUID>, to |
387 | print out the UID of the process. | |
a0d0e21e | 388 | |
84f709e7 JH |
389 | sub PrintUID |
390 | { | |
4358a253 | 391 | print "UID is $<\n"; |
a0d0e21e LW |
392 | } |
393 | ||
d1b91892 | 394 | and here is a C function to call it |
a0d0e21e | 395 | |
d1b91892 | 396 | static void |
a0d0e21e LW |
397 | call_PrintUID() |
398 | { | |
4358a253 | 399 | dSP; |
a0d0e21e | 400 | |
4358a253 SS |
401 | PUSHMARK(SP); |
402 | call_pv("PrintUID", G_DISCARD|G_NOARGS); | |
a0d0e21e LW |
403 | } |
404 | ||
02f6dca1 | 405 | Simple, eh? |
a0d0e21e | 406 | |
02f6dca1 | 407 | A few points to note about this example: |
a0d0e21e LW |
408 | |
409 | =over 5 | |
410 | ||
d1b91892 | 411 | =item 1. |
a0d0e21e | 412 | |
924508f0 | 413 | Ignore C<dSP> and C<PUSHMARK(SP)> for now. They will be discussed in |
d1b91892 | 414 | the next example. |
a0d0e21e LW |
415 | |
416 | =item 2. | |
417 | ||
d1b91892 AD |
418 | We aren't passing any parameters to I<PrintUID> so G_NOARGS can be |
419 | specified. | |
a0d0e21e | 420 | |
d1b91892 | 421 | =item 3. |
a0d0e21e LW |
422 | |
423 | We aren't interested in anything returned from I<PrintUID>, so | |
5f05dabc | 424 | G_DISCARD is specified. Even if I<PrintUID> was changed to |
a0d0e21e | 425 | return some value(s), having specified G_DISCARD will mean that they |
4929bf7b | 426 | will be wiped by the time control returns from I<call_pv>. |
a0d0e21e | 427 | |
d1b91892 | 428 | =item 4. |
a0d0e21e | 429 | |
4929bf7b | 430 | As I<call_pv> is being used, the Perl subroutine is specified as a |
d1b91892 AD |
431 | C string. In this case the subroutine name has been 'hard-wired' into the |
432 | code. | |
a0d0e21e LW |
433 | |
434 | =item 5. | |
435 | ||
d1b91892 | 436 | Because we specified G_DISCARD, it is not necessary to check the value |
4929bf7b | 437 | returned from I<call_pv>. It will always be 0. |
a0d0e21e LW |
438 | |
439 | =back | |
440 | ||
d1b91892 | 441 | =head2 Passing Parameters |
a0d0e21e | 442 | |
d1b91892 | 443 | Now let's make a slightly more complex example. This time we want to |
19799a22 GS |
444 | call a Perl subroutine, C<LeftString>, which will take 2 parameters--a |
445 | string ($s) and an integer ($n). The subroutine will simply | |
446 | print the first $n characters of the string. | |
a0d0e21e | 447 | |
02f6dca1 | 448 | So the Perl subroutine would look like this: |
a0d0e21e | 449 | |
84f709e7 JH |
450 | sub LeftString |
451 | { | |
4358a253 SS |
452 | my($s, $n) = @_; |
453 | print substr($s, 0, $n), "\n"; | |
a0d0e21e LW |
454 | } |
455 | ||
02f6dca1 | 456 | The C function required to call I<LeftString> would look like this: |
a0d0e21e LW |
457 | |
458 | static void | |
459 | call_LeftString(a, b) | |
4358a253 SS |
460 | char * a; |
461 | int b; | |
a0d0e21e | 462 | { |
4358a253 | 463 | dSP; |
a0d0e21e | 464 | |
4358a253 SS |
465 | ENTER; |
466 | SAVETMPS; | |
9b6570b4 | 467 | |
4358a253 | 468 | PUSHMARK(SP); |
a0d0e21e LW |
469 | XPUSHs(sv_2mortal(newSVpv(a, 0))); |
470 | XPUSHs(sv_2mortal(newSViv(b))); | |
4358a253 | 471 | PUTBACK; |
a0d0e21e | 472 | |
4929bf7b | 473 | call_pv("LeftString", G_DISCARD); |
9b6570b4 | 474 | |
4358a253 SS |
475 | FREETMPS; |
476 | LEAVE; | |
a0d0e21e LW |
477 | } |
478 | ||
a0d0e21e LW |
479 | Here are a few notes on the C function I<call_LeftString>. |
480 | ||
481 | =over 5 | |
482 | ||
d1b91892 | 483 | =item 1. |
a0d0e21e | 484 | |
d1b91892 AD |
485 | Parameters are passed to the Perl subroutine using the Perl stack. |
486 | This is the purpose of the code beginning with the line C<dSP> and | |
1e62ac33 | 487 | ending with the line C<PUTBACK>. The C<dSP> declares a local copy |
924508f0 GS |
488 | of the stack pointer. This local copy should B<always> be accessed |
489 | as C<SP>. | |
a0d0e21e | 490 | |
d1b91892 | 491 | =item 2. |
a0d0e21e LW |
492 | |
493 | If you are going to put something onto the Perl stack, you need to know | |
19799a22 | 494 | where to put it. This is the purpose of the macro C<dSP>--it declares |
d1b91892 | 495 | and initializes a I<local> copy of the Perl stack pointer. |
a0d0e21e LW |
496 | |
497 | All the other macros which will be used in this example require you to | |
d1b91892 | 498 | have used this macro. |
a0d0e21e | 499 | |
d1b91892 AD |
500 | The exception to this rule is if you are calling a Perl subroutine |
501 | directly from an XSUB function. In this case it is not necessary to | |
19799a22 | 502 | use the C<dSP> macro explicitly--it will be declared for you |
d1b91892 | 503 | automatically. |
a0d0e21e | 504 | |
d1b91892 | 505 | =item 3. |
a0d0e21e LW |
506 | |
507 | Any parameters to be pushed onto the stack should be bracketed by the | |
d1b91892 | 508 | C<PUSHMARK> and C<PUTBACK> macros. The purpose of these two macros, in |
5f05dabc | 509 | this context, is to count the number of parameters you are |
510 | pushing automatically. Then whenever Perl is creating the C<@_> array for the | |
d1b91892 AD |
511 | subroutine, it knows how big to make it. |
512 | ||
513 | The C<PUSHMARK> macro tells Perl to make a mental note of the current | |
514 | stack pointer. Even if you aren't passing any parameters (like the | |
02f6dca1 | 515 | example shown in the section I<No Parameters, Nothing Returned>) you |
d1b91892 | 516 | must still call the C<PUSHMARK> macro before you can call any of the |
4929bf7b | 517 | I<call_*> functions--Perl still needs to know that there are no |
d1b91892 AD |
518 | parameters. |
519 | ||
520 | The C<PUTBACK> macro sets the global copy of the stack pointer to be | |
02f6dca1 | 521 | the same as our local copy. If we didn't do this, I<call_pv> |
19799a22 | 522 | wouldn't know where the two parameters we pushed were--remember that |
d1b91892 AD |
523 | up to now all the stack pointer manipulation we have done is with our |
524 | local copy, I<not> the global copy. | |
525 | ||
526 | =item 4. | |
527 | ||
a0d0e21e | 528 | Next, we come to XPUSHs. This is where the parameters actually get |
d1b91892 AD |
529 | pushed onto the stack. In this case we are pushing a string and an |
530 | integer. | |
a0d0e21e | 531 | |
54310121 | 532 | See L<perlguts/"XSUBs and the Argument Stack"> for details |
d1b91892 | 533 | on how the XPUSH macros work. |
a0d0e21e | 534 | |
087fe227 | 535 | =item 5. |
a0d0e21e | 536 | |
9b6570b4 AB |
537 | Because we created temporary values (by means of sv_2mortal() calls) |
538 | we will have to tidy up the Perl stack and dispose of mortal SVs. | |
539 | ||
540 | This is the purpose of | |
541 | ||
4358a253 SS |
542 | ENTER; |
543 | SAVETMPS; | |
9b6570b4 AB |
544 | |
545 | at the start of the function, and | |
546 | ||
4358a253 SS |
547 | FREETMPS; |
548 | LEAVE; | |
9b6570b4 AB |
549 | |
550 | at the end. The C<ENTER>/C<SAVETMPS> pair creates a boundary for any | |
551 | temporaries we create. This means that the temporaries we get rid of | |
552 | will be limited to those which were created after these calls. | |
553 | ||
554 | The C<FREETMPS>/C<LEAVE> pair will get rid of any values returned by | |
555 | the Perl subroutine (see next example), plus it will also dump the | |
556 | mortal SVs we have created. Having C<ENTER>/C<SAVETMPS> at the | |
557 | beginning of the code makes sure that no other mortals are destroyed. | |
558 | ||
02f6dca1 | 559 | Think of these macros as working a bit like C<{> and C<}> in Perl |
9b6570b4 AB |
560 | to limit the scope of local variables. |
561 | ||
02f6dca1 | 562 | See the section I<Using Perl to Dispose of Temporaries> for details of |
9b6570b4 AB |
563 | an alternative to using these macros. |
564 | ||
087fe227 | 565 | =item 6. |
9b6570b4 | 566 | |
087fe227 JA |
567 | Finally, I<LeftString> can now be called via the I<call_pv> function. |
568 | The only flag specified this time is G_DISCARD. Because we are passing | |
569 | 2 parameters to the Perl subroutine this time, we have not specified | |
570 | G_NOARGS. | |
a0d0e21e LW |
571 | |
572 | =back | |
573 | ||
d1b91892 | 574 | =head2 Returning a Scalar |
a0d0e21e | 575 | |
d1b91892 AD |
576 | Now for an example of dealing with the items returned from a Perl |
577 | subroutine. | |
a0d0e21e | 578 | |
5f05dabc | 579 | Here is a Perl subroutine, I<Adder>, that takes 2 integer parameters |
d1b91892 | 580 | and simply returns their sum. |
a0d0e21e | 581 | |
84f709e7 JH |
582 | sub Adder |
583 | { | |
4358a253 SS |
584 | my($a, $b) = @_; |
585 | $a + $b; | |
a0d0e21e LW |
586 | } |
587 | ||
5f05dabc | 588 | Because we are now concerned with the return value from I<Adder>, the C |
d1b91892 | 589 | function required to call it is now a bit more complex. |
a0d0e21e LW |
590 | |
591 | static void | |
592 | call_Adder(a, b) | |
4358a253 SS |
593 | int a; |
594 | int b; | |
a0d0e21e | 595 | { |
4358a253 SS |
596 | dSP; |
597 | int count; | |
a0d0e21e | 598 | |
4358a253 | 599 | ENTER; |
a0d0e21e LW |
600 | SAVETMPS; |
601 | ||
4358a253 | 602 | PUSHMARK(SP); |
a0d0e21e LW |
603 | XPUSHs(sv_2mortal(newSViv(a))); |
604 | XPUSHs(sv_2mortal(newSViv(b))); | |
4358a253 | 605 | PUTBACK; |
a0d0e21e | 606 | |
4929bf7b | 607 | count = call_pv("Adder", G_SCALAR); |
a0d0e21e | 608 | |
4358a253 | 609 | SPAGAIN; |
a0d0e21e | 610 | |
d1b91892 | 611 | if (count != 1) |
4358a253 | 612 | croak("Big trouble\n"); |
a0d0e21e | 613 | |
4358a253 | 614 | printf ("The sum of %d and %d is %d\n", a, b, POPi); |
a0d0e21e | 615 | |
4358a253 SS |
616 | PUTBACK; |
617 | FREETMPS; | |
618 | LEAVE; | |
a0d0e21e LW |
619 | } |
620 | ||
a0d0e21e LW |
621 | Points to note this time are |
622 | ||
623 | =over 5 | |
624 | ||
54310121 | 625 | =item 1. |
a0d0e21e | 626 | |
02f6dca1 | 627 | The only flag specified this time was G_SCALAR. That means that the C<@_> |
d1b91892 | 628 | array will be created and that the value returned by I<Adder> will |
4929bf7b | 629 | still exist after the call to I<call_pv>. |
a0d0e21e | 630 | |
a0d0e21e LW |
631 | =item 2. |
632 | ||
a0d0e21e LW |
633 | The purpose of the macro C<SPAGAIN> is to refresh the local copy of the |
634 | stack pointer. This is necessary because it is possible that the memory | |
ed874981 | 635 | allocated to the Perl stack has been reallocated during the |
4929bf7b | 636 | I<call_pv> call. |
a0d0e21e | 637 | |
d1b91892 | 638 | If you are making use of the Perl stack pointer in your code you must |
54310121 | 639 | always refresh the local copy using SPAGAIN whenever you make use |
4929bf7b | 640 | of the I<call_*> functions or any other Perl internal function. |
a0d0e21e | 641 | |
9b6570b4 | 642 | =item 3. |
a0d0e21e | 643 | |
d1b91892 | 644 | Although only a single value was expected to be returned from I<Adder>, |
4929bf7b | 645 | it is still good practice to check the return code from I<call_pv> |
d1b91892 | 646 | anyway. |
a0d0e21e | 647 | |
d1b91892 AD |
648 | Expecting a single value is not quite the same as knowing that there |
649 | will be one. If someone modified I<Adder> to return a list and we | |
650 | didn't check for that possibility and take appropriate action the Perl | |
651 | stack would end up in an inconsistent state. That is something you | |
5f05dabc | 652 | I<really> don't want to happen ever. |
a0d0e21e | 653 | |
9b6570b4 | 654 | =item 4. |
a0d0e21e | 655 | |
d1b91892 AD |
656 | The C<POPi> macro is used here to pop the return value from the stack. |
657 | In this case we wanted an integer, so C<POPi> was used. | |
a0d0e21e LW |
658 | |
659 | ||
d1b91892 AD |
660 | Here is the complete list of POP macros available, along with the types |
661 | they return. | |
a0d0e21e | 662 | |
d1b91892 AD |
663 | POPs SV |
664 | POPp pointer | |
665 | POPn double | |
666 | POPi integer | |
667 | POPl long | |
a0d0e21e | 668 | |
9b6570b4 | 669 | =item 5. |
a0d0e21e | 670 | |
d1b91892 AD |
671 | The final C<PUTBACK> is used to leave the Perl stack in a consistent |
672 | state before exiting the function. This is necessary because when we | |
673 | popped the return value from the stack with C<POPi> it updated only our | |
674 | local copy of the stack pointer. Remember, C<PUTBACK> sets the global | |
675 | stack pointer to be the same as our local copy. | |
a0d0e21e LW |
676 | |
677 | =back | |
678 | ||
679 | ||
02f6dca1 | 680 | =head2 Returning a List of Values |
a0d0e21e | 681 | |
d1b91892 AD |
682 | Now, let's extend the previous example to return both the sum of the |
683 | parameters and the difference. | |
a0d0e21e | 684 | |
d1b91892 | 685 | Here is the Perl subroutine |
a0d0e21e | 686 | |
84f709e7 JH |
687 | sub AddSubtract |
688 | { | |
4358a253 SS |
689 | my($a, $b) = @_; |
690 | ($a+$b, $a-$b); | |
a0d0e21e LW |
691 | } |
692 | ||
a0d0e21e LW |
693 | and this is the C function |
694 | ||
695 | static void | |
696 | call_AddSubtract(a, b) | |
4358a253 SS |
697 | int a; |
698 | int b; | |
a0d0e21e | 699 | { |
4358a253 SS |
700 | dSP; |
701 | int count; | |
a0d0e21e | 702 | |
4358a253 | 703 | ENTER; |
a0d0e21e LW |
704 | SAVETMPS; |
705 | ||
4358a253 | 706 | PUSHMARK(SP); |
a0d0e21e LW |
707 | XPUSHs(sv_2mortal(newSViv(a))); |
708 | XPUSHs(sv_2mortal(newSViv(b))); | |
4358a253 | 709 | PUTBACK; |
a0d0e21e | 710 | |
4929bf7b | 711 | count = call_pv("AddSubtract", G_ARRAY); |
a0d0e21e | 712 | |
4358a253 | 713 | SPAGAIN; |
a0d0e21e | 714 | |
d1b91892 | 715 | if (count != 2) |
4358a253 | 716 | croak("Big trouble\n"); |
a0d0e21e | 717 | |
4358a253 SS |
718 | printf ("%d - %d = %d\n", a, b, POPi); |
719 | printf ("%d + %d = %d\n", a, b, POPi); | |
a0d0e21e | 720 | |
4358a253 SS |
721 | PUTBACK; |
722 | FREETMPS; | |
723 | LEAVE; | |
a0d0e21e LW |
724 | } |
725 | ||
d1b91892 AD |
726 | If I<call_AddSubtract> is called like this |
727 | ||
4358a253 | 728 | call_AddSubtract(7, 4); |
d1b91892 AD |
729 | |
730 | then here is the output | |
731 | ||
732 | 7 - 4 = 3 | |
733 | 7 + 4 = 11 | |
a0d0e21e LW |
734 | |
735 | Notes | |
736 | ||
737 | =over 5 | |
738 | ||
739 | =item 1. | |
740 | ||
90fdbbb7 | 741 | We wanted list context, so G_ARRAY was used. |
a0d0e21e LW |
742 | |
743 | =item 2. | |
744 | ||
d1b91892 AD |
745 | Not surprisingly C<POPi> is used twice this time because we were |
746 | retrieving 2 values from the stack. The important thing to note is that | |
747 | when using the C<POP*> macros they come off the stack in I<reverse> | |
748 | order. | |
a0d0e21e LW |
749 | |
750 | =back | |
751 | ||
02f6dca1 | 752 | =head2 Returning a List in a Scalar Context |
d1b91892 AD |
753 | |
754 | Say the Perl subroutine in the previous section was called in a scalar | |
755 | context, like this | |
756 | ||
757 | static void | |
758 | call_AddSubScalar(a, b) | |
4358a253 SS |
759 | int a; |
760 | int b; | |
d1b91892 | 761 | { |
4358a253 SS |
762 | dSP; |
763 | int count; | |
764 | int i; | |
d1b91892 | 765 | |
4358a253 | 766 | ENTER; |
d1b91892 AD |
767 | SAVETMPS; |
768 | ||
4358a253 | 769 | PUSHMARK(SP); |
d1b91892 AD |
770 | XPUSHs(sv_2mortal(newSViv(a))); |
771 | XPUSHs(sv_2mortal(newSViv(b))); | |
4358a253 | 772 | PUTBACK; |
d1b91892 | 773 | |
4929bf7b | 774 | count = call_pv("AddSubtract", G_SCALAR); |
d1b91892 | 775 | |
4358a253 | 776 | SPAGAIN; |
d1b91892 | 777 | |
4358a253 | 778 | printf ("Items Returned = %d\n", count); |
d1b91892 | 779 | |
4358a253 SS |
780 | for (i = 1; i <= count; ++i) |
781 | printf ("Value %d = %d\n", i, POPi); | |
d1b91892 | 782 | |
4358a253 SS |
783 | PUTBACK; |
784 | FREETMPS; | |
785 | LEAVE; | |
d1b91892 AD |
786 | } |
787 | ||
788 | The other modification made is that I<call_AddSubScalar> will print the | |
789 | number of items returned from the Perl subroutine and their value (for | |
790 | simplicity it assumes that they are integer). So if | |
791 | I<call_AddSubScalar> is called | |
792 | ||
4358a253 | 793 | call_AddSubScalar(7, 4); |
d1b91892 AD |
794 | |
795 | then the output will be | |
796 | ||
797 | Items Returned = 1 | |
798 | Value 1 = 3 | |
799 | ||
800 | In this case the main point to note is that only the last item in the | |
48052fe5 | 801 | list is returned from the subroutine. I<AddSubtract> actually made it back to |
d1b91892 AD |
802 | I<call_AddSubScalar>. |
803 | ||
804 | ||
02f6dca1 | 805 | =head2 Returning Data from Perl via the Parameter List |
a0d0e21e | 806 | |
48052fe5 FC |
807 | It is also possible to return values directly via the parameter |
808 | list--whether it is actually desirable to do it is another matter entirely. | |
a0d0e21e | 809 | |
d1b91892 AD |
810 | The Perl subroutine, I<Inc>, below takes 2 parameters and increments |
811 | each directly. | |
a0d0e21e | 812 | |
84f709e7 JH |
813 | sub Inc |
814 | { | |
4358a253 SS |
815 | ++ $_[0]; |
816 | ++ $_[1]; | |
a0d0e21e LW |
817 | } |
818 | ||
819 | and here is a C function to call it. | |
820 | ||
821 | static void | |
822 | call_Inc(a, b) | |
4358a253 SS |
823 | int a; |
824 | int b; | |
a0d0e21e | 825 | { |
4358a253 SS |
826 | dSP; |
827 | int count; | |
828 | SV * sva; | |
829 | SV * svb; | |
a0d0e21e | 830 | |
4358a253 | 831 | ENTER; |
a0d0e21e LW |
832 | SAVETMPS; |
833 | ||
4358a253 SS |
834 | sva = sv_2mortal(newSViv(a)); |
835 | svb = sv_2mortal(newSViv(b)); | |
a0d0e21e | 836 | |
4358a253 | 837 | PUSHMARK(SP); |
a0d0e21e LW |
838 | XPUSHs(sva); |
839 | XPUSHs(svb); | |
4358a253 | 840 | PUTBACK; |
a0d0e21e | 841 | |
4929bf7b | 842 | count = call_pv("Inc", G_DISCARD); |
a0d0e21e LW |
843 | |
844 | if (count != 0) | |
d1b91892 | 845 | croak ("call_Inc: expected 0 values from 'Inc', got %d\n", |
4358a253 | 846 | count); |
a0d0e21e | 847 | |
4358a253 SS |
848 | printf ("%d + 1 = %d\n", a, SvIV(sva)); |
849 | printf ("%d + 1 = %d\n", b, SvIV(svb)); | |
a0d0e21e | 850 | |
4358a253 SS |
851 | FREETMPS; |
852 | LEAVE; | |
a0d0e21e LW |
853 | } |
854 | ||
d1b91892 | 855 | To be able to access the two parameters that were pushed onto the stack |
4929bf7b | 856 | after they return from I<call_pv> it is necessary to make a note |
19799a22 | 857 | of their addresses--thus the two variables C<sva> and C<svb>. |
a0d0e21e | 858 | |
d1b91892 AD |
859 | The reason this is necessary is that the area of the Perl stack which |
860 | held them will very likely have been overwritten by something else by | |
4929bf7b | 861 | the time control returns from I<call_pv>. |
a0d0e21e LW |
862 | |
863 | ||
864 | ||
865 | ||
d1b91892 | 866 | =head2 Using G_EVAL |
a0d0e21e | 867 | |
d1b91892 AD |
868 | Now an example using G_EVAL. Below is a Perl subroutine which computes |
869 | the difference of its 2 parameters. If this would result in a negative | |
870 | result, the subroutine calls I<die>. | |
a0d0e21e | 871 | |
84f709e7 JH |
872 | sub Subtract |
873 | { | |
4358a253 | 874 | my ($a, $b) = @_; |
a0d0e21e | 875 | |
4358a253 | 876 | die "death can be fatal\n" if $a < $b; |
a0d0e21e | 877 | |
4358a253 | 878 | $a - $b; |
a0d0e21e LW |
879 | } |
880 | ||
881 | and some C to call it | |
882 | ||
883 | static void | |
884 | call_Subtract(a, b) | |
4358a253 SS |
885 | int a; |
886 | int b; | |
a0d0e21e | 887 | { |
4358a253 SS |
888 | dSP; |
889 | int count; | |
a0d0e21e | 890 | |
4358a253 | 891 | ENTER; |
a0d0e21e LW |
892 | SAVETMPS; |
893 | ||
4358a253 | 894 | PUSHMARK(SP); |
a0d0e21e LW |
895 | XPUSHs(sv_2mortal(newSViv(a))); |
896 | XPUSHs(sv_2mortal(newSViv(b))); | |
4358a253 | 897 | PUTBACK; |
a0d0e21e | 898 | |
4929bf7b | 899 | count = call_pv("Subtract", G_EVAL|G_SCALAR); |
a0d0e21e | 900 | |
4358a253 | 901 | SPAGAIN; |
d1b91892 AD |
902 | |
903 | /* Check the eval first */ | |
9cde0e7f | 904 | if (SvTRUE(ERRSV)) |
d1b91892 | 905 | { |
1c5b513e | 906 | printf ("Uh oh - %s\n", SvPV_nolen(ERRSV)); |
4358a253 | 907 | POPs; |
d1b91892 AD |
908 | } |
909 | else | |
910 | { | |
911 | if (count != 1) | |
912 | croak("call_Subtract: wanted 1 value from 'Subtract', got %d\n", | |
4358a253 | 913 | count); |
a0d0e21e | 914 | |
4358a253 | 915 | printf ("%d - %d = %d\n", a, b, POPi); |
d1b91892 | 916 | } |
a0d0e21e | 917 | |
4358a253 SS |
918 | PUTBACK; |
919 | FREETMPS; | |
920 | LEAVE; | |
a0d0e21e LW |
921 | } |
922 | ||
923 | If I<call_Subtract> is called thus | |
924 | ||
d1b91892 | 925 | call_Subtract(4, 5) |
a0d0e21e LW |
926 | |
927 | the following will be printed | |
928 | ||
d1b91892 | 929 | Uh oh - death can be fatal |
a0d0e21e LW |
930 | |
931 | Notes | |
932 | ||
933 | =over 5 | |
934 | ||
935 | =item 1. | |
936 | ||
d1b91892 AD |
937 | We want to be able to catch the I<die> so we have used the G_EVAL |
938 | flag. Not specifying this flag would mean that the program would | |
939 | terminate immediately at the I<die> statement in the subroutine | |
940 | I<Subtract>. | |
a0d0e21e LW |
941 | |
942 | =item 2. | |
943 | ||
54310121 | 944 | The code |
a0d0e21e | 945 | |
9cde0e7f | 946 | if (SvTRUE(ERRSV)) |
d1b91892 | 947 | { |
1c5b513e | 948 | printf ("Uh oh - %s\n", SvPV_nolen(ERRSV)); |
4358a253 | 949 | POPs; |
d1b91892 | 950 | } |
a0d0e21e | 951 | |
d1b91892 | 952 | is the direct equivalent of this bit of Perl |
a0d0e21e | 953 | |
4358a253 | 954 | print "Uh oh - $@\n" if $@; |
a0d0e21e | 955 | |
9cde0e7f GS |
956 | C<PL_errgv> is a perl global of type C<GV *> that points to the |
957 | symbol table entry containing the error. C<ERRSV> therefore | |
c07a80fd | 958 | refers to the C equivalent of C<$@>. |
959 | ||
d1b91892 | 960 | =item 3. |
a0d0e21e | 961 | |
d1b91892 | 962 | Note that the stack is popped using C<POPs> in the block where |
9cde0e7f | 963 | C<SvTRUE(ERRSV)> is true. This is necessary because whenever a |
4929bf7b | 964 | I<call_*> function invoked with G_EVAL|G_SCALAR returns an error, |
5f05dabc | 965 | the top of the stack holds the value I<undef>. Because we want the |
d1b91892 | 966 | program to continue after detecting this error, it is essential that |
6c818a50 | 967 | the stack be tidied up by removing the I<undef>. |
a0d0e21e LW |
968 | |
969 | =back | |
970 | ||
971 | ||
c07a80fd | 972 | =head2 Using G_KEEPERR |
973 | ||
974 | Consider this rather facetious example, where we have used an XS | |
975 | version of the call_Subtract example above inside a destructor: | |
976 | ||
977 | package Foo; | |
84f709e7 | 978 | sub new { bless {}, $_[0] } |
54310121 | 979 | sub Subtract { |
84f709e7 | 980 | my($a,$b) = @_; |
4358a253 | 981 | die "death can be fatal" if $a < $b; |
84f709e7 | 982 | $a - $b; |
c07a80fd | 983 | } |
84f709e7 JH |
984 | sub DESTROY { call_Subtract(5, 4); } |
985 | sub foo { die "foo dies"; } | |
c07a80fd | 986 | |
987 | package main; | |
7ce09284 Z |
988 | { |
989 | my $foo = Foo->new; | |
990 | eval { $foo->foo }; | |
991 | } | |
c07a80fd | 992 | print "Saw: $@" if $@; # should be, but isn't |
993 | ||
994 | This example will fail to recognize that an error occurred inside the | |
995 | C<eval {}>. Here's why: the call_Subtract code got executed while perl | |
7ce09284 | 996 | was cleaning up temporaries when exiting the outer braced block, and because |
4929bf7b | 997 | call_Subtract is implemented with I<call_pv> using the G_EVAL |
c07a80fd | 998 | flag, it promptly reset C<$@>. This results in the failure of the |
999 | outermost test for C<$@>, and thereby the failure of the error trap. | |
1000 | ||
4929bf7b | 1001 | Appending the G_KEEPERR flag, so that the I<call_pv> call in |
c07a80fd | 1002 | call_Subtract reads: |
1003 | ||
4929bf7b | 1004 | count = call_pv("Subtract", G_EVAL|G_SCALAR|G_KEEPERR); |
c07a80fd | 1005 | |
1006 | will preserve the error and restore reliable error handling. | |
1007 | ||
4929bf7b | 1008 | =head2 Using call_sv |
a0d0e21e | 1009 | |
d1b91892 AD |
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. | |
a0d0e21e LW |
1014 | |
1015 | Consider the Perl code below | |
1016 | ||
84f709e7 JH |
1017 | sub fred |
1018 | { | |
4358a253 | 1019 | print "Hello there\n"; |
d1b91892 AD |
1020 | } |
1021 | ||
4358a253 | 1022 | CallSubPV("fred"); |
d1b91892 AD |
1023 | |
1024 | Here is a snippet of XSUB which defines I<CallSubPV>. | |
1025 | ||
1026 | void | |
1027 | CallSubPV(name) | |
1028 | char * name | |
1029 | CODE: | |
4358a253 SS |
1030 | PUSHMARK(SP); |
1031 | call_pv(name, G_DISCARD|G_NOARGS); | |
a0d0e21e | 1032 | |
54310121 | 1033 | That is fine as far as it goes. The thing is, the Perl subroutine |
94a37a2f BF |
1034 | can be specified as only a string, however, Perl allows references |
1035 | to subroutines and anonymous subroutines. | |
4929bf7b | 1036 | This is where I<call_sv> is useful. |
d1b91892 AD |
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 | |
4929bf7b | 1040 | I<call_sv> instead of I<call_pv>. |
d1b91892 AD |
1041 | |
1042 | void | |
1043 | CallSubSV(name) | |
1044 | SV * name | |
1045 | CODE: | |
4358a253 SS |
1046 | PUSHMARK(SP); |
1047 | call_sv(name, G_DISCARD|G_NOARGS); | |
a0d0e21e | 1048 | |
1d45ec27 | 1049 | Because we are using an SV to call I<fred> the following can all be used: |
a0d0e21e | 1050 | |
4358a253 SS |
1051 | CallSubSV("fred"); |
1052 | CallSubSV(\&fred); | |
1053 | $ref = \&fred; | |
1054 | CallSubSV($ref); | |
1055 | CallSubSV( sub { print "Hello there\n" } ); | |
a0d0e21e | 1056 | |
4929bf7b | 1057 | As you can see, I<call_sv> gives you much greater flexibility in |
d1b91892 AD |
1058 | how you can specify the Perl subroutine. |
1059 | ||
1d45ec27 | 1060 | You should note that, if it is necessary to store the SV (C<name> in the |
d1b91892 | 1061 | example above) which corresponds to the Perl subroutine so that it can |
5f05dabc | 1062 | be used later in the program, it not enough just to store a copy of the |
1d45ec27 | 1063 | pointer to the SV. Say the code above had been like this: |
d1b91892 | 1064 | |
4358a253 | 1065 | static SV * rememberSub; |
d1b91892 AD |
1066 | |
1067 | void | |
1068 | SaveSub1(name) | |
1069 | SV * name | |
1070 | CODE: | |
4358a253 | 1071 | rememberSub = name; |
d1b91892 AD |
1072 | |
1073 | void | |
1074 | CallSavedSub1() | |
1075 | CODE: | |
4358a253 SS |
1076 | PUSHMARK(SP); |
1077 | call_sv(rememberSub, G_DISCARD|G_NOARGS); | |
a0d0e21e | 1078 | |
1d45ec27 | 1079 | The reason this is wrong is that, by the time you come to use the |
d1b91892 AD |
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 | |
1d45ec27 | 1082 | particularly true for these cases: |
a0d0e21e | 1083 | |
4358a253 SS |
1084 | SaveSub1(\&fred); |
1085 | CallSavedSub1(); | |
a0d0e21e | 1086 | |
4358a253 SS |
1087 | SaveSub1( sub { print "Hello there\n" } ); |
1088 | CallSavedSub1(); | |
a0d0e21e | 1089 | |
1d45ec27 | 1090 | By the time each of the C<SaveSub1> statements above has been executed, |
54310121 | 1091 | the SV*s which corresponded to the parameters will no longer exist. |
d1b91892 | 1092 | Expect an error message from Perl of the form |
a0d0e21e | 1093 | |
d1b91892 | 1094 | Can't use an undefined value as a subroutine reference at ... |
a0d0e21e | 1095 | |
d1b91892 | 1096 | for each of the C<CallSavedSub1> lines. |
a0d0e21e | 1097 | |
54310121 | 1098 | Similarly, with this code |
a0d0e21e | 1099 | |
4358a253 SS |
1100 | $ref = \&fred; |
1101 | SaveSub1($ref); | |
1102 | $ref = 47; | |
1103 | CallSavedSub1(); | |
a0d0e21e | 1104 | |
54310121 | 1105 | you can expect one of these messages (which you actually get is dependent on |
1106 | the version of Perl you are using) | |
a0d0e21e | 1107 | |
d1b91892 AD |
1108 | Not a CODE reference at ... |
1109 | Undefined subroutine &main::47 called ... | |
a0d0e21e | 1110 | |
19799a22 | 1111 | The variable $ref may have referred to the subroutine C<fred> |
d1b91892 | 1112 | whenever the call to C<SaveSub1> was made but by the time |
5f05dabc | 1113 | C<CallSavedSub1> gets called it now holds the number C<47>. Because we |
d1b91892 | 1114 | saved only a pointer to the original SV in C<SaveSub1>, any changes to |
19799a22 | 1115 | $ref will be tracked by the pointer C<rememberSub>. This means that |
d1b91892 AD |
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. | |
a0d0e21e | 1120 | |
1d45ec27 | 1121 | A similar but more subtle problem is illustrated with this code: |
a0d0e21e | 1122 | |
4358a253 SS |
1123 | $ref = \&fred; |
1124 | SaveSub1($ref); | |
1125 | $ref = \&joe; | |
1126 | CallSavedSub1(); | |
a0d0e21e | 1127 | |
1d45ec27 | 1128 | This time whenever C<CallSavedSub1> gets called it will execute the Perl |
54310121 | 1129 | subroutine C<joe> (assuming it exists) rather than C<fred> as was |
d1b91892 | 1130 | originally requested in the call to C<SaveSub1>. |
a0d0e21e | 1131 | |
d1b91892 | 1132 | To get around these problems it is necessary to take a full copy of the |
1d45ec27 | 1133 | SV. The code below shows C<SaveSub2> modified to do that. |
a0d0e21e | 1134 | |
4358a253 | 1135 | static SV * keepSub = (SV*)NULL; |
d1b91892 AD |
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 */ | |
4358a253 | 1144 | keepSub = newSVsv(name); |
d1b91892 AD |
1145 | else |
1146 | /* Been here before, so overwrite */ | |
4358a253 | 1147 | SvSetSV(keepSub, name); |
d1b91892 AD |
1148 | |
1149 | void | |
1150 | CallSavedSub2() | |
1151 | CODE: | |
4358a253 SS |
1152 | PUSHMARK(SP); |
1153 | call_sv(keepSub, G_DISCARD|G_NOARGS); | |
d1b91892 | 1154 | |
5f05dabc | 1155 | To avoid creating a new SV every time C<SaveSub2> is called, |
d1b91892 AD |
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 | |
1d45ec27 FC |
1158 | subroutine C<name> is copied to the variable C<keepSub> in one |
1159 | operation using C<newSVsv>. Thereafter, whenever C<SaveSub2> is called, | |
d1b91892 AD |
1160 | the existing SV, C<keepSub>, is overwritten with the new value using |
1161 | C<SvSetSV>. | |
1162 | ||
4929bf7b | 1163 | =head2 Using call_argv |
d1b91892 AD |
1164 | |
1165 | Here is a Perl subroutine which prints whatever parameters are passed | |
1166 | to it. | |
1167 | ||
84f709e7 JH |
1168 | sub PrintList |
1169 | { | |
4358a253 | 1170 | my(@list) = @_; |
d1b91892 | 1171 | |
84f709e7 | 1172 | foreach (@list) { print "$_\n" } |
d1b91892 AD |
1173 | } |
1174 | ||
1d45ec27 | 1175 | And here is an example of I<call_argv> which will call |
d1b91892 AD |
1176 | I<PrintList>. |
1177 | ||
4358a253 | 1178 | static char * words[] = {"alpha", "beta", "gamma", "delta", NULL}; |
d1b91892 AD |
1179 | |
1180 | static void | |
1181 | call_PrintList() | |
1182 | { | |
4358a253 | 1183 | dSP; |
d1b91892 | 1184 | |
4358a253 | 1185 | call_argv("PrintList", G_DISCARD, words); |
d1b91892 AD |
1186 | } |
1187 | ||
1188 | Note that it is not necessary to call C<PUSHMARK> in this instance. | |
4929bf7b | 1189 | This is because I<call_argv> will do it for you. |
d1b91892 | 1190 | |
4929bf7b | 1191 | =head2 Using call_method |
a0d0e21e | 1192 | |
1d45ec27 | 1193 | Consider the following Perl code: |
a0d0e21e | 1194 | |
d1b91892 | 1195 | { |
4358a253 | 1196 | package Mine; |
84f709e7 JH |
1197 | |
1198 | sub new | |
1199 | { | |
4358a253 | 1200 | my($type) = shift; |
84f709e7 JH |
1201 | bless [@_] |
1202 | } | |
1203 | ||
1204 | sub Display | |
1205 | { | |
4358a253 SS |
1206 | my ($self, $index) = @_; |
1207 | print "$index: $$self[$index]\n"; | |
84f709e7 JH |
1208 | } |
1209 | ||
1210 | sub PrintID | |
1211 | { | |
4358a253 SS |
1212 | my($class) = @_; |
1213 | print "This is Class $class version 1.0\n"; | |
84f709e7 | 1214 | } |
d1b91892 AD |
1215 | } |
1216 | ||
5f05dabc | 1217 | It implements just a very simple class to manage an array. Apart from |
d1b91892 | 1218 | the constructor, C<new>, it declares methods, one static and one |
5f05dabc | 1219 | virtual. The static method, C<PrintID>, prints out simply the class |
d1b91892 | 1220 | name and a version number. The virtual method, C<Display>, prints out a |
1d45ec27 | 1221 | single element of the array. Here is an all-Perl example of using it. |
d1b91892 | 1222 | |
797f796a | 1223 | $a = Mine->new('red', 'green', 'blue'); |
4358a253 | 1224 | $a->Display(1); |
797f796a | 1225 | Mine->PrintID; |
a0d0e21e | 1226 | |
d1b91892 | 1227 | will print |
a0d0e21e | 1228 | |
d1b91892 | 1229 | 1: green |
54310121 | 1230 | This is Class Mine version 1.0 |
a0d0e21e | 1231 | |
d1b91892 | 1232 | Calling a Perl method from C is fairly straightforward. The following |
1d45ec27 | 1233 | things are required: |
a0d0e21e | 1234 | |
d1b91892 AD |
1235 | =over 5 |
1236 | ||
1237 | =item * | |
1238 | ||
1d45ec27 FC |
1239 | A reference to the object for a virtual method or the name of the class |
1240 | for a static method | |
d1b91892 AD |
1241 | |
1242 | =item * | |
1243 | ||
1d45ec27 | 1244 | The name of the method |
d1b91892 AD |
1245 | |
1246 | =item * | |
1247 | ||
1d45ec27 | 1248 | Any other parameters specific to the method |
d1b91892 AD |
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: | |
924508f0 | 1261 | PUSHMARK(SP); |
d1b91892 | 1262 | XPUSHs(ref); |
4358a253 | 1263 | XPUSHs(sv_2mortal(newSViv(index))); |
d1b91892 AD |
1264 | PUTBACK; |
1265 | ||
4358a253 | 1266 | call_method(method, G_DISCARD); |
d1b91892 AD |
1267 | |
1268 | void | |
1269 | call_PrintID(class, method) | |
1270 | char * class | |
1271 | char * method | |
1272 | CODE: | |
924508f0 | 1273 | PUSHMARK(SP); |
4358a253 | 1274 | XPUSHs(sv_2mortal(newSVpv(class, 0))); |
d1b91892 AD |
1275 | PUTBACK; |
1276 | ||
4358a253 | 1277 | call_method(method, G_DISCARD); |
d1b91892 AD |
1278 | |
1279 | ||
1d45ec27 | 1280 | So the methods C<PrintID> and C<Display> can be invoked like this: |
d1b91892 | 1281 | |
797f796a | 1282 | $a = Mine->new('red', 'green', 'blue'); |
4358a253 SS |
1283 | call_Method($a, 'Display', 1); |
1284 | call_PrintID('Mine', 'PrintID'); | |
d1b91892 | 1285 | |
1d45ec27 | 1286 | The only thing to note is that, in both the static and virtual methods, |
19799a22 | 1287 | the method name is not passed via the stack--it is used as the first |
4929bf7b | 1288 | parameter to I<call_method>. |
d1b91892 | 1289 | |
54310121 | 1290 | =head2 Using GIMME_V |
d1b91892 | 1291 | |
54310121 | 1292 | Here is a trivial XSUB which prints the context in which it is |
d1b91892 AD |
1293 | currently executing. |
1294 | ||
1295 | void | |
1296 | PrintContext() | |
1297 | CODE: | |
54310121 | 1298 | I32 gimme = GIMME_V; |
1299 | if (gimme == G_VOID) | |
4358a253 | 1300 | printf ("Context is Void\n"); |
54310121 | 1301 | else if (gimme == G_SCALAR) |
4358a253 | 1302 | printf ("Context is Scalar\n"); |
d1b91892 | 1303 | else |
4358a253 | 1304 | printf ("Context is Array\n"); |
d1b91892 | 1305 | |
1d45ec27 | 1306 | And here is some Perl to test it. |
d1b91892 | 1307 | |
4358a253 SS |
1308 | PrintContext; |
1309 | $a = PrintContext; | |
1310 | @a = PrintContext; | |
d1b91892 AD |
1311 | |
1312 | The output from that will be | |
1313 | ||
54310121 | 1314 | Context is Void |
d1b91892 AD |
1315 | Context is Scalar |
1316 | Context is Array | |
1317 | ||
02f6dca1 | 1318 | =head2 Using Perl to Dispose of Temporaries |
d1b91892 AD |
1319 | |
1320 | In the examples given to date, any temporaries created in the callback | |
4929bf7b | 1321 | (i.e., parameters passed on the stack to the I<call_*> function or |
1d45ec27 | 1322 | values returned via the stack) have been freed by one of these methods: |
d1b91892 AD |
1323 | |
1324 | =over 5 | |
1325 | ||
1326 | =item * | |
1327 | ||
1d45ec27 | 1328 | Specifying the G_DISCARD flag with I<call_*> |
d1b91892 AD |
1329 | |
1330 | =item * | |
1331 | ||
1d45ec27 | 1332 | Explicitly using the C<ENTER>/C<SAVETMPS>--C<FREETMPS>/C<LEAVE> pairing |
d1b91892 AD |
1333 | |
1334 | =back | |
1335 | ||
1336 | There is another method which can be used, namely letting Perl do it | |
1337 | for you automatically whenever it regains control after the callback | |
1338 | has terminated. This is done by simply not using the | |
1339 | ||
4358a253 SS |
1340 | ENTER; |
1341 | SAVETMPS; | |
d1b91892 | 1342 | ... |
4358a253 SS |
1343 | FREETMPS; |
1344 | LEAVE; | |
d1b91892 AD |
1345 | |
1346 | sequence in the callback (and not, of course, specifying the G_DISCARD | |
1347 | flag). | |
1348 | ||
1349 | If you are going to use this method you have to be aware of a possible | |
1350 | memory leak which can arise under very specific circumstances. To | |
1351 | explain these circumstances you need to know a bit about the flow of | |
1352 | control between Perl and the callback routine. | |
1353 | ||
1354 | The examples given at the start of the document (an error handler and | |
1355 | an event driven program) are typical of the two main sorts of flow | |
1356 | control that you are likely to encounter with callbacks. There is a | |
1357 | very important distinction between them, so pay attention. | |
1358 | ||
1359 | In the first example, an error handler, the flow of control could be as | |
1360 | follows. You have created an interface to an external library. | |
1361 | Control can reach the external library like this | |
1362 | ||
1363 | perl --> XSUB --> external library | |
1364 | ||
1365 | Whilst control is in the library, an error condition occurs. You have | |
1366 | previously set up a Perl callback to handle this situation, so it will | |
1367 | get executed. Once the callback has finished, control will drop back to | |
1368 | Perl again. Here is what the flow of control will be like in that | |
1369 | situation | |
1370 | ||
1371 | perl --> XSUB --> external library | |
1372 | ... | |
1373 | error occurs | |
1374 | ... | |
4929bf7b | 1375 | external library --> call_* --> perl |
d1b91892 | 1376 | | |
4929bf7b | 1377 | perl <-- XSUB <-- external library <-- call_* <----+ |
d1b91892 | 1378 | |
4929bf7b | 1379 | After processing of the error using I<call_*> is completed, |
d1b91892 AD |
1380 | control reverts back to Perl more or less immediately. |
1381 | ||
1382 | In the diagram, the further right you go the more deeply nested the | |
1383 | scope is. It is only when control is back with perl on the extreme | |
1384 | left of the diagram that you will have dropped back to the enclosing | |
1385 | scope and any temporaries you have left hanging around will be freed. | |
1386 | ||
1387 | In the second example, an event driven program, the flow of control | |
1388 | will be more like this | |
1389 | ||
1390 | perl --> XSUB --> event handler | |
1391 | ... | |
4929bf7b | 1392 | event handler --> call_* --> perl |
d1b91892 | 1393 | | |
4929bf7b | 1394 | event handler <-- call_* <----+ |
d1b91892 | 1395 | ... |
4929bf7b | 1396 | event handler --> call_* --> perl |
d1b91892 | 1397 | | |
4929bf7b | 1398 | event handler <-- call_* <----+ |
d1b91892 | 1399 | ... |
4929bf7b | 1400 | event handler --> call_* --> perl |
d1b91892 | 1401 | | |
4929bf7b | 1402 | event handler <-- call_* <----+ |
d1b91892 AD |
1403 | |
1404 | In this case the flow of control can consist of only the repeated | |
1405 | sequence | |
1406 | ||
4929bf7b | 1407 | event handler --> call_* --> perl |
d1b91892 | 1408 | |
54310121 | 1409 | for practically the complete duration of the program. This means that |
1410 | control may I<never> drop back to the surrounding scope in Perl at the | |
1411 | extreme left. | |
d1b91892 AD |
1412 | |
1413 | So what is the big problem? Well, if you are expecting Perl to tidy up | |
1414 | those temporaries for you, you might be in for a long wait. For Perl | |
5f05dabc | 1415 | to dispose of your temporaries, control must drop back to the |
d1b91892 | 1416 | enclosing scope at some stage. In the event driven scenario that may |
1d45ec27 | 1417 | never happen. This means that, as time goes on, your program will |
d1b91892 AD |
1418 | create more and more temporaries, none of which will ever be freed. As |
1419 | each of these temporaries consumes some memory your program will | |
19799a22 | 1420 | eventually consume all the available memory in your system--kapow! |
d1b91892 | 1421 | |
19799a22 | 1422 | So here is the bottom line--if you are sure that control will revert |
d1b91892 | 1423 | back to the enclosing Perl scope fairly quickly after the end of your |
5f05dabc | 1424 | callback, then it isn't absolutely necessary to dispose explicitly of |
d1b91892 AD |
1425 | any temporaries you may have created. Mind you, if you are at all |
1426 | uncertain about what to do, it doesn't do any harm to tidy up anyway. | |
1427 | ||
1428 | ||
02f6dca1 | 1429 | =head2 Strategies for Storing Callback Context Information |
d1b91892 AD |
1430 | |
1431 | ||
1432 | Potentially one of the trickiest problems to overcome when designing a | |
1433 | callback interface can be figuring out how to store the mapping between | |
1434 | the C callback function and the Perl equivalent. | |
1435 | ||
1436 | To help understand why this can be a real problem first consider how a | |
1437 | callback is set up in an all C environment. Typically a C API will | |
1438 | provide a function to register a callback. This will expect a pointer | |
1439 | to a function as one of its parameters. Below is a call to a | |
1440 | hypothetical function C<register_fatal> which registers the C function | |
1441 | to get called when a fatal error occurs. | |
1442 | ||
4358a253 | 1443 | register_fatal(cb1); |
d1b91892 AD |
1444 | |
1445 | The single parameter C<cb1> is a pointer to a function, so you must | |
1446 | have defined C<cb1> in your code, say something like this | |
1447 | ||
1448 | static void | |
1449 | cb1() | |
1450 | { | |
4358a253 SS |
1451 | printf ("Fatal Error\n"); |
1452 | exit(1); | |
d1b91892 AD |
1453 | } |
1454 | ||
1455 | Now change that to call a Perl subroutine instead | |
1456 | ||
1457 | static SV * callback = (SV*)NULL; | |
1458 | ||
1459 | static void | |
1460 | cb1() | |
1461 | { | |
4358a253 | 1462 | dSP; |
d1b91892 | 1463 | |
4358a253 | 1464 | PUSHMARK(SP); |
d1b91892 AD |
1465 | |
1466 | /* Call the Perl sub to process the callback */ | |
4358a253 | 1467 | call_sv(callback, G_DISCARD); |
d1b91892 AD |
1468 | } |
1469 | ||
1470 | ||
1471 | void | |
1472 | register_fatal(fn) | |
1473 | SV * fn | |
1474 | CODE: | |
1475 | /* Remember the Perl sub */ | |
1476 | if (callback == (SV*)NULL) | |
4358a253 | 1477 | callback = newSVsv(fn); |
d1b91892 | 1478 | else |
4358a253 | 1479 | SvSetSV(callback, fn); |
d1b91892 AD |
1480 | |
1481 | /* register the callback with the external library */ | |
4358a253 | 1482 | register_fatal(cb1); |
d1b91892 AD |
1483 | |
1484 | where the Perl equivalent of C<register_fatal> and the callback it | |
1485 | registers, C<pcb1>, might look like this | |
1486 | ||
1487 | # Register the sub pcb1 | |
4358a253 | 1488 | register_fatal(\&pcb1); |
d1b91892 | 1489 | |
84f709e7 JH |
1490 | sub pcb1 |
1491 | { | |
4358a253 | 1492 | die "I'm dying...\n"; |
d1b91892 AD |
1493 | } |
1494 | ||
1495 | The mapping between the C callback and the Perl equivalent is stored in | |
1496 | the global variable C<callback>. | |
1497 | ||
5f05dabc | 1498 | This will be adequate if you ever need to have only one callback |
d1b91892 AD |
1499 | registered at any time. An example could be an error handler like the |
1500 | code sketched out above. Remember though, repeated calls to | |
1501 | C<register_fatal> will replace the previously registered callback | |
1502 | function with the new one. | |
1503 | ||
1504 | Say for example you want to interface to a library which allows asynchronous | |
1505 | file i/o. In this case you may be able to register a callback whenever | |
1506 | a read operation has completed. To be of any use we want to be able to | |
1507 | call separate Perl subroutines for each file that is opened. As it | |
1508 | stands, the error handler example above would not be adequate as it | |
1509 | allows only a single callback to be defined at any time. What we | |
1510 | require is a means of storing the mapping between the opened file and | |
1511 | the Perl subroutine we want to be called for that file. | |
1512 | ||
1513 | Say the i/o library has a function C<asynch_read> which associates a C | |
19799a22 | 1514 | function C<ProcessRead> with a file handle C<fh>--this assumes that it |
d1b91892 AD |
1515 | has also provided some routine to open the file and so obtain the file |
1516 | handle. | |
1517 | ||
1518 | asynch_read(fh, ProcessRead) | |
1519 | ||
1520 | This may expect the C I<ProcessRead> function of this form | |
1521 | ||
1522 | void | |
1523 | ProcessRead(fh, buffer) | |
4358a253 SS |
1524 | int fh; |
1525 | char * buffer; | |
d1b91892 | 1526 | { |
54310121 | 1527 | ... |
d1b91892 AD |
1528 | } |
1529 | ||
1530 | To provide a Perl interface to this library we need to be able to map | |
1531 | between the C<fh> parameter and the Perl subroutine we want called. A | |
1532 | hash is a convenient mechanism for storing this mapping. The code | |
1533 | below shows a possible implementation | |
1534 | ||
4358a253 | 1535 | static HV * Mapping = (HV*)NULL; |
a0d0e21e | 1536 | |
d1b91892 AD |
1537 | void |
1538 | asynch_read(fh, callback) | |
1539 | int fh | |
1540 | SV * callback | |
1541 | CODE: | |
1542 | /* If the hash doesn't already exist, create it */ | |
1543 | if (Mapping == (HV*)NULL) | |
4358a253 | 1544 | Mapping = newHV(); |
d1b91892 AD |
1545 | |
1546 | /* Save the fh -> callback mapping */ | |
4358a253 | 1547 | hv_store(Mapping, (char*)&fh, sizeof(fh), newSVsv(callback), 0); |
d1b91892 AD |
1548 | |
1549 | /* Register with the C Library */ | |
4358a253 | 1550 | asynch_read(fh, asynch_read_if); |
d1b91892 AD |
1551 | |
1552 | and C<asynch_read_if> could look like this | |
1553 | ||
1554 | static void | |
1555 | asynch_read_if(fh, buffer) | |
4358a253 SS |
1556 | int fh; |
1557 | char * buffer; | |
d1b91892 | 1558 | { |
4358a253 SS |
1559 | dSP; |
1560 | SV ** sv; | |
d1b91892 AD |
1561 | |
1562 | /* Get the callback associated with fh */ | |
4358a253 | 1563 | sv = hv_fetch(Mapping, (char*)&fh , sizeof(fh), FALSE); |
d1b91892 | 1564 | if (sv == (SV**)NULL) |
4358a253 | 1565 | croak("Internal error...\n"); |
d1b91892 | 1566 | |
4358a253 SS |
1567 | PUSHMARK(SP); |
1568 | XPUSHs(sv_2mortal(newSViv(fh))); | |
1569 | XPUSHs(sv_2mortal(newSVpv(buffer, 0))); | |
1570 | PUTBACK; | |
d1b91892 AD |
1571 | |
1572 | /* Call the Perl sub */ | |
4358a253 | 1573 | call_sv(*sv, G_DISCARD); |
d1b91892 AD |
1574 | } |
1575 | ||
1576 | For completeness, here is C<asynch_close>. This shows how to remove | |
1577 | the entry from the hash C<Mapping>. | |
1578 | ||
1579 | void | |
1580 | asynch_close(fh) | |
1581 | int fh | |
1582 | CODE: | |
1583 | /* Remove the entry from the hash */ | |
4358a253 | 1584 | (void) hv_delete(Mapping, (char*)&fh, sizeof(fh), G_DISCARD); |
a0d0e21e | 1585 | |
d1b91892 | 1586 | /* Now call the real asynch_close */ |
4358a253 | 1587 | asynch_close(fh); |
a0d0e21e | 1588 | |
d1b91892 AD |
1589 | So the Perl interface would look like this |
1590 | ||
84f709e7 JH |
1591 | sub callback1 |
1592 | { | |
4358a253 | 1593 | my($handle, $buffer) = @_; |
d1b91892 | 1594 | } |
a0d0e21e | 1595 | |
d1b91892 | 1596 | # Register the Perl callback |
4358a253 | 1597 | asynch_read($fh, \&callback1); |
a0d0e21e | 1598 | |
4358a253 | 1599 | asynch_close($fh); |
d1b91892 AD |
1600 | |
1601 | The mapping between the C callback and Perl is stored in the global | |
1602 | hash C<Mapping> this time. Using a hash has the distinct advantage that | |
1603 | it allows an unlimited number of callbacks to be registered. | |
1604 | ||
1605 | What if the interface provided by the C callback doesn't contain a | |
1606 | parameter which allows the file handle to Perl subroutine mapping? Say | |
1607 | in the asynchronous i/o package, the callback function gets passed only | |
1608 | the C<buffer> parameter like this | |
1609 | ||
1610 | void | |
1611 | ProcessRead(buffer) | |
4358a253 | 1612 | char * buffer; |
d1b91892 AD |
1613 | { |
1614 | ... | |
1615 | } | |
a0d0e21e | 1616 | |
d1b91892 AD |
1617 | Without the file handle there is no straightforward way to map from the |
1618 | C callback to the Perl subroutine. | |
a0d0e21e | 1619 | |
54310121 | 1620 | In this case a possible way around this problem is to predefine a |
d1b91892 AD |
1621 | series of C functions to act as the interface to Perl, thus |
1622 | ||
1623 | #define MAX_CB 3 | |
1624 | #define NULL_HANDLE -1 | |
4358a253 | 1625 | typedef void (*FnMap)(); |
d1b91892 AD |
1626 | |
1627 | struct MapStruct { | |
4358a253 SS |
1628 | FnMap Function; |
1629 | SV * PerlSub; | |
1630 | int Handle; | |
1631 | }; | |
d1b91892 | 1632 | |
4358a253 SS |
1633 | static void fn1(); |
1634 | static void fn2(); | |
1635 | static void fn3(); | |
d1b91892 AD |
1636 | |
1637 | static struct MapStruct Map [MAX_CB] = | |
1638 | { | |
1639 | { fn1, NULL, NULL_HANDLE }, | |
1640 | { fn2, NULL, NULL_HANDLE }, | |
1641 | { fn3, NULL, NULL_HANDLE } | |
4358a253 | 1642 | }; |
d1b91892 AD |
1643 | |
1644 | static void | |
1645 | Pcb(index, buffer) | |
4358a253 SS |
1646 | int index; |
1647 | char * buffer; | |
d1b91892 | 1648 | { |
4358a253 | 1649 | dSP; |
d1b91892 | 1650 | |
4358a253 SS |
1651 | PUSHMARK(SP); |
1652 | XPUSHs(sv_2mortal(newSVpv(buffer, 0))); | |
1653 | PUTBACK; | |
d1b91892 AD |
1654 | |
1655 | /* Call the Perl sub */ | |
4358a253 | 1656 | call_sv(Map[index].PerlSub, G_DISCARD); |
d1b91892 AD |
1657 | } |
1658 | ||
1659 | static void | |
1660 | fn1(buffer) | |
4358a253 | 1661 | char * buffer; |
d1b91892 | 1662 | { |
4358a253 | 1663 | Pcb(0, buffer); |
d1b91892 AD |
1664 | } |
1665 | ||
1666 | static void | |
1667 | fn2(buffer) | |
4358a253 | 1668 | char * buffer; |
d1b91892 | 1669 | { |
4358a253 | 1670 | Pcb(1, buffer); |
d1b91892 AD |
1671 | } |
1672 | ||
1673 | static void | |
1674 | fn3(buffer) | |
4358a253 | 1675 | char * buffer; |
d1b91892 | 1676 | { |
4358a253 | 1677 | Pcb(2, buffer); |
d1b91892 AD |
1678 | } |
1679 | ||
1680 | void | |
1681 | array_asynch_read(fh, callback) | |
1682 | int fh | |
1683 | SV * callback | |
1684 | CODE: | |
4358a253 SS |
1685 | int index; |
1686 | int null_index = MAX_CB; | |
d1b91892 AD |
1687 | |
1688 | /* Find the same handle or an empty entry */ | |
4358a253 | 1689 | for (index = 0; index < MAX_CB; ++index) |
d1b91892 AD |
1690 | { |
1691 | if (Map[index].Handle == fh) | |
4358a253 | 1692 | break; |
d1b91892 AD |
1693 | |
1694 | if (Map[index].Handle == NULL_HANDLE) | |
4358a253 | 1695 | null_index = index; |
d1b91892 AD |
1696 | } |
1697 | ||
1698 | if (index == MAX_CB && null_index == MAX_CB) | |
4358a253 | 1699 | croak ("Too many callback functions registered\n"); |
d1b91892 AD |
1700 | |
1701 | if (index == MAX_CB) | |
4358a253 | 1702 | index = null_index; |
d1b91892 AD |
1703 | |
1704 | /* Save the file handle */ | |
4358a253 | 1705 | Map[index].Handle = fh; |
d1b91892 AD |
1706 | |
1707 | /* Remember the Perl sub */ | |
1708 | if (Map[index].PerlSub == (SV*)NULL) | |
4358a253 | 1709 | Map[index].PerlSub = newSVsv(callback); |
d1b91892 | 1710 | else |
4358a253 | 1711 | SvSetSV(Map[index].PerlSub, callback); |
d1b91892 | 1712 | |
4358a253 | 1713 | asynch_read(fh, Map[index].Function); |
d1b91892 AD |
1714 | |
1715 | void | |
1716 | array_asynch_close(fh) | |
1717 | int fh | |
1718 | CODE: | |
4358a253 | 1719 | int index; |
d1b91892 AD |
1720 | |
1721 | /* Find the file handle */ | |
4358a253 | 1722 | for (index = 0; index < MAX_CB; ++ index) |
d1b91892 | 1723 | if (Map[index].Handle == fh) |
4358a253 | 1724 | break; |
d1b91892 AD |
1725 | |
1726 | if (index == MAX_CB) | |
4358a253 | 1727 | croak ("could not close fh %d\n", fh); |
d1b91892 | 1728 | |
4358a253 SS |
1729 | Map[index].Handle = NULL_HANDLE; |
1730 | SvREFCNT_dec(Map[index].PerlSub); | |
1731 | Map[index].PerlSub = (SV*)NULL; | |
d1b91892 | 1732 | |
4358a253 | 1733 | asynch_close(fh); |
d1b91892 | 1734 | |
5f05dabc | 1735 | In this case the functions C<fn1>, C<fn2>, and C<fn3> are used to |
d1b91892 | 1736 | remember the Perl subroutine to be called. Each of the functions holds |
4a6725af | 1737 | a separate hard-wired index which is used in the function C<Pcb> to |
d1b91892 AD |
1738 | access the C<Map> array and actually call the Perl subroutine. |
1739 | ||
1740 | There are some obvious disadvantages with this technique. | |
1741 | ||
1742 | Firstly, the code is considerably more complex than with the previous | |
1743 | example. | |
1744 | ||
4a6725af | 1745 | Secondly, there is a hard-wired limit (in this case 3) to the number of |
d1b91892 AD |
1746 | callbacks that can exist simultaneously. The only way to increase the |
1747 | limit is by modifying the code to add more functions and then | |
54310121 | 1748 | recompiling. None the less, as long as the number of functions is |
d1b91892 AD |
1749 | chosen with some care, it is still a workable solution and in some |
1750 | cases is the only one available. | |
1751 | ||
1752 | To summarize, here are a number of possible methods for you to consider | |
1753 | for storing the mapping between C and the Perl callback | |
1754 | ||
1755 | =over 5 | |
1756 | ||
1757 | =item 1. Ignore the problem - Allow only 1 callback | |
1758 | ||
1759 | For a lot of situations, like interfacing to an error handler, this may | |
1760 | be a perfectly adequate solution. | |
1761 | ||
1762 | =item 2. Create a sequence of callbacks - hard wired limit | |
1763 | ||
1764 | If it is impossible to tell from the parameters passed back from the C | |
1765 | callback what the context is, then you may need to create a sequence of C | |
1766 | callback interface functions, and store pointers to each in an array. | |
1767 | ||
1768 | =item 3. Use a parameter to map to the Perl callback | |
1769 | ||
1770 | A hash is an ideal mechanism to store the mapping between C and Perl. | |
1771 | ||
1772 | =back | |
a0d0e21e | 1773 | |
a0d0e21e LW |
1774 | |
1775 | =head2 Alternate Stack Manipulation | |
1776 | ||
a0d0e21e | 1777 | |
d1b91892 AD |
1778 | Although I have made use of only the C<POP*> macros to access values |
1779 | returned from Perl subroutines, it is also possible to bypass these | |
8e07c86e | 1780 | macros and read the stack using the C<ST> macro (See L<perlxs> for a |
d1b91892 AD |
1781 | full description of the C<ST> macro). |
1782 | ||
1d45ec27 | 1783 | Most of the time the C<POP*> macros should be adequate; the main |
d1b91892 AD |
1784 | problem with them is that they force you to process the returned values |
1785 | in sequence. This may not be the most suitable way to process the | |
1786 | values in some cases. What we want is to be able to access the stack in | |
1787 | a random order. The C<ST> macro as used when coding an XSUB is ideal | |
1788 | for this purpose. | |
1789 | ||
1d45ec27 FC |
1790 | The code below is the example given in the section I<Returning a List |
1791 | of Values> recoded to use C<ST> instead of C<POP*>. | |
d1b91892 AD |
1792 | |
1793 | static void | |
1794 | call_AddSubtract2(a, b) | |
4358a253 SS |
1795 | int a; |
1796 | int b; | |
d1b91892 | 1797 | { |
4358a253 SS |
1798 | dSP; |
1799 | I32 ax; | |
1800 | int count; | |
d1b91892 | 1801 | |
4358a253 | 1802 | ENTER; |
d1b91892 AD |
1803 | SAVETMPS; |
1804 | ||
4358a253 | 1805 | PUSHMARK(SP); |
d1b91892 AD |
1806 | XPUSHs(sv_2mortal(newSViv(a))); |
1807 | XPUSHs(sv_2mortal(newSViv(b))); | |
4358a253 | 1808 | PUTBACK; |
d1b91892 | 1809 | |
4929bf7b | 1810 | count = call_pv("AddSubtract", G_ARRAY); |
d1b91892 | 1811 | |
4358a253 SS |
1812 | SPAGAIN; |
1813 | SP -= count; | |
1814 | ax = (SP - PL_stack_base) + 1; | |
d1b91892 AD |
1815 | |
1816 | if (count != 2) | |
4358a253 | 1817 | croak("Big trouble\n"); |
a0d0e21e | 1818 | |
4358a253 SS |
1819 | printf ("%d + %d = %d\n", a, b, SvIV(ST(0))); |
1820 | printf ("%d - %d = %d\n", a, b, SvIV(ST(1))); | |
d1b91892 | 1821 | |
4358a253 SS |
1822 | PUTBACK; |
1823 | FREETMPS; | |
1824 | LEAVE; | |
d1b91892 AD |
1825 | } |
1826 | ||
1827 | Notes | |
1828 | ||
1829 | =over 5 | |
1830 | ||
1831 | =item 1. | |
1832 | ||
1833 | Notice that it was necessary to define the variable C<ax>. This is | |
1834 | because the C<ST> macro expects it to exist. If we were in an XSUB it | |
1835 | would not be necessary to define C<ax> as it is already defined for | |
1d45ec27 | 1836 | us. |
d1b91892 AD |
1837 | |
1838 | =item 2. | |
1839 | ||
1840 | The code | |
1841 | ||
4358a253 SS |
1842 | SPAGAIN; |
1843 | SP -= count; | |
1844 | ax = (SP - PL_stack_base) + 1; | |
d1b91892 AD |
1845 | |
1846 | sets the stack up so that we can use the C<ST> macro. | |
1847 | ||
1848 | =item 3. | |
1849 | ||
1850 | Unlike the original coding of this example, the returned | |
1851 | values are not accessed in reverse order. So C<ST(0)> refers to the | |
54310121 | 1852 | first value returned by the Perl subroutine and C<ST(count-1)> |
d1b91892 AD |
1853 | refers to the last. |
1854 | ||
1855 | =back | |
a0d0e21e | 1856 | |
02f6dca1 | 1857 | =head2 Creating and Calling an Anonymous Subroutine in C |
8f183262 | 1858 | |
4929bf7b | 1859 | As we've already shown, C<call_sv> can be used to invoke an |
c2611fb3 GS |
1860 | anonymous subroutine. However, our example showed a Perl script |
1861 | invoking an XSUB to perform this operation. Let's see how it can be | |
8f183262 DM |
1862 | done inside our C code: |
1863 | ||
8f183262 DM |
1864 | ... |
1865 | ||
4929bf7b | 1866 | SV *cvrv = eval_pv("sub { print 'You will not find me cluttering any namespace!' }", TRUE); |
8f183262 DM |
1867 | |
1868 | ... | |
1869 | ||
4929bf7b | 1870 | call_sv(cvrv, G_VOID|G_NOARGS); |
8f183262 | 1871 | |
4929bf7b GS |
1872 | C<eval_pv> is used to compile the anonymous subroutine, which |
1873 | will be the return value as well (read more about C<eval_pv> in | |
4a4eefd0 | 1874 | L<perlapi/eval_pv>). Once this code reference is in hand, it |
8f183262 DM |
1875 | can be mixed in with all the previous examples we've shown. |
1876 | ||
9850bf21 RH |
1877 | =head1 LIGHTWEIGHT CALLBACKS |
1878 | ||
1879 | Sometimes you need to invoke the same subroutine repeatedly. | |
1880 | This usually happens with a function that acts on a list of | |
1881 | values, such as Perl's built-in sort(). You can pass a | |
1882 | comparison function to sort(), which will then be invoked | |
1883 | for every pair of values that needs to be compared. The first() | |
1884 | and reduce() functions from L<List::Util> follow a similar | |
1885 | pattern. | |
1886 | ||
1887 | In this case it is possible to speed up the routine (often | |
1888 | quite substantially) by using the lightweight callback API. | |
1889 | The idea is that the calling context only needs to be | |
1890 | created and destroyed once, and the sub can be called | |
1891 | arbitrarily many times in between. | |
1892 | ||
ac036724 | 1893 | It is usual to pass parameters using global variables (typically |
1894 | $_ for one parameter, or $a and $b for two parameters) rather | |
9850bf21 RH |
1895 | than via @_. (It is possible to use the @_ mechanism if you know |
1896 | what you're doing, though there is as yet no supported API for | |
1897 | it. It's also inherently slower.) | |
1898 | ||
1899 | The pattern of macro calls is like this: | |
1900 | ||
82f35e8b | 1901 | dMULTICALL; /* Declare local variables */ |
9850bf21 | 1902 | I32 gimme = G_SCALAR; /* context of the call: G_SCALAR, |
782a81f5 | 1903 | * G_ARRAY, or G_VOID */ |
9850bf21 | 1904 | |
82f35e8b RH |
1905 | PUSH_MULTICALL(cv); /* Set up the context for calling cv, |
1906 | and set local vars appropriately */ | |
9850bf21 RH |
1907 | |
1908 | /* loop */ { | |
1909 | /* set the value(s) af your parameter variables */ | |
1910 | MULTICALL; /* Make the actual call */ | |
1911 | } /* end of loop */ | |
1912 | ||
1913 | POP_MULTICALL; /* Tear down the calling context */ | |
1914 | ||
1915 | For some concrete examples, see the implementation of the | |
1916 | first() and reduce() functions of List::Util 1.18. There you | |
1917 | will also find a header file that emulates the multicall API | |
1918 | on older versions of perl. | |
1919 | ||
a0d0e21e LW |
1920 | =head1 SEE ALSO |
1921 | ||
8e07c86e | 1922 | L<perlxs>, L<perlguts>, L<perlembed> |
a0d0e21e LW |
1923 | |
1924 | =head1 AUTHOR | |
1925 | ||
0536e0eb | 1926 | Paul Marquess |
a0d0e21e | 1927 | |
d1b91892 AD |
1928 | Special thanks to the following people who assisted in the creation of |
1929 | the document. | |
a0d0e21e | 1930 | |
c07a80fd | 1931 | Jeff Okamoto, Tim Bunce, Nick Gianniotis, Steve Kelem, Gurusamy Sarathy |
1932 | and Larry Wall. | |
a0d0e21e LW |
1933 | |
1934 | =head1 DATE | |
1935 | ||
137443ea | 1936 | Version 1.3, 14th Apr 1997 |