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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 |
95e84303 | 31 | event driven program, such as for an X windows 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); | |
59 | I32 call_argv(char *subname, I32 flags, register 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 | |
a0d0e21e | 206 | |
d1b91892 | 207 | =item 2. |
a0d0e21e | 208 | |
184e9718 | 209 | It ensures that all items returned from the subroutine will be |
4929bf7b | 210 | accessible when control returns from the I<call_*> function. |
a0d0e21e | 211 | |
d1b91892 | 212 | =back |
a0d0e21e | 213 | |
4929bf7b | 214 | The value returned by the I<call_*> function indicates how many |
d1b91892 | 215 | items have been returned by the Perl subroutine. |
a0d0e21e | 216 | |
184e9718 | 217 | If 0, then you have specified the G_DISCARD flag. |
a0d0e21e | 218 | |
d1b91892 AD |
219 | If not 0, then it will be a count of the number of items returned by |
220 | the subroutine. These items will be stored on the Perl stack. The | |
221 | section I<Returning a list of values> gives an example of using the | |
222 | G_ARRAY flag and the mechanics of accessing the returned items from the | |
223 | Perl stack. | |
a0d0e21e | 224 | |
d1b91892 | 225 | =head2 G_DISCARD |
a0d0e21e | 226 | |
4929bf7b | 227 | By default, the I<call_*> functions place the items returned from |
d1b91892 AD |
228 | by the Perl subroutine on the stack. If you are not interested in |
229 | these items, then setting this flag will make Perl get rid of them | |
230 | automatically for you. Note that it is still possible to indicate a | |
231 | context to the Perl subroutine by using either G_SCALAR or G_ARRAY. | |
a0d0e21e | 232 | |
d1b91892 | 233 | If you do not set this flag then it is I<very> important that you make |
5f05dabc | 234 | sure that any temporaries (i.e., parameters passed to the Perl |
d1b91892 AD |
235 | subroutine and values returned from the subroutine) are disposed of |
236 | yourself. The section I<Returning a Scalar> gives details of how to | |
5f05dabc | 237 | dispose of these temporaries explicitly and the section I<Using Perl to |
d1b91892 AD |
238 | dispose of temporaries> discusses the specific circumstances where you |
239 | can ignore the problem and let Perl deal with it for you. | |
a0d0e21e | 240 | |
d1b91892 | 241 | =head2 G_NOARGS |
a0d0e21e | 242 | |
4929bf7b | 243 | Whenever a Perl subroutine is called using one of the I<call_*> |
d1b91892 AD |
244 | functions, it is assumed by default that parameters are to be passed to |
245 | the subroutine. If you are not passing any parameters to the Perl | |
246 | subroutine, you can save a bit of time by setting this flag. It has | |
247 | the effect of not creating the C<@_> array for the Perl subroutine. | |
a0d0e21e | 248 | |
d1b91892 AD |
249 | Although the functionality provided by this flag may seem |
250 | straightforward, it should be used only if there is a good reason to do | |
02f6dca1 | 251 | so. The reason for being cautious is that, even if you have specified |
d1b91892 AD |
252 | the G_NOARGS flag, it is still possible for the Perl subroutine that |
253 | has been called to think that you have passed it parameters. | |
a0d0e21e | 254 | |
d1b91892 AD |
255 | In fact, what can happen is that the Perl subroutine you have called |
256 | can access the C<@_> array from a previous Perl subroutine. This will | |
4929bf7b | 257 | occur when the code that is executing the I<call_*> function has |
d1b91892 AD |
258 | itself been called from another Perl subroutine. The code below |
259 | illustrates this | |
a0d0e21e | 260 | |
84f709e7 JH |
261 | sub fred |
262 | { print "@_\n" } | |
a0d0e21e | 263 | |
84f709e7 JH |
264 | sub joe |
265 | { &fred } | |
a0d0e21e | 266 | |
4358a253 | 267 | &joe(1,2,3); |
a0d0e21e LW |
268 | |
269 | This will print | |
270 | ||
d1b91892 AD |
271 | 1 2 3 |
272 | ||
273 | What has happened is that C<fred> accesses the C<@_> array which | |
274 | belongs to C<joe>. | |
a0d0e21e | 275 | |
a0d0e21e | 276 | |
54310121 | 277 | =head2 G_EVAL |
a0d0e21e | 278 | |
d1b91892 | 279 | It is possible for the Perl subroutine you are calling to terminate |
5f05dabc | 280 | abnormally, e.g., by calling I<die> explicitly or by not actually |
268118b2 HS |
281 | existing. By default, when either of these events occurs, the |
282 | process will terminate immediately. If you want to trap this | |
d1b91892 AD |
283 | type of event, specify the G_EVAL flag. It will put an I<eval { }> |
284 | around the subroutine call. | |
a0d0e21e | 285 | |
4929bf7b | 286 | Whenever control returns from the I<call_*> function you need to |
d1b91892 AD |
287 | check the C<$@> variable as you would in a normal Perl script. |
288 | ||
4929bf7b | 289 | The value returned from the I<call_*> function is dependent on |
d1b91892 | 290 | what other flags have been specified and whether an error has |
184e9718 | 291 | occurred. Here are all the different cases that can occur: |
d1b91892 AD |
292 | |
293 | =over 5 | |
294 | ||
295 | =item * | |
296 | ||
4929bf7b | 297 | If the I<call_*> function returns normally, then the value |
d1b91892 AD |
298 | returned is as specified in the previous sections. |
299 | ||
300 | =item * | |
301 | ||
302 | If G_DISCARD is specified, the return value will always be 0. | |
303 | ||
304 | =item * | |
305 | ||
306 | If G_ARRAY is specified I<and> an error has occurred, the return value | |
307 | will always be 0. | |
308 | ||
309 | =item * | |
a0d0e21e | 310 | |
d1b91892 AD |
311 | If G_SCALAR is specified I<and> an error has occurred, the return value |
312 | will be 1 and the value on the top of the stack will be I<undef>. This | |
313 | means that if you have already detected the error by checking C<$@> and | |
314 | you want the program to continue, you must remember to pop the I<undef> | |
315 | from the stack. | |
a0d0e21e LW |
316 | |
317 | =back | |
318 | ||
54310121 | 319 | See I<Using G_EVAL> for details on using G_EVAL. |
d1b91892 | 320 | |
c07a80fd | 321 | =head2 G_KEEPERR |
322 | ||
7ce09284 Z |
323 | Using the G_EVAL flag described above will always set C<$@>: clearing |
324 | it if there was no error, and setting it to describe the error if there | |
325 | was an error in the called code. This is what you want if your intention | |
326 | is to handle possible errors, but sometimes you just want to trap errors | |
327 | and stop them interfering with the rest of the program. | |
328 | ||
329 | This scenario will mostly be applicable to code that is meant to be called | |
330 | from within destructors, asynchronous callbacks, and signal handlers. | |
331 | In such situations, where the code being called has little relation to the | |
332 | surrounding dynamic context, the main program needs to be insulated from | |
333 | errors in the called code, even if they can't be handled intelligently. | |
334 | It may also be useful to do this with code for C<__DIE__> or C<__WARN__> | |
335 | hooks, and C<tie> functions. | |
c07a80fd | 336 | |
337 | The G_KEEPERR flag is meant to be used in conjunction with G_EVAL in | |
be064c4a DM |
338 | I<call_*> functions that are used to implement such code, or with |
339 | C<eval_sv>. This flag has no effect on the C<call_*> functions when | |
340 | G_EVAL is not used. | |
c07a80fd | 341 | |
7ce09284 Z |
342 | When G_KEEPERR is used, any error in the called code will terminate the |
343 | call as usual, and the error will not propagate beyond the call (as usual | |
344 | for G_EVAL), but it will not go into C<$@>. Instead the error will be | |
345 | converted into a warning, prefixed with the string "\t(in cleanup)". | |
346 | This can be disabled using C<no warnings 'misc'>. If there is no error, | |
347 | C<$@> will not be cleared. | |
c07a80fd | 348 | |
be064c4a DM |
349 | Note that the G_KEEPERR flag does not propagate into inner evals; these |
350 | may still set C<$@>. | |
351 | ||
c07a80fd | 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 | ||
54310121 | 357 | =head2 Determining the Context |
d1b91892 AD |
358 | |
359 | As mentioned above, you can determine the context of the currently | |
54310121 | 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 | |
90fdbbb7 | 362 | C<G_ARRAY> if you have been called in a list context, C<G_SCALAR> if |
02f6dca1 | 363 | in a scalar context, or C<G_VOID> if in a void context (i.e., the |
54310121 | 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>. | |
d1b91892 | 368 | |
a0d0e21e LW |
369 | =head1 EXAMPLES |
370 | ||
02f6dca1 | 371 | Enough of the definition talk! Let's have a few examples. |
a0d0e21e | 372 | |
d1b91892 AD |
373 | Perl provides many macros to assist in accessing the Perl stack. |
374 | Wherever possible, these macros should always be used when interfacing | |
5f05dabc | 375 | to Perl internals. We hope this should make the code less vulnerable |
d1b91892 | 376 | to any changes made to Perl in the future. |
a0d0e21e | 377 | |
d1b91892 | 378 | Another point worth noting is that in the first series of examples I |
4929bf7b | 379 | have made use of only the I<call_pv> function. This has been done |
d1b91892 | 380 | to keep the code simpler and ease you into the topic. Wherever |
4929bf7b GS |
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. | |
a0d0e21e | 384 | |
02f6dca1 | 385 | =head2 No Parameters, Nothing Returned |
a0d0e21e | 386 | |
d1b91892 AD |
387 | This first trivial example will call a Perl subroutine, I<PrintUID>, to |
388 | print out the UID of the process. | |
a0d0e21e | 389 | |
84f709e7 JH |
390 | sub PrintUID |
391 | { | |
4358a253 | 392 | print "UID is $<\n"; |
a0d0e21e LW |
393 | } |
394 | ||
d1b91892 | 395 | and here is a C function to call it |
a0d0e21e | 396 | |
d1b91892 | 397 | static void |
a0d0e21e LW |
398 | call_PrintUID() |
399 | { | |
4358a253 | 400 | dSP; |
a0d0e21e | 401 | |
4358a253 SS |
402 | PUSHMARK(SP); |
403 | call_pv("PrintUID", G_DISCARD|G_NOARGS); | |
a0d0e21e LW |
404 | } |
405 | ||
02f6dca1 | 406 | Simple, eh? |
a0d0e21e | 407 | |
02f6dca1 | 408 | A few points to note about this example: |
a0d0e21e LW |
409 | |
410 | =over 5 | |
411 | ||
d1b91892 | 412 | =item 1. |
a0d0e21e | 413 | |
924508f0 | 414 | Ignore C<dSP> and C<PUSHMARK(SP)> for now. They will be discussed in |
d1b91892 | 415 | the next example. |
a0d0e21e LW |
416 | |
417 | =item 2. | |
418 | ||
d1b91892 AD |
419 | We aren't passing any parameters to I<PrintUID> so G_NOARGS can be |
420 | specified. | |
a0d0e21e | 421 | |
d1b91892 | 422 | =item 3. |
a0d0e21e LW |
423 | |
424 | We aren't interested in anything returned from I<PrintUID>, so | |
5f05dabc | 425 | G_DISCARD is specified. Even if I<PrintUID> was changed to |
a0d0e21e | 426 | return some value(s), having specified G_DISCARD will mean that they |
4929bf7b | 427 | will be wiped by the time control returns from I<call_pv>. |
a0d0e21e | 428 | |
d1b91892 | 429 | =item 4. |
a0d0e21e | 430 | |
4929bf7b | 431 | As I<call_pv> is being used, the Perl subroutine is specified as a |
d1b91892 AD |
432 | C string. In this case the subroutine name has been 'hard-wired' into the |
433 | code. | |
a0d0e21e LW |
434 | |
435 | =item 5. | |
436 | ||
d1b91892 | 437 | Because we specified G_DISCARD, it is not necessary to check the value |
4929bf7b | 438 | returned from I<call_pv>. It will always be 0. |
a0d0e21e LW |
439 | |
440 | =back | |
441 | ||
d1b91892 | 442 | =head2 Passing Parameters |
a0d0e21e | 443 | |
d1b91892 | 444 | Now let's make a slightly more complex example. This time we want to |
19799a22 GS |
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 $n characters of the string. | |
a0d0e21e | 448 | |
02f6dca1 | 449 | So the Perl subroutine would look like this: |
a0d0e21e | 450 | |
84f709e7 JH |
451 | sub LeftString |
452 | { | |
4358a253 SS |
453 | my($s, $n) = @_; |
454 | print substr($s, 0, $n), "\n"; | |
a0d0e21e LW |
455 | } |
456 | ||
02f6dca1 | 457 | The C function required to call I<LeftString> would look like this: |
a0d0e21e LW |
458 | |
459 | static void | |
460 | call_LeftString(a, b) | |
4358a253 SS |
461 | char * a; |
462 | int b; | |
a0d0e21e | 463 | { |
4358a253 | 464 | dSP; |
a0d0e21e | 465 | |
4358a253 SS |
466 | ENTER; |
467 | SAVETMPS; | |
9b6570b4 | 468 | |
4358a253 | 469 | PUSHMARK(SP); |
a0d0e21e LW |
470 | XPUSHs(sv_2mortal(newSVpv(a, 0))); |
471 | XPUSHs(sv_2mortal(newSViv(b))); | |
4358a253 | 472 | PUTBACK; |
a0d0e21e | 473 | |
4929bf7b | 474 | call_pv("LeftString", G_DISCARD); |
9b6570b4 | 475 | |
4358a253 SS |
476 | FREETMPS; |
477 | LEAVE; | |
a0d0e21e LW |
478 | } |
479 | ||
a0d0e21e LW |
480 | Here are a few notes on the C function I<call_LeftString>. |
481 | ||
482 | =over 5 | |
483 | ||
d1b91892 | 484 | =item 1. |
a0d0e21e | 485 | |
d1b91892 AD |
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 | |
1e62ac33 | 488 | ending with the line C<PUTBACK>. The C<dSP> declares a local copy |
924508f0 GS |
489 | of the stack pointer. This local copy should B<always> be accessed |
490 | as C<SP>. | |
a0d0e21e | 491 | |
d1b91892 | 492 | =item 2. |
a0d0e21e LW |
493 | |
494 | If you are going to put something onto the Perl stack, you need to know | |
19799a22 | 495 | where to put it. This is the purpose of the macro C<dSP>--it declares |
d1b91892 | 496 | and initializes a I<local> copy of the Perl stack pointer. |
a0d0e21e LW |
497 | |
498 | All the other macros which will be used in this example require you to | |
d1b91892 | 499 | have used this macro. |
a0d0e21e | 500 | |
d1b91892 AD |
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 | |
19799a22 | 503 | use the C<dSP> macro explicitly--it will be declared for you |
d1b91892 | 504 | automatically. |
a0d0e21e | 505 | |
d1b91892 | 506 | =item 3. |
a0d0e21e LW |
507 | |
508 | Any parameters to be pushed onto the stack should be bracketed by the | |
d1b91892 | 509 | C<PUSHMARK> and C<PUTBACK> macros. The purpose of these two macros, in |
5f05dabc | 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 | |
d1b91892 AD |
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 | |
02f6dca1 | 516 | example shown in the section I<No Parameters, Nothing Returned>) you |
d1b91892 | 517 | must still call the C<PUSHMARK> macro before you can call any of the |
4929bf7b | 518 | I<call_*> functions--Perl still needs to know that there are no |
d1b91892 AD |
519 | parameters. |
520 | ||
521 | The C<PUTBACK> macro sets the global copy of the stack pointer to be | |
02f6dca1 | 522 | the same as our local copy. If we didn't do this, I<call_pv> |
19799a22 | 523 | wouldn't know where the two parameters we pushed were--remember that |
d1b91892 AD |
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 | ||
a0d0e21e | 529 | Next, we come to XPUSHs. This is where the parameters actually get |
d1b91892 AD |
530 | pushed onto the stack. In this case we are pushing a string and an |
531 | integer. | |
a0d0e21e | 532 | |
54310121 | 533 | See L<perlguts/"XSUBs and the Argument Stack"> for details |
d1b91892 | 534 | on how the XPUSH macros work. |
a0d0e21e | 535 | |
087fe227 | 536 | =item 5. |
a0d0e21e | 537 | |
9b6570b4 AB |
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 | ||
4358a253 SS |
543 | ENTER; |
544 | SAVETMPS; | |
9b6570b4 AB |
545 | |
546 | at the start of the function, and | |
547 | ||
4358a253 SS |
548 | FREETMPS; |
549 | LEAVE; | |
9b6570b4 AB |
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 | ||
02f6dca1 | 560 | Think of these macros as working a bit like C<{> and C<}> in Perl |
9b6570b4 AB |
561 | to limit the scope of local variables. |
562 | ||
02f6dca1 | 563 | See the section I<Using Perl to Dispose of Temporaries> for details of |
9b6570b4 AB |
564 | an alternative to using these macros. |
565 | ||
087fe227 | 566 | =item 6. |
9b6570b4 | 567 | |
087fe227 JA |
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. | |
a0d0e21e LW |
572 | |
573 | =back | |
574 | ||
d1b91892 | 575 | =head2 Returning a Scalar |
a0d0e21e | 576 | |
d1b91892 AD |
577 | Now for an example of dealing with the items returned from a Perl |
578 | subroutine. | |
a0d0e21e | 579 | |
5f05dabc | 580 | Here is a Perl subroutine, I<Adder>, that takes 2 integer parameters |
d1b91892 | 581 | and simply returns their sum. |
a0d0e21e | 582 | |
84f709e7 JH |
583 | sub Adder |
584 | { | |
4358a253 SS |
585 | my($a, $b) = @_; |
586 | $a + $b; | |
a0d0e21e LW |
587 | } |
588 | ||
5f05dabc | 589 | Because we are now concerned with the return value from I<Adder>, the C |
d1b91892 | 590 | function required to call it is now a bit more complex. |
a0d0e21e LW |
591 | |
592 | static void | |
593 | call_Adder(a, b) | |
4358a253 SS |
594 | int a; |
595 | int b; | |
a0d0e21e | 596 | { |
4358a253 SS |
597 | dSP; |
598 | int count; | |
a0d0e21e | 599 | |
4358a253 | 600 | ENTER; |
a0d0e21e LW |
601 | SAVETMPS; |
602 | ||
4358a253 | 603 | PUSHMARK(SP); |
a0d0e21e LW |
604 | XPUSHs(sv_2mortal(newSViv(a))); |
605 | XPUSHs(sv_2mortal(newSViv(b))); | |
4358a253 | 606 | PUTBACK; |
a0d0e21e | 607 | |
4929bf7b | 608 | count = call_pv("Adder", G_SCALAR); |
a0d0e21e | 609 | |
4358a253 | 610 | SPAGAIN; |
a0d0e21e | 611 | |
d1b91892 | 612 | if (count != 1) |
4358a253 | 613 | croak("Big trouble\n"); |
a0d0e21e | 614 | |
4358a253 | 615 | printf ("The sum of %d and %d is %d\n", a, b, POPi); |
a0d0e21e | 616 | |
4358a253 SS |
617 | PUTBACK; |
618 | FREETMPS; | |
619 | LEAVE; | |
a0d0e21e LW |
620 | } |
621 | ||
a0d0e21e LW |
622 | Points to note this time are |
623 | ||
624 | =over 5 | |
625 | ||
54310121 | 626 | =item 1. |
a0d0e21e | 627 | |
02f6dca1 | 628 | The only flag specified this time was G_SCALAR. That means that the C<@_> |
d1b91892 | 629 | array will be created and that the value returned by I<Adder> will |
4929bf7b | 630 | still exist after the call to I<call_pv>. |
a0d0e21e | 631 | |
a0d0e21e LW |
632 | =item 2. |
633 | ||
a0d0e21e LW |
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 | |
ed874981 | 636 | allocated to the Perl stack has been reallocated during the |
4929bf7b | 637 | I<call_pv> call. |
a0d0e21e | 638 | |
d1b91892 | 639 | If you are making use of the Perl stack pointer in your code you must |
54310121 | 640 | always refresh the local copy using SPAGAIN whenever you make use |
4929bf7b | 641 | of the I<call_*> functions or any other Perl internal function. |
a0d0e21e | 642 | |
9b6570b4 | 643 | =item 3. |
a0d0e21e | 644 | |
d1b91892 | 645 | Although only a single value was expected to be returned from I<Adder>, |
4929bf7b | 646 | it is still good practice to check the return code from I<call_pv> |
d1b91892 | 647 | anyway. |
a0d0e21e | 648 | |
d1b91892 AD |
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 | |
5f05dabc | 653 | I<really> don't want to happen ever. |
a0d0e21e | 654 | |
9b6570b4 | 655 | =item 4. |
a0d0e21e | 656 | |
d1b91892 AD |
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. | |
a0d0e21e LW |
659 | |
660 | ||
d1b91892 AD |
661 | Here is the complete list of POP macros available, along with the types |
662 | they return. | |
a0d0e21e | 663 | |
d1b91892 AD |
664 | POPs SV |
665 | POPp pointer | |
666 | POPn double | |
667 | POPi integer | |
668 | POPl long | |
a0d0e21e | 669 | |
9b6570b4 | 670 | =item 5. |
a0d0e21e | 671 | |
d1b91892 AD |
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. | |
a0d0e21e LW |
677 | |
678 | =back | |
679 | ||
680 | ||
02f6dca1 | 681 | =head2 Returning a List of Values |
a0d0e21e | 682 | |
d1b91892 AD |
683 | Now, let's extend the previous example to return both the sum of the |
684 | parameters and the difference. | |
a0d0e21e | 685 | |
d1b91892 | 686 | Here is the Perl subroutine |
a0d0e21e | 687 | |
84f709e7 JH |
688 | sub AddSubtract |
689 | { | |
4358a253 SS |
690 | my($a, $b) = @_; |
691 | ($a+$b, $a-$b); | |
a0d0e21e LW |
692 | } |
693 | ||
a0d0e21e LW |
694 | and this is the C function |
695 | ||
696 | static void | |
697 | call_AddSubtract(a, b) | |
4358a253 SS |
698 | int a; |
699 | int b; | |
a0d0e21e | 700 | { |
4358a253 SS |
701 | dSP; |
702 | int count; | |
a0d0e21e | 703 | |
4358a253 | 704 | ENTER; |
a0d0e21e LW |
705 | SAVETMPS; |
706 | ||
4358a253 | 707 | PUSHMARK(SP); |
a0d0e21e LW |
708 | XPUSHs(sv_2mortal(newSViv(a))); |
709 | XPUSHs(sv_2mortal(newSViv(b))); | |
4358a253 | 710 | PUTBACK; |
a0d0e21e | 711 | |
4929bf7b | 712 | count = call_pv("AddSubtract", G_ARRAY); |
a0d0e21e | 713 | |
4358a253 | 714 | SPAGAIN; |
a0d0e21e | 715 | |
d1b91892 | 716 | if (count != 2) |
4358a253 | 717 | croak("Big trouble\n"); |
a0d0e21e | 718 | |
4358a253 SS |
719 | printf ("%d - %d = %d\n", a, b, POPi); |
720 | printf ("%d + %d = %d\n", a, b, POPi); | |
a0d0e21e | 721 | |
4358a253 SS |
722 | PUTBACK; |
723 | FREETMPS; | |
724 | LEAVE; | |
a0d0e21e LW |
725 | } |
726 | ||
d1b91892 AD |
727 | If I<call_AddSubtract> is called like this |
728 | ||
4358a253 | 729 | call_AddSubtract(7, 4); |
d1b91892 AD |
730 | |
731 | then here is the output | |
732 | ||
733 | 7 - 4 = 3 | |
734 | 7 + 4 = 11 | |
a0d0e21e LW |
735 | |
736 | Notes | |
737 | ||
738 | =over 5 | |
739 | ||
740 | =item 1. | |
741 | ||
90fdbbb7 | 742 | We wanted list context, so G_ARRAY was used. |
a0d0e21e LW |
743 | |
744 | =item 2. | |
745 | ||
d1b91892 AD |
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. | |
a0d0e21e LW |
750 | |
751 | =back | |
752 | ||
02f6dca1 | 753 | =head2 Returning a List in a Scalar Context |
d1b91892 AD |
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) | |
4358a253 SS |
760 | int a; |
761 | int b; | |
d1b91892 | 762 | { |
4358a253 SS |
763 | dSP; |
764 | int count; | |
765 | int i; | |
d1b91892 | 766 | |
4358a253 | 767 | ENTER; |
d1b91892 AD |
768 | SAVETMPS; |
769 | ||
4358a253 | 770 | PUSHMARK(SP); |
d1b91892 AD |
771 | XPUSHs(sv_2mortal(newSViv(a))); |
772 | XPUSHs(sv_2mortal(newSViv(b))); | |
4358a253 | 773 | PUTBACK; |
d1b91892 | 774 | |
4929bf7b | 775 | count = call_pv("AddSubtract", G_SCALAR); |
d1b91892 | 776 | |
4358a253 | 777 | SPAGAIN; |
d1b91892 | 778 | |
4358a253 | 779 | printf ("Items Returned = %d\n", count); |
d1b91892 | 780 | |
4358a253 SS |
781 | for (i = 1; i <= count; ++i) |
782 | printf ("Value %d = %d\n", i, POPi); | |
d1b91892 | 783 | |
4358a253 SS |
784 | PUTBACK; |
785 | FREETMPS; | |
786 | LEAVE; | |
d1b91892 AD |
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 | ||
4358a253 | 794 | call_AddSubScalar(7, 4); |
d1b91892 AD |
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 | |
48052fe5 | 802 | list is returned from the subroutine. I<AddSubtract> actually made it back to |
d1b91892 AD |
803 | I<call_AddSubScalar>. |
804 | ||
805 | ||
02f6dca1 | 806 | =head2 Returning Data from Perl via the Parameter List |
a0d0e21e | 807 | |
48052fe5 FC |
808 | It is also possible to return values directly via the parameter |
809 | list--whether it is actually desirable to do it is another matter entirely. | |
a0d0e21e | 810 | |
d1b91892 AD |
811 | The Perl subroutine, I<Inc>, below takes 2 parameters and increments |
812 | each directly. | |
a0d0e21e | 813 | |
84f709e7 JH |
814 | sub Inc |
815 | { | |
4358a253 SS |
816 | ++ $_[0]; |
817 | ++ $_[1]; | |
a0d0e21e LW |
818 | } |
819 | ||
820 | and here is a C function to call it. | |
821 | ||
822 | static void | |
823 | call_Inc(a, b) | |
4358a253 SS |
824 | int a; |
825 | int b; | |
a0d0e21e | 826 | { |
4358a253 SS |
827 | dSP; |
828 | int count; | |
829 | SV * sva; | |
830 | SV * svb; | |
a0d0e21e | 831 | |
4358a253 | 832 | ENTER; |
a0d0e21e LW |
833 | SAVETMPS; |
834 | ||
4358a253 SS |
835 | sva = sv_2mortal(newSViv(a)); |
836 | svb = sv_2mortal(newSViv(b)); | |
a0d0e21e | 837 | |
4358a253 | 838 | PUSHMARK(SP); |
a0d0e21e LW |
839 | XPUSHs(sva); |
840 | XPUSHs(svb); | |
4358a253 | 841 | PUTBACK; |
a0d0e21e | 842 | |
4929bf7b | 843 | count = call_pv("Inc", G_DISCARD); |
a0d0e21e LW |
844 | |
845 | if (count != 0) | |
d1b91892 | 846 | croak ("call_Inc: expected 0 values from 'Inc', got %d\n", |
4358a253 | 847 | count); |
a0d0e21e | 848 | |
4358a253 SS |
849 | printf ("%d + 1 = %d\n", a, SvIV(sva)); |
850 | printf ("%d + 1 = %d\n", b, SvIV(svb)); | |
a0d0e21e | 851 | |
4358a253 SS |
852 | FREETMPS; |
853 | LEAVE; | |
a0d0e21e LW |
854 | } |
855 | ||
d1b91892 | 856 | To be able to access the two parameters that were pushed onto the stack |
4929bf7b | 857 | after they return from I<call_pv> it is necessary to make a note |
19799a22 | 858 | of their addresses--thus the two variables C<sva> and C<svb>. |
a0d0e21e | 859 | |
d1b91892 AD |
860 | The reason this is necessary is that the area of the Perl stack which |
861 | held them will very likely have been overwritten by something else by | |
4929bf7b | 862 | the time control returns from I<call_pv>. |
a0d0e21e LW |
863 | |
864 | ||
865 | ||
866 | ||
d1b91892 | 867 | =head2 Using G_EVAL |
a0d0e21e | 868 | |
d1b91892 AD |
869 | Now an example using G_EVAL. Below is a Perl subroutine which computes |
870 | the difference of its 2 parameters. If this would result in a negative | |
871 | result, the subroutine calls I<die>. | |
a0d0e21e | 872 | |
84f709e7 JH |
873 | sub Subtract |
874 | { | |
4358a253 | 875 | my ($a, $b) = @_; |
a0d0e21e | 876 | |
4358a253 | 877 | die "death can be fatal\n" if $a < $b; |
a0d0e21e | 878 | |
4358a253 | 879 | $a - $b; |
a0d0e21e LW |
880 | } |
881 | ||
882 | and some C to call it | |
883 | ||
884 | static void | |
885 | call_Subtract(a, b) | |
4358a253 SS |
886 | int a; |
887 | int b; | |
a0d0e21e | 888 | { |
4358a253 SS |
889 | dSP; |
890 | int count; | |
a0d0e21e | 891 | |
4358a253 | 892 | ENTER; |
a0d0e21e LW |
893 | SAVETMPS; |
894 | ||
4358a253 | 895 | PUSHMARK(SP); |
a0d0e21e LW |
896 | XPUSHs(sv_2mortal(newSViv(a))); |
897 | XPUSHs(sv_2mortal(newSViv(b))); | |
4358a253 | 898 | PUTBACK; |
a0d0e21e | 899 | |
4929bf7b | 900 | count = call_pv("Subtract", G_EVAL|G_SCALAR); |
a0d0e21e | 901 | |
4358a253 | 902 | SPAGAIN; |
d1b91892 AD |
903 | |
904 | /* Check the eval first */ | |
9cde0e7f | 905 | if (SvTRUE(ERRSV)) |
d1b91892 | 906 | { |
1c5b513e | 907 | printf ("Uh oh - %s\n", SvPV_nolen(ERRSV)); |
4358a253 | 908 | POPs; |
d1b91892 AD |
909 | } |
910 | else | |
911 | { | |
912 | if (count != 1) | |
913 | croak("call_Subtract: wanted 1 value from 'Subtract', got %d\n", | |
4358a253 | 914 | count); |
a0d0e21e | 915 | |
4358a253 | 916 | printf ("%d - %d = %d\n", a, b, POPi); |
d1b91892 | 917 | } |
a0d0e21e | 918 | |
4358a253 SS |
919 | PUTBACK; |
920 | FREETMPS; | |
921 | LEAVE; | |
a0d0e21e LW |
922 | } |
923 | ||
924 | If I<call_Subtract> is called thus | |
925 | ||
d1b91892 | 926 | call_Subtract(4, 5) |
a0d0e21e LW |
927 | |
928 | the following will be printed | |
929 | ||
d1b91892 | 930 | Uh oh - death can be fatal |
a0d0e21e LW |
931 | |
932 | Notes | |
933 | ||
934 | =over 5 | |
935 | ||
936 | =item 1. | |
937 | ||
d1b91892 AD |
938 | We want to be able to catch the I<die> so we have used the G_EVAL |
939 | flag. Not specifying this flag would mean that the program would | |
940 | terminate immediately at the I<die> statement in the subroutine | |
941 | I<Subtract>. | |
a0d0e21e LW |
942 | |
943 | =item 2. | |
944 | ||
54310121 | 945 | The code |
a0d0e21e | 946 | |
9cde0e7f | 947 | if (SvTRUE(ERRSV)) |
d1b91892 | 948 | { |
1c5b513e | 949 | printf ("Uh oh - %s\n", SvPV_nolen(ERRSV)); |
4358a253 | 950 | POPs; |
d1b91892 | 951 | } |
a0d0e21e | 952 | |
d1b91892 | 953 | is the direct equivalent of this bit of Perl |
a0d0e21e | 954 | |
4358a253 | 955 | print "Uh oh - $@\n" if $@; |
a0d0e21e | 956 | |
9cde0e7f GS |
957 | C<PL_errgv> is a perl global of type C<GV *> that points to the |
958 | symbol table entry containing the error. C<ERRSV> therefore | |
c07a80fd | 959 | refers to the C equivalent of C<$@>. |
960 | ||
d1b91892 | 961 | =item 3. |
a0d0e21e | 962 | |
d1b91892 | 963 | Note that the stack is popped using C<POPs> in the block where |
9cde0e7f | 964 | C<SvTRUE(ERRSV)> is true. This is necessary because whenever a |
4929bf7b | 965 | I<call_*> function invoked with G_EVAL|G_SCALAR returns an error, |
5f05dabc | 966 | the top of the stack holds the value I<undef>. Because we want the |
d1b91892 | 967 | program to continue after detecting this error, it is essential that |
6c818a50 | 968 | the stack be tidied up by removing the I<undef>. |
a0d0e21e LW |
969 | |
970 | =back | |
971 | ||
972 | ||
c07a80fd | 973 | =head2 Using G_KEEPERR |
974 | ||
975 | Consider this rather facetious example, where we have used an XS | |
976 | version of the call_Subtract example above inside a destructor: | |
977 | ||
978 | package Foo; | |
84f709e7 | 979 | sub new { bless {}, $_[0] } |
54310121 | 980 | sub Subtract { |
84f709e7 | 981 | my($a,$b) = @_; |
4358a253 | 982 | die "death can be fatal" if $a < $b; |
84f709e7 | 983 | $a - $b; |
c07a80fd | 984 | } |
84f709e7 JH |
985 | sub DESTROY { call_Subtract(5, 4); } |
986 | sub foo { die "foo dies"; } | |
c07a80fd | 987 | |
988 | package main; | |
7ce09284 Z |
989 | { |
990 | my $foo = Foo->new; | |
991 | eval { $foo->foo }; | |
992 | } | |
c07a80fd | 993 | print "Saw: $@" if $@; # should be, but isn't |
994 | ||
995 | This example will fail to recognize that an error occurred inside the | |
996 | C<eval {}>. Here's why: the call_Subtract code got executed while perl | |
7ce09284 | 997 | was cleaning up temporaries when exiting the outer braced block, and because |
4929bf7b | 998 | call_Subtract is implemented with I<call_pv> using the G_EVAL |
c07a80fd | 999 | flag, it promptly reset C<$@>. This results in the failure of the |
1000 | outermost test for C<$@>, and thereby the failure of the error trap. | |
1001 | ||
4929bf7b | 1002 | Appending the G_KEEPERR flag, so that the I<call_pv> call in |
c07a80fd | 1003 | call_Subtract reads: |
1004 | ||
4929bf7b | 1005 | count = call_pv("Subtract", G_EVAL|G_SCALAR|G_KEEPERR); |
c07a80fd | 1006 | |
1007 | will preserve the error and restore reliable error handling. | |
1008 | ||
4929bf7b | 1009 | =head2 Using call_sv |
a0d0e21e | 1010 | |
d1b91892 AD |
1011 | In all the previous examples I have 'hard-wired' the name of the Perl |
1012 | subroutine to be called from C. Most of the time though, it is more | |
1013 | convenient to be able to specify the name of the Perl subroutine from | |
1014 | within the Perl script. | |
a0d0e21e LW |
1015 | |
1016 | Consider the Perl code below | |
1017 | ||
84f709e7 JH |
1018 | sub fred |
1019 | { | |
4358a253 | 1020 | print "Hello there\n"; |
d1b91892 AD |
1021 | } |
1022 | ||
4358a253 | 1023 | CallSubPV("fred"); |
d1b91892 AD |
1024 | |
1025 | Here is a snippet of XSUB which defines I<CallSubPV>. | |
1026 | ||
1027 | void | |
1028 | CallSubPV(name) | |
1029 | char * name | |
1030 | CODE: | |
4358a253 SS |
1031 | PUSHMARK(SP); |
1032 | call_pv(name, G_DISCARD|G_NOARGS); | |
a0d0e21e | 1033 | |
54310121 | 1034 | That is fine as far as it goes. The thing is, the Perl subroutine |
5f05dabc | 1035 | can be specified as only a string. For Perl 4 this was adequate, |
d1b91892 | 1036 | but Perl 5 allows references to subroutines and anonymous subroutines. |
4929bf7b | 1037 | This is where I<call_sv> is useful. |
d1b91892 AD |
1038 | |
1039 | The code below for I<CallSubSV> is identical to I<CallSubPV> except | |
1040 | that the C<name> parameter is now defined as an SV* and we use | |
4929bf7b | 1041 | I<call_sv> instead of I<call_pv>. |
d1b91892 AD |
1042 | |
1043 | void | |
1044 | CallSubSV(name) | |
1045 | SV * name | |
1046 | CODE: | |
4358a253 SS |
1047 | PUSHMARK(SP); |
1048 | call_sv(name, G_DISCARD|G_NOARGS); | |
a0d0e21e | 1049 | |
1d45ec27 | 1050 | Because we are using an SV to call I<fred> the following can all be used: |
a0d0e21e | 1051 | |
4358a253 SS |
1052 | CallSubSV("fred"); |
1053 | CallSubSV(\&fred); | |
1054 | $ref = \&fred; | |
1055 | CallSubSV($ref); | |
1056 | CallSubSV( sub { print "Hello there\n" } ); | |
a0d0e21e | 1057 | |
4929bf7b | 1058 | As you can see, I<call_sv> gives you much greater flexibility in |
d1b91892 AD |
1059 | how you can specify the Perl subroutine. |
1060 | ||
1d45ec27 | 1061 | You should note that, if it is necessary to store the SV (C<name> in the |
d1b91892 | 1062 | example above) which corresponds to the Perl subroutine so that it can |
5f05dabc | 1063 | be used later in the program, it not enough just to store a copy of the |
1d45ec27 | 1064 | pointer to the SV. Say the code above had been like this: |
d1b91892 | 1065 | |
4358a253 | 1066 | static SV * rememberSub; |
d1b91892 AD |
1067 | |
1068 | void | |
1069 | SaveSub1(name) | |
1070 | SV * name | |
1071 | CODE: | |
4358a253 | 1072 | rememberSub = name; |
d1b91892 AD |
1073 | |
1074 | void | |
1075 | CallSavedSub1() | |
1076 | CODE: | |
4358a253 SS |
1077 | PUSHMARK(SP); |
1078 | call_sv(rememberSub, G_DISCARD|G_NOARGS); | |
a0d0e21e | 1079 | |
1d45ec27 | 1080 | The reason this is wrong is that, by the time you come to use the |
d1b91892 AD |
1081 | pointer C<rememberSub> in C<CallSavedSub1>, it may or may not still refer |
1082 | to the Perl subroutine that was recorded in C<SaveSub1>. This is | |
1d45ec27 | 1083 | particularly true for these cases: |
a0d0e21e | 1084 | |
4358a253 SS |
1085 | SaveSub1(\&fred); |
1086 | CallSavedSub1(); | |
a0d0e21e | 1087 | |
4358a253 SS |
1088 | SaveSub1( sub { print "Hello there\n" } ); |
1089 | CallSavedSub1(); | |
a0d0e21e | 1090 | |
1d45ec27 | 1091 | By the time each of the C<SaveSub1> statements above has been executed, |
54310121 | 1092 | the SV*s which corresponded to the parameters will no longer exist. |
d1b91892 | 1093 | Expect an error message from Perl of the form |
a0d0e21e | 1094 | |
d1b91892 | 1095 | Can't use an undefined value as a subroutine reference at ... |
a0d0e21e | 1096 | |
d1b91892 | 1097 | for each of the C<CallSavedSub1> lines. |
a0d0e21e | 1098 | |
54310121 | 1099 | Similarly, with this code |
a0d0e21e | 1100 | |
4358a253 SS |
1101 | $ref = \&fred; |
1102 | SaveSub1($ref); | |
1103 | $ref = 47; | |
1104 | CallSavedSub1(); | |
a0d0e21e | 1105 | |
54310121 | 1106 | you can expect one of these messages (which you actually get is dependent on |
1107 | the version of Perl you are using) | |
a0d0e21e | 1108 | |
d1b91892 AD |
1109 | Not a CODE reference at ... |
1110 | Undefined subroutine &main::47 called ... | |
a0d0e21e | 1111 | |
19799a22 | 1112 | The variable $ref may have referred to the subroutine C<fred> |
d1b91892 | 1113 | whenever the call to C<SaveSub1> was made but by the time |
5f05dabc | 1114 | C<CallSavedSub1> gets called it now holds the number C<47>. Because we |
d1b91892 | 1115 | saved only a pointer to the original SV in C<SaveSub1>, any changes to |
19799a22 | 1116 | $ref will be tracked by the pointer C<rememberSub>. This means that |
d1b91892 AD |
1117 | whenever C<CallSavedSub1> gets called, it will attempt to execute the |
1118 | code which is referenced by the SV* C<rememberSub>. In this case | |
1119 | though, it now refers to the integer C<47>, so expect Perl to complain | |
1120 | loudly. | |
a0d0e21e | 1121 | |
1d45ec27 | 1122 | A similar but more subtle problem is illustrated with this code: |
a0d0e21e | 1123 | |
4358a253 SS |
1124 | $ref = \&fred; |
1125 | SaveSub1($ref); | |
1126 | $ref = \&joe; | |
1127 | CallSavedSub1(); | |
a0d0e21e | 1128 | |
1d45ec27 | 1129 | This time whenever C<CallSavedSub1> gets called it will execute the Perl |
54310121 | 1130 | subroutine C<joe> (assuming it exists) rather than C<fred> as was |
d1b91892 | 1131 | originally requested in the call to C<SaveSub1>. |
a0d0e21e | 1132 | |
d1b91892 | 1133 | To get around these problems it is necessary to take a full copy of the |
1d45ec27 | 1134 | SV. The code below shows C<SaveSub2> modified to do that. |
a0d0e21e | 1135 | |
4358a253 | 1136 | static SV * keepSub = (SV*)NULL; |
d1b91892 AD |
1137 | |
1138 | void | |
1139 | SaveSub2(name) | |
1140 | SV * name | |
1141 | CODE: | |
1142 | /* Take a copy of the callback */ | |
1143 | if (keepSub == (SV*)NULL) | |
1144 | /* First time, so create a new SV */ | |
4358a253 | 1145 | keepSub = newSVsv(name); |
d1b91892 AD |
1146 | else |
1147 | /* Been here before, so overwrite */ | |
4358a253 | 1148 | SvSetSV(keepSub, name); |
d1b91892 AD |
1149 | |
1150 | void | |
1151 | CallSavedSub2() | |
1152 | CODE: | |
4358a253 SS |
1153 | PUSHMARK(SP); |
1154 | call_sv(keepSub, G_DISCARD|G_NOARGS); | |
d1b91892 | 1155 | |
5f05dabc | 1156 | To avoid creating a new SV every time C<SaveSub2> is called, |
d1b91892 AD |
1157 | the function first checks to see if it has been called before. If not, |
1158 | then space for a new SV is allocated and the reference to the Perl | |
1d45ec27 FC |
1159 | subroutine C<name> is copied to the variable C<keepSub> in one |
1160 | operation using C<newSVsv>. Thereafter, whenever C<SaveSub2> is called, | |
d1b91892 AD |
1161 | the existing SV, C<keepSub>, is overwritten with the new value using |
1162 | C<SvSetSV>. | |
1163 | ||
4929bf7b | 1164 | =head2 Using call_argv |
d1b91892 AD |
1165 | |
1166 | Here is a Perl subroutine which prints whatever parameters are passed | |
1167 | to it. | |
1168 | ||
84f709e7 JH |
1169 | sub PrintList |
1170 | { | |
4358a253 | 1171 | my(@list) = @_; |
d1b91892 | 1172 | |
84f709e7 | 1173 | foreach (@list) { print "$_\n" } |
d1b91892 AD |
1174 | } |
1175 | ||
1d45ec27 | 1176 | And here is an example of I<call_argv> which will call |
d1b91892 AD |
1177 | I<PrintList>. |
1178 | ||
4358a253 | 1179 | static char * words[] = {"alpha", "beta", "gamma", "delta", NULL}; |
d1b91892 AD |
1180 | |
1181 | static void | |
1182 | call_PrintList() | |
1183 | { | |
4358a253 | 1184 | dSP; |
d1b91892 | 1185 | |
4358a253 | 1186 | call_argv("PrintList", G_DISCARD, words); |
d1b91892 AD |
1187 | } |
1188 | ||
1189 | Note that it is not necessary to call C<PUSHMARK> in this instance. | |
4929bf7b | 1190 | This is because I<call_argv> will do it for you. |
d1b91892 | 1191 | |
4929bf7b | 1192 | =head2 Using call_method |
a0d0e21e | 1193 | |
1d45ec27 | 1194 | Consider the following Perl code: |
a0d0e21e | 1195 | |
d1b91892 | 1196 | { |
4358a253 | 1197 | package Mine; |
84f709e7 JH |
1198 | |
1199 | sub new | |
1200 | { | |
4358a253 | 1201 | my($type) = shift; |
84f709e7 JH |
1202 | bless [@_] |
1203 | } | |
1204 | ||
1205 | sub Display | |
1206 | { | |
4358a253 SS |
1207 | my ($self, $index) = @_; |
1208 | print "$index: $$self[$index]\n"; | |
84f709e7 JH |
1209 | } |
1210 | ||
1211 | sub PrintID | |
1212 | { | |
4358a253 SS |
1213 | my($class) = @_; |
1214 | print "This is Class $class version 1.0\n"; | |
84f709e7 | 1215 | } |
d1b91892 AD |
1216 | } |
1217 | ||
5f05dabc | 1218 | It implements just a very simple class to manage an array. Apart from |
d1b91892 | 1219 | the constructor, C<new>, it declares methods, one static and one |
5f05dabc | 1220 | virtual. The static method, C<PrintID>, prints out simply the class |
d1b91892 | 1221 | name and a version number. The virtual method, C<Display>, prints out a |
1d45ec27 | 1222 | single element of the array. Here is an all-Perl example of using it. |
d1b91892 | 1223 | |
797f796a | 1224 | $a = Mine->new('red', 'green', 'blue'); |
4358a253 | 1225 | $a->Display(1); |
797f796a | 1226 | Mine->PrintID; |
a0d0e21e | 1227 | |
d1b91892 | 1228 | will print |
a0d0e21e | 1229 | |
d1b91892 | 1230 | 1: green |
54310121 | 1231 | This is Class Mine version 1.0 |
a0d0e21e | 1232 | |
d1b91892 | 1233 | Calling a Perl method from C is fairly straightforward. The following |
1d45ec27 | 1234 | things are required: |
a0d0e21e | 1235 | |
d1b91892 AD |
1236 | =over 5 |
1237 | ||
1238 | =item * | |
1239 | ||
1d45ec27 FC |
1240 | A reference to the object for a virtual method or the name of the class |
1241 | for a static method | |
d1b91892 AD |
1242 | |
1243 | =item * | |
1244 | ||
1d45ec27 | 1245 | The name of the method |
d1b91892 AD |
1246 | |
1247 | =item * | |
1248 | ||
1d45ec27 | 1249 | Any other parameters specific to the method |
d1b91892 AD |
1250 | |
1251 | =back | |
1252 | ||
1253 | Here is a simple XSUB which illustrates the mechanics of calling both | |
1254 | the C<PrintID> and C<Display> methods from C. | |
1255 | ||
1256 | void | |
1257 | call_Method(ref, method, index) | |
1258 | SV * ref | |
1259 | char * method | |
1260 | int index | |
1261 | CODE: | |
924508f0 | 1262 | PUSHMARK(SP); |
d1b91892 | 1263 | XPUSHs(ref); |
4358a253 | 1264 | XPUSHs(sv_2mortal(newSViv(index))); |
d1b91892 AD |
1265 | PUTBACK; |
1266 | ||
4358a253 | 1267 | call_method(method, G_DISCARD); |
d1b91892 AD |
1268 | |
1269 | void | |
1270 | call_PrintID(class, method) | |
1271 | char * class | |
1272 | char * method | |
1273 | CODE: | |
924508f0 | 1274 | PUSHMARK(SP); |
4358a253 | 1275 | XPUSHs(sv_2mortal(newSVpv(class, 0))); |
d1b91892 AD |
1276 | PUTBACK; |
1277 | ||
4358a253 | 1278 | call_method(method, G_DISCARD); |
d1b91892 AD |
1279 | |
1280 | ||
1d45ec27 | 1281 | So the methods C<PrintID> and C<Display> can be invoked like this: |
d1b91892 | 1282 | |
797f796a | 1283 | $a = Mine->new('red', 'green', 'blue'); |
4358a253 SS |
1284 | call_Method($a, 'Display', 1); |
1285 | call_PrintID('Mine', 'PrintID'); | |
d1b91892 | 1286 | |
1d45ec27 | 1287 | The only thing to note is that, in both the static and virtual methods, |
19799a22 | 1288 | the method name is not passed via the stack--it is used as the first |
4929bf7b | 1289 | parameter to I<call_method>. |
d1b91892 | 1290 | |
54310121 | 1291 | =head2 Using GIMME_V |
d1b91892 | 1292 | |
54310121 | 1293 | Here is a trivial XSUB which prints the context in which it is |
d1b91892 AD |
1294 | currently executing. |
1295 | ||
1296 | void | |
1297 | PrintContext() | |
1298 | CODE: | |
54310121 | 1299 | I32 gimme = GIMME_V; |
1300 | if (gimme == G_VOID) | |
4358a253 | 1301 | printf ("Context is Void\n"); |
54310121 | 1302 | else if (gimme == G_SCALAR) |
4358a253 | 1303 | printf ("Context is Scalar\n"); |
d1b91892 | 1304 | else |
4358a253 | 1305 | printf ("Context is Array\n"); |
d1b91892 | 1306 | |
1d45ec27 | 1307 | And here is some Perl to test it. |
d1b91892 | 1308 | |
4358a253 SS |
1309 | PrintContext; |
1310 | $a = PrintContext; | |
1311 | @a = PrintContext; | |
d1b91892 AD |
1312 | |
1313 | The output from that will be | |
1314 | ||
54310121 | 1315 | Context is Void |
d1b91892 AD |
1316 | Context is Scalar |
1317 | Context is Array | |
1318 | ||
02f6dca1 | 1319 | =head2 Using Perl to Dispose of Temporaries |
d1b91892 AD |
1320 | |
1321 | In the examples given to date, any temporaries created in the callback | |
4929bf7b | 1322 | (i.e., parameters passed on the stack to the I<call_*> function or |
1d45ec27 | 1323 | values returned via the stack) have been freed by one of these methods: |
d1b91892 AD |
1324 | |
1325 | =over 5 | |
1326 | ||
1327 | =item * | |
1328 | ||
1d45ec27 | 1329 | Specifying the G_DISCARD flag with I<call_*> |
d1b91892 AD |
1330 | |
1331 | =item * | |
1332 | ||
1d45ec27 | 1333 | Explicitly using the C<ENTER>/C<SAVETMPS>--C<FREETMPS>/C<LEAVE> pairing |
d1b91892 AD |
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 | ||
4358a253 SS |
1341 | ENTER; |
1342 | SAVETMPS; | |
d1b91892 | 1343 | ... |
4358a253 SS |
1344 | FREETMPS; |
1345 | LEAVE; | |
d1b91892 AD |
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 | ... | |
4929bf7b | 1376 | external library --> call_* --> perl |
d1b91892 | 1377 | | |
4929bf7b | 1378 | perl <-- XSUB <-- external library <-- call_* <----+ |
d1b91892 | 1379 | |
4929bf7b | 1380 | After processing of the error using I<call_*> is completed, |
d1b91892 AD |
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 | ... | |
4929bf7b | 1393 | event handler --> call_* --> perl |
d1b91892 | 1394 | | |
4929bf7b | 1395 | event handler <-- call_* <----+ |
d1b91892 | 1396 | ... |
4929bf7b | 1397 | event handler --> call_* --> perl |
d1b91892 | 1398 | | |
4929bf7b | 1399 | event handler <-- call_* <----+ |
d1b91892 | 1400 | ... |
4929bf7b | 1401 | event handler --> call_* --> perl |
d1b91892 | 1402 | | |
4929bf7b | 1403 | event handler <-- call_* <----+ |
d1b91892 AD |
1404 | |
1405 | In this case the flow of control can consist of only the repeated | |
1406 | sequence | |
1407 | ||
4929bf7b | 1408 | event handler --> call_* --> perl |
d1b91892 | 1409 | |
54310121 | 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. | |
d1b91892 AD |
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 | |
5f05dabc | 1416 | to dispose of your temporaries, control must drop back to the |
d1b91892 | 1417 | enclosing scope at some stage. In the event driven scenario that may |
1d45ec27 | 1418 | never happen. This means that, as time goes on, your program will |
d1b91892 AD |
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 | |
19799a22 | 1421 | eventually consume all the available memory in your system--kapow! |
d1b91892 | 1422 | |
19799a22 | 1423 | So here is the bottom line--if you are sure that control will revert |
d1b91892 | 1424 | back to the enclosing Perl scope fairly quickly after the end of your |
5f05dabc | 1425 | callback, then it isn't absolutely necessary to dispose explicitly of |
d1b91892 AD |
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 | ||
02f6dca1 | 1430 | =head2 Strategies for Storing Callback Context Information |
d1b91892 AD |
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 | ||
4358a253 | 1444 | register_fatal(cb1); |
d1b91892 AD |
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 | { | |
4358a253 SS |
1452 | printf ("Fatal Error\n"); |
1453 | exit(1); | |
d1b91892 AD |
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 | { | |
4358a253 | 1463 | dSP; |
d1b91892 | 1464 | |
4358a253 | 1465 | PUSHMARK(SP); |
d1b91892 AD |
1466 | |
1467 | /* Call the Perl sub to process the callback */ | |
4358a253 | 1468 | call_sv(callback, G_DISCARD); |
d1b91892 AD |
1469 | } |
1470 | ||
1471 | ||
1472 | void | |
1473 | register_fatal(fn) | |
1474 | SV * fn | |
1475 | CODE: | |
1476 | /* Remember the Perl sub */ | |
1477 | if (callback == (SV*)NULL) | |
4358a253 | 1478 | callback = newSVsv(fn); |
d1b91892 | 1479 | else |
4358a253 | 1480 | SvSetSV(callback, fn); |
d1b91892 AD |
1481 | |
1482 | /* register the callback with the external library */ | |
4358a253 | 1483 | register_fatal(cb1); |
d1b91892 AD |
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 | |
4358a253 | 1489 | register_fatal(\&pcb1); |
d1b91892 | 1490 | |
84f709e7 JH |
1491 | sub pcb1 |
1492 | { | |
4358a253 | 1493 | die "I'm dying...\n"; |
d1b91892 AD |
1494 | } |
1495 | ||
1496 | The mapping between the C callback and the Perl equivalent is stored in | |
1497 | the global variable C<callback>. | |
1498 | ||
5f05dabc | 1499 | This will be adequate if you ever need to have only one callback |
d1b91892 AD |
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 | |
19799a22 | 1515 | function C<ProcessRead> with a file handle C<fh>--this assumes that it |
d1b91892 AD |
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) | |
4358a253 SS |
1525 | int fh; |
1526 | char * buffer; | |
d1b91892 | 1527 | { |
54310121 | 1528 | ... |
d1b91892 AD |
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 | ||
4358a253 | 1536 | static HV * Mapping = (HV*)NULL; |
a0d0e21e | 1537 | |
d1b91892 AD |
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) | |
4358a253 | 1545 | Mapping = newHV(); |
d1b91892 AD |
1546 | |
1547 | /* Save the fh -> callback mapping */ | |
4358a253 | 1548 | hv_store(Mapping, (char*)&fh, sizeof(fh), newSVsv(callback), 0); |
d1b91892 AD |
1549 | |
1550 | /* Register with the C Library */ | |
4358a253 | 1551 | asynch_read(fh, asynch_read_if); |
d1b91892 AD |
1552 | |
1553 | and C<asynch_read_if> could look like this | |
1554 | ||
1555 | static void | |
1556 | asynch_read_if(fh, buffer) | |
4358a253 SS |
1557 | int fh; |
1558 | char * buffer; | |
d1b91892 | 1559 | { |
4358a253 SS |
1560 | dSP; |
1561 | SV ** sv; | |
d1b91892 AD |
1562 | |
1563 | /* Get the callback associated with fh */ | |
4358a253 | 1564 | sv = hv_fetch(Mapping, (char*)&fh , sizeof(fh), FALSE); |
d1b91892 | 1565 | if (sv == (SV**)NULL) |
4358a253 | 1566 | croak("Internal error...\n"); |
d1b91892 | 1567 | |
4358a253 SS |
1568 | PUSHMARK(SP); |
1569 | XPUSHs(sv_2mortal(newSViv(fh))); | |
1570 | XPUSHs(sv_2mortal(newSVpv(buffer, 0))); | |
1571 | PUTBACK; | |
d1b91892 AD |
1572 | |
1573 | /* Call the Perl sub */ | |
4358a253 | 1574 | call_sv(*sv, G_DISCARD); |
d1b91892 AD |
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 */ | |
4358a253 | 1585 | (void) hv_delete(Mapping, (char*)&fh, sizeof(fh), G_DISCARD); |
a0d0e21e | 1586 | |
d1b91892 | 1587 | /* Now call the real asynch_close */ |
4358a253 | 1588 | asynch_close(fh); |
a0d0e21e | 1589 | |
d1b91892 AD |
1590 | So the Perl interface would look like this |
1591 | ||
84f709e7 JH |
1592 | sub callback1 |
1593 | { | |
4358a253 | 1594 | my($handle, $buffer) = @_; |
d1b91892 | 1595 | } |
a0d0e21e | 1596 | |
d1b91892 | 1597 | # Register the Perl callback |
4358a253 | 1598 | asynch_read($fh, \&callback1); |
a0d0e21e | 1599 | |
4358a253 | 1600 | asynch_close($fh); |
d1b91892 AD |
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) | |
4358a253 | 1613 | char * buffer; |
d1b91892 AD |
1614 | { |
1615 | ... | |
1616 | } | |
a0d0e21e | 1617 | |
d1b91892 AD |
1618 | Without the file handle there is no straightforward way to map from the |
1619 | C callback to the Perl subroutine. | |
a0d0e21e | 1620 | |
54310121 | 1621 | In this case a possible way around this problem is to predefine a |
d1b91892 AD |
1622 | series of C functions to act as the interface to Perl, thus |
1623 | ||
1624 | #define MAX_CB 3 | |
1625 | #define NULL_HANDLE -1 | |
4358a253 | 1626 | typedef void (*FnMap)(); |
d1b91892 AD |
1627 | |
1628 | struct MapStruct { | |
4358a253 SS |
1629 | FnMap Function; |
1630 | SV * PerlSub; | |
1631 | int Handle; | |
1632 | }; | |
d1b91892 | 1633 | |
4358a253 SS |
1634 | static void fn1(); |
1635 | static void fn2(); | |
1636 | static void fn3(); | |
d1b91892 AD |
1637 | |
1638 | static struct MapStruct Map [MAX_CB] = | |
1639 | { | |
1640 | { fn1, NULL, NULL_HANDLE }, | |
1641 | { fn2, NULL, NULL_HANDLE }, | |
1642 | { fn3, NULL, NULL_HANDLE } | |
4358a253 | 1643 | }; |
d1b91892 AD |
1644 | |
1645 | static void | |
1646 | Pcb(index, buffer) | |
4358a253 SS |
1647 | int index; |
1648 | char * buffer; | |
d1b91892 | 1649 | { |
4358a253 | 1650 | dSP; |
d1b91892 | 1651 | |
4358a253 SS |
1652 | PUSHMARK(SP); |
1653 | XPUSHs(sv_2mortal(newSVpv(buffer, 0))); | |
1654 | PUTBACK; | |
d1b91892 AD |
1655 | |
1656 | /* Call the Perl sub */ | |
4358a253 | 1657 | call_sv(Map[index].PerlSub, G_DISCARD); |
d1b91892 AD |
1658 | } |
1659 | ||
1660 | static void | |
1661 | fn1(buffer) | |
4358a253 | 1662 | char * buffer; |
d1b91892 | 1663 | { |
4358a253 | 1664 | Pcb(0, buffer); |
d1b91892 AD |
1665 | } |
1666 | ||
1667 | static void | |
1668 | fn2(buffer) | |
4358a253 | 1669 | char * buffer; |
d1b91892 | 1670 | { |
4358a253 | 1671 | Pcb(1, buffer); |
d1b91892 AD |
1672 | } |
1673 | ||
1674 | static void | |
1675 | fn3(buffer) | |
4358a253 | 1676 | char * buffer; |
d1b91892 | 1677 | { |
4358a253 | 1678 | Pcb(2, buffer); |
d1b91892 AD |
1679 | } |
1680 | ||
1681 | void | |
1682 | array_asynch_read(fh, callback) | |
1683 | int fh | |
1684 | SV * callback | |
1685 | CODE: | |
4358a253 SS |
1686 | int index; |
1687 | int null_index = MAX_CB; | |
d1b91892 AD |
1688 | |
1689 | /* Find the same handle or an empty entry */ | |
4358a253 | 1690 | for (index = 0; index < MAX_CB; ++index) |
d1b91892 AD |
1691 | { |
1692 | if (Map[index].Handle == fh) | |
4358a253 | 1693 | break; |
d1b91892 AD |
1694 | |
1695 | if (Map[index].Handle == NULL_HANDLE) | |
4358a253 | 1696 | null_index = index; |
d1b91892 AD |
1697 | } |
1698 | ||
1699 | if (index == MAX_CB && null_index == MAX_CB) | |
4358a253 | 1700 | croak ("Too many callback functions registered\n"); |
d1b91892 AD |
1701 | |
1702 | if (index == MAX_CB) | |
4358a253 | 1703 | index = null_index; |
d1b91892 AD |
1704 | |
1705 | /* Save the file handle */ | |
4358a253 | 1706 | Map[index].Handle = fh; |
d1b91892 AD |
1707 | |
1708 | /* Remember the Perl sub */ | |
1709 | if (Map[index].PerlSub == (SV*)NULL) | |
4358a253 | 1710 | Map[index].PerlSub = newSVsv(callback); |
d1b91892 | 1711 | else |
4358a253 | 1712 | SvSetSV(Map[index].PerlSub, callback); |
d1b91892 | 1713 | |
4358a253 | 1714 | asynch_read(fh, Map[index].Function); |
d1b91892 AD |
1715 | |
1716 | void | |
1717 | array_asynch_close(fh) | |
1718 | int fh | |
1719 | CODE: | |
4358a253 | 1720 | int index; |
d1b91892 AD |
1721 | |
1722 | /* Find the file handle */ | |
4358a253 | 1723 | for (index = 0; index < MAX_CB; ++ index) |
d1b91892 | 1724 | if (Map[index].Handle == fh) |
4358a253 | 1725 | break; |
d1b91892 AD |
1726 | |
1727 | if (index == MAX_CB) | |
4358a253 | 1728 | croak ("could not close fh %d\n", fh); |
d1b91892 | 1729 | |
4358a253 SS |
1730 | Map[index].Handle = NULL_HANDLE; |
1731 | SvREFCNT_dec(Map[index].PerlSub); | |
1732 | Map[index].PerlSub = (SV*)NULL; | |
d1b91892 | 1733 | |
4358a253 | 1734 | asynch_close(fh); |
d1b91892 | 1735 | |
5f05dabc | 1736 | In this case the functions C<fn1>, C<fn2>, and C<fn3> are used to |
d1b91892 | 1737 | remember the Perl subroutine to be called. Each of the functions holds |
4a6725af | 1738 | a separate hard-wired index which is used in the function C<Pcb> to |
d1b91892 AD |
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 | ||
4a6725af | 1746 | Secondly, there is a hard-wired limit (in this case 3) to the number of |
d1b91892 AD |
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 | |
54310121 | 1749 | recompiling. None the less, as long as the number of functions is |
d1b91892 AD |
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 | |
a0d0e21e | 1774 | |
a0d0e21e LW |
1775 | |
1776 | =head2 Alternate Stack Manipulation | |
1777 | ||
a0d0e21e | 1778 | |
d1b91892 AD |
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 | |
8e07c86e | 1781 | macros and read the stack using the C<ST> macro (See L<perlxs> for a |
d1b91892 AD |
1782 | full description of the C<ST> macro). |
1783 | ||
1d45ec27 | 1784 | Most of the time the C<POP*> macros should be adequate; the main |
d1b91892 AD |
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 | ||
1d45ec27 FC |
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*>. | |
d1b91892 AD |
1793 | |
1794 | static void | |
1795 | call_AddSubtract2(a, b) | |
4358a253 SS |
1796 | int a; |
1797 | int b; | |
d1b91892 | 1798 | { |
4358a253 SS |
1799 | dSP; |
1800 | I32 ax; | |
1801 | int count; | |
d1b91892 | 1802 | |
4358a253 | 1803 | ENTER; |
d1b91892 AD |
1804 | SAVETMPS; |
1805 | ||
4358a253 | 1806 | PUSHMARK(SP); |
d1b91892 AD |
1807 | XPUSHs(sv_2mortal(newSViv(a))); |
1808 | XPUSHs(sv_2mortal(newSViv(b))); | |
4358a253 | 1809 | PUTBACK; |
d1b91892 | 1810 | |
4929bf7b | 1811 | count = call_pv("AddSubtract", G_ARRAY); |
d1b91892 | 1812 | |
4358a253 SS |
1813 | SPAGAIN; |
1814 | SP -= count; | |
1815 | ax = (SP - PL_stack_base) + 1; | |
d1b91892 AD |
1816 | |
1817 | if (count != 2) | |
4358a253 | 1818 | croak("Big trouble\n"); |
a0d0e21e | 1819 | |
4358a253 SS |
1820 | printf ("%d + %d = %d\n", a, b, SvIV(ST(0))); |
1821 | printf ("%d - %d = %d\n", a, b, SvIV(ST(1))); | |
d1b91892 | 1822 | |
4358a253 SS |
1823 | PUTBACK; |
1824 | FREETMPS; | |
1825 | LEAVE; | |
d1b91892 AD |
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 | |
1d45ec27 | 1837 | us. |
d1b91892 AD |
1838 | |
1839 | =item 2. | |
1840 | ||
1841 | The code | |
1842 | ||
4358a253 SS |
1843 | SPAGAIN; |
1844 | SP -= count; | |
1845 | ax = (SP - PL_stack_base) + 1; | |
d1b91892 AD |
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 | |
54310121 | 1853 | first value returned by the Perl subroutine and C<ST(count-1)> |
d1b91892 AD |
1854 | refers to the last. |
1855 | ||
1856 | =back | |
a0d0e21e | 1857 | |
02f6dca1 | 1858 | =head2 Creating and Calling an Anonymous Subroutine in C |
8f183262 | 1859 | |
4929bf7b | 1860 | As we've already shown, C<call_sv> can be used to invoke an |
c2611fb3 GS |
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 | |
8f183262 DM |
1863 | done inside our C code: |
1864 | ||
8f183262 DM |
1865 | ... |
1866 | ||
4929bf7b | 1867 | SV *cvrv = eval_pv("sub { print 'You will not find me cluttering any namespace!' }", TRUE); |
8f183262 DM |
1868 | |
1869 | ... | |
1870 | ||
4929bf7b | 1871 | call_sv(cvrv, G_VOID|G_NOARGS); |
8f183262 | 1872 | |
4929bf7b GS |
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 | |
4a4eefd0 | 1875 | L<perlapi/eval_pv>). Once this code reference is in hand, it |
8f183262 DM |
1876 | can be mixed in with all the previous examples we've shown. |
1877 | ||
9850bf21 RH |
1878 | =head1 LIGHTWEIGHT CALLBACKS |
1879 | ||
1880 | Sometimes you need to invoke the same subroutine repeatedly. | |
1881 | This usually happens with a function that acts on a list of | |
1882 | values, such as Perl's built-in sort(). You can pass a | |
1883 | comparison function to sort(), which will then be invoked | |
1884 | for every pair of values that needs to be compared. The first() | |
1885 | and reduce() functions from L<List::Util> follow a similar | |
1886 | pattern. | |
1887 | ||
1888 | In this case it is possible to speed up the routine (often | |
1889 | quite substantially) by using the lightweight callback API. | |
1890 | The idea is that the calling context only needs to be | |
1891 | created and destroyed once, and the sub can be called | |
1892 | arbitrarily many times in between. | |
1893 | ||
ac036724 | 1894 | It is usual to pass parameters using global variables (typically |
1895 | $_ for one parameter, or $a and $b for two parameters) rather | |
9850bf21 RH |
1896 | than via @_. (It is possible to use the @_ mechanism if you know |
1897 | what you're doing, though there is as yet no supported API for | |
1898 | it. It's also inherently slower.) | |
1899 | ||
1900 | The pattern of macro calls is like this: | |
1901 | ||
82f35e8b | 1902 | dMULTICALL; /* Declare local variables */ |
9850bf21 RH |
1903 | I32 gimme = G_SCALAR; /* context of the call: G_SCALAR, |
1904 | * G_LIST, or G_VOID */ | |
1905 | ||
82f35e8b RH |
1906 | PUSH_MULTICALL(cv); /* Set up the context for calling cv, |
1907 | and set local vars appropriately */ | |
9850bf21 RH |
1908 | |
1909 | /* loop */ { | |
1910 | /* set the value(s) af your parameter variables */ | |
1911 | MULTICALL; /* Make the actual call */ | |
1912 | } /* end of loop */ | |
1913 | ||
1914 | POP_MULTICALL; /* Tear down the calling context */ | |
1915 | ||
1916 | For some concrete examples, see the implementation of the | |
1917 | first() and reduce() functions of List::Util 1.18. There you | |
1918 | will also find a header file that emulates the multicall API | |
1919 | on older versions of perl. | |
1920 | ||
a0d0e21e LW |
1921 | =head1 SEE ALSO |
1922 | ||
8e07c86e | 1923 | L<perlxs>, L<perlguts>, L<perlembed> |
a0d0e21e LW |
1924 | |
1925 | =head1 AUTHOR | |
1926 | ||
0536e0eb | 1927 | Paul Marquess |
a0d0e21e | 1928 | |
d1b91892 AD |
1929 | Special thanks to the following people who assisted in the creation of |
1930 | the document. | |
a0d0e21e | 1931 | |
c07a80fd | 1932 | Jeff Okamoto, Tim Bunce, Nick Gianniotis, Steve Kelem, Gurusamy Sarathy |
1933 | and Larry Wall. | |
a0d0e21e LW |
1934 | |
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137443ea | 1937 | Version 1.3, 14th Apr 1997 |