3 perlcall - Perl calling conventions from C
7 The purpose of this document is to show you how to call Perl subroutines
8 directly from C, i.e., how to write I<callbacks>.
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.
15 Examples where callbacks are necessary include
19 =item * An Error Handler
21 You have created an XSUB interface to an application's C API.
23 A fairly common feature in applications is to allow you to define a C
24 function that will be called whenever something nasty occurs. What we
25 would like is to be able to specify a Perl subroutine that will be
28 =item * An Event-Driven Program
30 The classic example of where callbacks are used is when writing an
31 event driven program, such as for an X11 application. In this case
32 you register functions to be called whenever specific events occur,
33 e.g., a mouse button is pressed, the cursor moves into a window or a
34 menu item is selected.
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
44 Before you launch yourself head first into the rest of this document,
45 it would be a good idea to have read the following two documents--L<perlxs>
48 =head1 THE CALL_ FUNCTIONS
50 Although this stuff is easier to explain using examples, you first need
51 be aware of a few important definitions.
53 Perl has a number of C functions that allow you to call Perl
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, char **argv);
61 The key function is I<call_sv>. All the other functions are
62 fairly simple wrappers which make it easier to call Perl subroutines in
63 special cases. At the end of the day they will all call I<call_sv>
64 to invoke the Perl subroutine.
66 All the I<call_*> functions have a C<flags> parameter which is
67 used to pass a bit mask of options to Perl. This bit mask operates
68 identically for each of the functions. The settings available in the
69 bit mask are discussed in L</FLAG VALUES>.
71 Each of the functions will now be discussed in turn.
77 I<call_sv> takes two parameters. The first, C<sv>, is an SV*.
78 This allows you to specify the Perl subroutine to be called either as a
79 C string (which has first been converted to an SV) or a reference to a
80 subroutine. The section, L</Using call_sv>, shows how you can make
85 The function, I<call_pv>, is similar to I<call_sv> except it
86 expects its first parameter to be a C char* which identifies the Perl
87 subroutine you want to call, e.g., C<call_pv("fred", 0)>. If the
88 subroutine you want to call is in another package, just include the
89 package name in the string, e.g., C<"pkg::fred">.
93 The function I<call_method> is used to call a method from a Perl
94 class. The parameter C<methname> corresponds to the name of the method
95 to be called. Note that the class that the method belongs to is passed
96 on the Perl stack rather than in the parameter list. This class can be
97 either the name of the class (for a static method) or a reference to an
98 object (for a virtual method). See L<perlobj> for more information on
99 static and virtual methods and L</Using call_method> for an example
100 of using I<call_method>.
104 I<call_argv> calls the Perl subroutine specified by the C string
105 stored in the C<subname> parameter. It also takes the usual C<flags>
106 parameter. The final parameter, C<argv>, consists of a NULL-terminated
107 list of C strings to be passed as parameters to the Perl subroutine.
108 See L</Using call_argv>.
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.
116 As a general rule you should I<always> check the return value from
117 these functions. Even if you are expecting only a particular number of
118 values to be returned from the Perl subroutine, there is nothing to
119 stop someone from doing something unexpected--don't say you haven't
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.
130 Calls the Perl subroutine in a void context.
132 This flag has 2 effects:
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
144 It ensures that nothing is actually returned from the subroutine.
148 The value returned by the I<call_*> function indicates how many
149 items have been returned by the Perl subroutine--in this case it will
155 Calls the Perl subroutine in a scalar context. This is the default
156 context flag setting for all the I<call_*> functions.
158 This flag has 2 effects:
164 It indicates to the subroutine being called that it is executing in a
165 scalar context (if it executes I<wantarray> the result will be false).
169 It ensures that only a scalar is actually returned from the subroutine.
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
176 The value returned by the I<call_*> function indicates how many
177 items have been returned by the Perl subroutine - in this case it will
180 If 0, then you have specified the G_DISCARD flag.
182 If 1, then the item actually returned by the Perl subroutine will be
183 stored on the Perl stack - the section L</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
189 I<call_*> function. The section L</Returning a List in Scalar
190 Context> shows an example of this behavior.
195 Calls the Perl subroutine in a list context.
197 As with G_SCALAR, this flag has 2 effects:
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).
208 It ensures that all items returned from the subroutine will be
209 accessible when control returns from the I<call_*> function.
213 The value returned by the I<call_*> function indicates how many
214 items have been returned by the Perl subroutine.
216 If 0, then you have specified the G_DISCARD flag.
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 L</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
226 By default, the I<call_*> functions place the items returned from
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.
232 If you do not set this flag then it is I<very> important that you make
233 sure that any temporaries (i.e., parameters passed to the Perl
234 subroutine and values returned from the subroutine) are disposed of
235 yourself. The section L</Returning a Scalar> gives details of how to
236 dispose of these temporaries explicitly and the section L</Using Perl to
237 Dispose of Temporaries> discusses the specific circumstances where you
238 can ignore the problem and let Perl deal with it for you.
242 Whenever a Perl subroutine is called using one of the I<call_*>
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.
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
250 so. The reason for being cautious is that, even if you have specified
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.
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
256 occur when the code that is executing the I<call_*> function has
257 itself been called from another Perl subroutine. The code below
272 What has happened is that C<fred> accesses the C<@_> array which
278 It is possible for the Perl subroutine you are calling to terminate
279 abnormally, e.g., by calling I<die> explicitly or by not actually
280 existing. By default, when either of these events occurs, the
281 process will terminate immediately. If you want to trap this
282 type of event, specify the G_EVAL flag. It will put an I<eval { }>
283 around the subroutine call.
285 Whenever control returns from the I<call_*> function you need to
286 check the C<$@> variable as you would in a normal Perl script.
288 The value returned from the I<call_*> function is dependent on
289 what other flags have been specified and whether an error has
290 occurred. Here are all the different cases that can occur:
296 If the I<call_*> function returns normally, then the value
297 returned is as specified in the previous sections.
301 If G_DISCARD is specified, the return value will always be 0.
305 If G_ARRAY is specified I<and> an error has occurred, the return value
310 If G_SCALAR is specified I<and> an error has occurred, the return value
311 will be 1 and the value on the top of the stack will be I<undef>. This
312 means that if you have already detected the error by checking C<$@> and
313 you want the program to continue, you must remember to pop the I<undef>
318 See L</Using G_EVAL> for details on using G_EVAL.
322 Using the G_EVAL flag described above will always set C<$@>: clearing
323 it if there was no error, and setting it to describe the error if there
324 was an error in the called code. This is what you want if your intention
325 is to handle possible errors, but sometimes you just want to trap errors
326 and stop them interfering with the rest of the program.
328 This scenario will mostly be applicable to code that is meant to be called
329 from within destructors, asynchronous callbacks, and signal handlers.
330 In such situations, where the code being called has little relation to the
331 surrounding dynamic context, the main program needs to be insulated from
332 errors in the called code, even if they can't be handled intelligently.
333 It may also be useful to do this with code for C<__DIE__> or C<__WARN__>
334 hooks, and C<tie> functions.
336 The G_KEEPERR flag is meant to be used in conjunction with G_EVAL in
337 I<call_*> functions that are used to implement such code, or with
338 C<eval_sv>. This flag has no effect on the C<call_*> functions when
341 When G_KEEPERR is used, any error in the called code will terminate the
342 call as usual, and the error will not propagate beyond the call (as usual
343 for G_EVAL), but it will not go into C<$@>. 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.
348 Note that the G_KEEPERR flag does not propagate into inner evals; these
351 The G_KEEPERR flag was introduced in Perl version 5.002.
353 See L</Using G_KEEPERR> for an example of a situation that warrants the
356 =head2 Determining the Context
358 As mentioned above, you can determine the context of the currently
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
361 C<G_ARRAY> if you have been called in a list context, C<G_SCALAR> if
362 in a scalar context, or C<G_VOID> if in a void context (i.e., the
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 L</Using GIMME_V>.
370 Enough of the definition talk! Let's have a few examples.
372 Perl provides many macros to assist in accessing the Perl stack.
373 Wherever possible, these macros should always be used when interfacing
374 to Perl internals. We hope this should make the code less vulnerable
375 to any changes made to Perl in the future.
377 Another point worth noting is that in the first series of examples I
378 have made use of only the I<call_pv> function. This has been done
379 to keep the code simpler and ease you into the topic. Wherever
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 L</Using call_sv> for details.
384 =head2 No Parameters, Nothing Returned
386 This first trivial example will call a Perl subroutine, I<PrintUID>, to
387 print out the UID of the process.
394 and here is a C function to call it
402 call_pv("PrintUID", G_DISCARD|G_NOARGS);
407 A few points to note about this example:
413 Ignore C<dSP> and C<PUSHMARK(SP)> for now. They will be discussed in
418 We aren't passing any parameters to I<PrintUID> so G_NOARGS can be
423 We aren't interested in anything returned from I<PrintUID>, so
424 G_DISCARD is specified. Even if I<PrintUID> was changed to
425 return some value(s), having specified G_DISCARD will mean that they
426 will be wiped by the time control returns from I<call_pv>.
430 As I<call_pv> is being used, the Perl subroutine is specified as a
431 C string. In this case the subroutine name has been 'hard-wired' into the
436 Because we specified G_DISCARD, it is not necessary to check the value
437 returned from I<call_pv>. It will always be 0.
441 =head2 Passing Parameters
443 Now let's make a slightly more complex example. This time we want to
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 $n characters of the string.
448 So the Perl subroutine would look like this:
453 print substr($s, 0, $n), "\n";
456 The C function required to call I<LeftString> would look like this:
459 call_LeftString(a, b)
470 PUSHs(sv_2mortal(newSVpv(a, 0)));
471 PUSHs(sv_2mortal(newSViv(b)));
474 call_pv("LeftString", G_DISCARD);
480 Here are a few notes on the C function I<call_LeftString>.
486 Parameters are passed to the Perl subroutine using the Perl stack.
487 This is the purpose of the code beginning with the line C<dSP> and
488 ending with the line C<PUTBACK>. The C<dSP> declares a local copy
489 of the stack pointer. This local copy should B<always> be accessed
494 If you are going to put something onto the Perl stack, you need to know
495 where to put it. This is the purpose of the macro C<dSP>--it declares
496 and initializes a I<local> copy of the Perl stack pointer.
498 All the other macros which will be used in this example require you to
499 have used this macro.
501 The exception to this rule is if you are calling a Perl subroutine
502 directly from an XSUB function. In this case it is not necessary to
503 use the C<dSP> macro explicitly--it will be declared for you
508 Any parameters to be pushed onto the stack should be bracketed by the
509 C<PUSHMARK> and C<PUTBACK> macros. The purpose of these two macros, in
510 this context, is to count the number of parameters you are
511 pushing automatically. Then whenever Perl is creating the C<@_> array for the
512 subroutine, it knows how big to make it.
514 The C<PUSHMARK> macro tells Perl to make a mental note of the current
515 stack pointer. Even if you aren't passing any parameters (like the
516 example shown in the section L</No Parameters, Nothing Returned>) you
517 must still call the C<PUSHMARK> macro before you can call any of the
518 I<call_*> functions--Perl still needs to know that there are no
521 The C<PUTBACK> macro sets the global copy of the stack pointer to be
522 the same as our local copy. If we didn't do this, I<call_pv>
523 wouldn't know where the two parameters we pushed were--remember that
524 up to now all the stack pointer manipulation we have done is with our
525 local copy, I<not> the global copy.
529 Next, we come to EXTEND and PUSHs. This is where the parameters
530 actually get pushed onto the stack. In this case we are pushing a
531 string and an integer.
533 Alternatively you can use the XPUSHs() macro, which combines a
534 C<EXTEND(SP, 1)> and C<PUSHs()>. This is less efficient if you're
535 pushing multiple values.
537 See L<perlguts/"XSUBs and the Argument Stack"> for details
538 on how the PUSH macros work.
542 Because we created temporary values (by means of sv_2mortal() calls)
543 we will have to tidy up the Perl stack and dispose of mortal SVs.
545 This is the purpose of
550 at the start of the function, and
555 at the end. The C<ENTER>/C<SAVETMPS> pair creates a boundary for any
556 temporaries we create. This means that the temporaries we get rid of
557 will be limited to those which were created after these calls.
559 The C<FREETMPS>/C<LEAVE> pair will get rid of any values returned by
560 the Perl subroutine (see next example), plus it will also dump the
561 mortal SVs we have created. Having C<ENTER>/C<SAVETMPS> at the
562 beginning of the code makes sure that no other mortals are destroyed.
564 Think of these macros as working a bit like C<{> and C<}> in Perl
565 to limit the scope of local variables.
567 See the section L</Using Perl to Dispose of Temporaries> for details of
568 an alternative to using these macros.
572 Finally, I<LeftString> can now be called via the I<call_pv> function.
573 The only flag specified this time is G_DISCARD. Because we are passing
574 2 parameters to the Perl subroutine this time, we have not specified
579 =head2 Returning a Scalar
581 Now for an example of dealing with the items returned from a Perl
584 Here is a Perl subroutine, I<Adder>, that takes 2 integer parameters
585 and simply returns their sum.
593 Because we are now concerned with the return value from I<Adder>, the C
594 function required to call it is now a bit more complex.
609 PUSHs(sv_2mortal(newSViv(a)));
610 PUSHs(sv_2mortal(newSViv(b)));
613 count = call_pv("Adder", G_SCALAR);
618 croak("Big trouble\n");
620 printf ("The sum of %d and %d is %d\n", a, b, POPi);
627 Points to note this time are
633 The only flag specified this time was G_SCALAR. That means that the C<@_>
634 array will be created and that the value returned by I<Adder> will
635 still exist after the call to I<call_pv>.
639 The purpose of the macro C<SPAGAIN> is to refresh the local copy of the
640 stack pointer. This is necessary because it is possible that the memory
641 allocated to the Perl stack has been reallocated during the
644 If you are making use of the Perl stack pointer in your code you must
645 always refresh the local copy using SPAGAIN whenever you make use
646 of the I<call_*> functions or any other Perl internal function.
650 Although only a single value was expected to be returned from I<Adder>,
651 it is still good practice to check the return code from I<call_pv>
654 Expecting a single value is not quite the same as knowing that there
655 will be one. If someone modified I<Adder> to return a list and we
656 didn't check for that possibility and take appropriate action the Perl
657 stack would end up in an inconsistent state. That is something you
658 I<really> don't want to happen ever.
662 The C<POPi> macro is used here to pop the return value from the stack.
663 In this case we wanted an integer, so C<POPi> was used.
666 Here is the complete list of POP macros available, along with the types
671 POPpbytex pointer to bytes (PV)
674 POPu unsigned integer (UV)
678 Since these macros have side-effects don't use them as arguments to
679 macros that may evaluate their argument several times, for example:
681 /* Bad idea, don't do this */
683 const char *s = SvPV(POPs, len);
685 Instead, use a temporary:
689 const char *s = SvPV(sv, len);
691 or a macro that guarantees it will evaluate its arguments only once:
694 const char *s = SvPVx(POPs, len);
698 The final C<PUTBACK> is used to leave the Perl stack in a consistent
699 state before exiting the function. This is necessary because when we
700 popped the return value from the stack with C<POPi> it updated only our
701 local copy of the stack pointer. Remember, C<PUTBACK> sets the global
702 stack pointer to be the same as our local copy.
707 =head2 Returning a List of Values
709 Now, let's extend the previous example to return both the sum of the
710 parameters and the difference.
712 Here is the Perl subroutine
720 and this is the C function
723 call_AddSubtract(a, b)
735 PUSHs(sv_2mortal(newSViv(a)));
736 PUSHs(sv_2mortal(newSViv(b)));
739 count = call_pv("AddSubtract", G_ARRAY);
744 croak("Big trouble\n");
746 printf ("%d - %d = %d\n", a, b, POPi);
747 printf ("%d + %d = %d\n", a, b, POPi);
754 If I<call_AddSubtract> is called like this
756 call_AddSubtract(7, 4);
758 then here is the output
769 We wanted list context, so G_ARRAY was used.
773 Not surprisingly C<POPi> is used twice this time because we were
774 retrieving 2 values from the stack. The important thing to note is that
775 when using the C<POP*> macros they come off the stack in I<reverse>
780 =head2 Returning a List in Scalar Context
782 Say the Perl subroutine in the previous section was called in a scalar
786 call_AddSubScalar(a, b)
799 PUSHs(sv_2mortal(newSViv(a)));
800 PUSHs(sv_2mortal(newSViv(b)));
803 count = call_pv("AddSubtract", G_SCALAR);
807 printf ("Items Returned = %d\n", count);
809 for (i = 1; i <= count; ++i)
810 printf ("Value %d = %d\n", i, POPi);
817 The other modification made is that I<call_AddSubScalar> will print the
818 number of items returned from the Perl subroutine and their value (for
819 simplicity it assumes that they are integer). So if
820 I<call_AddSubScalar> is called
822 call_AddSubScalar(7, 4);
824 then the output will be
829 In this case the main point to note is that only the last item in the
830 list is returned from the subroutine. I<AddSubtract> actually made it back to
831 I<call_AddSubScalar>.
834 =head2 Returning Data from Perl via the Parameter List
836 It is also possible to return values directly via the parameter
837 list--whether it is actually desirable to do it is another matter entirely.
839 The Perl subroutine, I<Inc>, below takes 2 parameters and increments
848 and here is a C function to call it.
863 sva = sv_2mortal(newSViv(a));
864 svb = sv_2mortal(newSViv(b));
872 count = call_pv("Inc", G_DISCARD);
875 croak ("call_Inc: expected 0 values from 'Inc', got %d\n",
878 printf ("%d + 1 = %d\n", a, SvIV(sva));
879 printf ("%d + 1 = %d\n", b, SvIV(svb));
885 To be able to access the two parameters that were pushed onto the stack
886 after they return from I<call_pv> it is necessary to make a note
887 of their addresses--thus the two variables C<sva> and C<svb>.
889 The reason this is necessary is that the area of the Perl stack which
890 held them will very likely have been overwritten by something else by
891 the time control returns from I<call_pv>.
898 Now an example using G_EVAL. Below is a Perl subroutine which computes
899 the difference of its 2 parameters. If this would result in a negative
900 result, the subroutine calls I<die>.
906 die "death can be fatal\n" if $a < $b;
911 and some C to call it
927 PUSHs(sv_2mortal(newSViv(a)));
928 PUSHs(sv_2mortal(newSViv(b)));
931 count = call_pv("Subtract", G_EVAL|G_SCALAR);
935 /* Check the eval first */
939 printf ("Uh oh - %s\n", SvPV_nolen(err_tmp));
945 croak("call_Subtract: wanted 1 value from 'Subtract', got %d\n",
948 printf ("%d - %d = %d\n", a, b, POPi);
956 If I<call_Subtract> is called thus
960 the following will be printed
962 Uh oh - death can be fatal
970 We want to be able to catch the I<die> so we have used the G_EVAL
971 flag. Not specifying this flag would mean that the program would
972 terminate immediately at the I<die> statement in the subroutine
982 printf ("Uh oh - %s\n", SvPV_nolen(err_tmp));
986 is the direct equivalent of this bit of Perl
988 print "Uh oh - $@\n" if $@;
990 C<PL_errgv> is a perl global of type C<GV *> that points to the symbol
991 table entry containing the error. C<ERRSV> therefore refers to the C
992 equivalent of C<$@>. We use a local temporary, C<err_tmp>, since
993 C<ERRSV> is a macro that calls a function, and C<SvTRUE(ERRSV)> would
994 end up calling that function multiple times.
998 Note that the stack is popped using C<POPs> in the block where
999 C<SvTRUE(err_tmp)> is true. This is necessary because whenever a
1000 I<call_*> function invoked with G_EVAL|G_SCALAR returns an error,
1001 the top of the stack holds the value I<undef>. Because we want the
1002 program to continue after detecting this error, it is essential that
1003 the stack be tidied up by removing the I<undef>.
1008 =head2 Using G_KEEPERR
1010 Consider this rather facetious example, where we have used an XS
1011 version of the call_Subtract example above inside a destructor:
1014 sub new { bless {}, $_[0] }
1017 die "death can be fatal" if $a < $b;
1020 sub DESTROY { call_Subtract(5, 4); }
1021 sub foo { die "foo dies"; }
1028 print "Saw: $@" if $@; # should be, but isn't
1030 This example will fail to recognize that an error occurred inside the
1031 C<eval {}>. Here's why: the call_Subtract code got executed while perl
1032 was cleaning up temporaries when exiting the outer braced block, and because
1033 call_Subtract is implemented with I<call_pv> using the G_EVAL
1034 flag, it promptly reset C<$@>. This results in the failure of the
1035 outermost test for C<$@>, and thereby the failure of the error trap.
1037 Appending the G_KEEPERR flag, so that the I<call_pv> call in
1038 call_Subtract reads:
1040 count = call_pv("Subtract", G_EVAL|G_SCALAR|G_KEEPERR);
1042 will preserve the error and restore reliable error handling.
1044 =head2 Using call_sv
1046 In all the previous examples I have 'hard-wired' the name of the Perl
1047 subroutine to be called from C. Most of the time though, it is more
1048 convenient to be able to specify the name of the Perl subroutine from
1049 within the Perl script, and you'll want to use
1050 L<call_sv|perlapi/call_sv>.
1052 Consider the Perl code below
1056 print "Hello there\n";
1061 Here is a snippet of XSUB which defines I<CallSubPV>.
1068 call_pv(name, G_DISCARD|G_NOARGS);
1070 That is fine as far as it goes. The thing is, the Perl subroutine
1071 can be specified as only a string, however, Perl allows references
1072 to subroutines and anonymous subroutines.
1073 This is where I<call_sv> is useful.
1075 The code below for I<CallSubSV> is identical to I<CallSubPV> except
1076 that the C<name> parameter is now defined as an SV* and we use
1077 I<call_sv> instead of I<call_pv>.
1084 call_sv(name, G_DISCARD|G_NOARGS);
1086 Because we are using an SV to call I<fred> the following can all be used:
1092 CallSubSV( sub { print "Hello there\n" } );
1094 As you can see, I<call_sv> gives you much greater flexibility in
1095 how you can specify the Perl subroutine.
1097 You should note that, if it is necessary to store the SV (C<name> in the
1098 example above) which corresponds to the Perl subroutine so that it can
1099 be used later in the program, it not enough just to store a copy of the
1100 pointer to the SV. Say the code above had been like this:
1102 static SV * rememberSub;
1114 call_sv(rememberSub, G_DISCARD|G_NOARGS);
1116 The reason this is wrong is that, by the time you come to use the
1117 pointer C<rememberSub> in C<CallSavedSub1>, it may or may not still refer
1118 to the Perl subroutine that was recorded in C<SaveSub1>. This is
1119 particularly true for these cases:
1124 SaveSub1( sub { print "Hello there\n" } );
1127 By the time each of the C<SaveSub1> statements above has been executed,
1128 the SV*s which corresponded to the parameters will no longer exist.
1129 Expect an error message from Perl of the form
1131 Can't use an undefined value as a subroutine reference at ...
1133 for each of the C<CallSavedSub1> lines.
1135 Similarly, with this code
1142 you can expect one of these messages (which you actually get is dependent on
1143 the version of Perl you are using)
1145 Not a CODE reference at ...
1146 Undefined subroutine &main::47 called ...
1148 The variable $ref may have referred to the subroutine C<fred>
1149 whenever the call to C<SaveSub1> was made but by the time
1150 C<CallSavedSub1> gets called it now holds the number C<47>. Because we
1151 saved only a pointer to the original SV in C<SaveSub1>, any changes to
1152 $ref will be tracked by the pointer C<rememberSub>. This means that
1153 whenever C<CallSavedSub1> gets called, it will attempt to execute the
1154 code which is referenced by the SV* C<rememberSub>. In this case
1155 though, it now refers to the integer C<47>, so expect Perl to complain
1158 A similar but more subtle problem is illustrated with this code:
1165 This time whenever C<CallSavedSub1> gets called it will execute the Perl
1166 subroutine C<joe> (assuming it exists) rather than C<fred> as was
1167 originally requested in the call to C<SaveSub1>.
1169 To get around these problems it is necessary to take a full copy of the
1170 SV. The code below shows C<SaveSub2> modified to do that.
1172 /* this isn't thread-safe */
1173 static SV * keepSub = (SV*)NULL;
1179 /* Take a copy of the callback */
1180 if (keepSub == (SV*)NULL)
1181 /* First time, so create a new SV */
1182 keepSub = newSVsv(name);
1184 /* Been here before, so overwrite */
1185 SvSetSV(keepSub, name);
1191 call_sv(keepSub, G_DISCARD|G_NOARGS);
1193 To avoid creating a new SV every time C<SaveSub2> is called,
1194 the function first checks to see if it has been called before. If not,
1195 then space for a new SV is allocated and the reference to the Perl
1196 subroutine C<name> is copied to the variable C<keepSub> in one
1197 operation using C<newSVsv>. Thereafter, whenever C<SaveSub2> is called,
1198 the existing SV, C<keepSub>, is overwritten with the new value using
1201 Note: using a static or global variable to store the SV isn't
1202 thread-safe. You can either use the C<MY_CXT> mechanism documented in
1203 L<perlxs/Safely Storing Static Data in XS> which is fast, or store the
1204 values in perl global variables, using get_sv(), which is much slower.
1206 =head2 Using call_argv
1208 Here is a Perl subroutine which prints whatever parameters are passed
1215 foreach (@list) { print "$_\n" }
1218 And here is an example of I<call_argv> which will call
1221 static char * words[] = {"alpha", "beta", "gamma", "delta", NULL};
1226 call_argv("PrintList", G_DISCARD, words);
1229 Note that it is not necessary to call C<PUSHMARK> in this instance.
1230 This is because I<call_argv> will do it for you.
1232 =head2 Using call_method
1234 Consider the following Perl code:
1247 my ($self, $index) = @_;
1248 print "$index: $$self[$index]\n";
1254 print "This is Class $class version 1.0\n";
1258 It implements just a very simple class to manage an array. Apart from
1259 the constructor, C<new>, it declares methods, one static and one
1260 virtual. The static method, C<PrintID>, prints out simply the class
1261 name and a version number. The virtual method, C<Display>, prints out a
1262 single element of the array. Here is an all-Perl example of using it.
1264 $a = Mine->new('red', 'green', 'blue');
1271 This is Class Mine version 1.0
1273 Calling a Perl method from C is fairly straightforward. The following
1274 things are required:
1280 A reference to the object for a virtual method or the name of the class
1285 The name of the method
1289 Any other parameters specific to the method
1293 Here is a simple XSUB which illustrates the mechanics of calling both
1294 the C<PrintID> and C<Display> methods from C.
1297 call_Method(ref, method, index)
1305 PUSHs(sv_2mortal(newSViv(index)));
1308 call_method(method, G_DISCARD);
1311 call_PrintID(class, method)
1316 XPUSHs(sv_2mortal(newSVpv(class, 0)));
1319 call_method(method, G_DISCARD);
1322 So the methods C<PrintID> and C<Display> can be invoked like this:
1324 $a = Mine->new('red', 'green', 'blue');
1325 call_Method($a, 'Display', 1);
1326 call_PrintID('Mine', 'PrintID');
1328 The only thing to note is that, in both the static and virtual methods,
1329 the method name is not passed via the stack--it is used as the first
1330 parameter to I<call_method>.
1332 =head2 Using GIMME_V
1334 Here is a trivial XSUB which prints the context in which it is
1335 currently executing.
1341 if (gimme == G_VOID)
1342 printf ("Context is Void\n");
1343 else if (gimme == G_SCALAR)
1344 printf ("Context is Scalar\n");
1346 printf ("Context is Array\n");
1348 And here is some Perl to test it.
1354 The output from that will be
1360 =head2 Using Perl to Dispose of Temporaries
1362 In the examples given to date, any temporaries created in the callback
1363 (i.e., parameters passed on the stack to the I<call_*> function or
1364 values returned via the stack) have been freed by one of these methods:
1370 Specifying the G_DISCARD flag with I<call_*>
1374 Explicitly using the C<ENTER>/C<SAVETMPS>--C<FREETMPS>/C<LEAVE> pairing
1378 There is another method which can be used, namely letting Perl do it
1379 for you automatically whenever it regains control after the callback
1380 has terminated. This is done by simply not using the
1388 sequence in the callback (and not, of course, specifying the G_DISCARD
1391 If you are going to use this method you have to be aware of a possible
1392 memory leak which can arise under very specific circumstances. To
1393 explain these circumstances you need to know a bit about the flow of
1394 control between Perl and the callback routine.
1396 The examples given at the start of the document (an error handler and
1397 an event driven program) are typical of the two main sorts of flow
1398 control that you are likely to encounter with callbacks. There is a
1399 very important distinction between them, so pay attention.
1401 In the first example, an error handler, the flow of control could be as
1402 follows. You have created an interface to an external library.
1403 Control can reach the external library like this
1405 perl --> XSUB --> external library
1407 Whilst control is in the library, an error condition occurs. You have
1408 previously set up a Perl callback to handle this situation, so it will
1409 get executed. Once the callback has finished, control will drop back to
1410 Perl again. Here is what the flow of control will be like in that
1413 perl --> XSUB --> external library
1417 external library --> call_* --> perl
1419 perl <-- XSUB <-- external library <-- call_* <----+
1421 After processing of the error using I<call_*> is completed,
1422 control reverts back to Perl more or less immediately.
1424 In the diagram, the further right you go the more deeply nested the
1425 scope is. It is only when control is back with perl on the extreme
1426 left of the diagram that you will have dropped back to the enclosing
1427 scope and any temporaries you have left hanging around will be freed.
1429 In the second example, an event driven program, the flow of control
1430 will be more like this
1432 perl --> XSUB --> event handler
1434 event handler --> call_* --> perl
1436 event handler <-- call_* <----+
1438 event handler --> call_* --> perl
1440 event handler <-- call_* <----+
1442 event handler --> call_* --> perl
1444 event handler <-- call_* <----+
1446 In this case the flow of control can consist of only the repeated
1449 event handler --> call_* --> perl
1451 for practically the complete duration of the program. This means that
1452 control may I<never> drop back to the surrounding scope in Perl at the
1455 So what is the big problem? Well, if you are expecting Perl to tidy up
1456 those temporaries for you, you might be in for a long wait. For Perl
1457 to dispose of your temporaries, control must drop back to the
1458 enclosing scope at some stage. In the event driven scenario that may
1459 never happen. This means that, as time goes on, your program will
1460 create more and more temporaries, none of which will ever be freed. As
1461 each of these temporaries consumes some memory your program will
1462 eventually consume all the available memory in your system--kapow!
1464 So here is the bottom line--if you are sure that control will revert
1465 back to the enclosing Perl scope fairly quickly after the end of your
1466 callback, then it isn't absolutely necessary to dispose explicitly of
1467 any temporaries you may have created. Mind you, if you are at all
1468 uncertain about what to do, it doesn't do any harm to tidy up anyway.
1471 =head2 Strategies for Storing Callback Context Information
1474 Potentially one of the trickiest problems to overcome when designing a
1475 callback interface can be figuring out how to store the mapping between
1476 the C callback function and the Perl equivalent.
1478 To help understand why this can be a real problem first consider how a
1479 callback is set up in an all C environment. Typically a C API will
1480 provide a function to register a callback. This will expect a pointer
1481 to a function as one of its parameters. Below is a call to a
1482 hypothetical function C<register_fatal> which registers the C function
1483 to get called when a fatal error occurs.
1485 register_fatal(cb1);
1487 The single parameter C<cb1> is a pointer to a function, so you must
1488 have defined C<cb1> in your code, say something like this
1493 printf ("Fatal Error\n");
1497 Now change that to call a Perl subroutine instead
1499 static SV * callback = (SV*)NULL;
1508 /* Call the Perl sub to process the callback */
1509 call_sv(callback, G_DISCARD);
1517 /* Remember the Perl sub */
1518 if (callback == (SV*)NULL)
1519 callback = newSVsv(fn);
1521 SvSetSV(callback, fn);
1523 /* register the callback with the external library */
1524 register_fatal(cb1);
1526 where the Perl equivalent of C<register_fatal> and the callback it
1527 registers, C<pcb1>, might look like this
1529 # Register the sub pcb1
1530 register_fatal(\&pcb1);
1534 die "I'm dying...\n";
1537 The mapping between the C callback and the Perl equivalent is stored in
1538 the global variable C<callback>.
1540 This will be adequate if you ever need to have only one callback
1541 registered at any time. An example could be an error handler like the
1542 code sketched out above. Remember though, repeated calls to
1543 C<register_fatal> will replace the previously registered callback
1544 function with the new one.
1546 Say for example you want to interface to a library which allows asynchronous
1547 file i/o. In this case you may be able to register a callback whenever
1548 a read operation has completed. To be of any use we want to be able to
1549 call separate Perl subroutines for each file that is opened. As it
1550 stands, the error handler example above would not be adequate as it
1551 allows only a single callback to be defined at any time. What we
1552 require is a means of storing the mapping between the opened file and
1553 the Perl subroutine we want to be called for that file.
1555 Say the i/o library has a function C<asynch_read> which associates a C
1556 function C<ProcessRead> with a file handle C<fh>--this assumes that it
1557 has also provided some routine to open the file and so obtain the file
1560 asynch_read(fh, ProcessRead)
1562 This may expect the C I<ProcessRead> function of this form
1565 ProcessRead(fh, buffer)
1572 To provide a Perl interface to this library we need to be able to map
1573 between the C<fh> parameter and the Perl subroutine we want called. A
1574 hash is a convenient mechanism for storing this mapping. The code
1575 below shows a possible implementation
1577 static HV * Mapping = (HV*)NULL;
1580 asynch_read(fh, callback)
1584 /* If the hash doesn't already exist, create it */
1585 if (Mapping == (HV*)NULL)
1588 /* Save the fh -> callback mapping */
1589 hv_store(Mapping, (char*)&fh, sizeof(fh), newSVsv(callback), 0);
1591 /* Register with the C Library */
1592 asynch_read(fh, asynch_read_if);
1594 and C<asynch_read_if> could look like this
1597 asynch_read_if(fh, buffer)
1604 /* Get the callback associated with fh */
1605 sv = hv_fetch(Mapping, (char*)&fh , sizeof(fh), FALSE);
1606 if (sv == (SV**)NULL)
1607 croak("Internal error...\n");
1611 PUSHs(sv_2mortal(newSViv(fh)));
1612 PUSHs(sv_2mortal(newSVpv(buffer, 0)));
1615 /* Call the Perl sub */
1616 call_sv(*sv, G_DISCARD);
1619 For completeness, here is C<asynch_close>. This shows how to remove
1620 the entry from the hash C<Mapping>.
1626 /* Remove the entry from the hash */
1627 (void) hv_delete(Mapping, (char*)&fh, sizeof(fh), G_DISCARD);
1629 /* Now call the real asynch_close */
1632 So the Perl interface would look like this
1636 my($handle, $buffer) = @_;
1639 # Register the Perl callback
1640 asynch_read($fh, \&callback1);
1644 The mapping between the C callback and Perl is stored in the global
1645 hash C<Mapping> this time. Using a hash has the distinct advantage that
1646 it allows an unlimited number of callbacks to be registered.
1648 What if the interface provided by the C callback doesn't contain a
1649 parameter which allows the file handle to Perl subroutine mapping? Say
1650 in the asynchronous i/o package, the callback function gets passed only
1651 the C<buffer> parameter like this
1660 Without the file handle there is no straightforward way to map from the
1661 C callback to the Perl subroutine.
1663 In this case a possible way around this problem is to predefine a
1664 series of C functions to act as the interface to Perl, thus
1667 #define NULL_HANDLE -1
1668 typedef void (*FnMap)();
1680 static struct MapStruct Map [MAX_CB] =
1682 { fn1, NULL, NULL_HANDLE },
1683 { fn2, NULL, NULL_HANDLE },
1684 { fn3, NULL, NULL_HANDLE }
1695 XPUSHs(sv_2mortal(newSVpv(buffer, 0)));
1698 /* Call the Perl sub */
1699 call_sv(Map[index].PerlSub, G_DISCARD);
1724 array_asynch_read(fh, callback)
1729 int null_index = MAX_CB;
1731 /* Find the same handle or an empty entry */
1732 for (index = 0; index < MAX_CB; ++index)
1734 if (Map[index].Handle == fh)
1737 if (Map[index].Handle == NULL_HANDLE)
1741 if (index == MAX_CB && null_index == MAX_CB)
1742 croak ("Too many callback functions registered\n");
1744 if (index == MAX_CB)
1747 /* Save the file handle */
1748 Map[index].Handle = fh;
1750 /* Remember the Perl sub */
1751 if (Map[index].PerlSub == (SV*)NULL)
1752 Map[index].PerlSub = newSVsv(callback);
1754 SvSetSV(Map[index].PerlSub, callback);
1756 asynch_read(fh, Map[index].Function);
1759 array_asynch_close(fh)
1764 /* Find the file handle */
1765 for (index = 0; index < MAX_CB; ++ index)
1766 if (Map[index].Handle == fh)
1769 if (index == MAX_CB)
1770 croak ("could not close fh %d\n", fh);
1772 Map[index].Handle = NULL_HANDLE;
1773 SvREFCNT_dec(Map[index].PerlSub);
1774 Map[index].PerlSub = (SV*)NULL;
1778 In this case the functions C<fn1>, C<fn2>, and C<fn3> are used to
1779 remember the Perl subroutine to be called. Each of the functions holds
1780 a separate hard-wired index which is used in the function C<Pcb> to
1781 access the C<Map> array and actually call the Perl subroutine.
1783 There are some obvious disadvantages with this technique.
1785 Firstly, the code is considerably more complex than with the previous
1788 Secondly, there is a hard-wired limit (in this case 3) to the number of
1789 callbacks that can exist simultaneously. The only way to increase the
1790 limit is by modifying the code to add more functions and then
1791 recompiling. None the less, as long as the number of functions is
1792 chosen with some care, it is still a workable solution and in some
1793 cases is the only one available.
1795 To summarize, here are a number of possible methods for you to consider
1796 for storing the mapping between C and the Perl callback
1800 =item 1. Ignore the problem - Allow only 1 callback
1802 For a lot of situations, like interfacing to an error handler, this may
1803 be a perfectly adequate solution.
1805 =item 2. Create a sequence of callbacks - hard wired limit
1807 If it is impossible to tell from the parameters passed back from the C
1808 callback what the context is, then you may need to create a sequence of C
1809 callback interface functions, and store pointers to each in an array.
1811 =item 3. Use a parameter to map to the Perl callback
1813 A hash is an ideal mechanism to store the mapping between C and Perl.
1818 =head2 Alternate Stack Manipulation
1821 Although I have made use of only the C<POP*> macros to access values
1822 returned from Perl subroutines, it is also possible to bypass these
1823 macros and read the stack using the C<ST> macro (See L<perlxs> for a
1824 full description of the C<ST> macro).
1826 Most of the time the C<POP*> macros should be adequate; the main
1827 problem with them is that they force you to process the returned values
1828 in sequence. This may not be the most suitable way to process the
1829 values in some cases. What we want is to be able to access the stack in
1830 a random order. The C<ST> macro as used when coding an XSUB is ideal
1833 The code below is the example given in the section L</Returning a List
1834 of Values> recoded to use C<ST> instead of C<POP*>.
1837 call_AddSubtract2(a, b)
1850 PUSHs(sv_2mortal(newSViv(a)));
1851 PUSHs(sv_2mortal(newSViv(b)));
1854 count = call_pv("AddSubtract", G_ARRAY);
1858 ax = (SP - PL_stack_base) + 1;
1861 croak("Big trouble\n");
1863 printf ("%d + %d = %d\n", a, b, SvIV(ST(0)));
1864 printf ("%d - %d = %d\n", a, b, SvIV(ST(1)));
1877 Notice that it was necessary to define the variable C<ax>. This is
1878 because the C<ST> macro expects it to exist. If we were in an XSUB it
1879 would not be necessary to define C<ax> as it is already defined for
1888 ax = (SP - PL_stack_base) + 1;
1890 sets the stack up so that we can use the C<ST> macro.
1894 Unlike the original coding of this example, the returned
1895 values are not accessed in reverse order. So C<ST(0)> refers to the
1896 first value returned by the Perl subroutine and C<ST(count-1)>
1901 =head2 Creating and Calling an Anonymous Subroutine in C
1903 As we've already shown, C<call_sv> can be used to invoke an
1904 anonymous subroutine. However, our example showed a Perl script
1905 invoking an XSUB to perform this operation. Let's see how it can be
1906 done inside our C code:
1912 print 'You will not find me cluttering any namespace!'
1917 call_sv(cvrv, G_VOID|G_NOARGS);
1919 C<eval_pv> is used to compile the anonymous subroutine, which
1920 will be the return value as well (read more about C<eval_pv> in
1921 L<perlapi/eval_pv>). Once this code reference is in hand, it
1922 can be mixed in with all the previous examples we've shown.
1924 =head1 LIGHTWEIGHT CALLBACKS
1926 Sometimes you need to invoke the same subroutine repeatedly.
1927 This usually happens with a function that acts on a list of
1928 values, such as Perl's built-in sort(). You can pass a
1929 comparison function to sort(), which will then be invoked
1930 for every pair of values that needs to be compared. The first()
1931 and reduce() functions from L<List::Util> follow a similar
1934 In this case it is possible to speed up the routine (often
1935 quite substantially) by using the lightweight callback API.
1936 The idea is that the calling context only needs to be
1937 created and destroyed once, and the sub can be called
1938 arbitrarily many times in between.
1940 It is usual to pass parameters using global variables (typically
1941 $_ for one parameter, or $a and $b for two parameters) rather
1942 than via @_. (It is possible to use the @_ mechanism if you know
1943 what you're doing, though there is as yet no supported API for
1944 it. It's also inherently slower.)
1946 The pattern of macro calls is like this:
1948 dMULTICALL; /* Declare local variables */
1949 U8 gimme = G_SCALAR; /* context of the call: G_SCALAR,
1950 * G_ARRAY, or G_VOID */
1952 PUSH_MULTICALL(cv); /* Set up the context for calling cv,
1953 and set local vars appropriately */
1956 /* set the value(s) af your parameter variables */
1957 MULTICALL; /* Make the actual call */
1960 POP_MULTICALL; /* Tear down the calling context */
1962 For some concrete examples, see the implementation of the
1963 first() and reduce() functions of List::Util 1.18. There you
1964 will also find a header file that emulates the multicall API
1965 on older versions of perl.
1969 L<perlxs>, L<perlguts>, L<perlembed>
1975 Special thanks to the following people who assisted in the creation of
1978 Jeff Okamoto, Tim Bunce, Nick Gianniotis, Steve Kelem, Gurusamy Sarathy
1983 Last updated for perl 5.23.1.