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 like for an X windows application. In this case
32 you register functions to be called whenever specific events occur,
33 e.g., a mouse button is pressed, the cursor moves into a window or a
34 menu item is selected.
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 -
46 L<perlxs> and L<perlguts>.
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, register 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, I<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 I<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 a bit mask
125 which can consist of any combination of the symbols defined below,
131 Calls the Perl subroutine in a void context.
133 This flag has 2 effects:
139 It indicates to the subroutine being called that it is executing in
140 a void context (if it executes I<wantarray> the result will be the
145 It ensures that nothing is actually returned from the subroutine.
149 The value returned by the I<call_*> function indicates how many
150 items have been returned by the Perl subroutine - in this case it will
156 Calls the Perl subroutine in a scalar context. This is the default
157 context flag setting for all the I<call_*> functions.
159 This flag has 2 effects:
165 It indicates to the subroutine being called that it is executing in a
166 scalar context (if it executes I<wantarray> the result will be false).
170 It ensures that only a scalar is actually returned from the subroutine.
171 The subroutine can, of course, ignore the I<wantarray> and return a
172 list anyway. If so, then only the last element of the list will be
177 The value returned by the I<call_*> function indicates how many
178 items have been returned by the Perl subroutine - in this case it will
181 If 0, then you have specified the G_DISCARD flag.
183 If 1, then the item actually returned by the Perl subroutine will be
184 stored on the Perl stack - the section I<Returning a Scalar> shows how
185 to access this value on the stack. Remember that regardless of how
186 many items the Perl subroutine returns, only the last one will be
187 accessible from the stack - think of the case where only one value is
188 returned as being a list with only one element. Any other items that
189 were returned will not exist by the time control returns from the
190 I<call_*> function. The section I<Returning a list in a scalar
191 context> shows an example of this behavior.
196 Calls the Perl subroutine in a list context.
198 As with G_SCALAR, this flag has 2 effects:
204 It indicates to the subroutine being called that it is executing in a
205 list context (if it executes I<wantarray> the result will be true).
210 It ensures that all items returned from the subroutine will be
211 accessible when control returns from the I<call_*> function.
215 The value returned by the I<call_*> function indicates how many
216 items have been returned by the Perl subroutine.
218 If 0, then you have specified the G_DISCARD flag.
220 If not 0, then it will be a count of the number of items returned by
221 the subroutine. These items will be stored on the Perl stack. The
222 section I<Returning a list of values> gives an example of using the
223 G_ARRAY flag and the mechanics of accessing the returned items from the
228 By default, the I<call_*> functions place the items returned from
229 by the Perl subroutine on the stack. If you are not interested in
230 these items, then setting this flag will make Perl get rid of them
231 automatically for you. Note that it is still possible to indicate a
232 context to the Perl subroutine by using either G_SCALAR or G_ARRAY.
234 If you do not set this flag then it is I<very> important that you make
235 sure that any temporaries (i.e., parameters passed to the Perl
236 subroutine and values returned from the subroutine) are disposed of
237 yourself. The section I<Returning a Scalar> gives details of how to
238 dispose of these temporaries explicitly and the section I<Using Perl to
239 dispose of temporaries> discusses the specific circumstances where you
240 can ignore the problem and let Perl deal with it for you.
244 Whenever a Perl subroutine is called using one of the I<call_*>
245 functions, it is assumed by default that parameters are to be passed to
246 the subroutine. If you are not passing any parameters to the Perl
247 subroutine, you can save a bit of time by setting this flag. It has
248 the effect of not creating the C<@_> array for the Perl subroutine.
250 Although the functionality provided by this flag may seem
251 straightforward, it should be used only if there is a good reason to do
252 so. The reason for being cautious is that even if you have specified
253 the G_NOARGS flag, it is still possible for the Perl subroutine that
254 has been called to think that you have passed it parameters.
256 In fact, what can happen is that the Perl subroutine you have called
257 can access the C<@_> array from a previous Perl subroutine. This will
258 occur when the code that is executing the I<call_*> function has
259 itself been called from another Perl subroutine. The code below
274 What has happened is that C<fred> accesses the C<@_> array which
280 It is possible for the Perl subroutine you are calling to terminate
281 abnormally, e.g., by calling I<die> explicitly or by not actually
282 existing. By default, when either of these events occurs, the
283 process will terminate immediately. If you want to trap this
284 type of event, specify the G_EVAL flag. It will put an I<eval { }>
285 around the subroutine call.
287 Whenever control returns from the I<call_*> function you need to
288 check the C<$@> variable as you would in a normal Perl script.
290 The value returned from the I<call_*> function is dependent on
291 what other flags have been specified and whether an error has
292 occurred. Here are all the different cases that can occur:
298 If the I<call_*> function returns normally, then the value
299 returned is as specified in the previous sections.
303 If G_DISCARD is specified, the return value will always be 0.
307 If G_ARRAY is specified I<and> an error has occurred, the return value
312 If G_SCALAR is specified I<and> an error has occurred, the return value
313 will be 1 and the value on the top of the stack will be I<undef>. This
314 means that if you have already detected the error by checking C<$@> and
315 you want the program to continue, you must remember to pop the I<undef>
320 See I<Using G_EVAL> for details on using G_EVAL.
324 Using the G_EVAL flag described above will always set C<$@>: clearing
325 it if there was no error, and setting it to describe the error if there
326 was an error in the called code. This is what you want if your intention
327 is to handle possible errors, but sometimes you just want to trap errors
328 and stop them interfering with the rest of the program.
330 This scenario will mostly be applicable to code that is meant to be called
331 from within destructors, asynchronous callbacks, and signal handlers.
332 In such situations, where the code being called has little relation to the
333 surrounding dynamic context, the main program needs to be insulated from
334 errors in the called code, even if they can't be handled intelligently.
335 It may also be useful to do this with code for C<__DIE__> or C<__WARN__>
336 hooks, and C<tie> functions.
338 The G_KEEPERR flag is meant to be used in conjunction with G_EVAL in
339 I<call_*> functions that are used to implement such code, or with
340 C<eval_sv>. This flag has no effect on the C<call_*> functions when
343 When G_KEEPERR is used, any error in the called code will terminate the
344 call as usual, and the error will not propagate beyond the call (as usual
345 for G_EVAL), but it will not go into C<$@>. Instead the error will be
346 converted into a warning, prefixed with the string "\t(in cleanup)".
347 This can be disabled using C<no warnings 'misc'>. If there is no error,
348 C<$@> will not be cleared.
350 Note that the G_KEEPERR flag does not propagate into inner evals; these
353 The G_KEEPERR flag was introduced in Perl version 5.002.
355 See I<Using G_KEEPERR> for an example of a situation that warrants the
358 =head2 Determining the Context
360 As mentioned above, you can determine the context of the currently
361 executing subroutine in Perl with I<wantarray>. The equivalent test
362 can be made in C by using the C<GIMME_V> macro, which returns
363 C<G_ARRAY> if you have been called in a list context, C<G_SCALAR> if
364 in a scalar context, or C<G_VOID> if in a void context (i.e. the
365 return value will not be used). An older version of this macro is
366 called C<GIMME>; in a void context it returns C<G_SCALAR> instead of
367 C<G_VOID>. An example of using the C<GIMME_V> macro is shown in
368 section I<Using GIMME_V>.
372 Enough of the definition talk, let's have a few examples.
374 Perl provides many macros to assist in accessing the Perl stack.
375 Wherever possible, these macros should always be used when interfacing
376 to Perl internals. We hope this should make the code less vulnerable
377 to any changes made to Perl in the future.
379 Another point worth noting is that in the first series of examples I
380 have made use of only the I<call_pv> function. This has been done
381 to keep the code simpler and ease you into the topic. Wherever
382 possible, if the choice is between using I<call_pv> and
383 I<call_sv>, you should always try to use I<call_sv>. See
384 I<Using call_sv> for details.
386 =head2 No Parameters, Nothing returned
388 This first trivial example will call a Perl subroutine, I<PrintUID>, to
389 print out the UID of the process.
396 and here is a C function to call it
404 call_pv("PrintUID", G_DISCARD|G_NOARGS);
409 A few points to note about this example.
415 Ignore C<dSP> and C<PUSHMARK(SP)> for now. They will be discussed in
420 We aren't passing any parameters to I<PrintUID> so G_NOARGS can be
425 We aren't interested in anything returned from I<PrintUID>, so
426 G_DISCARD is specified. Even if I<PrintUID> was changed to
427 return some value(s), having specified G_DISCARD will mean that they
428 will be wiped by the time control returns from I<call_pv>.
432 As I<call_pv> is being used, the Perl subroutine is specified as a
433 C string. In this case the subroutine name has been 'hard-wired' into the
438 Because we specified G_DISCARD, it is not necessary to check the value
439 returned from I<call_pv>. It will always be 0.
443 =head2 Passing Parameters
445 Now let's make a slightly more complex example. This time we want to
446 call a Perl subroutine, C<LeftString>, which will take 2 parameters--a
447 string ($s) and an integer ($n). The subroutine will simply
448 print the first $n characters of the string.
450 So the Perl subroutine would look like this
455 print substr($s, 0, $n), "\n";
458 The C function required to call I<LeftString> would look like this.
461 call_LeftString(a, b)
471 XPUSHs(sv_2mortal(newSVpv(a, 0)));
472 XPUSHs(sv_2mortal(newSViv(b)));
475 call_pv("LeftString", G_DISCARD);
481 Here are a few notes on the C function I<call_LeftString>.
487 Parameters are passed to the Perl subroutine using the Perl stack.
488 This is the purpose of the code beginning with the line C<dSP> and
489 ending with the line C<PUTBACK>. The C<dSP> declares a local copy
490 of the stack pointer. This local copy should B<always> be accessed
495 If you are going to put something onto the Perl stack, you need to know
496 where to put it. This is the purpose of the macro C<dSP>--it declares
497 and initializes a I<local> copy of the Perl stack pointer.
499 All the other macros which will be used in this example require you to
500 have used this macro.
502 The exception to this rule is if you are calling a Perl subroutine
503 directly from an XSUB function. In this case it is not necessary to
504 use the C<dSP> macro explicitly--it will be declared for you
509 Any parameters to be pushed onto the stack should be bracketed by the
510 C<PUSHMARK> and C<PUTBACK> macros. The purpose of these two macros, in
511 this context, is to count the number of parameters you are
512 pushing automatically. Then whenever Perl is creating the C<@_> array for the
513 subroutine, it knows how big to make it.
515 The C<PUSHMARK> macro tells Perl to make a mental note of the current
516 stack pointer. Even if you aren't passing any parameters (like the
517 example shown in the section I<No Parameters, Nothing returned>) you
518 must still call the C<PUSHMARK> macro before you can call any of the
519 I<call_*> functions--Perl still needs to know that there are no
522 The C<PUTBACK> macro sets the global copy of the stack pointer to be
523 the same as our local copy. If we didn't do this I<call_pv>
524 wouldn't know where the two parameters we pushed were--remember that
525 up to now all the stack pointer manipulation we have done is with our
526 local copy, I<not> the global copy.
530 Next, we come to XPUSHs. This is where the parameters actually get
531 pushed onto the stack. In this case we are pushing a string and an
534 See L<perlguts/"XSUBs and the Argument Stack"> for details
535 on how the XPUSH macros work.
539 Because we created temporary values (by means of sv_2mortal() calls)
540 we will have to tidy up the Perl stack and dispose of mortal SVs.
542 This is the purpose of
547 at the start of the function, and
552 at the end. The C<ENTER>/C<SAVETMPS> pair creates a boundary for any
553 temporaries we create. This means that the temporaries we get rid of
554 will be limited to those which were created after these calls.
556 The C<FREETMPS>/C<LEAVE> pair will get rid of any values returned by
557 the Perl subroutine (see next example), plus it will also dump the
558 mortal SVs we have created. Having C<ENTER>/C<SAVETMPS> at the
559 beginning of the code makes sure that no other mortals are destroyed.
561 Think of these macros as working a bit like using C<{> and C<}> in Perl
562 to limit the scope of local variables.
564 See the section I<Using Perl to dispose of temporaries> for details of
565 an alternative to using these macros.
569 Finally, I<LeftString> can now be called via the I<call_pv> function.
570 The only flag specified this time is G_DISCARD. Because we are passing
571 2 parameters to the Perl subroutine this time, we have not specified
576 =head2 Returning a Scalar
578 Now for an example of dealing with the items returned from a Perl
581 Here is a Perl subroutine, I<Adder>, that takes 2 integer parameters
582 and simply returns their sum.
590 Because we are now concerned with the return value from I<Adder>, the C
591 function required to call it is now a bit more complex.
605 XPUSHs(sv_2mortal(newSViv(a)));
606 XPUSHs(sv_2mortal(newSViv(b)));
609 count = call_pv("Adder", G_SCALAR);
614 croak("Big trouble\n");
616 printf ("The sum of %d and %d is %d\n", a, b, POPi);
623 Points to note this time are
629 The only flag specified this time was G_SCALAR. That means the C<@_>
630 array will be created and that the value returned by I<Adder> will
631 still exist after the call to I<call_pv>.
635 The purpose of the macro C<SPAGAIN> is to refresh the local copy of the
636 stack pointer. This is necessary because it is possible that the memory
637 allocated to the Perl stack has been reallocated whilst in the
640 If you are making use of the Perl stack pointer in your code you must
641 always refresh the local copy using SPAGAIN whenever you make use
642 of the I<call_*> functions or any other Perl internal function.
646 Although only a single value was expected to be returned from I<Adder>,
647 it is still good practice to check the return code from I<call_pv>
650 Expecting a single value is not quite the same as knowing that there
651 will be one. If someone modified I<Adder> to return a list and we
652 didn't check for that possibility and take appropriate action the Perl
653 stack would end up in an inconsistent state. That is something you
654 I<really> don't want to happen ever.
658 The C<POPi> macro is used here to pop the return value from the stack.
659 In this case we wanted an integer, so C<POPi> was used.
662 Here is the complete list of POP macros available, along with the types
673 The final C<PUTBACK> is used to leave the Perl stack in a consistent
674 state before exiting the function. This is necessary because when we
675 popped the return value from the stack with C<POPi> it updated only our
676 local copy of the stack pointer. Remember, C<PUTBACK> sets the global
677 stack pointer to be the same as our local copy.
682 =head2 Returning a list of values
684 Now, let's extend the previous example to return both the sum of the
685 parameters and the difference.
687 Here is the Perl subroutine
695 and this is the C function
698 call_AddSubtract(a, b)
709 XPUSHs(sv_2mortal(newSViv(a)));
710 XPUSHs(sv_2mortal(newSViv(b)));
713 count = call_pv("AddSubtract", G_ARRAY);
718 croak("Big trouble\n");
720 printf ("%d - %d = %d\n", a, b, POPi);
721 printf ("%d + %d = %d\n", a, b, POPi);
728 If I<call_AddSubtract> is called like this
730 call_AddSubtract(7, 4);
732 then here is the output
743 We wanted list context, so G_ARRAY was used.
747 Not surprisingly C<POPi> is used twice this time because we were
748 retrieving 2 values from the stack. The important thing to note is that
749 when using the C<POP*> macros they come off the stack in I<reverse>
754 =head2 Returning a list in a scalar context
756 Say the Perl subroutine in the previous section was called in a scalar
760 call_AddSubScalar(a, b)
772 XPUSHs(sv_2mortal(newSViv(a)));
773 XPUSHs(sv_2mortal(newSViv(b)));
776 count = call_pv("AddSubtract", G_SCALAR);
780 printf ("Items Returned = %d\n", count);
782 for (i = 1; i <= count; ++i)
783 printf ("Value %d = %d\n", i, POPi);
790 The other modification made is that I<call_AddSubScalar> will print the
791 number of items returned from the Perl subroutine and their value (for
792 simplicity it assumes that they are integer). So if
793 I<call_AddSubScalar> is called
795 call_AddSubScalar(7, 4);
797 then the output will be
802 In this case the main point to note is that only the last item in the
803 list is returned from the subroutine, I<AddSubtract> actually made it back to
804 I<call_AddSubScalar>.
807 =head2 Returning Data from Perl via the parameter list
809 It is also possible to return values directly via the parameter list -
810 whether it is actually desirable to do it is another matter entirely.
812 The Perl subroutine, I<Inc>, below takes 2 parameters and increments
821 and here is a C function to call it.
836 sva = sv_2mortal(newSViv(a));
837 svb = sv_2mortal(newSViv(b));
844 count = call_pv("Inc", G_DISCARD);
847 croak ("call_Inc: expected 0 values from 'Inc', got %d\n",
850 printf ("%d + 1 = %d\n", a, SvIV(sva));
851 printf ("%d + 1 = %d\n", b, SvIV(svb));
857 To be able to access the two parameters that were pushed onto the stack
858 after they return from I<call_pv> it is necessary to make a note
859 of their addresses--thus the two variables C<sva> and C<svb>.
861 The reason this is necessary is that the area of the Perl stack which
862 held them will very likely have been overwritten by something else by
863 the time control returns from I<call_pv>.
870 Now an example using G_EVAL. Below is a Perl subroutine which computes
871 the difference of its 2 parameters. If this would result in a negative
872 result, the subroutine calls I<die>.
878 die "death can be fatal\n" if $a < $b;
883 and some C to call it
897 XPUSHs(sv_2mortal(newSViv(a)));
898 XPUSHs(sv_2mortal(newSViv(b)));
901 count = call_pv("Subtract", G_EVAL|G_SCALAR);
905 /* Check the eval first */
908 printf ("Uh oh - %s\n", SvPV_nolen(ERRSV));
914 croak("call_Subtract: wanted 1 value from 'Subtract', got %d\n",
917 printf ("%d - %d = %d\n", a, b, POPi);
925 If I<call_Subtract> is called thus
929 the following will be printed
931 Uh oh - death can be fatal
939 We want to be able to catch the I<die> so we have used the G_EVAL
940 flag. Not specifying this flag would mean that the program would
941 terminate immediately at the I<die> statement in the subroutine
950 printf ("Uh oh - %s\n", SvPV_nolen(ERRSV));
954 is the direct equivalent of this bit of Perl
956 print "Uh oh - $@\n" if $@;
958 C<PL_errgv> is a perl global of type C<GV *> that points to the
959 symbol table entry containing the error. C<ERRSV> therefore
960 refers to the C equivalent of C<$@>.
964 Note that the stack is popped using C<POPs> in the block where
965 C<SvTRUE(ERRSV)> is true. This is necessary because whenever a
966 I<call_*> function invoked with G_EVAL|G_SCALAR returns an error,
967 the top of the stack holds the value I<undef>. Because we want the
968 program to continue after detecting this error, it is essential that
969 the stack is tidied up by removing the I<undef>.
974 =head2 Using G_KEEPERR
976 Consider this rather facetious example, where we have used an XS
977 version of the call_Subtract example above inside a destructor:
980 sub new { bless {}, $_[0] }
983 die "death can be fatal" if $a < $b;
986 sub DESTROY { call_Subtract(5, 4); }
987 sub foo { die "foo dies"; }
994 print "Saw: $@" if $@; # should be, but isn't
996 This example will fail to recognize that an error occurred inside the
997 C<eval {}>. Here's why: the call_Subtract code got executed while perl
998 was cleaning up temporaries when exiting the outer braced block, and because
999 call_Subtract is implemented with I<call_pv> using the G_EVAL
1000 flag, it promptly reset C<$@>. This results in the failure of the
1001 outermost test for C<$@>, and thereby the failure of the error trap.
1003 Appending the G_KEEPERR flag, so that the I<call_pv> call in
1004 call_Subtract reads:
1006 count = call_pv("Subtract", G_EVAL|G_SCALAR|G_KEEPERR);
1008 will preserve the error and restore reliable error handling.
1010 =head2 Using call_sv
1012 In all the previous examples I have 'hard-wired' the name of the Perl
1013 subroutine to be called from C. Most of the time though, it is more
1014 convenient to be able to specify the name of the Perl subroutine from
1015 within the Perl script.
1017 Consider the Perl code below
1021 print "Hello there\n";
1026 Here is a snippet of XSUB which defines I<CallSubPV>.
1033 call_pv(name, G_DISCARD|G_NOARGS);
1035 That is fine as far as it goes. The thing is, the Perl subroutine
1036 can be specified as only a string. For Perl 4 this was adequate,
1037 but Perl 5 allows references to subroutines and anonymous subroutines.
1038 This is where I<call_sv> is useful.
1040 The code below for I<CallSubSV> is identical to I<CallSubPV> except
1041 that the C<name> parameter is now defined as an SV* and we use
1042 I<call_sv> instead of I<call_pv>.
1049 call_sv(name, G_DISCARD|G_NOARGS);
1051 Because we are using an SV to call I<fred> the following can all be used
1057 CallSubSV( sub { print "Hello there\n" } );
1059 As you can see, I<call_sv> gives you much greater flexibility in
1060 how you can specify the Perl subroutine.
1062 You should note that if it is necessary to store the SV (C<name> in the
1063 example above) which corresponds to the Perl subroutine so that it can
1064 be used later in the program, it not enough just to store a copy of the
1065 pointer to the SV. Say the code above had been like this
1067 static SV * rememberSub;
1079 call_sv(rememberSub, G_DISCARD|G_NOARGS);
1081 The reason this is wrong is that by the time you come to use the
1082 pointer C<rememberSub> in C<CallSavedSub1>, it may or may not still refer
1083 to the Perl subroutine that was recorded in C<SaveSub1>. This is
1084 particularly true for these cases
1089 SaveSub1( sub { print "Hello there\n" } );
1092 By the time each of the C<SaveSub1> statements above have been executed,
1093 the SV*s which corresponded to the parameters will no longer exist.
1094 Expect an error message from Perl of the form
1096 Can't use an undefined value as a subroutine reference at ...
1098 for each of the C<CallSavedSub1> lines.
1100 Similarly, with this code
1107 you can expect one of these messages (which you actually get is dependent on
1108 the version of Perl you are using)
1110 Not a CODE reference at ...
1111 Undefined subroutine &main::47 called ...
1113 The variable $ref may have referred to the subroutine C<fred>
1114 whenever the call to C<SaveSub1> was made but by the time
1115 C<CallSavedSub1> gets called it now holds the number C<47>. Because we
1116 saved only a pointer to the original SV in C<SaveSub1>, any changes to
1117 $ref will be tracked by the pointer C<rememberSub>. This means that
1118 whenever C<CallSavedSub1> gets called, it will attempt to execute the
1119 code which is referenced by the SV* C<rememberSub>. In this case
1120 though, it now refers to the integer C<47>, so expect Perl to complain
1123 A similar but more subtle problem is illustrated with this code
1130 This time whenever C<CallSavedSub1> get called it will execute the Perl
1131 subroutine C<joe> (assuming it exists) rather than C<fred> as was
1132 originally requested in the call to C<SaveSub1>.
1134 To get around these problems it is necessary to take a full copy of the
1135 SV. The code below shows C<SaveSub2> modified to do that
1137 static SV * keepSub = (SV*)NULL;
1143 /* Take a copy of the callback */
1144 if (keepSub == (SV*)NULL)
1145 /* First time, so create a new SV */
1146 keepSub = newSVsv(name);
1148 /* Been here before, so overwrite */
1149 SvSetSV(keepSub, name);
1155 call_sv(keepSub, G_DISCARD|G_NOARGS);
1157 To avoid creating a new SV every time C<SaveSub2> is called,
1158 the function first checks to see if it has been called before. If not,
1159 then space for a new SV is allocated and the reference to the Perl
1160 subroutine, C<name> is copied to the variable C<keepSub> in one
1161 operation using C<newSVsv>. Thereafter, whenever C<SaveSub2> is called
1162 the existing SV, C<keepSub>, is overwritten with the new value using
1165 =head2 Using call_argv
1167 Here is a Perl subroutine which prints whatever parameters are passed
1174 foreach (@list) { print "$_\n" }
1177 and here is an example of I<call_argv> which will call
1180 static char * words[] = {"alpha", "beta", "gamma", "delta", NULL};
1187 call_argv("PrintList", G_DISCARD, words);
1190 Note that it is not necessary to call C<PUSHMARK> in this instance.
1191 This is because I<call_argv> will do it for you.
1193 =head2 Using call_method
1195 Consider the following Perl code
1208 my ($self, $index) = @_;
1209 print "$index: $$self[$index]\n";
1215 print "This is Class $class version 1.0\n";
1219 It implements just a very simple class to manage an array. Apart from
1220 the constructor, C<new>, it declares methods, one static and one
1221 virtual. The static method, C<PrintID>, prints out simply the class
1222 name and a version number. The virtual method, C<Display>, prints out a
1223 single element of the array. Here is an all Perl example of using it.
1225 $a = Mine->new('red', 'green', 'blue');
1232 This is Class Mine version 1.0
1234 Calling a Perl method from C is fairly straightforward. The following
1241 a reference to the object for a virtual method or the name of the class
1242 for a static method.
1246 the name of the method.
1250 any other parameters specific to the method.
1254 Here is a simple XSUB which illustrates the mechanics of calling both
1255 the C<PrintID> and C<Display> methods from C.
1258 call_Method(ref, method, index)
1265 XPUSHs(sv_2mortal(newSViv(index)));
1268 call_method(method, G_DISCARD);
1271 call_PrintID(class, method)
1276 XPUSHs(sv_2mortal(newSVpv(class, 0)));
1279 call_method(method, G_DISCARD);
1282 So the methods C<PrintID> and C<Display> can be invoked like this
1284 $a = Mine->new('red', 'green', 'blue');
1285 call_Method($a, 'Display', 1);
1286 call_PrintID('Mine', 'PrintID');
1288 The only thing to note is that in both the static and virtual methods,
1289 the method name is not passed via the stack--it is used as the first
1290 parameter to I<call_method>.
1292 =head2 Using GIMME_V
1294 Here is a trivial XSUB which prints the context in which it is
1295 currently executing.
1300 I32 gimme = GIMME_V;
1301 if (gimme == G_VOID)
1302 printf ("Context is Void\n");
1303 else if (gimme == G_SCALAR)
1304 printf ("Context is Scalar\n");
1306 printf ("Context is Array\n");
1308 and here is some Perl to test it
1314 The output from that will be
1320 =head2 Using Perl to dispose of temporaries
1322 In the examples given to date, any temporaries created in the callback
1323 (i.e., parameters passed on the stack to the I<call_*> function or
1324 values returned via the stack) have been freed by one of these methods
1330 specifying the G_DISCARD flag with I<call_*>.
1334 explicitly disposed of using the C<ENTER>/C<SAVETMPS> -
1335 C<FREETMPS>/C<LEAVE> pairing.
1339 There is another method which can be used, namely letting Perl do it
1340 for you automatically whenever it regains control after the callback
1341 has terminated. This is done by simply not using the
1349 sequence in the callback (and not, of course, specifying the G_DISCARD
1352 If you are going to use this method you have to be aware of a possible
1353 memory leak which can arise under very specific circumstances. To
1354 explain these circumstances you need to know a bit about the flow of
1355 control between Perl and the callback routine.
1357 The examples given at the start of the document (an error handler and
1358 an event driven program) are typical of the two main sorts of flow
1359 control that you are likely to encounter with callbacks. There is a
1360 very important distinction between them, so pay attention.
1362 In the first example, an error handler, the flow of control could be as
1363 follows. You have created an interface to an external library.
1364 Control can reach the external library like this
1366 perl --> XSUB --> external library
1368 Whilst control is in the library, an error condition occurs. You have
1369 previously set up a Perl callback to handle this situation, so it will
1370 get executed. Once the callback has finished, control will drop back to
1371 Perl again. Here is what the flow of control will be like in that
1374 perl --> XSUB --> external library
1378 external library --> call_* --> perl
1380 perl <-- XSUB <-- external library <-- call_* <----+
1382 After processing of the error using I<call_*> is completed,
1383 control reverts back to Perl more or less immediately.
1385 In the diagram, the further right you go the more deeply nested the
1386 scope is. It is only when control is back with perl on the extreme
1387 left of the diagram that you will have dropped back to the enclosing
1388 scope and any temporaries you have left hanging around will be freed.
1390 In the second example, an event driven program, the flow of control
1391 will be more like this
1393 perl --> XSUB --> event handler
1395 event handler --> call_* --> perl
1397 event handler <-- call_* <----+
1399 event handler --> call_* --> perl
1401 event handler <-- call_* <----+
1403 event handler --> call_* --> perl
1405 event handler <-- call_* <----+
1407 In this case the flow of control can consist of only the repeated
1410 event handler --> call_* --> perl
1412 for practically the complete duration of the program. This means that
1413 control may I<never> drop back to the surrounding scope in Perl at the
1416 So what is the big problem? Well, if you are expecting Perl to tidy up
1417 those temporaries for you, you might be in for a long wait. For Perl
1418 to dispose of your temporaries, control must drop back to the
1419 enclosing scope at some stage. In the event driven scenario that may
1420 never happen. This means that as time goes on, your program will
1421 create more and more temporaries, none of which will ever be freed. As
1422 each of these temporaries consumes some memory your program will
1423 eventually consume all the available memory in your system--kapow!
1425 So here is the bottom line--if you are sure that control will revert
1426 back to the enclosing Perl scope fairly quickly after the end of your
1427 callback, then it isn't absolutely necessary to dispose explicitly of
1428 any temporaries you may have created. Mind you, if you are at all
1429 uncertain about what to do, it doesn't do any harm to tidy up anyway.
1432 =head2 Strategies for storing Callback Context Information
1435 Potentially one of the trickiest problems to overcome when designing a
1436 callback interface can be figuring out how to store the mapping between
1437 the C callback function and the Perl equivalent.
1439 To help understand why this can be a real problem first consider how a
1440 callback is set up in an all C environment. Typically a C API will
1441 provide a function to register a callback. This will expect a pointer
1442 to a function as one of its parameters. Below is a call to a
1443 hypothetical function C<register_fatal> which registers the C function
1444 to get called when a fatal error occurs.
1446 register_fatal(cb1);
1448 The single parameter C<cb1> is a pointer to a function, so you must
1449 have defined C<cb1> in your code, say something like this
1454 printf ("Fatal Error\n");
1458 Now change that to call a Perl subroutine instead
1460 static SV * callback = (SV*)NULL;
1469 /* Call the Perl sub to process the callback */
1470 call_sv(callback, G_DISCARD);
1478 /* Remember the Perl sub */
1479 if (callback == (SV*)NULL)
1480 callback = newSVsv(fn);
1482 SvSetSV(callback, fn);
1484 /* register the callback with the external library */
1485 register_fatal(cb1);
1487 where the Perl equivalent of C<register_fatal> and the callback it
1488 registers, C<pcb1>, might look like this
1490 # Register the sub pcb1
1491 register_fatal(\&pcb1);
1495 die "I'm dying...\n";
1498 The mapping between the C callback and the Perl equivalent is stored in
1499 the global variable C<callback>.
1501 This will be adequate if you ever need to have only one callback
1502 registered at any time. An example could be an error handler like the
1503 code sketched out above. Remember though, repeated calls to
1504 C<register_fatal> will replace the previously registered callback
1505 function with the new one.
1507 Say for example you want to interface to a library which allows asynchronous
1508 file i/o. In this case you may be able to register a callback whenever
1509 a read operation has completed. To be of any use we want to be able to
1510 call separate Perl subroutines for each file that is opened. As it
1511 stands, the error handler example above would not be adequate as it
1512 allows only a single callback to be defined at any time. What we
1513 require is a means of storing the mapping between the opened file and
1514 the Perl subroutine we want to be called for that file.
1516 Say the i/o library has a function C<asynch_read> which associates a C
1517 function C<ProcessRead> with a file handle C<fh>--this assumes that it
1518 has also provided some routine to open the file and so obtain the file
1521 asynch_read(fh, ProcessRead)
1523 This may expect the C I<ProcessRead> function of this form
1526 ProcessRead(fh, buffer)
1533 To provide a Perl interface to this library we need to be able to map
1534 between the C<fh> parameter and the Perl subroutine we want called. A
1535 hash is a convenient mechanism for storing this mapping. The code
1536 below shows a possible implementation
1538 static HV * Mapping = (HV*)NULL;
1541 asynch_read(fh, callback)
1545 /* If the hash doesn't already exist, create it */
1546 if (Mapping == (HV*)NULL)
1549 /* Save the fh -> callback mapping */
1550 hv_store(Mapping, (char*)&fh, sizeof(fh), newSVsv(callback), 0);
1552 /* Register with the C Library */
1553 asynch_read(fh, asynch_read_if);
1555 and C<asynch_read_if> could look like this
1558 asynch_read_if(fh, buffer)
1565 /* Get the callback associated with fh */
1566 sv = hv_fetch(Mapping, (char*)&fh , sizeof(fh), FALSE);
1567 if (sv == (SV**)NULL)
1568 croak("Internal error...\n");
1571 XPUSHs(sv_2mortal(newSViv(fh)));
1572 XPUSHs(sv_2mortal(newSVpv(buffer, 0)));
1575 /* Call the Perl sub */
1576 call_sv(*sv, G_DISCARD);
1579 For completeness, here is C<asynch_close>. This shows how to remove
1580 the entry from the hash C<Mapping>.
1586 /* Remove the entry from the hash */
1587 (void) hv_delete(Mapping, (char*)&fh, sizeof(fh), G_DISCARD);
1589 /* Now call the real asynch_close */
1592 So the Perl interface would look like this
1596 my($handle, $buffer) = @_;
1599 # Register the Perl callback
1600 asynch_read($fh, \&callback1);
1604 The mapping between the C callback and Perl is stored in the global
1605 hash C<Mapping> this time. Using a hash has the distinct advantage that
1606 it allows an unlimited number of callbacks to be registered.
1608 What if the interface provided by the C callback doesn't contain a
1609 parameter which allows the file handle to Perl subroutine mapping? Say
1610 in the asynchronous i/o package, the callback function gets passed only
1611 the C<buffer> parameter like this
1620 Without the file handle there is no straightforward way to map from the
1621 C callback to the Perl subroutine.
1623 In this case a possible way around this problem is to predefine a
1624 series of C functions to act as the interface to Perl, thus
1627 #define NULL_HANDLE -1
1628 typedef void (*FnMap)();
1640 static struct MapStruct Map [MAX_CB] =
1642 { fn1, NULL, NULL_HANDLE },
1643 { fn2, NULL, NULL_HANDLE },
1644 { fn3, NULL, NULL_HANDLE }
1655 XPUSHs(sv_2mortal(newSVpv(buffer, 0)));
1658 /* Call the Perl sub */
1659 call_sv(Map[index].PerlSub, G_DISCARD);
1684 array_asynch_read(fh, callback)
1689 int null_index = MAX_CB;
1691 /* Find the same handle or an empty entry */
1692 for (index = 0; index < MAX_CB; ++index)
1694 if (Map[index].Handle == fh)
1697 if (Map[index].Handle == NULL_HANDLE)
1701 if (index == MAX_CB && null_index == MAX_CB)
1702 croak ("Too many callback functions registered\n");
1704 if (index == MAX_CB)
1707 /* Save the file handle */
1708 Map[index].Handle = fh;
1710 /* Remember the Perl sub */
1711 if (Map[index].PerlSub == (SV*)NULL)
1712 Map[index].PerlSub = newSVsv(callback);
1714 SvSetSV(Map[index].PerlSub, callback);
1716 asynch_read(fh, Map[index].Function);
1719 array_asynch_close(fh)
1724 /* Find the file handle */
1725 for (index = 0; index < MAX_CB; ++ index)
1726 if (Map[index].Handle == fh)
1729 if (index == MAX_CB)
1730 croak ("could not close fh %d\n", fh);
1732 Map[index].Handle = NULL_HANDLE;
1733 SvREFCNT_dec(Map[index].PerlSub);
1734 Map[index].PerlSub = (SV*)NULL;
1738 In this case the functions C<fn1>, C<fn2>, and C<fn3> are used to
1739 remember the Perl subroutine to be called. Each of the functions holds
1740 a separate hard-wired index which is used in the function C<Pcb> to
1741 access the C<Map> array and actually call the Perl subroutine.
1743 There are some obvious disadvantages with this technique.
1745 Firstly, the code is considerably more complex than with the previous
1748 Secondly, there is a hard-wired limit (in this case 3) to the number of
1749 callbacks that can exist simultaneously. The only way to increase the
1750 limit is by modifying the code to add more functions and then
1751 recompiling. None the less, as long as the number of functions is
1752 chosen with some care, it is still a workable solution and in some
1753 cases is the only one available.
1755 To summarize, here are a number of possible methods for you to consider
1756 for storing the mapping between C and the Perl callback
1760 =item 1. Ignore the problem - Allow only 1 callback
1762 For a lot of situations, like interfacing to an error handler, this may
1763 be a perfectly adequate solution.
1765 =item 2. Create a sequence of callbacks - hard wired limit
1767 If it is impossible to tell from the parameters passed back from the C
1768 callback what the context is, then you may need to create a sequence of C
1769 callback interface functions, and store pointers to each in an array.
1771 =item 3. Use a parameter to map to the Perl callback
1773 A hash is an ideal mechanism to store the mapping between C and Perl.
1778 =head2 Alternate Stack Manipulation
1781 Although I have made use of only the C<POP*> macros to access values
1782 returned from Perl subroutines, it is also possible to bypass these
1783 macros and read the stack using the C<ST> macro (See L<perlxs> for a
1784 full description of the C<ST> macro).
1786 Most of the time the C<POP*> macros should be adequate, the main
1787 problem with them is that they force you to process the returned values
1788 in sequence. This may not be the most suitable way to process the
1789 values in some cases. What we want is to be able to access the stack in
1790 a random order. The C<ST> macro as used when coding an XSUB is ideal
1793 The code below is the example given in the section I<Returning a list
1794 of values> recoded to use C<ST> instead of C<POP*>.
1797 call_AddSubtract2(a, b)
1809 XPUSHs(sv_2mortal(newSViv(a)));
1810 XPUSHs(sv_2mortal(newSViv(b)));
1813 count = call_pv("AddSubtract", G_ARRAY);
1817 ax = (SP - PL_stack_base) + 1;
1820 croak("Big trouble\n");
1822 printf ("%d + %d = %d\n", a, b, SvIV(ST(0)));
1823 printf ("%d - %d = %d\n", a, b, SvIV(ST(1)));
1836 Notice that it was necessary to define the variable C<ax>. This is
1837 because the C<ST> macro expects it to exist. If we were in an XSUB it
1838 would not be necessary to define C<ax> as it is already defined for
1847 ax = (SP - PL_stack_base) + 1;
1849 sets the stack up so that we can use the C<ST> macro.
1853 Unlike the original coding of this example, the returned
1854 values are not accessed in reverse order. So C<ST(0)> refers to the
1855 first value returned by the Perl subroutine and C<ST(count-1)>
1860 =head2 Creating and calling an anonymous subroutine in C
1862 As we've already shown, C<call_sv> can be used to invoke an
1863 anonymous subroutine. However, our example showed a Perl script
1864 invoking an XSUB to perform this operation. Let's see how it can be
1865 done inside our C code:
1869 SV *cvrv = eval_pv("sub { print 'You will not find me cluttering any namespace!' }", TRUE);
1873 call_sv(cvrv, G_VOID|G_NOARGS);
1875 C<eval_pv> is used to compile the anonymous subroutine, which
1876 will be the return value as well (read more about C<eval_pv> in
1877 L<perlapi/eval_pv>). Once this code reference is in hand, it
1878 can be mixed in with all the previous examples we've shown.
1880 =head1 LIGHTWEIGHT CALLBACKS
1882 Sometimes you need to invoke the same subroutine repeatedly.
1883 This usually happens with a function that acts on a list of
1884 values, such as Perl's built-in sort(). You can pass a
1885 comparison function to sort(), which will then be invoked
1886 for every pair of values that needs to be compared. The first()
1887 and reduce() functions from L<List::Util> follow a similar
1890 In this case it is possible to speed up the routine (often
1891 quite substantially) by using the lightweight callback API.
1892 The idea is that the calling context only needs to be
1893 created and destroyed once, and the sub can be called
1894 arbitrarily many times in between.
1896 It is usual to pass parameters using global variables (typically
1897 $_ for one parameter, or $a and $b for two parameters) rather
1898 than via @_. (It is possible to use the @_ mechanism if you know
1899 what you're doing, though there is as yet no supported API for
1900 it. It's also inherently slower.)
1902 The pattern of macro calls is like this:
1904 dMULTICALL; /* Declare local variables */
1905 I32 gimme = G_SCALAR; /* context of the call: G_SCALAR,
1906 * G_LIST, or G_VOID */
1908 PUSH_MULTICALL(cv); /* Set up the context for calling cv,
1909 and set local vars appropriately */
1912 /* set the value(s) af your parameter variables */
1913 MULTICALL; /* Make the actual call */
1916 POP_MULTICALL; /* Tear down the calling context */
1918 For some concrete examples, see the implementation of the
1919 first() and reduce() functions of List::Util 1.18. There you
1920 will also find a header file that emulates the multicall API
1921 on older versions of perl.
1925 L<perlxs>, L<perlguts>, L<perlembed>
1931 Special thanks to the following people who assisted in the creation of
1934 Jeff Okamoto, Tim Bunce, Nick Gianniotis, Steve Kelem, Gurusamy Sarathy
1939 Version 1.3, 14th Apr 1997