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 PERL_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 perl_call_sv(SV* sv, I32 flags) ;
57 I32 perl_call_pv(char *subname, I32 flags) ;
58 I32 perl_call_method(char *methname, I32 flags) ;
59 I32 perl_call_argv(char *subname, I32 flags, register char **argv) ;
61 The key function is I<perl_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<perl_call_sv>
64 to invoke the Perl subroutine.
66 All the I<perl_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<perl_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 perl_call_sv>, shows how you can make
81 use of I<perl_call_sv>.
85 The function, I<perl_call_pv>, is similar to I<perl_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<perl_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">.
91 =item B<perl_call_method>
93 The function I<perl_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 perl_call_method> for an example
100 of using I<perl_call_method>.
102 =item B<perl_call_argv>
104 I<perl_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 perl_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<perl_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<perl_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<perl_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<perl_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<perl_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 an
205 array 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<perl_call_*> function.
215 The value returned by the I<perl_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<perl_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<perl_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<perl_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 of events occurs, the
283 process will terminate immediately. If though, 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<perl_call_*> function you need to
288 check the C<$@> variable as you would in a normal Perl script.
290 The value returned from the I<perl_call_*> function is dependent on
291 what other flags have been specified and whether an error has
292 occurred. Here are all the different cases that can occur:
298 If the I<perl_call_*> function returns normally, then the value
299 returned is as specified in the previous sections.
303 If G_DISCARD is specified, the return value will always be 0.
307 If G_ARRAY is specified I<and> an error has occurred, the return value
312 If G_SCALAR is specified I<and> an error has occurred, the return value
313 will be 1 and the value on the top of the stack will be I<undef>. This
314 means that if you have already detected the error by checking C<$@> 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 You may have noticed that using the G_EVAL flag described above will
325 B<always> clear the C<$@> variable and set it to a string describing
326 the error iff there was an error in the called code. This unqualified
327 resetting of C<$@> can be problematic in the reliable identification of
328 errors using the C<eval {}> mechanism, because the possibility exists
329 that perl will call other code (end of block processing code, for
330 example) between the time the error causes C<$@> to be set within
331 C<eval {}>, and the subsequent statement which checks for the value of
332 C<$@> gets executed in the user's script.
334 This scenario will mostly be applicable to code that is meant to be
335 called from within destructors, asynchronous callbacks, signal
336 handlers, C<__DIE__> or C<__WARN__> hooks, and C<tie> functions. In
337 such situations, you will not want to clear C<$@> at all, but simply to
338 append any new errors to any existing value of C<$@>.
340 The G_KEEPERR flag is meant to be used in conjunction with G_EVAL in
341 I<perl_call_*> functions that are used to implement such code. This flag
342 has no effect when G_EVAL is not used.
344 When G_KEEPERR is used, any errors in the called code will be prefixed
345 with the string "\t(in cleanup)", and appended to the current value
348 The G_KEEPERR flag was introduced in Perl version 5.002.
350 See I<Using G_KEEPERR> for an example of a situation that warrants the
353 =head2 Determining the Context
355 As mentioned above, you can determine the context of the currently
356 executing subroutine in Perl with I<wantarray>. The equivalent test
357 can be made in C by using the C<GIMME_V> macro, which returns
358 C<G_ARRAY> if you have been called in an array context, C<G_SCALAR> if
359 in a scalar context, or C<G_VOID> if in a void context (i.e. the
360 return value will not be used). An older version of this macro is
361 called C<GIMME>; in a void context it returns C<G_SCALAR> instead of
362 C<G_VOID>. An example of using the C<GIMME_V> macro is shown in
363 section I<Using GIMME_V>.
365 =head1 KNOWN PROBLEMS
367 This section outlines all known problems that exist in the
368 I<perl_call_*> functions.
374 If you are intending to make use of both the G_EVAL and G_SCALAR flags
375 in your code, use a version of Perl greater than 5.000. There is a bug
376 in version 5.000 of Perl which means that the combination of these two
377 flags will not work as described in the section I<FLAG VALUES>.
379 Specifically, if the two flags are used when calling a subroutine and
380 that subroutine does not call I<die>, the value returned by
381 I<perl_call_*> will be wrong.
386 In Perl 5.000 and 5.001 there is a problem with using I<perl_call_*> if
387 the Perl sub you are calling attempts to trap a I<die>.
389 The symptom of this problem is that the called Perl sub will continue
390 to completion, but whenever it attempts to pass control back to the
391 XSUB, the program will immediately terminate.
393 For example, say you want to call this Perl sub
397 eval { die "Fatal Error" ; }
398 print "Trapped error: $@\n"
408 perl_call_pv("fred", G_DISCARD|G_NOARGS) ;
409 fprintf(stderr, "back in Call_fred\n") ;
411 When C<Call_fred> is executed it will print
413 Trapped error: Fatal Error
415 As control never returns to C<Call_fred>, the C<"back in Call_fred">
416 string will not get printed.
418 To work around this problem, you can either upgrade to Perl 5.002 or
419 higher, or use the G_EVAL flag with I<perl_call_*> as shown below
425 perl_call_pv("fred", G_EVAL|G_DISCARD|G_NOARGS) ;
426 fprintf(stderr, "back in Call_fred\n") ;
434 Enough of the definition talk, let's have a few examples.
436 Perl provides many macros to assist in accessing the Perl stack.
437 Wherever possible, these macros should always be used when interfacing
438 to Perl internals. We hope this should make the code less vulnerable
439 to any changes made to Perl in the future.
441 Another point worth noting is that in the first series of examples I
442 have made use of only the I<perl_call_pv> function. This has been done
443 to keep the code simpler and ease you into the topic. Wherever
444 possible, if the choice is between using I<perl_call_pv> and
445 I<perl_call_sv>, you should always try to use I<perl_call_sv>. See
446 I<Using perl_call_sv> for details.
448 =head2 No Parameters, Nothing returned
450 This first trivial example will call a Perl subroutine, I<PrintUID>, to
451 print out the UID of the process.
455 print "UID is $<\n" ;
458 and here is a C function to call it
466 perl_call_pv("PrintUID", G_DISCARD|G_NOARGS) ;
471 A few points to note about this example.
477 Ignore C<dSP> and C<PUSHMARK(sp)> for now. They will be discussed in
482 We aren't passing any parameters to I<PrintUID> so G_NOARGS can be
487 We aren't interested in anything returned from I<PrintUID>, so
488 G_DISCARD is specified. Even if I<PrintUID> was changed to
489 return some value(s), having specified G_DISCARD will mean that they
490 will be wiped by the time control returns from I<perl_call_pv>.
494 As I<perl_call_pv> is being used, the Perl subroutine is specified as a
495 C string. In this case the subroutine name has been 'hard-wired' into the
500 Because we specified G_DISCARD, it is not necessary to check the value
501 returned from I<perl_call_pv>. It will always be 0.
505 =head2 Passing Parameters
507 Now let's make a slightly more complex example. This time we want to
508 call a Perl subroutine, C<LeftString>, which will take 2 parameters - a
509 string (C<$s>) and an integer (C<$n>). The subroutine will simply
510 print the first C<$n> characters of the string.
512 So the Perl subroutine would look like this
517 print substr($s, 0, $n), "\n" ;
520 The C function required to call I<LeftString> would look like this.
523 call_LeftString(a, b)
530 XPUSHs(sv_2mortal(newSVpv(a, 0)));
531 XPUSHs(sv_2mortal(newSViv(b)));
534 perl_call_pv("LeftString", G_DISCARD);
537 Here are a few notes on the C function I<call_LeftString>.
543 Parameters are passed to the Perl subroutine using the Perl stack.
544 This is the purpose of the code beginning with the line C<dSP> and
545 ending with the line C<PUTBACK>.
550 If you are going to put something onto the Perl stack, you need to know
551 where to put it. This is the purpose of the macro C<dSP> - it declares
552 and initializes a I<local> copy of the Perl stack pointer.
554 All the other macros which will be used in this example require you to
555 have used this macro.
557 The exception to this rule is if you are calling a Perl subroutine
558 directly from an XSUB function. In this case it is not necessary to
559 use the C<dSP> macro explicitly - it will be declared for you
564 Any parameters to be pushed onto the stack should be bracketed by the
565 C<PUSHMARK> and C<PUTBACK> macros. The purpose of these two macros, in
566 this context, is to count the number of parameters you are
567 pushing automatically. Then whenever Perl is creating the C<@_> array for the
568 subroutine, it knows how big to make it.
570 The C<PUSHMARK> macro tells Perl to make a mental note of the current
571 stack pointer. Even if you aren't passing any parameters (like the
572 example shown in the section I<No Parameters, Nothing returned>) you
573 must still call the C<PUSHMARK> macro before you can call any of the
574 I<perl_call_*> functions - Perl still needs to know that there are no
577 The C<PUTBACK> macro sets the global copy of the stack pointer to be
578 the same as our local copy. If we didn't do this I<perl_call_pv>
579 wouldn't know where the two parameters we pushed were - remember that
580 up to now all the stack pointer manipulation we have done is with our
581 local copy, I<not> the global copy.
585 The only flag specified this time is G_DISCARD. Because we are passing 2
586 parameters to the Perl subroutine this time, we have not specified
591 Next, we come to XPUSHs. This is where the parameters actually get
592 pushed onto the stack. In this case we are pushing a string and an
595 See L<perlguts/"XSUBs and the Argument Stack"> for details
596 on how the XPUSH macros work.
600 Finally, I<LeftString> can now be called via the I<perl_call_pv>
605 =head2 Returning a Scalar
607 Now for an example of dealing with the items returned from a Perl
610 Here is a Perl subroutine, I<Adder>, that takes 2 integer parameters
611 and simply returns their sum.
619 Because we are now concerned with the return value from I<Adder>, the C
620 function required to call it is now a bit more complex.
634 XPUSHs(sv_2mortal(newSViv(a)));
635 XPUSHs(sv_2mortal(newSViv(b)));
638 count = perl_call_pv("Adder", G_SCALAR);
643 croak("Big trouble\n") ;
645 printf ("The sum of %d and %d is %d\n", a, b, POPi) ;
652 Points to note this time are
658 The only flag specified this time was G_SCALAR. That means the C<@_>
659 array will be created and that the value returned by I<Adder> will
660 still exist after the call to I<perl_call_pv>.
666 Because we are interested in what is returned from I<Adder> we cannot
667 specify G_DISCARD. This means that we will have to tidy up the Perl
668 stack and dispose of any temporary values ourselves. This is the
674 at the start of the function, and
679 at the end. The C<ENTER>/C<SAVETMPS> pair creates a boundary for any
680 temporaries we create. This means that the temporaries we get rid of
681 will be limited to those which were created after these calls.
683 The C<FREETMPS>/C<LEAVE> pair will get rid of any values returned by
684 the Perl subroutine, plus it will also dump the mortal SVs we have
685 created. Having C<ENTER>/C<SAVETMPS> at the beginning of the code
686 makes sure that no other mortals are destroyed.
688 Think of these macros as working a bit like using C<{> and C<}> in Perl
689 to limit the scope of local variables.
691 See the section I<Using Perl to dispose of temporaries> for details of
692 an alternative to using these macros.
696 The purpose of the macro C<SPAGAIN> is to refresh the local copy of the
697 stack pointer. This is necessary because it is possible that the memory
698 allocated to the Perl stack has been reallocated whilst in the
699 I<perl_call_pv> call.
701 If you are making use of the Perl stack pointer in your code you must
702 always refresh the local copy using SPAGAIN whenever you make use
703 of the I<perl_call_*> functions or any other Perl internal function.
707 Although only a single value was expected to be returned from I<Adder>,
708 it is still good practice to check the return code from I<perl_call_pv>
711 Expecting a single value is not quite the same as knowing that there
712 will be one. If someone modified I<Adder> to return a list and we
713 didn't check for that possibility and take appropriate action the Perl
714 stack would end up in an inconsistent state. That is something you
715 I<really> don't want to happen ever.
719 The C<POPi> macro is used here to pop the return value from the stack.
720 In this case we wanted an integer, so C<POPi> was used.
723 Here is the complete list of POP macros available, along with the types
734 The final C<PUTBACK> is used to leave the Perl stack in a consistent
735 state before exiting the function. This is necessary because when we
736 popped the return value from the stack with C<POPi> it updated only our
737 local copy of the stack pointer. Remember, C<PUTBACK> sets the global
738 stack pointer to be the same as our local copy.
743 =head2 Returning a list of values
745 Now, let's extend the previous example to return both the sum of the
746 parameters and the difference.
748 Here is the Perl subroutine
756 and this is the C function
759 call_AddSubtract(a, b)
770 XPUSHs(sv_2mortal(newSViv(a)));
771 XPUSHs(sv_2mortal(newSViv(b)));
774 count = perl_call_pv("AddSubtract", G_ARRAY);
779 croak("Big trouble\n") ;
781 printf ("%d - %d = %d\n", a, b, POPi) ;
782 printf ("%d + %d = %d\n", a, b, POPi) ;
789 If I<call_AddSubtract> is called like this
791 call_AddSubtract(7, 4) ;
793 then here is the output
804 We wanted array context, so G_ARRAY was used.
808 Not surprisingly C<POPi> is used twice this time because we were
809 retrieving 2 values from the stack. The important thing to note is that
810 when using the C<POP*> macros they come off the stack in I<reverse>
815 =head2 Returning a list in a scalar context
817 Say the Perl subroutine in the previous section was called in a scalar
821 call_AddSubScalar(a, b)
833 XPUSHs(sv_2mortal(newSViv(a)));
834 XPUSHs(sv_2mortal(newSViv(b)));
837 count = perl_call_pv("AddSubtract", G_SCALAR);
841 printf ("Items Returned = %d\n", count) ;
843 for (i = 1 ; i <= count ; ++i)
844 printf ("Value %d = %d\n", i, POPi) ;
851 The other modification made is that I<call_AddSubScalar> will print the
852 number of items returned from the Perl subroutine and their value (for
853 simplicity it assumes that they are integer). So if
854 I<call_AddSubScalar> is called
856 call_AddSubScalar(7, 4) ;
858 then the output will be
863 In this case the main point to note is that only the last item in the
864 list is returned from the subroutine, I<AddSubtract> actually made it back to
865 I<call_AddSubScalar>.
868 =head2 Returning Data from Perl via the parameter list
870 It is also possible to return values directly via the parameter list -
871 whether it is actually desirable to do it is another matter entirely.
873 The Perl subroutine, I<Inc>, below takes 2 parameters and increments
882 and here is a C function to call it.
897 sva = sv_2mortal(newSViv(a)) ;
898 svb = sv_2mortal(newSViv(b)) ;
905 count = perl_call_pv("Inc", G_DISCARD);
908 croak ("call_Inc: expected 0 values from 'Inc', got %d\n",
911 printf ("%d + 1 = %d\n", a, SvIV(sva)) ;
912 printf ("%d + 1 = %d\n", b, SvIV(svb)) ;
918 To be able to access the two parameters that were pushed onto the stack
919 after they return from I<perl_call_pv> it is necessary to make a note
920 of their addresses - thus the two variables C<sva> and C<svb>.
922 The reason this is necessary is that the area of the Perl stack which
923 held them will very likely have been overwritten by something else by
924 the time control returns from I<perl_call_pv>.
931 Now an example using G_EVAL. Below is a Perl subroutine which computes
932 the difference of its 2 parameters. If this would result in a negative
933 result, the subroutine calls I<die>.
939 die "death can be fatal\n" if $a < $b ;
944 and some C to call it
958 XPUSHs(sv_2mortal(newSViv(a)));
959 XPUSHs(sv_2mortal(newSViv(b)));
962 count = perl_call_pv("Subtract", G_EVAL|G_SCALAR);
966 /* Check the eval first */
967 if (SvTRUE(GvSV(errgv)))
969 printf ("Uh oh - %s\n", SvPV(GvSV(errgv), na)) ;
975 croak("call_Subtract: wanted 1 value from 'Subtract', got %d\n",
978 printf ("%d - %d = %d\n", a, b, POPi) ;
986 If I<call_Subtract> is called thus
990 the following will be printed
992 Uh oh - death can be fatal
1000 We want to be able to catch the I<die> so we have used the G_EVAL
1001 flag. Not specifying this flag would mean that the program would
1002 terminate immediately at the I<die> statement in the subroutine
1009 if (SvTRUE(GvSV(errgv)))
1011 printf ("Uh oh - %s\n", SvPV(GvSV(errgv), na)) ;
1015 is the direct equivalent of this bit of Perl
1017 print "Uh oh - $@\n" if $@ ;
1019 C<errgv> is a perl global of type C<GV *> that points to the
1020 symbol table entry containing the error. C<GvSV(errgv)> therefore
1021 refers to the C equivalent of C<$@>.
1025 Note that the stack is popped using C<POPs> in the block where
1026 C<SvTRUE(GvSV(errgv))> is true. This is necessary because whenever a
1027 I<perl_call_*> function invoked with G_EVAL|G_SCALAR returns an error,
1028 the top of the stack holds the value I<undef>. Because we want the
1029 program to continue after detecting this error, it is essential that
1030 the stack is tidied up by removing the I<undef>.
1035 =head2 Using G_KEEPERR
1037 Consider this rather facetious example, where we have used an XS
1038 version of the call_Subtract example above inside a destructor:
1041 sub new { bless {}, $_[0] }
1044 die "death can be fatal" if $a < $b ;
1047 sub DESTROY { call_Subtract(5, 4); }
1048 sub foo { die "foo dies"; }
1051 eval { Foo->new->foo };
1052 print "Saw: $@" if $@; # should be, but isn't
1054 This example will fail to recognize that an error occurred inside the
1055 C<eval {}>. Here's why: the call_Subtract code got executed while perl
1056 was cleaning up temporaries when exiting the eval block, and because
1057 call_Subtract is implemented with I<perl_call_pv> using the G_EVAL
1058 flag, it promptly reset C<$@>. This results in the failure of the
1059 outermost test for C<$@>, and thereby the failure of the error trap.
1061 Appending the G_KEEPERR flag, so that the I<perl_call_pv> call in
1062 call_Subtract reads:
1064 count = perl_call_pv("Subtract", G_EVAL|G_SCALAR|G_KEEPERR);
1066 will preserve the error and restore reliable error handling.
1068 =head2 Using perl_call_sv
1070 In all the previous examples I have 'hard-wired' the name of the Perl
1071 subroutine to be called from C. Most of the time though, it is more
1072 convenient to be able to specify the name of the Perl subroutine from
1073 within the Perl script.
1075 Consider the Perl code below
1079 print "Hello there\n" ;
1084 Here is a snippet of XSUB which defines I<CallSubPV>.
1091 perl_call_pv(name, G_DISCARD|G_NOARGS) ;
1093 That is fine as far as it goes. The thing is, the Perl subroutine
1094 can be specified as only a string. For Perl 4 this was adequate,
1095 but Perl 5 allows references to subroutines and anonymous subroutines.
1096 This is where I<perl_call_sv> is useful.
1098 The code below for I<CallSubSV> is identical to I<CallSubPV> except
1099 that the C<name> parameter is now defined as an SV* and we use
1100 I<perl_call_sv> instead of I<perl_call_pv>.
1107 perl_call_sv(name, G_DISCARD|G_NOARGS) ;
1109 Because we are using an SV to call I<fred> the following can all be used
1115 CallSubSV( sub { print "Hello there\n" } ) ;
1117 As you can see, I<perl_call_sv> gives you much greater flexibility in
1118 how you can specify the Perl subroutine.
1120 You should note that if it is necessary to store the SV (C<name> in the
1121 example above) which corresponds to the Perl subroutine so that it can
1122 be used later in the program, it not enough just to store a copy of the
1123 pointer to the SV. Say the code above had been like this
1125 static SV * rememberSub ;
1131 rememberSub = name ;
1137 perl_call_sv(rememberSub, G_DISCARD|G_NOARGS) ;
1139 The reason this is wrong is that by the time you come to use the
1140 pointer C<rememberSub> in C<CallSavedSub1>, it may or may not still refer
1141 to the Perl subroutine that was recorded in C<SaveSub1>. This is
1142 particularly true for these cases
1147 SaveSub1( sub { print "Hello there\n" } ) ;
1150 By the time each of the C<SaveSub1> statements above have been executed,
1151 the SV*s which corresponded to the parameters will no longer exist.
1152 Expect an error message from Perl of the form
1154 Can't use an undefined value as a subroutine reference at ...
1156 for each of the C<CallSavedSub1> lines.
1158 Similarly, with this code
1165 you can expect one of these messages (which you actually get is dependent on
1166 the version of Perl you are using)
1168 Not a CODE reference at ...
1169 Undefined subroutine &main::47 called ...
1171 The variable C<$ref> may have referred to the subroutine C<fred>
1172 whenever the call to C<SaveSub1> was made but by the time
1173 C<CallSavedSub1> gets called it now holds the number C<47>. Because we
1174 saved only a pointer to the original SV in C<SaveSub1>, any changes to
1175 C<$ref> will be tracked by the pointer C<rememberSub>. This means that
1176 whenever C<CallSavedSub1> gets called, it will attempt to execute the
1177 code which is referenced by the SV* C<rememberSub>. In this case
1178 though, it now refers to the integer C<47>, so expect Perl to complain
1181 A similar but more subtle problem is illustrated with this code
1188 This time whenever C<CallSavedSub1> get called it will execute the Perl
1189 subroutine C<joe> (assuming it exists) rather than C<fred> as was
1190 originally requested in the call to C<SaveSub1>.
1192 To get around these problems it is necessary to take a full copy of the
1193 SV. The code below shows C<SaveSub2> modified to do that
1195 static SV * keepSub = (SV*)NULL ;
1201 /* Take a copy of the callback */
1202 if (keepSub == (SV*)NULL)
1203 /* First time, so create a new SV */
1204 keepSub = newSVsv(name) ;
1206 /* Been here before, so overwrite */
1207 SvSetSV(keepSub, name) ;
1213 perl_call_sv(keepSub, G_DISCARD|G_NOARGS) ;
1215 To avoid creating a new SV every time C<SaveSub2> is called,
1216 the function first checks to see if it has been called before. If not,
1217 then space for a new SV is allocated and the reference to the Perl
1218 subroutine, C<name> is copied to the variable C<keepSub> in one
1219 operation using C<newSVsv>. Thereafter, whenever C<SaveSub2> is called
1220 the existing SV, C<keepSub>, is overwritten with the new value using
1223 =head2 Using perl_call_argv
1225 Here is a Perl subroutine which prints whatever parameters are passed
1232 foreach (@list) { print "$_\n" }
1235 and here is an example of I<perl_call_argv> which will call
1238 static char * words[] = {"alpha", "beta", "gamma", "delta", NULL} ;
1245 perl_call_argv("PrintList", G_DISCARD, words) ;
1248 Note that it is not necessary to call C<PUSHMARK> in this instance.
1249 This is because I<perl_call_argv> will do it for you.
1251 =head2 Using perl_call_method
1253 Consider the following Perl code
1266 my ($self, $index) = @_ ;
1267 print "$index: $$self[$index]\n" ;
1273 print "This is Class $class version 1.0\n" ;
1277 It implements just a very simple class to manage an array. Apart from
1278 the constructor, C<new>, it declares methods, one static and one
1279 virtual. The static method, C<PrintID>, prints out simply the class
1280 name and a version number. The virtual method, C<Display>, prints out a
1281 single element of the array. Here is an all Perl example of using it.
1283 $a = new Mine ('red', 'green', 'blue') ;
1290 This is Class Mine version 1.0
1292 Calling a Perl method from C is fairly straightforward. The following
1299 a reference to the object for a virtual method or the name of the class
1300 for a static method.
1304 the name of the method.
1308 any other parameters specific to the method.
1312 Here is a simple XSUB which illustrates the mechanics of calling both
1313 the C<PrintID> and C<Display> methods from C.
1316 call_Method(ref, method, index)
1323 XPUSHs(sv_2mortal(newSViv(index))) ;
1326 perl_call_method(method, G_DISCARD) ;
1329 call_PrintID(class, method)
1334 XPUSHs(sv_2mortal(newSVpv(class, 0))) ;
1337 perl_call_method(method, G_DISCARD) ;
1340 So the methods C<PrintID> and C<Display> can be invoked like this
1342 $a = new Mine ('red', 'green', 'blue') ;
1343 call_Method($a, 'Display', 1) ;
1344 call_PrintID('Mine', 'PrintID') ;
1346 The only thing to note is that in both the static and virtual methods,
1347 the method name is not passed via the stack - it is used as the first
1348 parameter to I<perl_call_method>.
1350 =head2 Using GIMME_V
1352 Here is a trivial XSUB which prints the context in which it is
1353 currently executing.
1358 I32 gimme = GIMME_V;
1359 if (gimme == G_VOID)
1360 printf ("Context is Void\n") ;
1361 else if (gimme == G_SCALAR)
1362 printf ("Context is Scalar\n") ;
1364 printf ("Context is Array\n") ;
1366 and here is some Perl to test it
1372 The output from that will be
1378 =head2 Using Perl to dispose of temporaries
1380 In the examples given to date, any temporaries created in the callback
1381 (i.e., parameters passed on the stack to the I<perl_call_*> function or
1382 values returned via the stack) have been freed by one of these methods
1388 specifying the G_DISCARD flag with I<perl_call_*>.
1392 explicitly disposed of using the C<ENTER>/C<SAVETMPS> -
1393 C<FREETMPS>/C<LEAVE> pairing.
1397 There is another method which can be used, namely letting Perl do it
1398 for you automatically whenever it regains control after the callback
1399 has terminated. This is done by simply not using the
1407 sequence in the callback (and not, of course, specifying the G_DISCARD
1410 If you are going to use this method you have to be aware of a possible
1411 memory leak which can arise under very specific circumstances. To
1412 explain these circumstances you need to know a bit about the flow of
1413 control between Perl and the callback routine.
1415 The examples given at the start of the document (an error handler and
1416 an event driven program) are typical of the two main sorts of flow
1417 control that you are likely to encounter with callbacks. There is a
1418 very important distinction between them, so pay attention.
1420 In the first example, an error handler, the flow of control could be as
1421 follows. You have created an interface to an external library.
1422 Control can reach the external library like this
1424 perl --> XSUB --> external library
1426 Whilst control is in the library, an error condition occurs. You have
1427 previously set up a Perl callback to handle this situation, so it will
1428 get executed. Once the callback has finished, control will drop back to
1429 Perl again. Here is what the flow of control will be like in that
1432 perl --> XSUB --> external library
1436 external library --> perl_call --> perl
1438 perl <-- XSUB <-- external library <-- perl_call <----+
1440 After processing of the error using I<perl_call_*> is completed,
1441 control reverts back to Perl more or less immediately.
1443 In the diagram, the further right you go the more deeply nested the
1444 scope is. It is only when control is back with perl on the extreme
1445 left of the diagram that you will have dropped back to the enclosing
1446 scope and any temporaries you have left hanging around will be freed.
1448 In the second example, an event driven program, the flow of control
1449 will be more like this
1451 perl --> XSUB --> event handler
1453 event handler --> perl_call --> perl
1455 event handler <-- perl_call <----+
1457 event handler --> perl_call --> perl
1459 event handler <-- perl_call <----+
1461 event handler --> perl_call --> perl
1463 event handler <-- perl_call <----+
1465 In this case the flow of control can consist of only the repeated
1468 event handler --> perl_call --> perl
1470 for practically the complete duration of the program. This means that
1471 control may I<never> drop back to the surrounding scope in Perl at the
1474 So what is the big problem? Well, if you are expecting Perl to tidy up
1475 those temporaries for you, you might be in for a long wait. For Perl
1476 to dispose of your temporaries, control must drop back to the
1477 enclosing scope at some stage. In the event driven scenario that may
1478 never happen. This means that as time goes on, your program will
1479 create more and more temporaries, none of which will ever be freed. As
1480 each of these temporaries consumes some memory your program will
1481 eventually consume all the available memory in your system - kapow!
1483 So here is the bottom line - if you are sure that control will revert
1484 back to the enclosing Perl scope fairly quickly after the end of your
1485 callback, then it isn't absolutely necessary to dispose explicitly of
1486 any temporaries you may have created. Mind you, if you are at all
1487 uncertain about what to do, it doesn't do any harm to tidy up anyway.
1490 =head2 Strategies for storing Callback Context Information
1493 Potentially one of the trickiest problems to overcome when designing a
1494 callback interface can be figuring out how to store the mapping between
1495 the C callback function and the Perl equivalent.
1497 To help understand why this can be a real problem first consider how a
1498 callback is set up in an all C environment. Typically a C API will
1499 provide a function to register a callback. This will expect a pointer
1500 to a function as one of its parameters. Below is a call to a
1501 hypothetical function C<register_fatal> which registers the C function
1502 to get called when a fatal error occurs.
1504 register_fatal(cb1) ;
1506 The single parameter C<cb1> is a pointer to a function, so you must
1507 have defined C<cb1> in your code, say something like this
1512 printf ("Fatal Error\n") ;
1516 Now change that to call a Perl subroutine instead
1518 static SV * callback = (SV*)NULL;
1527 /* Call the Perl sub to process the callback */
1528 perl_call_sv(callback, G_DISCARD) ;
1536 /* Remember the Perl sub */
1537 if (callback == (SV*)NULL)
1538 callback = newSVsv(fn) ;
1540 SvSetSV(callback, fn) ;
1542 /* register the callback with the external library */
1543 register_fatal(cb1) ;
1545 where the Perl equivalent of C<register_fatal> and the callback it
1546 registers, C<pcb1>, might look like this
1548 # Register the sub pcb1
1549 register_fatal(\&pcb1) ;
1553 die "I'm dying...\n" ;
1556 The mapping between the C callback and the Perl equivalent is stored in
1557 the global variable C<callback>.
1559 This will be adequate if you ever need to have only one callback
1560 registered at any time. An example could be an error handler like the
1561 code sketched out above. Remember though, repeated calls to
1562 C<register_fatal> will replace the previously registered callback
1563 function with the new one.
1565 Say for example you want to interface to a library which allows asynchronous
1566 file i/o. In this case you may be able to register a callback whenever
1567 a read operation has completed. To be of any use we want to be able to
1568 call separate Perl subroutines for each file that is opened. As it
1569 stands, the error handler example above would not be adequate as it
1570 allows only a single callback to be defined at any time. What we
1571 require is a means of storing the mapping between the opened file and
1572 the Perl subroutine we want to be called for that file.
1574 Say the i/o library has a function C<asynch_read> which associates a C
1575 function C<ProcessRead> with a file handle C<fh> - this assumes that it
1576 has also provided some routine to open the file and so obtain the file
1579 asynch_read(fh, ProcessRead)
1581 This may expect the C I<ProcessRead> function of this form
1584 ProcessRead(fh, buffer)
1591 To provide a Perl interface to this library we need to be able to map
1592 between the C<fh> parameter and the Perl subroutine we want called. A
1593 hash is a convenient mechanism for storing this mapping. The code
1594 below shows a possible implementation
1596 static HV * Mapping = (HV*)NULL ;
1599 asynch_read(fh, callback)
1603 /* If the hash doesn't already exist, create it */
1604 if (Mapping == (HV*)NULL)
1607 /* Save the fh -> callback mapping */
1608 hv_store(Mapping, (char*)&fh, sizeof(fh), newSVsv(callback), 0) ;
1610 /* Register with the C Library */
1611 asynch_read(fh, asynch_read_if) ;
1613 and C<asynch_read_if> could look like this
1616 asynch_read_if(fh, buffer)
1623 /* Get the callback associated with fh */
1624 sv = hv_fetch(Mapping, (char*)&fh , sizeof(fh), FALSE) ;
1625 if (sv == (SV**)NULL)
1626 croak("Internal error...\n") ;
1629 XPUSHs(sv_2mortal(newSViv(fh))) ;
1630 XPUSHs(sv_2mortal(newSVpv(buffer, 0))) ;
1633 /* Call the Perl sub */
1634 perl_call_sv(*sv, G_DISCARD) ;
1637 For completeness, here is C<asynch_close>. This shows how to remove
1638 the entry from the hash C<Mapping>.
1644 /* Remove the entry from the hash */
1645 (void) hv_delete(Mapping, (char*)&fh, sizeof(fh), G_DISCARD) ;
1647 /* Now call the real asynch_close */
1650 So the Perl interface would look like this
1654 my($handle, $buffer) = @_ ;
1657 # Register the Perl callback
1658 asynch_read($fh, \&callback1) ;
1662 The mapping between the C callback and Perl is stored in the global
1663 hash C<Mapping> this time. Using a hash has the distinct advantage that
1664 it allows an unlimited number of callbacks to be registered.
1666 What if the interface provided by the C callback doesn't contain a
1667 parameter which allows the file handle to Perl subroutine mapping? Say
1668 in the asynchronous i/o package, the callback function gets passed only
1669 the C<buffer> parameter like this
1678 Without the file handle there is no straightforward way to map from the
1679 C callback to the Perl subroutine.
1681 In this case a possible way around this problem is to predefine a
1682 series of C functions to act as the interface to Perl, thus
1685 #define NULL_HANDLE -1
1686 typedef void (*FnMap)() ;
1698 static struct MapStruct Map [MAX_CB] =
1700 { fn1, NULL, NULL_HANDLE },
1701 { fn2, NULL, NULL_HANDLE },
1702 { fn3, NULL, NULL_HANDLE }
1713 XPUSHs(sv_2mortal(newSVpv(buffer, 0))) ;
1716 /* Call the Perl sub */
1717 perl_call_sv(Map[index].PerlSub, G_DISCARD) ;
1742 array_asynch_read(fh, callback)
1747 int null_index = MAX_CB ;
1749 /* Find the same handle or an empty entry */
1750 for (index = 0 ; index < MAX_CB ; ++index)
1752 if (Map[index].Handle == fh)
1755 if (Map[index].Handle == NULL_HANDLE)
1756 null_index = index ;
1759 if (index == MAX_CB && null_index == MAX_CB)
1760 croak ("Too many callback functions registered\n") ;
1762 if (index == MAX_CB)
1763 index = null_index ;
1765 /* Save the file handle */
1766 Map[index].Handle = fh ;
1768 /* Remember the Perl sub */
1769 if (Map[index].PerlSub == (SV*)NULL)
1770 Map[index].PerlSub = newSVsv(callback) ;
1772 SvSetSV(Map[index].PerlSub, callback) ;
1774 asynch_read(fh, Map[index].Function) ;
1777 array_asynch_close(fh)
1782 /* Find the file handle */
1783 for (index = 0; index < MAX_CB ; ++ index)
1784 if (Map[index].Handle == fh)
1787 if (index == MAX_CB)
1788 croak ("could not close fh %d\n", fh) ;
1790 Map[index].Handle = NULL_HANDLE ;
1791 SvREFCNT_dec(Map[index].PerlSub) ;
1792 Map[index].PerlSub = (SV*)NULL ;
1796 In this case the functions C<fn1>, C<fn2>, and C<fn3> are used to
1797 remember the Perl subroutine to be called. Each of the functions holds
1798 a separate hard-wired index which is used in the function C<Pcb> to
1799 access the C<Map> array and actually call the Perl subroutine.
1801 There are some obvious disadvantages with this technique.
1803 Firstly, the code is considerably more complex than with the previous
1806 Secondly, there is a hard-wired limit (in this case 3) to the number of
1807 callbacks that can exist simultaneously. The only way to increase the
1808 limit is by modifying the code to add more functions and then
1809 recompiling. None the less, as long as the number of functions is
1810 chosen with some care, it is still a workable solution and in some
1811 cases is the only one available.
1813 To summarize, here are a number of possible methods for you to consider
1814 for storing the mapping between C and the Perl callback
1818 =item 1. Ignore the problem - Allow only 1 callback
1820 For a lot of situations, like interfacing to an error handler, this may
1821 be a perfectly adequate solution.
1823 =item 2. Create a sequence of callbacks - hard wired limit
1825 If it is impossible to tell from the parameters passed back from the C
1826 callback what the context is, then you may need to create a sequence of C
1827 callback interface functions, and store pointers to each in an array.
1829 =item 3. Use a parameter to map to the Perl callback
1831 A hash is an ideal mechanism to store the mapping between C and Perl.
1836 =head2 Alternate Stack Manipulation
1839 Although I have made use of only the C<POP*> macros to access values
1840 returned from Perl subroutines, it is also possible to bypass these
1841 macros and read the stack using the C<ST> macro (See L<perlxs> for a
1842 full description of the C<ST> macro).
1844 Most of the time the C<POP*> macros should be adequate, the main
1845 problem with them is that they force you to process the returned values
1846 in sequence. This may not be the most suitable way to process the
1847 values in some cases. What we want is to be able to access the stack in
1848 a random order. The C<ST> macro as used when coding an XSUB is ideal
1851 The code below is the example given in the section I<Returning a list
1852 of values> recoded to use C<ST> instead of C<POP*>.
1855 call_AddSubtract2(a, b)
1867 XPUSHs(sv_2mortal(newSViv(a)));
1868 XPUSHs(sv_2mortal(newSViv(b)));
1871 count = perl_call_pv("AddSubtract", G_ARRAY);
1875 ax = (sp - stack_base) + 1 ;
1878 croak("Big trouble\n") ;
1880 printf ("%d + %d = %d\n", a, b, SvIV(ST(0))) ;
1881 printf ("%d - %d = %d\n", a, b, SvIV(ST(1))) ;
1894 Notice that it was necessary to define the variable C<ax>. This is
1895 because the C<ST> macro expects it to exist. If we were in an XSUB it
1896 would not be necessary to define C<ax> as it is already defined for
1905 ax = (sp - stack_base) + 1 ;
1907 sets the stack up so that we can use the C<ST> macro.
1911 Unlike the original coding of this example, the returned
1912 values are not accessed in reverse order. So C<ST(0)> refers to the
1913 first value returned by the Perl subroutine and C<ST(count-1)>
1918 =head2 Creating and calling an anonymous subroutine in C
1920 As we've already shown, L<perl_call_sv> can be used to invoke an
1921 anonymous subroutine. However, our example showed how Perl script
1922 invoking an XSUB to preform this operation. Let's see how it can be
1923 done inside our C code:
1927 SV *cvrv = perl_eval_pv("sub { print 'You will not find me cluttering any namespace!' }", TRUE);
1931 perl_call_sv(cvrv, G_VOID|G_NOARGS);
1933 L<perlguts/perl_eval_pv> is used to compile the anonymous subroutine, which
1934 will be the return value as well. Once this code reference is in hand, it
1935 can be mixed in with all the previous examples we've shown.
1939 L<perlxs>, L<perlguts>, L<perlembed>
1943 Paul Marquess <F<pmarquess@bfsec.bt.co.uk>>
1945 Special thanks to the following people who assisted in the creation of
1948 Jeff Okamoto, Tim Bunce, Nick Gianniotis, Steve Kelem, Gurusamy Sarathy
1953 Version 1.3, 14th Apr 1997