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 your 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 actually 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 scalar context. This is the default
132 context flag setting for all the I<perl_call_*> functions.
134 This flag has 2 effects
140 it indicates to the subroutine being called that it is executing in a
141 scalar context (if it executes I<wantarray> the result will be false).
146 it ensures that only a scalar is actually returned from the subroutine.
147 The subroutine can, of course, ignore the I<wantarray> and return a
148 list anyway. If so, then only the last element of the list will be
153 The value returned by the I<perl_call_*> function indicates how may
154 items have been returned by the Perl subroutine - in this case it will
157 If 0, then you have specified the G_DISCARD flag.
159 If 1, then the item actually returned by the Perl subroutine will be
160 stored on the Perl stack - the section I<Returning a Scalar> shows how
161 to access this value on the stack. Remember that regardless of how
162 many items the Perl subroutine returns, only the last one will be
163 accessible from the stack - think of the case where only one value is
164 returned as being a list with only one element. Any other items that
165 were returned will not exist by the time control returns from the
166 I<perl_call_*> function. The section I<Returning a list in a scalar
167 context> shows an example of this behaviour.
172 Calls the Perl subroutine in a list context.
174 As with G_SCALAR, this flag has 2 effects
180 it indicates to the subroutine being called that it is executing in an
181 array context (if it executes I<wantarray> the result will be true).
186 it ensures that all items returned from the subroutine will be
187 accessible when control returns from the I<perl_call_*> function.
191 The value returned by the I<perl_call_*> function indicates how may
192 items have been returned by the Perl subroutine.
194 If 0, the you have specified the G_DISCARD flag.
196 If not 0, then it will be a count of the number of items returned by
197 the subroutine. These items will be stored on the Perl stack. The
198 section I<Returning a list of values> gives an example of using the
199 G_ARRAY flag and the mechanics of accessing the returned items from the
204 By default, the I<perl_call_*> functions place the items returned from
205 by the Perl subroutine on the stack. If you are not interested in
206 these items, then setting this flag will make Perl get rid of them
207 automatically for you. Note that it is still possible to indicate a
208 context to the Perl subroutine by using either G_SCALAR or G_ARRAY.
210 If you do not set this flag then it is I<very> important that you make
211 sure that any temporaries (i.e. parameters passed to the Perl
212 subroutine and values returned from the subroutine) are disposed of
213 yourself. The section I<Returning a Scalar> gives details of how to
214 explicitly dispose of these temporaries and the section I<Using Perl to
215 dispose of temporaries> discusses the specific circumstances where you
216 can ignore the problem and let Perl deal with it for you.
220 Whenever a Perl subroutine is called using one of the I<perl_call_*>
221 functions, it is assumed by default that parameters are to be passed to
222 the subroutine. If you are not passing any parameters to the Perl
223 subroutine, you can save a bit of time by setting this flag. It has
224 the effect of not creating the C<@_> array for the Perl subroutine.
226 Although the functionality provided by this flag may seem
227 straightforward, it should be used only if there is a good reason to do
228 so. The reason for being cautious is that even if you have specified
229 the G_NOARGS flag, it is still possible for the Perl subroutine that
230 has been called to think that you have passed it parameters.
232 In fact, what can happen is that the Perl subroutine you have called
233 can access the C<@_> array from a previous Perl subroutine. This will
234 occur when the code that is executing the I<perl_call_*> function has
235 itself been called from another Perl subroutine. The code below
250 What has happened is that C<fred> accesses the C<@_> array which
256 It is possible for the Perl subroutine you are calling to terminate
257 abnormally, e.g. by calling I<die> explicitly or by not actually
258 existing. By default, when either of these of events occurs, the
259 process will terminate immediately. If though, you want to trap this
260 type of event, specify the G_EVAL flag. It will put an I<eval { }>
261 around the subroutine call.
263 Whenever control returns from the I<perl_call_*> function you need to
264 check the C<$@> variable as you would in a normal Perl script.
266 The value returned from the I<perl_call_*> function is dependent on
267 what other flags have been specified and whether an error has
268 occurred. Here are all the different cases that can occur
274 If the I<perl_call_*> function returns normally, then the value
275 returned is as specified in the previous sections.
279 If G_DISCARD is specified, the return value will always be 0.
283 If G_ARRAY is specified I<and> an error has occurred, the return value
288 If G_SCALAR is specified I<and> an error has occurred, the return value
289 will be 1 and the value on the top of the stack will be I<undef>. This
290 means that if you have already detected the error by checking C<$@> and
291 you want the program to continue, you must remember to pop the I<undef>
296 See I<Using G_EVAL> for details of using G_EVAL.
300 You may have noticed that using the G_EVAL flag described above will
301 B<always> clear the C<$@> variable and set it to a string describing
302 the error iff there was an error in the called code. This unqualified
303 resetting of C<$@> can be problematic in the reliable identification of
304 errors using the C<eval {}> mechanism, because the possibility exists
305 that perl will call other code (end of block processing code, for
306 example) between the time the error causes C<$@> to be set within
307 C<eval {}>, and the subsequent statement which checks for the value of
308 C<$@> gets executed in the user's script.
310 This scenario will mostly be applicable to code that is meant to be
311 called from within destructors, asynchronous callbacks, signal
312 handlers, C<__DIE__> or C<__WARN__> hooks, and C<tie> functions. In
313 such situations, you will not want to clear C<$@> at all, but simply to
314 append any new errors to any existing value of C<$@>.
316 The G_KEEPERR flag is meant to be used in conjunction with G_EVAL in
317 I<perl_call_*> functions that are used to implement such code. This flag
318 has no effect when G_EVAL is not used.
320 When G_KEEPERR is used, any errors in the called code will be prefixed
321 with the string "\t(in cleanup)", and appended to the current value
324 The G_KEEPERR flag was introduced in Perl version 5.002.
326 See I<Using G_KEEPERR> for an example of a situation that warrants the
329 =head2 Determining the Context
331 As mentioned above, you can determine the context of the currently
332 executing subroutine in Perl with I<wantarray>. The equivalent test can
333 be made in C by using the C<GIMME> macro. This will return C<G_SCALAR>
334 if you have been called in a scalar context and C<G_ARRAY> if in an
335 array context. An example of using the C<GIMME> macro is shown in
336 section I<Using GIMME>.
338 =head1 KNOWN PROBLEMS
340 This section outlines all known problems that exist in the
341 I<perl_call_*> functions.
347 If you are intending to make use of both the G_EVAL and G_SCALAR flags
348 in your code, use a version of Perl greater than 5.000. There is a bug
349 in version 5.000 of Perl which means that the combination of these two
350 flags will not work as described in the section I<FLAG VALUES>.
352 Specifically, if the two flags are used when calling a subroutine and
353 that subroutine does not call I<die>, the value returned by
354 I<perl_call_*> will be wrong.
359 In Perl 5.000 and 5.001 there is a problem with using I<perl_call_*> if
360 the Perl sub you are calling attempts to trap a I<die>.
362 The symptom of this problem is that the called Perl sub will continue
363 to completion, but whenever it attempts to pass control back to the
364 XSUB, the program will immediately terminate.
366 For example, say you want to call this Perl sub
370 eval { die "Fatal Error" ; }
371 print "Trapped error: $@\n"
381 perl_call_pv("fred", G_DISCARD|G_NOARGS) ;
382 fprintf(stderr, "back in Call_fred\n") ;
384 When C<Call_fred> is executed it will print
386 Trapped error: Fatal Error
388 As control never returns to C<Call_fred>, the C<"back in Call_fred">
389 string will not get printed.
391 To work around this problem, you can either upgrade to Perl 5.002 (or
392 later), or use the G_EVAL flag with I<perl_call_*> as shown below
398 perl_call_pv("fred", G_EVAL|G_DISCARD|G_NOARGS) ;
399 fprintf(stderr, "back in Call_fred\n") ;
407 Enough of the definition talk, let's have a few examples.
409 Perl provides many macros to assist in accessing the Perl stack.
410 Wherever possible, these macros should always be used when interfacing
411 to Perl internals. Hopefully this should make the code less vulnerable
412 to any changes made to Perl in the future.
414 Another point worth noting is that in the first series of examples I
415 have made use of only the I<perl_call_pv> function. This has been done
416 to keep the code simpler and ease you into the topic. Wherever
417 possible, if the choice is between using I<perl_call_pv> and
418 I<perl_call_sv>, you should always try to use I<perl_call_sv>. See
419 I<Using perl_call_sv> for details.
421 =head2 No Parameters, Nothing returned
423 This first trivial example will call a Perl subroutine, I<PrintUID>, to
424 print out the UID of the process.
428 print "UID is $<\n" ;
431 and here is a C function to call it
439 perl_call_pv("PrintUID", G_DISCARD|G_NOARGS) ;
444 A few points to note about this example.
450 Ignore C<dSP> and C<PUSHMARK(sp)> for now. They will be discussed in
455 We aren't passing any parameters to I<PrintUID> so G_NOARGS can be
460 We aren't interested in anything returned from I<PrintUID>, so
461 G_DISCARD is specified. Even if I<PrintUID> was changed to actually
462 return some value(s), having specified G_DISCARD will mean that they
463 will be wiped by the time control returns from I<perl_call_pv>.
467 As I<perl_call_pv> is being used, the Perl subroutine is specified as a
468 C string. In this case the subroutine name has been 'hard-wired' into the
473 Because we specified G_DISCARD, it is not necessary to check the value
474 returned from I<perl_call_pv>. It will always be 0.
478 =head2 Passing Parameters
480 Now let's make a slightly more complex example. This time we want to
481 call a Perl subroutine, C<LeftString>, which will take 2 parameters - a
482 string (C<$s>) and an integer (C<$n>). The subroutine will simply
483 print the first C<$n> characters of the string.
485 So the Perl subroutine would look like this
490 print substr($s, 0, $n), "\n" ;
493 The C function required to call I<LeftString> would look like this.
496 call_LeftString(a, b)
503 XPUSHs(sv_2mortal(newSVpv(a, 0)));
504 XPUSHs(sv_2mortal(newSViv(b)));
507 perl_call_pv("LeftString", G_DISCARD);
510 Here are a few notes on the C function I<call_LeftString>.
516 Parameters are passed to the Perl subroutine using the Perl stack.
517 This is the purpose of the code beginning with the line C<dSP> and
518 ending with the line C<PUTBACK>.
523 If you are going to put something onto the Perl stack, you need to know
524 where to put it. This is the purpose of the macro C<dSP> - it declares
525 and initializes a I<local> copy of the Perl stack pointer.
527 All the other macros which will be used in this example require you to
528 have used this macro.
530 The exception to this rule is if you are calling a Perl subroutine
531 directly from an XSUB function. In this case it is not necessary to
532 explicitly use the C<dSP> macro - it will be declared for you
537 Any parameters to be pushed onto the stack should be bracketed by the
538 C<PUSHMARK> and C<PUTBACK> macros. The purpose of these two macros, in
539 this context, is to automatically count the number of parameters you
540 are pushing. Then whenever Perl is creating the C<@_> array for the
541 subroutine, it knows how big to make it.
543 The C<PUSHMARK> macro tells Perl to make a mental note of the current
544 stack pointer. Even if you aren't passing any parameters (like the
545 example shown in the section I<No Parameters, Nothing returned>) you
546 must still call the C<PUSHMARK> macro before you can call any of the
547 I<perl_call_*> functions - Perl still needs to know that there are no
550 The C<PUTBACK> macro sets the global copy of the stack pointer to be
551 the same as our local copy. If we didn't do this I<perl_call_pv>
552 wouldn't know where the two parameters we pushed were - remember that
553 up to now all the stack pointer manipulation we have done is with our
554 local copy, I<not> the global copy.
558 The only flag specified this time is G_DISCARD. Since we are passing 2
559 parameters to the Perl subroutine this time, we have not specified
564 Next, we come to XPUSHs. This is where the parameters actually get
565 pushed onto the stack. In this case we are pushing a string and an
568 See the section L<perlguts/"XSUB'S and the Argument Stack"> for details
569 on how the XPUSH macros work.
573 Finally, I<LeftString> can now be called via the I<perl_call_pv>
578 =head2 Returning a Scalar
580 Now for an example of dealing with the items returned from a Perl
583 Here is a Perl subroutine, I<Adder>, which takes 2 integer parameters
584 and simply returns their sum.
592 Since we are now concerned with the return value from I<Adder>, the C
593 function required to call it is now a bit more complex.
607 XPUSHs(sv_2mortal(newSViv(a)));
608 XPUSHs(sv_2mortal(newSViv(b)));
611 count = perl_call_pv("Adder", G_SCALAR);
616 croak("Big trouble\n") ;
618 printf ("The sum of %d and %d is %d\n", a, b, POPi) ;
625 Points to note this time are
631 The only flag specified this time was G_SCALAR. That means the C<@_>
632 array will be created and that the value returned by I<Adder> will
633 still exist after the call to I<perl_call_pv>.
639 Because we are interested in what is returned from I<Adder> we cannot
640 specify G_DISCARD. This means that we will have to tidy up the Perl
641 stack and dispose of any temporary values ourselves. This is the
647 at the start of the function, and
652 at the end. The C<ENTER>/C<SAVETMPS> pair creates a boundary for any
653 temporaries we create. This means that the temporaries we get rid of
654 will be limited to those which were created after these calls.
656 The C<FREETMPS>/C<LEAVE> pair will get rid of any values returned by
657 the Perl subroutine, plus it will also dump the mortal SV's we have
658 created. Having C<ENTER>/C<SAVETMPS> at the beginning of the code
659 makes sure that no other mortals are destroyed.
661 Think of these macros as working a bit like using C<{> and C<}> in Perl
662 to limit the scope of local variables.
664 See the section I<Using Perl to dispose of temporaries> for details of
665 an alternative to using these macros.
669 The purpose of the macro C<SPAGAIN> is to refresh the local copy of the
670 stack pointer. This is necessary because it is possible that the memory
671 allocated to the Perl stack has been re-allocated whilst in the
672 I<perl_call_pv> call.
674 If you are making use of the Perl stack pointer in your code you must
675 always refresh the your local copy using SPAGAIN whenever you make use
676 of the I<perl_call_*> functions or any other Perl internal function.
680 Although only a single value was expected to be returned from I<Adder>,
681 it is still good practice to check the return code from I<perl_call_pv>
684 Expecting a single value is not quite the same as knowing that there
685 will be one. If someone modified I<Adder> to return a list and we
686 didn't check for that possibility and take appropriate action the Perl
687 stack would end up in an inconsistent state. That is something you
688 I<really> don't want to ever happen.
692 The C<POPi> macro is used here to pop the return value from the stack.
693 In this case we wanted an integer, so C<POPi> was used.
696 Here is the complete list of POP macros available, along with the types
707 The final C<PUTBACK> is used to leave the Perl stack in a consistent
708 state before exiting the function. This is necessary because when we
709 popped the return value from the stack with C<POPi> it updated only our
710 local copy of the stack pointer. Remember, C<PUTBACK> sets the global
711 stack pointer to be the same as our local copy.
716 =head2 Returning a list of values
718 Now, let's extend the previous example to return both the sum of the
719 parameters and the difference.
721 Here is the Perl subroutine
729 and this is the C function
732 call_AddSubtract(a, b)
743 XPUSHs(sv_2mortal(newSViv(a)));
744 XPUSHs(sv_2mortal(newSViv(b)));
747 count = perl_call_pv("AddSubtract", G_ARRAY);
752 croak("Big trouble\n") ;
754 printf ("%d - %d = %d\n", a, b, POPi) ;
755 printf ("%d + %d = %d\n", a, b, POPi) ;
762 If I<call_AddSubtract> is called like this
764 call_AddSubtract(7, 4) ;
766 then here is the output
777 We wanted array context, so G_ARRAY was used.
781 Not surprisingly C<POPi> is used twice this time because we were
782 retrieving 2 values from the stack. The important thing to note is that
783 when using the C<POP*> macros they come off the stack in I<reverse>
788 =head2 Returning a list in a scalar context
790 Say the Perl subroutine in the previous section was called in a scalar
794 call_AddSubScalar(a, b)
806 XPUSHs(sv_2mortal(newSViv(a)));
807 XPUSHs(sv_2mortal(newSViv(b)));
810 count = perl_call_pv("AddSubtract", G_SCALAR);
814 printf ("Items Returned = %d\n", count) ;
816 for (i = 1 ; i <= count ; ++i)
817 printf ("Value %d = %d\n", i, POPi) ;
824 The other modification made is that I<call_AddSubScalar> will print the
825 number of items returned from the Perl subroutine and their value (for
826 simplicity it assumes that they are integer). So if
827 I<call_AddSubScalar> is called
829 call_AddSubScalar(7, 4) ;
831 then the output will be
836 In this case the main point to note is that only the last item in the
837 list returned from the subroutine, I<Adder> actually made it back to
838 I<call_AddSubScalar>.
841 =head2 Returning Data from Perl via the parameter list
843 It is also possible to return values directly via the parameter list -
844 whether it is actually desirable to do it is another matter entirely.
846 The Perl subroutine, I<Inc>, below takes 2 parameters and increments
855 and here is a C function to call it.
870 sva = sv_2mortal(newSViv(a)) ;
871 svb = sv_2mortal(newSViv(b)) ;
878 count = perl_call_pv("Inc", G_DISCARD);
881 croak ("call_Inc: expected 0 values from 'Inc', got %d\n",
884 printf ("%d + 1 = %d\n", a, SvIV(sva)) ;
885 printf ("%d + 1 = %d\n", b, SvIV(svb)) ;
891 To be able to access the two parameters that were pushed onto the stack
892 after they return from I<perl_call_pv> it is necessary to make a note
893 of their addresses - thus the two variables C<sva> and C<svb>.
895 The reason this is necessary is that the area of the Perl stack which
896 held them will very likely have been overwritten by something else by
897 the time control returns from I<perl_call_pv>.
904 Now an example using G_EVAL. Below is a Perl subroutine which computes
905 the difference of its 2 parameters. If this would result in a negative
906 result, the subroutine calls I<die>.
912 die "death can be fatal\n" if $a < $b ;
917 and some C to call it
931 XPUSHs(sv_2mortal(newSViv(a)));
932 XPUSHs(sv_2mortal(newSViv(b)));
935 count = perl_call_pv("Subtract", G_EVAL|G_SCALAR);
939 /* Check the eval first */
940 if (SvTRUE(GvSV(errgv)))
942 printf ("Uh oh - %s\n", SvPV(GvSV(errgv), na)) ;
948 croak("call_Subtract: wanted 1 value from 'Subtract', got %d\n",
951 printf ("%d - %d = %d\n", a, b, POPi) ;
959 If I<call_Subtract> is called thus
963 the following will be printed
965 Uh oh - death can be fatal
973 We want to be able to catch the I<die> so we have used the G_EVAL
974 flag. Not specifying this flag would mean that the program would
975 terminate immediately at the I<die> statement in the subroutine
982 if (SvTRUE(GvSV(errgv)))
984 printf ("Uh oh - %s\n", SvPV(GvSV(errgv), na)) ;
988 is the direct equivalent of this bit of Perl
990 print "Uh oh - $@\n" if $@ ;
992 C<errgv> is a perl global of type C<GV *> that points to the
993 symbol table entry containing the error. C<GvSV(errgv)> therefore
994 refers to the C equivalent of C<$@>.
998 Note that the stack is popped using C<POPs> in the block where
999 C<SvTRUE(GvSV(errgv))> is true. This is necessary because whenever a
1000 I<perl_call_*> function invoked with G_EVAL|G_SCALAR returns an error,
1001 the top of the stack holds the value I<undef>. Since we want the
1002 program to continue after detecting this error, it is essential that
1003 the stack is tidied up by removing the I<undef>.
1008 =head2 Using G_KEEPERR
1010 Consider this rather facetious example, where we have used an XS
1011 version of the call_Subtract example above inside a destructor:
1014 sub new { bless {}, $_[0] }
1017 die "death can be fatal" if $a < $b ;
1020 sub DESTROY { call_Subtract(5, 4); }
1021 sub foo { die "foo dies"; }
1024 eval { Foo->new->foo };
1025 print "Saw: $@" if $@; # should be, but isn't
1027 This example will fail to recognize that an error occurred inside the
1028 C<eval {}>. Here's why: the call_Subtract code got executed while perl
1029 was cleaning up temporaries when exiting the eval block, and since
1030 call_Subtract is implemented with I<perl_call_pv> using the G_EVAL
1031 flag, it promptly reset C<$@>. This results in the failure of the
1032 outermost test for C<$@>, and thereby the failure of the error trap.
1034 Appending the G_KEEPERR flag, so that the I<perl_call_pv> call in
1035 call_Subtract reads:
1037 count = perl_call_pv("Subtract", G_EVAL|G_SCALAR|G_KEEPERR);
1039 will preserve the error and restore reliable error handling.
1041 =head2 Using perl_call_sv
1043 In all the previous examples I have 'hard-wired' the name of the Perl
1044 subroutine to be called from C. Most of the time though, it is more
1045 convenient to be able to specify the name of the Perl subroutine from
1046 within the Perl script.
1048 Consider the Perl code below
1052 print "Hello there\n" ;
1057 Here is a snippet of XSUB which defines I<CallSubPV>.
1064 perl_call_pv(name, G_DISCARD|G_NOARGS) ;
1066 That is fine as far as it goes. The thing is, the Perl subroutine
1067 can be specified only as a string. For Perl 4 this was adequate,
1068 but Perl 5 allows references to subroutines and anonymous subroutines.
1069 This is where I<perl_call_sv> is useful.
1071 The code below for I<CallSubSV> is identical to I<CallSubPV> except
1072 that the C<name> parameter is now defined as an SV* and we use
1073 I<perl_call_sv> instead of I<perl_call_pv>.
1080 perl_call_sv(name, G_DISCARD|G_NOARGS) ;
1082 Since we are using an SV to call I<fred> the following can all be used
1088 CallSubSV( sub { print "Hello there\n" } ) ;
1090 As you can see, I<perl_call_sv> gives you much greater flexibility in
1091 how you can specify the Perl subroutine.
1093 You should note that if it is necessary to store the SV (C<name> in the
1094 example above) which corresponds to the Perl subroutine so that it can
1095 be used later in the program, it not enough to just store a copy of the
1096 pointer to the SV. Say the code above had been like this
1098 static SV * rememberSub ;
1104 rememberSub = name ;
1110 perl_call_sv(rememberSub, G_DISCARD|G_NOARGS) ;
1112 The reason this is wrong is that by the time you come to use the
1113 pointer C<rememberSub> in C<CallSavedSub1>, it may or may not still refer
1114 to the Perl subroutine that was recorded in C<SaveSub1>. This is
1115 particularly true for these cases
1120 SaveSub1( sub { print "Hello there\n" } ) ;
1123 By the time each of the C<SaveSub1> statements above have been executed,
1124 the SV*'s which corresponded to the parameters will no longer exist.
1125 Expect an error message from Perl of the form
1127 Can't use an undefined value as a subroutine reference at ...
1129 for each of the C<CallSavedSub1> lines.
1131 Similarly, with this code
1138 you can expect one of these messages (which you actually get is dependant on
1139 the version of Perl you are using)
1141 Not a CODE reference at ...
1142 Undefined subroutine &main::47 called ...
1144 The variable C<$ref> may have referred to the subroutine C<fred>
1145 whenever the call to C<SaveSub1> was made but by the time
1146 C<CallSavedSub1> gets called it now holds the number C<47>. Since we
1147 saved only a pointer to the original SV in C<SaveSub1>, any changes to
1148 C<$ref> will be tracked by the pointer C<rememberSub>. This means that
1149 whenever C<CallSavedSub1> gets called, it will attempt to execute the
1150 code which is referenced by the SV* C<rememberSub>. In this case
1151 though, it now refers to the integer C<47>, so expect Perl to complain
1154 A similar but more subtle problem is illustrated with this code
1161 This time whenever C<CallSavedSub1> get called it will execute the Perl
1162 subroutine C<joe> (assuming it exists) rather than C<fred> as was
1163 originally requested in the call to C<SaveSub1>.
1165 To get around these problems it is necessary to take a full copy of the
1166 SV. The code below shows C<SaveSub2> modified to do that
1168 static SV * keepSub = (SV*)NULL ;
1174 /* Take a copy of the callback */
1175 if (keepSub == (SV*)NULL)
1176 /* First time, so create a new SV */
1177 keepSub = newSVsv(name) ;
1179 /* Been here before, so overwrite */
1180 SvSetSV(keepSub, name) ;
1186 perl_call_sv(keepSub, G_DISCARD|G_NOARGS) ;
1188 In order to avoid creating a new SV every time C<SaveSub2> is called,
1189 the function first checks to see if it has been called before. If not,
1190 then space for a new SV is allocated and the reference to the Perl
1191 subroutine, C<name> is copied to the variable C<keepSub> in one
1192 operation using C<newSVsv>. Thereafter, whenever C<SaveSub2> is called
1193 the existing SV, C<keepSub>, is overwritten with the new value using
1196 =head2 Using perl_call_argv
1198 Here is a Perl subroutine which prints whatever parameters are passed
1205 foreach (@list) { print "$_\n" }
1208 and here is an example of I<perl_call_argv> which will call
1211 static char * words[] = {"alpha", "beta", "gamma", "delta", NULL} ;
1218 perl_call_argv("PrintList", G_DISCARD, words) ;
1221 Note that it is not necessary to call C<PUSHMARK> in this instance.
1222 This is because I<perl_call_argv> will do it for you.
1224 =head2 Using perl_call_method
1226 Consider the following Perl code
1239 my ($self, $index) = @_ ;
1240 print "$index: $$self[$index]\n" ;
1246 print "This is Class $class version 1.0\n" ;
1250 It just implements a very simple class to manage an array. Apart from
1251 the constructor, C<new>, it declares methods, one static and one
1252 virtual. The static method, C<PrintID>, simply prints out the class
1253 name and a version number. The virtual method, C<Display>, prints out a
1254 single element of the array. Here is an all Perl example of using it.
1256 $a = new Mine ('red', 'green', 'blue') ;
1263 This is Class Mine version 1.0
1265 Calling a Perl method from C is fairly straightforward. The following
1272 a reference to the object for a virtual method or the name of the class
1273 for a static method.
1277 the name of the method.
1281 any other parameters specific to the method.
1285 Here is a simple XSUB which illustrates the mechanics of calling both
1286 the C<PrintID> and C<Display> methods from C.
1289 call_Method(ref, method, index)
1296 XPUSHs(sv_2mortal(newSViv(index))) ;
1299 perl_call_method(method, G_DISCARD) ;
1302 call_PrintID(class, method)
1307 XPUSHs(sv_2mortal(newSVpv(class, 0))) ;
1310 perl_call_method(method, G_DISCARD) ;
1313 So the methods C<PrintID> and C<Display> can be invoked like this
1315 $a = new Mine ('red', 'green', 'blue') ;
1316 call_Method($a, 'Display', 1) ;
1317 call_PrintID('Mine', 'PrintID') ;
1319 The only thing to note is that in both the static and virtual methods,
1320 the method name is not passed via the stack - it is used as the first
1321 parameter to I<perl_call_method>.
1325 Here is a trivial XSUB which prints the context in which it is
1326 currently executing.
1331 if (GIMME == G_SCALAR)
1332 printf ("Context is Scalar\n") ;
1334 printf ("Context is Array\n") ;
1336 and here is some Perl to test it
1341 The output from that will be
1346 =head2 Using Perl to dispose of temporaries
1348 In the examples given to date, any temporaries created in the callback
1349 (i.e. parameters passed on the stack to the I<perl_call_*> function or
1350 values returned via the stack) have been freed by one of these methods
1356 specifying the G_DISCARD flag with I<perl_call_*>.
1360 explicitly disposed of using the C<ENTER>/C<SAVETMPS> -
1361 C<FREETMPS>/C<LEAVE> pairing.
1365 There is another method which can be used, namely letting Perl do it
1366 for you automatically whenever it regains control after the callback
1367 has terminated. This is done by simply not using the
1375 sequence in the callback (and not, of course, specifying the G_DISCARD
1378 If you are going to use this method you have to be aware of a possible
1379 memory leak which can arise under very specific circumstances. To
1380 explain these circumstances you need to know a bit about the flow of
1381 control between Perl and the callback routine.
1383 The examples given at the start of the document (an error handler and
1384 an event driven program) are typical of the two main sorts of flow
1385 control that you are likely to encounter with callbacks. There is a
1386 very important distinction between them, so pay attention.
1388 In the first example, an error handler, the flow of control could be as
1389 follows. You have created an interface to an external library.
1390 Control can reach the external library like this
1392 perl --> XSUB --> external library
1394 Whilst control is in the library, an error condition occurs. You have
1395 previously set up a Perl callback to handle this situation, so it will
1396 get executed. Once the callback has finished, control will drop back to
1397 Perl again. Here is what the flow of control will be like in that
1400 perl --> XSUB --> external library
1404 external library --> perl_call --> perl
1406 perl <-- XSUB <-- external library <-- perl_call <----+
1408 After processing of the error using I<perl_call_*> is completed,
1409 control reverts back to Perl more or less immediately.
1411 In the diagram, the further right you go the more deeply nested the
1412 scope is. It is only when control is back with perl on the extreme
1413 left of the diagram that you will have dropped back to the enclosing
1414 scope and any temporaries you have left hanging around will be freed.
1416 In the second example, an event driven program, the flow of control
1417 will be more like this
1419 perl --> XSUB --> event handler
1421 event handler --> perl_call --> perl
1423 event handler <-- perl_call --<--+
1425 event handler --> perl_call --> perl
1427 event handler <-- perl_call --<--+
1429 event handler --> perl_call --> perl
1431 event handler <-- perl_call --<--+
1433 In this case the flow of control can consist of only the repeated
1436 event handler --> perl_call --> perl
1438 for the practically the complete duration of the program. This means
1439 that control may I<never> drop back to the surrounding scope in Perl at
1442 So what is the big problem? Well, if you are expecting Perl to tidy up
1443 those temporaries for you, you might be in for a long wait. For Perl
1444 to actually dispose of your temporaries, control must drop back to the
1445 enclosing scope at some stage. In the event driven scenario that may
1446 never happen. This means that as time goes on, your program will
1447 create more and more temporaries, none of which will ever be freed. As
1448 each of these temporaries consumes some memory your program will
1449 eventually consume all the available memory in your system - kapow!
1451 So here is the bottom line - if you are sure that control will revert
1452 back to the enclosing Perl scope fairly quickly after the end of your
1453 callback, then it isn't absolutely necessary to explicitly dispose of
1454 any temporaries you may have created. Mind you, if you are at all
1455 uncertain about what to do, it doesn't do any harm to tidy up anyway.
1458 =head2 Strategies for storing Callback Context Information
1461 Potentially one of the trickiest problems to overcome when designing a
1462 callback interface can be figuring out how to store the mapping between
1463 the C callback function and the Perl equivalent.
1465 To help understand why this can be a real problem first consider how a
1466 callback is set up in an all C environment. Typically a C API will
1467 provide a function to register a callback. This will expect a pointer
1468 to a function as one of its parameters. Below is a call to a
1469 hypothetical function C<register_fatal> which registers the C function
1470 to get called when a fatal error occurs.
1472 register_fatal(cb1) ;
1474 The single parameter C<cb1> is a pointer to a function, so you must
1475 have defined C<cb1> in your code, say something like this
1480 printf ("Fatal Error\n") ;
1484 Now change that to call a Perl subroutine instead
1486 static SV * callback = (SV*)NULL;
1495 /* Call the Perl sub to process the callback */
1496 perl_call_sv(callback, G_DISCARD) ;
1504 /* Remember the Perl sub */
1505 if (callback == (SV*)NULL)
1506 callback = newSVsv(fn) ;
1508 SvSetSV(callback, fn) ;
1510 /* register the callback with the external library */
1511 register_fatal(cb1) ;
1513 where the Perl equivalent of C<register_fatal> and the callback it
1514 registers, C<pcb1>, might look like this
1516 # Register the sub pcb1
1517 register_fatal(\&pcb1) ;
1521 die "I'm dying...\n" ;
1524 The mapping between the C callback and the Perl equivalent is stored in
1525 the global variable C<callback>.
1527 This will be adequate if you ever need to have only 1 callback
1528 registered at any time. An example could be an error handler like the
1529 code sketched out above. Remember though, repeated calls to
1530 C<register_fatal> will replace the previously registered callback
1531 function with the new one.
1533 Say for example you want to interface to a library which allows asynchronous
1534 file i/o. In this case you may be able to register a callback whenever
1535 a read operation has completed. To be of any use we want to be able to
1536 call separate Perl subroutines for each file that is opened. As it
1537 stands, the error handler example above would not be adequate as it
1538 allows only a single callback to be defined at any time. What we
1539 require is a means of storing the mapping between the opened file and
1540 the Perl subroutine we want to be called for that file.
1542 Say the i/o library has a function C<asynch_read> which associates a C
1543 function C<ProcessRead> with a file handle C<fh> - this assumes that it
1544 has also provided some routine to open the file and so obtain the file
1547 asynch_read(fh, ProcessRead)
1549 This may expect the C I<ProcessRead> function of this form
1552 ProcessRead(fh, buffer)
1559 To provide a Perl interface to this library we need to be able to map
1560 between the C<fh> parameter and the Perl subroutine we want called. A
1561 hash is a convenient mechanism for storing this mapping. The code
1562 below shows a possible implementation
1564 static HV * Mapping = (HV*)NULL ;
1567 asynch_read(fh, callback)
1571 /* If the hash doesn't already exist, create it */
1572 if (Mapping == (HV*)NULL)
1575 /* Save the fh -> callback mapping */
1576 hv_store(Mapping, (char*)&fh, sizeof(fh), newSVsv(callback), 0) ;
1578 /* Register with the C Library */
1579 asynch_read(fh, asynch_read_if) ;
1581 and C<asynch_read_if> could look like this
1584 asynch_read_if(fh, buffer)
1591 /* Get the callback associated with fh */
1592 sv = hv_fetch(Mapping, (char*)&fh , sizeof(fh), FALSE) ;
1593 if (sv == (SV**)NULL)
1594 croak("Internal error...\n") ;
1597 XPUSHs(sv_2mortal(newSViv(fh))) ;
1598 XPUSHs(sv_2mortal(newSVpv(buffer, 0))) ;
1601 /* Call the Perl sub */
1602 perl_call_sv(*sv, G_DISCARD) ;
1605 For completeness, here is C<asynch_close>. This shows how to remove
1606 the entry from the hash C<Mapping>.
1612 /* Remove the entry from the hash */
1613 (void) hv_delete(Mapping, (char*)&fh, sizeof(fh), G_DISCARD) ;
1615 /* Now call the real asynch_close */
1618 So the Perl interface would look like this
1622 my($handle, $buffer) = @_ ;
1625 # Register the Perl callback
1626 asynch_read($fh, \&callback1) ;
1630 The mapping between the C callback and Perl is stored in the global
1631 hash C<Mapping> this time. Using a hash has the distinct advantage that
1632 it allows an unlimited number of callbacks to be registered.
1634 What if the interface provided by the C callback doesn't contain a
1635 parameter which allows the file handle to Perl subroutine mapping? Say
1636 in the asynchronous i/o package, the callback function gets passed only
1637 the C<buffer> parameter like this
1646 Without the file handle there is no straightforward way to map from the
1647 C callback to the Perl subroutine.
1649 In this case a possible way around this problem is to pre-define a
1650 series of C functions to act as the interface to Perl, thus
1653 #define NULL_HANDLE -1
1654 typedef void (*FnMap)() ;
1666 static struct MapStruct Map [MAX_CB] =
1668 { fn1, NULL, NULL_HANDLE },
1669 { fn2, NULL, NULL_HANDLE },
1670 { fn3, NULL, NULL_HANDLE }
1681 XPUSHs(sv_2mortal(newSVpv(buffer, 0))) ;
1684 /* Call the Perl sub */
1685 perl_call_sv(Map[index].PerlSub, G_DISCARD) ;
1710 array_asynch_read(fh, callback)
1715 int null_index = MAX_CB ;
1717 /* Find the same handle or an empty entry */
1718 for (index = 0 ; index < MAX_CB ; ++index)
1720 if (Map[index].Handle == fh)
1723 if (Map[index].Handle == NULL_HANDLE)
1724 null_index = index ;
1727 if (index == MAX_CB && null_index == MAX_CB)
1728 croak ("Too many callback functions registered\n") ;
1730 if (index == MAX_CB)
1731 index = null_index ;
1733 /* Save the file handle */
1734 Map[index].Handle = fh ;
1736 /* Remember the Perl sub */
1737 if (Map[index].PerlSub == (SV*)NULL)
1738 Map[index].PerlSub = newSVsv(callback) ;
1740 SvSetSV(Map[index].PerlSub, callback) ;
1742 asynch_read(fh, Map[index].Function) ;
1745 array_asynch_close(fh)
1750 /* Find the file handle */
1751 for (index = 0; index < MAX_CB ; ++ index)
1752 if (Map[index].Handle == fh)
1755 if (index == MAX_CB)
1756 croak ("could not close fh %d\n", fh) ;
1758 Map[index].Handle = NULL_HANDLE ;
1759 SvREFCNT_dec(Map[index].PerlSub) ;
1760 Map[index].PerlSub = (SV*)NULL ;
1764 In this case the functions C<fn1>, C<fn2> and C<fn3> are used to
1765 remember the Perl subroutine to be called. Each of the functions holds
1766 a separate hard-wired index which is used in the function C<Pcb> to
1767 access the C<Map> array and actually call the Perl subroutine.
1769 There are some obvious disadvantages with this technique.
1771 Firstly, the code is considerably more complex than with the previous
1774 Secondly, there is a hard-wired limit (in this case 3) to the number of
1775 callbacks that can exist simultaneously. The only way to increase the
1776 limit is by modifying the code to add more functions and then
1777 re-compiling. None the less, as long as the number of functions is
1778 chosen with some care, it is still a workable solution and in some
1779 cases is the only one available.
1781 To summarize, here are a number of possible methods for you to consider
1782 for storing the mapping between C and the Perl callback
1786 =item 1. Ignore the problem - Allow only 1 callback
1788 For a lot of situations, like interfacing to an error handler, this may
1789 be a perfectly adequate solution.
1791 =item 2. Create a sequence of callbacks - hard wired limit
1793 If it is impossible to tell from the parameters passed back from the C
1794 callback what the context is, then you may need to create a sequence of C
1795 callback interface functions, and store pointers to each in an array.
1797 =item 3. Use a parameter to map to the Perl callback
1799 A hash is an ideal mechanism to store the mapping between C and Perl.
1804 =head2 Alternate Stack Manipulation
1807 Although I have made use of only the C<POP*> macros to access values
1808 returned from Perl subroutines, it is also possible to bypass these
1809 macros and read the stack using the C<ST> macro (See L<perlxs> for a
1810 full description of the C<ST> macro).
1812 Most of the time the C<POP*> macros should be adequate, the main
1813 problem with them is that they force you to process the returned values
1814 in sequence. This may not be the most suitable way to process the
1815 values in some cases. What we want is to be able to access the stack in
1816 a random order. The C<ST> macro as used when coding an XSUB is ideal
1819 The code below is the example given in the section I<Returning a list
1820 of values> recoded to use C<ST> instead of C<POP*>.
1823 call_AddSubtract2(a, b)
1835 XPUSHs(sv_2mortal(newSViv(a)));
1836 XPUSHs(sv_2mortal(newSViv(b)));
1839 count = perl_call_pv("AddSubtract", G_ARRAY);
1843 ax = (sp - stack_base) + 1 ;
1846 croak("Big trouble\n") ;
1848 printf ("%d + %d = %d\n", a, b, SvIV(ST(0))) ;
1849 printf ("%d - %d = %d\n", a, b, SvIV(ST(1))) ;
1862 Notice that it was necessary to define the variable C<ax>. This is
1863 because the C<ST> macro expects it to exist. If we were in an XSUB it
1864 would not be necessary to define C<ax> as it is already defined for
1873 ax = (sp - stack_base) + 1 ;
1875 sets the stack up so that we can use the C<ST> macro.
1879 Unlike the original coding of this example, the returned
1880 values are not accessed in reverse order. So C<ST(0)> refers to the
1881 first value returned by the Perl subroutine and C<ST(count-1)>
1888 L<perlxs>, L<perlguts>, L<perlembed>
1892 Paul Marquess <pmarquess@bfsec.bt.co.uk>
1894 Special thanks to the following people who assisted in the creation of
1897 Jeff Okamoto, Tim Bunce, Nick Gianniotis, Steve Kelem, Gurusamy Sarathy
1902 Version 1.2, 16th Jan 1996