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PERLCALL(1pm)          Perl Programmers Reference Guide          PERLCALL(1pm)


       perlcall - Perl calling conventions from C


       The purpose of this document is to show you how to call Perl
       subroutines directly from C, i.e., how to write callbacks.

       Apart from discussing the C interface provided by Perl for writing
       callbacks the document uses a series of examples to show how the
       interface actually works in practice.  In addition some techniques for
       coding callbacks are covered.

       Examples where callbacks are necessary include

       o    An Error Handler

            You have created an XSUB interface to an application's C API.

            A fairly common feature in applications is to allow you to define
            a C function that will be called whenever something nasty occurs.
            What we would like is to be able to specify a Perl subroutine that
            will be called instead.

       o    An Event-Driven Program

            The classic example of where callbacks are used is when writing an
            event driven program, such as for an X11 application.  In this
            case you register functions to be called whenever specific events
            occur, e.g., a mouse button is pressed, the cursor moves into a
            window or a menu item is selected.

       Although the techniques described here are applicable when embedding
       Perl in a C program, this is not the primary goal of this document.
       There are other details that must be considered and are specific to
       embedding Perl. For details on embedding Perl in C refer to perlembed.

       Before you launch yourself head first into the rest of this document,
       it would be a good idea to have read the following two
       documents--perlxs and perlguts.


       Although this stuff is easier to explain using examples, you first need
       be aware of a few important definitions.

       Perl has a number of C functions that allow you to call Perl
       subroutines.  They are

           I32 call_sv(SV* sv, I32 flags);
           I32 call_pv(char *subname, I32 flags);
           I32 call_method(char *methname, I32 flags);
           I32 call_argv(char *subname, I32 flags, char **argv);

       The key function is call_sv.  All the other functions are fairly simple
       wrappers which make it easier to call Perl subroutines in special
       cases. At the end of the day they will all call call_sv to invoke the
       Perl subroutine.

       All the call_* functions have a "flags" parameter which is used to pass
       a bit mask of options to Perl.  This bit mask operates identically for
       each of the functions.  The settings available in the bit mask are
       discussed in "FLAG VALUES".

       Each of the functions will now be discussed in turn.

            call_sv takes two parameters. The first, "sv", is an SV*.  This
            allows you to specify the Perl subroutine to be called either as a
            C string (which has first been converted to an SV) or a reference
            to a subroutine. The section, "Using call_sv", shows how you can
            make use of call_sv.

            The function, call_pv, is similar to call_sv except it expects its
            first parameter to be a C char* which identifies the Perl
            subroutine you want to call, e.g., "call_pv("fred", 0)".  If the
            subroutine you want to call is in another package, just include
            the package name in the string, e.g., "pkg::fred".

            The function call_method is used to call a method from a Perl
            class.  The parameter "methname" corresponds to the name of the
            method to be called.  Note that the class that the method belongs
            to is passed on the Perl stack rather than in the parameter list.
            This class can be either the name of the class (for a static
            method) or a reference to an object (for a virtual method).  See
            perlobj for more information on static and virtual methods and
            "Using call_method" for an example of using call_method.

            call_argv calls the Perl subroutine specified by the C string
            stored in the "subname" parameter. It also takes the usual "flags"
            parameter.  The final parameter, "argv", consists of a NULL-
            terminated list of C strings to be passed as parameters to the
            Perl subroutine.  See "Using call_argv".

       All the functions return an integer. This is a count of the number of
       items returned by the Perl subroutine. The actual items returned by the
       subroutine are stored on the Perl stack.

       As a general rule you should always check the return value from these
       functions.  Even if you are expecting only a particular number of
       values to be returned from the Perl subroutine, there is nothing to
       stop someone from doing something unexpected--don't say you haven't
       been warned.


       The "flags" parameter in all the call_* functions is one of "G_VOID",
       "G_SCALAR", or "G_ARRAY", which indicate the call context, OR'ed
       together with a bit mask of any combination of the other G_* symbols
       defined below.

       Calls the Perl subroutine in a void context.

       This flag has 2 effects:

       1.   It indicates to the subroutine being called that it is executing
            in a void context (if it executes wantarray the result will be the
            undefined value).

       2.   It ensures that nothing is actually returned from the subroutine.

       The value returned by the call_* function indicates how many items have
       been returned by the Perl subroutine--in this case it will be 0.

       Calls the Perl subroutine in a scalar context.  This is the default
       context flag setting for all the call_* functions.

       This flag has 2 effects:

       1.   It indicates to the subroutine being called that it is executing
            in a scalar context (if it executes wantarray the result will be

       2.   It ensures that only a scalar is actually returned from the
            subroutine.  The subroutine can, of course,  ignore the wantarray
            and return a list anyway. If so, then only the last element of the
            list will be returned.

       The value returned by the call_* function indicates how many items have
       been returned by the Perl subroutine - in this case it will be either 0
       or 1.

       If 0, then you have specified the G_DISCARD flag.

       If 1, then the item actually returned by the Perl subroutine will be
       stored on the Perl stack - the section "Returning a Scalar" shows how
       to access this value on the stack.  Remember that regardless of how
       many items the Perl subroutine returns, only the last one will be
       accessible from the stack - think of the case where only one value is
       returned as being a list with only one element.  Any other items that
       were returned will not exist by the time control returns from the
       call_* function.  The section "Returning a List in Scalar Context"
       shows an example of this behavior.

       Calls the Perl subroutine in a list context.

       As with G_SCALAR, this flag has 2 effects:

       1.   It indicates to the subroutine being called that it is executing
            in a list context (if it executes wantarray the result will be

       2.   It ensures that all items returned from the subroutine will be
            accessible when control returns from the call_* function.

       The value returned by the call_* function indicates how many items have
       been returned by the Perl subroutine.

       If 0, then you have specified the G_DISCARD flag.

       If not 0, then it will be a count of the number of items returned by
       the subroutine. These items will be stored on the Perl stack.  The
       section "Returning a List of Values" gives an example of using the
       G_ARRAY flag and the mechanics of accessing the returned items from the
       Perl stack.

       By default, the call_* functions place the items returned from by the
       Perl subroutine on the stack.  If you are not interested in these
       items, then setting this flag will make Perl get rid of them
       automatically for you.  Note that it is still possible to indicate a
       context to the Perl subroutine by using either G_SCALAR or G_ARRAY.

       If you do not set this flag then it is very important that you make
       sure that any temporaries (i.e., parameters passed to the Perl
       subroutine and values returned from the subroutine) are disposed of
       yourself.  The section "Returning a Scalar" gives details of how to
       dispose of these temporaries explicitly and the section "Using Perl to
       Dispose of Temporaries" discusses the specific circumstances where you
       can ignore the problem and let Perl deal with it for you.

       Whenever a Perl subroutine is called using one of the call_* functions,
       it is assumed by default that parameters are to be passed to the
       subroutine.  If you are not passing any parameters to the Perl
       subroutine, you can save a bit of time by setting this flag.  It has
       the effect of not creating the @_ array for the Perl subroutine.

       Although the functionality provided by this flag may seem
       straightforward, it should be used only if there is a good reason to do
       so.  The reason for being cautious is that, even if you have specified
       the G_NOARGS flag, it is still possible for the Perl subroutine that
       has been called to think that you have passed it parameters.

       In fact, what can happen is that the Perl subroutine you have called
       can access the @_ array from a previous Perl subroutine.  This will
       occur when the code that is executing the call_* function has itself
       been called from another Perl subroutine. The code below illustrates

           sub fred
             { print "@_\n"  }

           sub joe
             { &fred }


       This will print

           1 2 3

       What has happened is that "fred" accesses the @_ array which belongs to

       It is possible for the Perl subroutine you are calling to terminate
       abnormally, e.g., by calling die explicitly or by not actually
       existing.  By default, when either of these events occurs, the process
       will terminate immediately.  If you want to trap this type of event,
       specify the G_EVAL flag.  It will put an eval { } around the subroutine

       Whenever control returns from the call_* function you need to check the
       $@ variable as you would in a normal Perl script.

       The value returned from the call_* function is dependent on what other
       flags have been specified and whether an error has occurred.  Here are
       all the different cases that can occur:

       o    If the call_* function returns normally, then the value returned
            is as specified in the previous sections.

       o    If G_DISCARD is specified, the return value will always be 0.

       o    If G_ARRAY is specified and an error has occurred, the return
            value will always be 0.

       o    If G_SCALAR is specified and an error has occurred, the return
            value will be 1 and the value on the top of the stack will be
            undef. This means that if you have already detected the error by
            checking $@ and you want the program to continue, you must
            remember to pop the undef from the stack.

       See "Using G_EVAL" for details on using G_EVAL.

       Using the G_EVAL flag described above will always set $@: clearing it
       if there was no error, and setting it to describe the error if there
       was an error in the called code.  This is what you want if your
       intention is to handle possible errors, but sometimes you just want to
       trap errors and stop them interfering with the rest of the program.

       This scenario will mostly be applicable to code that is meant to be
       called from within destructors, asynchronous callbacks, and signal
       handlers.  In such situations, where the code being called has little
       relation to the surrounding dynamic context, the main program needs to
       be insulated from errors in the called code, even if they can't be
       handled intelligently.  It may also be useful to do this with code for
       "__DIE__" or "__WARN__" hooks, and "tie" functions.

       The G_KEEPERR flag is meant to be used in conjunction with G_EVAL in
       call_* functions that are used to implement such code, or with
       "eval_sv".  This flag has no effect on the "call_*" functions when
       G_EVAL is not used.

       When G_KEEPERR is used, any error in the called code will terminate the
       call as usual, and the error will not propagate beyond the call (as
       usual for G_EVAL), but it will not go into $@.  Instead the error will
       be converted into a warning, prefixed with the string "\t(in cleanup)".
       This can be disabled using "no warnings 'misc'".  If there is no error,
       $@ will not be cleared.

       Note that the G_KEEPERR flag does not propagate into inner evals; these
       may still set $@.

       The G_KEEPERR flag was introduced in Perl version 5.002.

       See "Using G_KEEPERR" for an example of a situation that warrants the
       use of this flag.

   Determining the Context
       As mentioned above, you can determine the context of the currently
       executing subroutine in Perl with wantarray.  The equivalent test can
       be made in C by using the "GIMME_V" macro, which returns "G_ARRAY" if
       you have been called in a list context, "G_SCALAR" if in a scalar
       context, or "G_VOID" if in a void context (i.e., the return value will
       not be used).  An older version of this macro is called "GIMME"; in a
       void context it returns "G_SCALAR" instead of "G_VOID".  An example of
       using the "GIMME_V" macro is shown in section "Using GIMME_V".


       Enough of the definition talk! Let's have a few examples.

       Perl provides many macros to assist in accessing the Perl stack.
       Wherever possible, these macros should always be used when interfacing
       to Perl internals.  We hope this should make the code less vulnerable
       to any changes made to Perl in the future.

       Another point worth noting is that in the first series of examples I
       have made use of only the call_pv function.  This has been done to keep
       the code simpler and ease you into the topic.  Wherever possible, if
       the choice is between using call_pv and call_sv, you should always try
       to use call_sv.  See "Using call_sv" for details.

   No Parameters, Nothing Returned
       This first trivial example will call a Perl subroutine, PrintUID, to
       print out the UID of the process.

           sub PrintUID
               print "UID is $<\n";

       and here is a C function to call it

           static void

               call_pv("PrintUID", G_DISCARD|G_NOARGS);

       Simple, eh?

       A few points to note about this example:

       1.   Ignore "dSP" and "PUSHMARK(SP)" for now. They will be discussed in
            the next example.

       2.   We aren't passing any parameters to PrintUID so G_NOARGS can be

       3.   We aren't interested in anything returned from PrintUID, so
            G_DISCARD is specified. Even if PrintUID was changed to return
            some value(s), having specified G_DISCARD will mean that they will
            be wiped by the time control returns from call_pv.

       4.   As call_pv is being used, the Perl subroutine is specified as a C
            string. In this case the subroutine name has been 'hard-wired'
            into the code.

       5.   Because we specified G_DISCARD, it is not necessary to check the
            value returned from call_pv. It will always be 0.

   Passing Parameters
       Now let's make a slightly more complex example. This time we want to
       call a Perl subroutine, "LeftString", which will take 2 parameters--a
       string ($s) and an integer ($n).  The subroutine will simply print the
       first $n characters of the string.

       So the Perl subroutine would look like this:

           sub LeftString
               my($s, $n) = @_;
               print substr($s, 0, $n), "\n";

       The C function required to call LeftString would look like this:

           static void
           call_LeftString(a, b)
           char * a;
           int b;


               EXTEND(SP, 2);
               PUSHs(sv_2mortal(newSVpv(a, 0)));

               call_pv("LeftString", G_DISCARD);


       Here are a few notes on the C function call_LeftString.

       1.   Parameters are passed to the Perl subroutine using the Perl stack.
            This is the purpose of the code beginning with the line "dSP" and
            ending with the line "PUTBACK".  The "dSP" declares a local copy
            of the stack pointer.  This local copy should always be accessed
            as "SP".

       2.   If you are going to put something onto the Perl stack, you need to
            know where to put it. This is the purpose of the macro "dSP"--it
            declares and initializes a local copy of the Perl stack pointer.

            All the other macros which will be used in this example require
            you to have used this macro.

            The exception to this rule is if you are calling a Perl subroutine
            directly from an XSUB function. In this case it is not necessary
            to use the "dSP" macro explicitly--it will be declared for you

       3.   Any parameters to be pushed onto the stack should be bracketed by
            the "PUSHMARK" and "PUTBACK" macros.  The purpose of these two
            macros, in this context, is to count the number of parameters you
            are pushing automatically.  Then whenever Perl is creating the @_
            array for the subroutine, it knows how big to make it.

            The "PUSHMARK" macro tells Perl to make a mental note of the
            current stack pointer. Even if you aren't passing any parameters
            (like the example shown in the section "No Parameters, Nothing
            Returned") you must still call the "PUSHMARK" macro before you can
            call any of the call_* functions--Perl still needs to know that
            there are no parameters.

            The "PUTBACK" macro sets the global copy of the stack pointer to
            be the same as our local copy. If we didn't do this, call_pv
            wouldn't know where the two parameters we pushed were--remember
            that up to now all the stack pointer manipulation we have done is
            with our local copy, not the global copy.

       4.   Next, we come to EXTEND and PUSHs. This is where the parameters
            actually get pushed onto the stack. In this case we are pushing a
            string and an integer.

            Alternatively you can use the XPUSHs() macro, which combines a
            "EXTEND(SP, 1)" and "PUSHs()".  This is less efficient if you're
            pushing multiple values.

            See "XSUBs and the Argument Stack" in perlguts for details on how
            the PUSH macros work.

       5.   Because we created temporary values (by means of sv_2mortal()
            calls) we will have to tidy up the Perl stack and dispose of
            mortal SVs.

            This is the purpose of


            at the start of the function, and


            at the end. The "ENTER"/"SAVETMPS" pair creates a boundary for any
            temporaries we create.  This means that the temporaries we get rid
            of will be limited to those which were created after these calls.

            The "FREETMPS"/"LEAVE" pair will get rid of any values returned by
            the Perl subroutine (see next example), plus it will also dump the
            mortal SVs we have created.  Having "ENTER"/"SAVETMPS" at the
            beginning of the code makes sure that no other mortals are

            Think of these macros as working a bit like "{" and "}" in Perl to
            limit the scope of local variables.

            See the section "Using Perl to Dispose of Temporaries" for details
            of an alternative to using these macros.

       6.   Finally, LeftString can now be called via the call_pv function.
            The only flag specified this time is G_DISCARD. Because we are
            passing 2 parameters to the Perl subroutine this time, we have not
            specified G_NOARGS.

   Returning a Scalar
       Now for an example of dealing with the items returned from a Perl

       Here is a Perl subroutine, Adder, that takes 2 integer parameters and
       simply returns their sum.

           sub Adder
               my($a, $b) = @_;
               $a + $b;

       Because we are now concerned with the return value from Adder, the C
       function required to call it is now a bit more complex.

           static void
           call_Adder(a, b)
           int a;
           int b;
               int count;


               EXTEND(SP, 2);

               count = call_pv("Adder", G_SCALAR);


               if (count != 1)
                   croak("Big trouble\n");

               printf ("The sum of %d and %d is %d\n", a, b, POPi);


       Points to note this time are

       1.   The only flag specified this time was G_SCALAR. That means that
            the @_ array will be created and that the value returned by Adder
            will still exist after the call to call_pv.

       2.   The purpose of the macro "SPAGAIN" is to refresh the local copy of
            the stack pointer. This is necessary because it is possible that
            the memory allocated to the Perl stack has been reallocated during
            the call_pv call.

            If you are making use of the Perl stack pointer in your code you
            must always refresh the local copy using SPAGAIN whenever you make
            use of the call_* functions or any other Perl internal function.

       3.   Although only a single value was expected to be returned from
            Adder, it is still good practice to check the return code from
            call_pv anyway.

            Expecting a single value is not quite the same as knowing that
            there will be one. If someone modified Adder to return a list and
            we didn't check for that possibility and take appropriate action
            the Perl stack would end up in an inconsistent state. That is
            something you really don't want to happen ever.

       4.   The "POPi" macro is used here to pop the return value from the
            stack.  In this case we wanted an integer, so "POPi" was used.

            Here is the complete list of POP macros available, along with the
            types they return.

                POPs        SV
                POPp        pointer (PV)
                POPpbytex   pointer to bytes (PV)
                POPn        double (NV)
                POPi        integer (IV)
                POPu        unsigned integer (UV)
                POPl        long
                POPul       unsigned long

            Since these macros have side-effects don't use them as arguments
            to macros that may evaluate their argument several times, for

              /* Bad idea, don't do this */
              STRLEN len;
              const char *s = SvPV(POPs, len);

            Instead, use a temporary:

              STRLEN len;
              SV *sv = POPs;
              const char *s = SvPV(sv, len);

            or a macro that guarantees it will evaluate its arguments only

              STRLEN len;
              const char *s = SvPVx(POPs, len);

       5.   The final "PUTBACK" is used to leave the Perl stack in a
            consistent state before exiting the function.  This is necessary
            because when we popped the return value from the stack with "POPi"
            it updated only our local copy of the stack pointer.  Remember,
            "PUTBACK" sets the global stack pointer to be the same as our
            local copy.

   Returning a List of Values
       Now, let's extend the previous example to return both the sum of the
       parameters and the difference.

       Here is the Perl subroutine

           sub AddSubtract
              my($a, $b) = @_;
              ($a+$b, $a-$b);

       and this is the C function

           static void
           call_AddSubtract(a, b)
           int a;
           int b;
               int count;


               EXTEND(SP, 2);

               count = call_pv("AddSubtract", G_ARRAY);


               if (count != 2)
                   croak("Big trouble\n");

               printf ("%d - %d = %d\n", a, b, POPi);
               printf ("%d + %d = %d\n", a, b, POPi);


       If call_AddSubtract is called like this

           call_AddSubtract(7, 4);

       then here is the output

           7 - 4 = 3
           7 + 4 = 11


       1.   We wanted list context, so G_ARRAY was used.

       2.   Not surprisingly "POPi" is used twice this time because we were
            retrieving 2 values from the stack. The important thing to note is
            that when using the "POP*" macros they come off the stack in
            reverse order.

   Returning a List in Scalar Context
       Say the Perl subroutine in the previous section was called in a scalar
       context, like this

           static void
           call_AddSubScalar(a, b)
           int a;
           int b;
               int count;
               int i;


               EXTEND(SP, 2);

               count = call_pv("AddSubtract", G_SCALAR);


               printf ("Items Returned = %d\n", count);

               for (i = 1; i <= count; ++i)
                   printf ("Value %d = %d\n", i, POPi);


       The other modification made is that call_AddSubScalar will print the
       number of items returned from the Perl subroutine and their value (for
       simplicity it assumes that they are integer).  So if call_AddSubScalar
       is called

           call_AddSubScalar(7, 4);

       then the output will be

           Items Returned = 1
           Value 1 = 3

       In this case the main point to note is that only the last item in the
       list is returned from the subroutine. AddSubtract actually made it back
       to call_AddSubScalar.

   Returning Data from Perl via the Parameter List
       It is also possible to return values directly via the parameter
       list--whether it is actually desirable to do it is another matter

       The Perl subroutine, Inc, below takes 2 parameters and increments each

           sub Inc
               ++ $_[0];
               ++ $_[1];

       and here is a C function to call it.

           static void
           call_Inc(a, b)
           int a;
           int b;
               int count;
               SV * sva;
               SV * svb;


               sva = sv_2mortal(newSViv(a));
               svb = sv_2mortal(newSViv(b));

               EXTEND(SP, 2);

               count = call_pv("Inc", G_DISCARD);

               if (count != 0)
                   croak ("call_Inc: expected 0 values from 'Inc', got %d\n",

               printf ("%d + 1 = %d\n", a, SvIV(sva));
               printf ("%d + 1 = %d\n", b, SvIV(svb));


       To be able to access the two parameters that were pushed onto the stack
       after they return from call_pv it is necessary to make a note of their
       addresses--thus the two variables "sva" and "svb".

       The reason this is necessary is that the area of the Perl stack which
       held them will very likely have been overwritten by something else by
       the time control returns from call_pv.

   Using G_EVAL
       Now an example using G_EVAL. Below is a Perl subroutine which computes
       the difference of its 2 parameters. If this would result in a negative
       result, the subroutine calls die.

           sub Subtract
               my ($a, $b) = @_;

               die "death can be fatal\n" if $a < $b;

               $a - $b;

       and some C to call it

        static void
        call_Subtract(a, b)
        int a;
        int b;
            int count;
            SV *err_tmp;


            EXTEND(SP, 2);

            count = call_pv("Subtract", G_EVAL|G_SCALAR);


            /* Check the eval first */
            err_tmp = ERRSV;
            if (SvTRUE(err_tmp))
                printf ("Uh oh - %s\n", SvPV_nolen(err_tmp));
              if (count != 1)
               croak("call_Subtract: wanted 1 value from 'Subtract', got %d\n",

                printf ("%d - %d = %d\n", a, b, POPi);


       If call_Subtract is called thus

           call_Subtract(4, 5)

       the following will be printed

           Uh oh - death can be fatal


       1.   We want to be able to catch the die so we have used the G_EVAL
            flag.  Not specifying this flag would mean that the program would
            terminate immediately at the die statement in the subroutine

       2.   The code

                err_tmp = ERRSV;
                if (SvTRUE(err_tmp))
                    printf ("Uh oh - %s\n", SvPV_nolen(err_tmp));

            is the direct equivalent of this bit of Perl

                print "Uh oh - $@\n" if $@;

            "PL_errgv" is a perl global of type "GV *" that points to the
            symbol table entry containing the error.  "ERRSV" therefore refers
            to the C equivalent of $@.  We use a local temporary, "err_tmp",
            since "ERRSV" is a macro that calls a function, and
            "SvTRUE(ERRSV)" would end up calling that function multiple times.

       3.   Note that the stack is popped using "POPs" in the block where
            "SvTRUE(err_tmp)" is true.  This is necessary because whenever a
            call_* function invoked with G_EVAL|G_SCALAR returns an error, the
            top of the stack holds the value undef. Because we want the
            program to continue after detecting this error, it is essential
            that the stack be tidied up by removing the undef.

   Using G_KEEPERR
       Consider this rather facetious example, where we have used an XS
       version of the call_Subtract example above inside a destructor:

           package Foo;
           sub new { bless {}, $_[0] }
           sub Subtract {
               my($a,$b) = @_;
               die "death can be fatal" if $a < $b;
               $a - $b;
           sub DESTROY { call_Subtract(5, 4); }
           sub foo { die "foo dies"; }

           package main;
               my $foo = Foo->new;
               eval { $foo->foo };
           print "Saw: $@" if $@;             # should be, but isn't

       This example will fail to recognize that an error occurred inside the
       "eval {}".  Here's why: the call_Subtract code got executed while perl
       was cleaning up temporaries when exiting the outer braced block, and
       because call_Subtract is implemented with call_pv using the G_EVAL
       flag, it promptly reset $@.  This results in the failure of the
       outermost test for $@, and thereby the failure of the error trap.

       Appending the G_KEEPERR flag, so that the call_pv call in call_Subtract

               count = call_pv("Subtract", G_EVAL|G_SCALAR|G_KEEPERR);

       will preserve the error and restore reliable error handling.

   Using call_sv
       In all the previous examples I have 'hard-wired' the name of the Perl
       subroutine to be called from C.  Most of the time though, it is more
       convenient to be able to specify the name of the Perl subroutine from
       within the Perl script, and you'll want to use call_sv.

       Consider the Perl code below

           sub fred
               print "Hello there\n";


       Here is a snippet of XSUB which defines CallSubPV.

               char *  name
               call_pv(name, G_DISCARD|G_NOARGS);

       That is fine as far as it goes. The thing is, the Perl subroutine can
       be specified as only a string, however, Perl allows references to
       subroutines and anonymous subroutines.  This is where call_sv is

       The code below for CallSubSV is identical to CallSubPV except that the
       "name" parameter is now defined as an SV* and we use call_sv instead of

               SV *    name
               call_sv(name, G_DISCARD|G_NOARGS);

       Because we are using an SV to call fred the following can all be used:

           $ref = \&fred;
           CallSubSV( sub { print "Hello there\n" } );

       As you can see, call_sv gives you much greater flexibility in how you
       can specify the Perl subroutine.

       You should note that, if it is necessary to store the SV ("name" in the
       example above) which corresponds to the Perl subroutine so that it can
       be used later in the program, it not enough just to store a copy of the
       pointer to the SV. Say the code above had been like this:

           static SV * rememberSub;

               SV *    name
               rememberSub = name;

               call_sv(rememberSub, G_DISCARD|G_NOARGS);

       The reason this is wrong is that, by the time you come to use the
       pointer "rememberSub" in "CallSavedSub1", it may or may not still refer
       to the Perl subroutine that was recorded in "SaveSub1".  This is
       particularly true for these cases:


           SaveSub1( sub { print "Hello there\n" } );

       By the time each of the "SaveSub1" statements above has been executed,
       the SV*s which corresponded to the parameters will no longer exist.
       Expect an error message from Perl of the form

           Can't use an undefined value as a subroutine reference at ...

       for each of the "CallSavedSub1" lines.

       Similarly, with this code

           $ref = \&fred;
           $ref = 47;

       you can expect one of these messages (which you actually get is
       dependent on the version of Perl you are using)

           Not a CODE reference at ...
           Undefined subroutine &main::47 called ...

       The variable $ref may have referred to the subroutine "fred" whenever
       the call to "SaveSub1" was made but by the time "CallSavedSub1" gets
       called it now holds the number 47. Because we saved only a pointer to
       the original SV in "SaveSub1", any changes to $ref will be tracked by
       the pointer "rememberSub". This means that whenever "CallSavedSub1"
       gets called, it will attempt to execute the code which is referenced by
       the SV* "rememberSub".  In this case though, it now refers to the
       integer 47, so expect Perl to complain loudly.

       A similar but more subtle problem is illustrated with this code:

           $ref = \&fred;
           $ref = \&joe;

       This time whenever "CallSavedSub1" gets called it will execute the Perl
       subroutine "joe" (assuming it exists) rather than "fred" as was
       originally requested in the call to "SaveSub1".

       To get around these problems it is necessary to take a full copy of the
       SV.  The code below shows "SaveSub2" modified to do that.

           /* this isn't thread-safe */
           static SV * keepSub = (SV*)NULL;

               SV *    name
               /* Take a copy of the callback */
               if (keepSub == (SV*)NULL)
                   /* First time, so create a new SV */
                   keepSub = newSVsv(name);
                   /* Been here before, so overwrite */
                   SvSetSV(keepSub, name);

               call_sv(keepSub, G_DISCARD|G_NOARGS);

       To avoid creating a new SV every time "SaveSub2" is called, the
       function first checks to see if it has been called before.  If not,
       then space for a new SV is allocated and the reference to the Perl
       subroutine "name" is copied to the variable "keepSub" in one operation
       using "newSVsv".  Thereafter, whenever "SaveSub2" is called, the
       existing SV, "keepSub", is overwritten with the new value using

       Note: using a static or global variable to store the SV isn't thread-
       safe.  You can either use the "MY_CXT" mechanism documented in "Safely
       Storing Static Data in XS" in perlxs which is fast, or store the values
       in perl global variables, using get_sv(), which is much slower.

   Using call_argv
       Here is a Perl subroutine which prints whatever parameters are passed
       to it.

           sub PrintList
               my(@list) = @_;

               foreach (@list) { print "$_\n" }

       And here is an example of call_argv which will call PrintList.

           static char * words[] = {"alpha", "beta", "gamma", "delta", NULL};

           static void
               call_argv("PrintList", G_DISCARD, words);

       Note that it is not necessary to call "PUSHMARK" in this instance.
       This is because call_argv will do it for you.

   Using call_method
       Consider the following Perl code:

               package Mine;

               sub new
                   my($type) = shift;
                   bless [@_]

               sub Display
                   my ($self, $index) = @_;
                   print "$index: $$self[$index]\n";

               sub PrintID
                   my($class) = @_;
                   print "This is Class $class version 1.0\n";

       It implements just a very simple class to manage an array.  Apart from
       the constructor, "new", it declares methods, one static and one
       virtual. The static method, "PrintID", prints out simply the class name
       and a version number. The virtual method, "Display", prints out a
       single element of the array.  Here is an all-Perl example of using it.

           $a = Mine->new('red', 'green', 'blue');

       will print

           1: green
           This is Class Mine version 1.0

       Calling a Perl method from C is fairly straightforward. The following
       things are required:

       o    A reference to the object for a virtual method or the name of the
            class for a static method

       o    The name of the method

       o    Any other parameters specific to the method

       Here is a simple XSUB which illustrates the mechanics of calling both
       the "PrintID" and "Display" methods from C.

           call_Method(ref, method, index)
               SV *    ref
               char *  method
               int             index
               EXTEND(SP, 2);

               call_method(method, G_DISCARD);

           call_PrintID(class, method)
               char *  class
               char *  method
               XPUSHs(sv_2mortal(newSVpv(class, 0)));

               call_method(method, G_DISCARD);

       So the methods "PrintID" and "Display" can be invoked like this:

           $a = Mine->new('red', 'green', 'blue');
           call_Method($a, 'Display', 1);
           call_PrintID('Mine', 'PrintID');

       The only thing to note is that, in both the static and virtual methods,
       the method name is not passed via the stack--it is used as the first
       parameter to call_method.

   Using GIMME_V
       Here is a trivial XSUB which prints the context in which it is
       currently executing.

               U8 gimme = GIMME_V;
               if (gimme == G_VOID)
                   printf ("Context is Void\n");
               else if (gimme == G_SCALAR)
                   printf ("Context is Scalar\n");
                   printf ("Context is Array\n");

       And here is some Perl to test it.

           $a = PrintContext;
           @a = PrintContext;

       The output from that will be

           Context is Void
           Context is Scalar
           Context is Array

   Using Perl to Dispose of Temporaries
       In the examples given to date, any temporaries created in the callback
       (i.e., parameters passed on the stack to the call_* function or values
       returned via the stack) have been freed by one of these methods:

       o    Specifying the G_DISCARD flag with call_*

       o    Explicitly using the "ENTER"/"SAVETMPS"--"FREETMPS"/"LEAVE"

       There is another method which can be used, namely letting Perl do it
       for you automatically whenever it regains control after the callback
       has terminated.  This is done by simply not using the


       sequence in the callback (and not, of course, specifying the G_DISCARD

       If you are going to use this method you have to be aware of a possible
       memory leak which can arise under very specific circumstances.  To
       explain these circumstances you need to know a bit about the flow of
       control between Perl and the callback routine.

       The examples given at the start of the document (an error handler and
       an event driven program) are typical of the two main sorts of flow
       control that you are likely to encounter with callbacks.  There is a
       very important distinction between them, so pay attention.

       In the first example, an error handler, the flow of control could be as
       follows.  You have created an interface to an external library.
       Control can reach the external library like this

           perl --> XSUB --> external library

       Whilst control is in the library, an error condition occurs. You have
       previously set up a Perl callback to handle this situation, so it will
       get executed. Once the callback has finished, control will drop back to
       Perl again.  Here is what the flow of control will be like in that

           perl --> XSUB --> external library
                             error occurs
                             external library --> call_* --> perl
           perl <-- XSUB <-- external library <-- call_* <----+

       After processing of the error using call_* is completed, control
       reverts back to Perl more or less immediately.

       In the diagram, the further right you go the more deeply nested the
       scope is.  It is only when control is back with perl on the extreme
       left of the diagram that you will have dropped back to the enclosing
       scope and any temporaries you have left hanging around will be freed.

       In the second example, an event driven program, the flow of control
       will be more like this

           perl --> XSUB --> event handler
                             event handler --> call_* --> perl
                             event handler <-- call_* <----+
                             event handler --> call_* --> perl
                             event handler <-- call_* <----+
                             event handler --> call_* --> perl
                             event handler <-- call_* <----+

       In this case the flow of control can consist of only the repeated

           event handler --> call_* --> perl

       for practically the complete duration of the program.  This means that
       control may never drop back to the surrounding scope in Perl at the
       extreme left.

       So what is the big problem? Well, if you are expecting Perl to tidy up
       those temporaries for you, you might be in for a long wait.  For Perl
       to dispose of your temporaries, control must drop back to the enclosing
       scope at some stage.  In the event driven scenario that may never
       happen.  This means that, as time goes on, your program will create
       more and more temporaries, none of which will ever be freed. As each of
       these temporaries consumes some memory your program will eventually
       consume all the available memory in your system--kapow!

       So here is the bottom line--if you are sure that control will revert
       back to the enclosing Perl scope fairly quickly after the end of your
       callback, then it isn't absolutely necessary to dispose explicitly of
       any temporaries you may have created. Mind you, if you are at all
       uncertain about what to do, it doesn't do any harm to tidy up anyway.

   Strategies for Storing Callback Context Information
       Potentially one of the trickiest problems to overcome when designing a
       callback interface can be figuring out how to store the mapping between
       the C callback function and the Perl equivalent.

       To help understand why this can be a real problem first consider how a
       callback is set up in an all C environment.  Typically a C API will
       provide a function to register a callback.  This will expect a pointer
       to a function as one of its parameters.  Below is a call to a
       hypothetical function "register_fatal" which registers the C function
       to get called when a fatal error occurs.


       The single parameter "cb1" is a pointer to a function, so you must have
       defined "cb1" in your code, say something like this

           static void
               printf ("Fatal Error\n");

       Now change that to call a Perl subroutine instead

           static SV * callback = (SV*)NULL;

           static void


               /* Call the Perl sub to process the callback */
               call_sv(callback, G_DISCARD);

               SV *    fn
               /* Remember the Perl sub */
               if (callback == (SV*)NULL)
                   callback = newSVsv(fn);
                   SvSetSV(callback, fn);

               /* register the callback with the external library */

       where the Perl equivalent of "register_fatal" and the callback it
       registers, "pcb1", might look like this

           # Register the sub pcb1

           sub pcb1
               die "I'm dying...\n";

       The mapping between the C callback and the Perl equivalent is stored in
       the global variable "callback".

       This will be adequate if you ever need to have only one callback
       registered at any time. An example could be an error handler like the
       code sketched out above. Remember though, repeated calls to
       "register_fatal" will replace the previously registered callback
       function with the new one.

       Say for example you want to interface to a library which allows
       asynchronous file i/o.  In this case you may be able to register a
       callback whenever a read operation has completed. To be of any use we
       want to be able to call separate Perl subroutines for each file that is
       opened.  As it stands, the error handler example above would not be
       adequate as it allows only a single callback to be defined at any time.
       What we require is a means of storing the mapping between the opened
       file and the Perl subroutine we want to be called for that file.

       Say the i/o library has a function "asynch_read" which associates a C
       function "ProcessRead" with a file handle "fh"--this assumes that it
       has also provided some routine to open the file and so obtain the file

           asynch_read(fh, ProcessRead)

       This may expect the C ProcessRead function of this form

           ProcessRead(fh, buffer)
           int fh;
           char *      buffer;

       To provide a Perl interface to this library we need to be able to map
       between the "fh" parameter and the Perl subroutine we want called.  A
       hash is a convenient mechanism for storing this mapping.  The code
       below shows a possible implementation

           static HV * Mapping = (HV*)NULL;

           asynch_read(fh, callback)
               int     fh
               SV *    callback
               /* If the hash doesn't already exist, create it */
               if (Mapping == (HV*)NULL)
                   Mapping = newHV();

               /* Save the fh -> callback mapping */
               hv_store(Mapping, (char*)&fh, sizeof(fh), newSVsv(callback), 0);

               /* Register with the C Library */
               asynch_read(fh, asynch_read_if);

       and "asynch_read_if" could look like this

           static void
           asynch_read_if(fh, buffer)
           int fh;
           char *      buffer;
               SV ** sv;

               /* Get the callback associated with fh */
               sv =  hv_fetch(Mapping, (char*)&fh , sizeof(fh), FALSE);
               if (sv == (SV**)NULL)
                   croak("Internal error...\n");

               EXTEND(SP, 2);
               PUSHs(sv_2mortal(newSVpv(buffer, 0)));

               /* Call the Perl sub */
               call_sv(*sv, G_DISCARD);

       For completeness, here is "asynch_close".  This shows how to remove the
       entry from the hash "Mapping".

               int     fh
               /* Remove the entry from the hash */
               (void) hv_delete(Mapping, (char*)&fh, sizeof(fh), G_DISCARD);

               /* Now call the real asynch_close */

       So the Perl interface would look like this

           sub callback1
               my($handle, $buffer) = @_;

           # Register the Perl callback
           asynch_read($fh, \&callback1);


       The mapping between the C callback and Perl is stored in the global
       hash "Mapping" this time. Using a hash has the distinct advantage that
       it allows an unlimited number of callbacks to be registered.

       What if the interface provided by the C callback doesn't contain a
       parameter which allows the file handle to Perl subroutine mapping?  Say
       in the asynchronous i/o package, the callback function gets passed only
       the "buffer" parameter like this

           char *      buffer;

       Without the file handle there is no straightforward way to map from the
       C callback to the Perl subroutine.

       In this case a possible way around this problem is to predefine a
       series of C functions to act as the interface to Perl, thus

           #define MAX_CB              3
           #define NULL_HANDLE -1
           typedef void (*FnMap)();

           struct MapStruct {
               FnMap    Function;
               SV *     PerlSub;
               int      Handle;

           static void  fn1();
           static void  fn2();
           static void  fn3();

           static struct MapStruct Map [MAX_CB] =
                   { fn1, NULL, NULL_HANDLE },
                   { fn2, NULL, NULL_HANDLE },
                   { fn3, NULL, NULL_HANDLE }

           static void
           Pcb(index, buffer)
           int index;
           char * buffer;

               XPUSHs(sv_2mortal(newSVpv(buffer, 0)));

               /* Call the Perl sub */
               call_sv(Map[index].PerlSub, G_DISCARD);

           static void
           char * buffer;
               Pcb(0, buffer);

           static void
           char * buffer;
               Pcb(1, buffer);

           static void
           char * buffer;
               Pcb(2, buffer);

           array_asynch_read(fh, callback)
               int             fh
               SV *    callback
               int index;
               int null_index = MAX_CB;

               /* Find the same handle or an empty entry */
               for (index = 0; index < MAX_CB; ++index)
                   if (Map[index].Handle == fh)

                   if (Map[index].Handle == NULL_HANDLE)
                       null_index = index;

               if (index == MAX_CB && null_index == MAX_CB)
                   croak ("Too many callback functions registered\n");

               if (index == MAX_CB)
                   index = null_index;

               /* Save the file handle */
               Map[index].Handle = fh;

               /* Remember the Perl sub */
               if (Map[index].PerlSub == (SV*)NULL)
                   Map[index].PerlSub = newSVsv(callback);
                   SvSetSV(Map[index].PerlSub, callback);

               asynch_read(fh, Map[index].Function);

               int     fh
               int index;

               /* Find the file handle */
               for (index = 0; index < MAX_CB; ++ index)
                   if (Map[index].Handle == fh)

               if (index == MAX_CB)
                   croak ("could not close fh %d\n", fh);

               Map[index].Handle = NULL_HANDLE;
               Map[index].PerlSub = (SV*)NULL;


       In this case the functions "fn1", "fn2", and "fn3" are used to remember
       the Perl subroutine to be called. Each of the functions holds a
       separate hard-wired index which is used in the function "Pcb" to access
       the "Map" array and actually call the Perl subroutine.

       There are some obvious disadvantages with this technique.

       Firstly, the code is considerably more complex than with the previous

       Secondly, there is a hard-wired limit (in this case 3) to the number of
       callbacks that can exist simultaneously. The only way to increase the
       limit is by modifying the code to add more functions and then
       recompiling.  None the less, as long as the number of functions is
       chosen with some care, it is still a workable solution and in some
       cases is the only one available.

       To summarize, here are a number of possible methods for you to consider
       for storing the mapping between C and the Perl callback

       1. Ignore the problem - Allow only 1 callback
            For a lot of situations, like interfacing to an error handler,
            this may be a perfectly adequate solution.

       2. Create a sequence of callbacks - hard wired limit
            If it is impossible to tell from the parameters passed back from
            the C callback what the context is, then you may need to create a
            sequence of C callback interface functions, and store pointers to
            each in an array.

       3. Use a parameter to map to the Perl callback
            A hash is an ideal mechanism to store the mapping between C and

   Alternate Stack Manipulation
       Although I have made use of only the "POP*" macros to access values
       returned from Perl subroutines, it is also possible to bypass these
       macros and read the stack using the "ST" macro (See perlxs for a full
       description of the "ST" macro).

       Most of the time the "POP*" macros should be adequate; the main problem
       with them is that they force you to process the returned values in
       sequence. This may not be the most suitable way to process the values
       in some cases. What we want is to be able to access the stack in a
       random order. The "ST" macro as used when coding an XSUB is ideal for
       this purpose.

       The code below is the example given in the section "Returning a List of
       Values" recoded to use "ST" instead of "POP*".

           static void
           call_AddSubtract2(a, b)
           int a;
           int b;
               I32 ax;
               int count;


               EXTEND(SP, 2);

               count = call_pv("AddSubtract", G_ARRAY);

               SP -= count;
               ax = (SP - PL_stack_base) + 1;

               if (count != 2)
                   croak("Big trouble\n");

               printf ("%d + %d = %d\n", a, b, SvIV(ST(0)));
               printf ("%d - %d = %d\n", a, b, SvIV(ST(1)));



       1.   Notice that it was necessary to define the variable "ax".  This is
            because the "ST" macro expects it to exist.  If we were in an XSUB
            it would not be necessary to define "ax" as it is already defined
            for us.

       2.   The code

                    SP -= count;
                    ax = (SP - PL_stack_base) + 1;

            sets the stack up so that we can use the "ST" macro.

       3.   Unlike the original coding of this example, the returned values
            are not accessed in reverse order.  So ST(0) refers to the first
            value returned by the Perl subroutine and "ST(count-1)" refers to
            the last.

   Creating and Calling an Anonymous Subroutine in C
       As we've already shown, "call_sv" can be used to invoke an anonymous
       subroutine.  However, our example showed a Perl script invoking an XSUB
       to perform this operation.  Let's see how it can be done inside our C


        SV *cvrv
           = eval_pv("sub {
                       print 'You will not find me cluttering any namespace!'
                      }", TRUE);


        call_sv(cvrv, G_VOID|G_NOARGS);

       "eval_pv" is used to compile the anonymous subroutine, which will be
       the return value as well (read more about "eval_pv" in "eval_pv" in
       perlapi).  Once this code reference is in hand, it can be mixed in with
       all the previous examples we've shown.


       Sometimes you need to invoke the same subroutine repeatedly.  This
       usually happens with a function that acts on a list of values, such as
       Perl's built-in sort(). You can pass a comparison function to sort(),
       which will then be invoked for every pair of values that needs to be
       compared. The first() and reduce() functions from List::Util follow a
       similar pattern.

       In this case it is possible to speed up the routine (often quite
       substantially) by using the lightweight callback API.  The idea is that
       the calling context only needs to be created and destroyed once, and
       the sub can be called arbitrarily many times in between.

       It is usual to pass parameters using global variables (typically $_ for
       one parameter, or $a and $b for two parameters) rather than via @_. (It
       is possible to use the @_ mechanism if you know what you're doing,
       though there is as yet no supported API for it. It's also inherently

       The pattern of macro calls is like this:

           dMULTICALL;                 /* Declare local variables */
           U8 gimme = G_SCALAR;        /* context of the call: G_SCALAR,
                                        * G_ARRAY, or G_VOID */

           PUSH_MULTICALL(cv);         /* Set up the context for calling cv,
                                          and set local vars appropriately */

           /* loop */ {
               /* set the value(s) af your parameter variables */
               MULTICALL;              /* Make the actual call */
           } /* end of loop */

           POP_MULTICALL;              /* Tear down the calling context */

       For some concrete examples, see the implementation of the first() and
       reduce() functions of List::Util 1.18. There you will also find a
       header file that emulates the multicall API on older versions of perl.


       perlxs(1), perlguts(1), perlembed(1)


       Paul Marquess

       Special thanks to the following people who assisted in the creation of
       the document.

       Jeff Okamoto, Tim Bunce, Nick Gianniotis, Steve Kelem, Gurusamy Sarathy
       and Larry Wall.


       Last updated for perl 5.23.1.

perl v5.34.0                      2020-11-18                     PERLCALL(1pm)

perl 5.34.0 - Generated Fri Feb 25 15:43:31 CST 2022
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