perlcall


DESCRIPTION
       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 like for an X windows 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.

THE CALL_ FUNCTIONS
       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, register 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.
            to a subroutine. The section, Using call_sv, shows how you can
            make use of call_sv.

       call_pv
            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".

       call_method
            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
            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.

FLAG VALUES
       The "flags" parameter in all the call_* functions is a bit mask which
       can consist of any combination of the symbols defined below, OR'ed
       together.

   G_VOID
       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
       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 a scalar context shows an example of
       this behavior.

   G_ARRAY
       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
            true).

       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.

   G_DISCARD
       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
       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
       this

           sub fred
             { print "@_\n"  }

           sub joe
             { &fred }

           &joe(1,2,3);

       This will print

           1 2 3

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

   G_EVAL
       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
       call.

       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

       {}", and the subsequent statement which checks for the value of $@ gets
       executed in the user's script.

       This scenario will mostly be applicable to code that is meant to be
       called from within destructors, asynchronous callbacks, signal
       handlers, "__DIE__" or "__WARN__" hooks, and "tie" functions.  In such
       situations, you will not want to clear $@ at all, but simply to append
       any new errors to any existing value of $@.

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

       When G_KEEPERR is used, any errors in the called code will be prefixed
       with the string "\t(in cleanup)", and appended to the current value of
       $@.  an error will not be appended if that same error string is already
       at the end of $@.

       In addition, a warning is generated using the appended string. This can
       be disabled using "no warnings 'misc'".

       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.

EXAMPLES
       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.

               PUSHMARK(SP);
               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
            specified.

       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;
           {
               dSP;

               ENTER;
               SAVETMPS;

               PUSHMARK(SP);
               XPUSHs(sv_2mortal(newSVpv(a, 0)));

            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
            automatically.

       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 XPUSHs. This is where the parameters actually get
            pushed onto the stack. In this case we are pushing a string and an
            integer.

            See "XSUBs and the Argument Stack" in perlguts for details on how
            the XPUSH 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

                ENTER;
                SAVETMPS;

            at the start of the function, and


            Think of these macros as working a bit like using "{" 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
       subroutine.

       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;
           {
               dSP;
               int count;

               ENTER;
               SAVETMPS;

               PUSHMARK(SP);
               XPUSHs(sv_2mortal(newSViv(a)));
               XPUSHs(sv_2mortal(newSViv(b)));
               PUTBACK;

               count = call_pv("Adder", G_SCALAR);

               SPAGAIN;

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

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

               PUTBACK;

            the memory allocated to the Perl stack has been reallocated whilst
            in 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
                POPn        double
                POPi        integer
                POPl        long

       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;
               count = call_pv("AddSubtract", G_ARRAY);

               SPAGAIN;

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

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

               PUTBACK;
               FREETMPS;
               LEAVE;
           }

       If call_AddSubtract is called like this

           call_AddSubtract(7, 4);

       then here is the output

           7 - 4 = 3
           7 + 4 = 11

       Notes

       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 a 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;
           {
               dSP;
               int count;
               int i;

               ENTER;
               SAVETMPS;

               PUSHMARK(SP);
               XPUSHs(sv_2mortal(newSViv(a)));
               XPUSHs(sv_2mortal(newSViv(b)));
               PUTBACK;

           }

       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 entirely.

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

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

       and here is a C function to call it.

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

               ENTER;
               SAVETMPS;

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

               PUSHMARK(SP);
               XPUSHs(sva);
               XPUSHs(svb);
               PUTBACK;


       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;
           {
               dSP;
               int count;

               ENTER;
               SAVETMPS;

               PUSHMARK(SP);
               XPUSHs(sv_2mortal(newSViv(a)));
               XPUSHs(sv_2mortal(newSViv(b)));
               PUTBACK;

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

               SPAGAIN;

               /* Check the eval first */
               if (SvTRUE(ERRSV))
               {
                   printf ("Uh oh - %s\n", SvPV_nolen(ERRSV));
                   POPs;
               }
               else
               {
                   if (count != 1)

           call_Subtract(4, 5)

       the following will be printed

           Uh oh - death can be fatal

       Notes

       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
            Subtract.

       2.   The code

                if (SvTRUE(ERRSV))
                {
                    printf ("Uh oh - %s\n", SvPV_nolen(ERRSV));
                    POPs;
                }

            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 $@.

       3.   Note that the stack is popped using "POPs" in the block where
            "SvTRUE(ERRSV)" 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 is 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;
           eval { Foo->new->foo };
           print "Saw: $@" if $@;             # should be, but isn't

       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.

       Consider the Perl code below

           sub fred
           {
               print "Hello there\n";
           }

           CallSubPV("fred");

       Here is a snippet of XSUB which defines CallSubPV.

           void
           CallSubPV(name)
               char *  name
               CODE:
               PUSHMARK(SP);
               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.  For Perl 4 this was adequate, but Perl
       5 allows references to subroutines and anonymous subroutines.  This is
       where call_sv is useful.

       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
       call_pv.

           void
           CallSubSV(name)
               SV *    name
               CODE:
               PUSHMARK(SP);
               call_sv(name, G_DISCARD|G_NOARGS);

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

           CallSubSV("fred");
           CallSubSV(\&fred);
           $ref = \&fred;
           CallSubSV($ref);
           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.


           void
           CallSavedSub1()
               CODE:
               PUSHMARK(SP);
               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(\&fred);
           CallSavedSub1();

           SaveSub1( sub { print "Hello there\n" } );
           CallSavedSub1();

       By the time each of the "SaveSub1" statements above have 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

           $ref = \&fred;
           SaveSub1($ref);
           $ref = 47;
           CallSavedSub1();

       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

           $ref = \&fred;
           SaveSub1($ref);
           $ref = \&joe;
           CallSavedSub1();
               SV *    name
               CODE:
               /* Take a copy of the callback */
               if (keepSub == (SV*)NULL)
                   /* First time, so create a new SV */
                   keepSub = newSVsv(name);
               else
                   /* Been here before, so overwrite */
                   SvSetSV(keepSub, name);

           void
           CallSavedSub2()
               CODE:
               PUSHMARK(SP);
               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
       "SvSetSV".

   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_PrintList()
           {
               dSP;

               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

           {
               }

               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');
           $a->Display(1);
           Mine->PrintID;

       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.

           void
           call_Method(ref, method, index)
               SV *    ref
               char *  method
               int             index
               CODE:
               PUSHMARK(SP);
               XPUSHs(ref);
               XPUSHs(sv_2mortal(newSViv(index)));
               PUTBACK;

               call_method(method, G_DISCARD);

           void
           call_PrintID(class, method)
               char *  class
               char *  method

       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.

           void
           PrintContext()
               CODE:
               I32 gimme = GIMME_V;
               if (gimme == G_VOID)
                   printf ("Context is Void\n");
               else if (gimme == G_SCALAR)
                   printf ("Context is Scalar\n");
               else
                   printf ("Context is Array\n");

       and here is some Perl to test it

           PrintContext;
           $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 disposed of using the "ENTER"/"SAVETMPS" -
            "FREETMPS"/"LEAVE" pairing.

       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

           ENTER;
           SAVETMPS;
           ...
           FREETMPS;
           LEAVE;

       sequence in the callback (and not, of course, specifying the G_DISCARD
       flag).
       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
       situation

           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
       sequence

           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
       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.

           register_fatal(cb1);

       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
           cb1()
           {
               printf ("Fatal Error\n");
               exit(1);
           }

       Now change that to call a Perl subroutine instead

           static SV * callback = (SV*)NULL;

           static void
           cb1()
           {
               dSP;

               PUSHMARK(SP);

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


           void
           register_fatal(fn)
               SV *    fn
               CODE:
               /* Remember the Perl sub */
               if (callback == (SV*)NULL)
                   callback = newSVsv(fn);
               else
                   SvSetSV(callback, fn);

               /* register the callback with the external library */
               register_fatal(cb1);

       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
       handle.

           asynch_read(fh, ProcessRead)

       This may expect the C ProcessRead function of this form

           void
           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;

           void
           asynch_read(fh, callback)
               int     fh
               SV *    callback
               CODE:
               /* 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);


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

               PUSHMARK(SP);
               XPUSHs(sv_2mortal(newSViv(fh)));
               XPUSHs(sv_2mortal(newSVpv(buffer, 0)));
               PUTBACK;

               /* 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".

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

               /* Now call the real asynch_close */
               asynch_close(fh);

       So the Perl interface would look like this

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

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

           asynch_close($fh);

       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

           void
           ProcessRead(buffer)
           char *      buffer;
           {
               ...

           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;
           {
               dSP;

               PUSHMARK(SP);
               XPUSHs(sv_2mortal(newSVpv(buffer, 0)));
               PUTBACK;

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

           static void
           fn1(buffer)
           char * buffer;
           {
               Pcb(0, buffer);
           }

           static void
           fn2(buffer)
           char * buffer;
           {
               Pcb(1, buffer);
           }

           static void
           fn3(buffer)
           char * buffer;
           {
               Pcb(2, buffer);
           }

           void

                   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);
               else
                   SvSetSV(Map[index].PerlSub, callback);

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

           void
           array_asynch_close(fh)
               int     fh
               CODE:
               int index;

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

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

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

               asynch_close(fh);

       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
       example.

       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
       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
            Perl.

   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;
           {
               dSP;
               I32 ax;
               int count;

               ENTER;
               SAVETMPS;

               PUSHMARK(SP);
               XPUSHs(sv_2mortal(newSViv(a)));
               XPUSHs(sv_2mortal(newSViv(b)));
               PUTBACK;

               count = call_pv("AddSubtract", G_ARRAY);

               SPAGAIN;
               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)));

       2.   The code

                    SPAGAIN;
                    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
       code:

        ...

        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.

LIGHTWEIGHT CALLBACKS
       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 slower.)

       The pattern of macro calls is like this:

           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.

SEE ALSO
       perlxs, perlguts, perlembed

AUTHOR
       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.

DATE
       Version 1.3, 14th Apr 1997



perl v5.10.1                      2009-04-12                       PERLCALL(1)
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