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perlcall



DESCRIPTION

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

       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

       * An Error Handler
            You have created an XSUB interface to an applica­
            tion's C API.

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

       * 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 func­
            tions 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) ;

            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 subrou­
            tine. 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" corre­
            sponds 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 parame­
            ter, "argv", consists of a NULL terminated list of C
            strings to be passed as parameters to the Perl sub­
            routine.  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
       this case it will be 0.

       G_SCALAR

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

       This flag has 2 effects:

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

       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 subrou­
       tine 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 acces­
       sible 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 sec­
       tion 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 wan­
            tarray the result will be true).

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

       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 Return­
       ing a Scalar gives details of how to dispose of these tem­
       poraries 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.

       G_NOARGS

       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 pass­
       ing 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 execut­
       ing 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".

       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:

       ·    If the call_* function returns normally, then the
            value returned is as specified in the previous sec­
            tions.

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

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

       ·    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.

       G_KEEPERR

       You may have noticed that using the G_EVAL flag described
       above will always clear the $@ variable and set it to a
       string describing the error iff there was an error in the
       called code.  This unqualified resetting of $@ can be
       problematic in the reliable identification of errors using
       the "eval {}" mechanism, because the possibility exists
       that perl will call other code (end of block processing
       code, for example) between the time the error causes $@ to
       be set within "eval {}", 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
       called in a list context, "G_SCALAR" if in a scalar con­
       text, 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.


KNOWN PROBLEMS

       This section outlines all known problems that exist in the
       call_* functions.

       1.   If you are intending to make use of both the G_EVAL
            and G_SCALAR flags in your code, use a version of
            Perl greater than 5.000.  There is a bug in version
            5.000 of Perl which means that the combination of
            these two flags will not work as described in the
            section FLAG VALUES.

            Specifically, if the two flags are used when calling
            a subroutine and that subroutine does not call die,
            the value returned by call_* will be wrong.

       2.   In Perl 5.000 and 5.001 there is a problem with using
            call_* if the Perl sub you are calling attempts to
            trap a die.

            The symptom of this problem is that the called Perl
            sub will continue to completion, but whenever it
            attempts to pass control back to the XSUB, the pro­
            gram will immediately terminate.

            For example, say you want to call this Perl sub

                sub fred
                {
                    eval { die "Fatal Error" ; }
                    print "Trapped error: $@\n"
                        if $@ ;
                }

            via this XSUB

                void
                Call_fred()
                    CODE:
                    PUSHMARK(SP) ;
                    call_pv("fred", G_DISCARD|G_NOARGS) ;
                    fprintf(stderr, "back in Call_fred\n") ;

            When "Call_fred" is executed it will print

                Trapped error: Fatal Error


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.

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

       and here is a C function to call it

           static void
           call_PrintUID()
           {
               dSP ;

               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 Print­
            UID, 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

       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)));
               XPUSHs(sv_2mortal(newSViv(b)));
               PUTBACK ;

               call_pv("LeftString", G_DISCARD);

               FREETMPS ;
               LEAVE ;
           }

       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 initial­
            izes a local copy of the Perl stack pointer.

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

            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

                FREETMPS ;
                LEAVE ;

            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 val­
            ues returned by the Perl subroutine (see next exam­
            ple), 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
            destroyed.

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

       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 ;
               FREETMPS ;
               LEAVE ;
           }

       Points to note this time are

       1.   The only flag specified this time was G_SCALAR. That
            means 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
            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 ;
           {
               dSP ;
               int count ;

               ENTER ;
               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 ;

               count = call_pv("AddSubtract", G_SCALAR);

               SPAGAIN ;

       tine 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, AddSub­
       tract 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 neces­
       sary to make a note of their addresses--thus the two vari­
       ables "sva" and "svb".

       The reason this is necessary is that the area of the Perl
       stack which held them will very likely have been overwrit­
       ten 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 ;

               else
               {
                   if (count != 1)
                      croak("call_Subtract: wanted 1 value from 'Subtract', got %d\n",
                               count) ;

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

               PUTBACK ;
               FREETMPS ;
               LEAVE ;
           }

       If call_Subtract is called thus

           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))
                {
                    STRLEN n_a;
                    printf ("Uh oh - %s\n", SvPV(ERRSV, n_a)) ;
                    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 neces­
            sary 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 pro­

           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

       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 eval 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 reads:

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

       will preserve the error and restore reliable error han­
       dling.

       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)

           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.

       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 pro­
       gram, 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 ;

           void
           SaveSub1(name)
               SV *    name
               CODE:
               rememberSub = name ;

           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() ;

           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 "Call­
       SavedSub1" 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() ;

       This time whenever "CallSavedSub1" get called it will exe­
       cute 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" mod­
       ified to do that

           static SV * keepSub = (SV*)NULL ;

           void
           SaveSub2(name)
               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()

       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 Print­
       List.

           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

           {
               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" ;
               }
           }
           1: green
           This is Class Mine version 1.0

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

       ·    a reference to the object for a virtual method or the
            name of the class for a static method.

       ·    the name of the method.

       ·    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
               CODE:
               PUSHMARK(SP);
               XPUSHs(sv_2mortal(newSVpv(class, 0))) ;
               PUTBACK;

               call_method(method, G_DISCARD) ;

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

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

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

       Using GIMME_V
                   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

       ·    specifying the G_DISCARD flag with call_*.

       ·    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 con­
       trol 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).

       If you are going to use this method you have to be aware
       of a possible memory leak which can arise under very spe­
       cific 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 dis­
       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 sur­
       rounding 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 hap­
       pen.  This means that as time goes on, your program will
       create more and more temporaries, none of which will ever
       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 */

       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 sub­
       routine 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)

           {
               dSP ;
               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") ;

               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" param­
       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 pre­
       define 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 ;
           {
               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)
           void
           array_asynch_read(fh, callback)
               int             fh
               SV *    callback
               CODE:
               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)
                       break ;

                   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 ;

       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 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 Return­
       ing a list of values recoded to use "ST" instead of
       "POP*".

           static void
           call_AddSubtract2(a, b)
           int a ;
           int b ;
           {
               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))) ;

               PUTBACK ;
               FREETMPS ;
               LEAVE ;
           }

       Notes

       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 neces­
            sary to define "ax" as it is already defined for you.

       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
       Gurusamy Sarathy and Larry Wall.


DATE

       Version 1.3, 14th Apr 1997

perl v5.8.1                 2003-09-02                PERLCALL(1)

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