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NAME
       perlcall - Perl calling conventions from C



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

       - 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.

       - 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 the perlembed
       manpage.

       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 - the perlxs manpage and the
       perlguts manpage.

THE PERL_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






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           I32 perl_call_sv(SV* sv, I32 flags) ;
           I32 perl_call_pv(char *subname, I32 flags) ;
           I32 perl_call_method(char *methname, I32 flags) ;
           I32 perl_call_argv(char *subname, I32 flags, register char **argv) ;

       The key function is perl_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 perl_call_sv to invoke the Perl
       subroutine.

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

       Each of the functions will now be discussed in turn.

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

       perl_call_pv
            The function, perl_call_pv, is similar to
            perl_call_sv except it expects its first parameter to
            be a C char* which identifies the Perl subroutine you
            want to call, e.g., perl_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".

       perl_call_method
            The function perl_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 the perlobj manpage for
            more information on static and virtual methods and
            the section on Using perl_call_method for an example
            of using perl_call_method.

       perl_call_argv
            perl_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



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            of C strings to be passed as parameters to the Perl
            subroutine.  See Using perl_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 perl_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 perl_call_* function indicates
       how many items have been returned by the Perl subroutine -
       in 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 perl_call_*
       functions.

       This flag has 2 effects:

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




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       The value returned by the perl_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 perl_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 an array 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 perl_call_* function.

       The value returned by the perl_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 perl_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



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       you make sure that any temporaries (i.e., parameters
       passed to the Perl subroutine and values returned from the
       subroutine) are disposed of yourself.  The section
       Returning a Scalar gives details of how to dispose of
       these temporaries explicitly and the section Using Perl to
       dispose of temporaries discusses the specific
       circumstances where you can ignore the problem and let
       Perl deal with it for you.

       G_NOARGS

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

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

       In fact, what can happen is that the Perl subroutine you
       have called can access the @_ array from a previous Perl
       subroutine.  This will occur when the code that is
       executing the perl_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



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       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 perl_call_* function you
       need to check the $@ variable as you would in a normal
       Perl script.

       The value returned from the perl_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 perl_call_* function returns normally, then
            the value returned is as specified in the previous
            sections.

       -    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



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       G_EVAL in perl_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 $@.

       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 an
       array 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.

KNOWN PROBLEMS
       This section outlines all known problems that exist in the
       perl_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 perl_call_* will be wrong.

       2.   In Perl 5.000 and 5.001 there is a problem with using
            perl_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
            program will immediately terminate.

            For example, say you want to call this Perl sub






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

            via this XSUB

                void
                Call_fred()
                    CODE:
                    PUSHMARK(SP) ;
                    perl_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

            As control never returns to Call_fred, the "back in
            Call_fred" string will not get printed.

            To work around this problem, you can either upgrade
            to Perl 5.002 or higher, or use the G_EVAL flag with
            perl_call_* as shown below

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


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 perl_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 perl_call_pv and perl_call_sv, you should
       always try to use perl_call_sv.  See Using perl_call_sv
       for details.






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       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) ;
               perl_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 perl_call_pv.

       4.   As perl_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 perl_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



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           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 ;

               perl_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
            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.





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       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 perl_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 perl_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.   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.

       5.   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 the section on XSUBs and the Argument Stack in
            the perlguts manpage for details on how the XPUSH
            macros work.

       6.   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.




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

            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.

       7.   Finally, LeftString can now be called via the
            perl_call_pv function.

       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 = perl_call_pv("Adder", G_SCALAR);

               SPAGAIN ;




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               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
            perl_call_pv.

       2.   The purpose of the macro SPAGAIN is to refresh the
            local copy of the stack pointer. This is necessary
            because it is possible that the memory allocated to
            the Perl stack has been reallocated whilst in the
            perl_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
            perl_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 perl_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





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       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 ;
               SAVETMPS;

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

               count = perl_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 ;
           }




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       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 array 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 = perl_call_pv("AddSubtract", G_SCALAR);

               SPAGAIN ;

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

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

               PUTBACK ;
               FREETMPS ;
               LEAVE ;
           }




29/Apr/1999            perl 5.005, patch 03                    15





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


       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 ;




29/Apr/1999            perl 5.005, patch 03                    16





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


               count = perl_call_pv("Inc", G_DISCARD);

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

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

               FREETMPS ;
               LEAVE ;
           }

       To be able to access the two parameters that were pushed
       onto the stack after they return from perl_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 perl_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;






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


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

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

               SPAGAIN ;

               /* Check the eval first */
               if (SvTRUE(ERRSV))
               {
                   STRLEN n_a;
                   printf ("Uh oh - %s\n", SvPV(ERRSV, n_a)) ;
                   POPs ;
               }
               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 ;
                }




29/Apr/1999            perl 5.005, patch 03                    18





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


            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 perl_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

       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 perl_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 perl_call_pv
       call in call_Subtract reads:

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

       will preserve the error and restore reliable error
       handling.




29/Apr/1999            perl 5.005, patch 03                    19





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


       Using perl_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) ;
               perl_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
       perl_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 perl_call_sv instead of perl_call_pv.

           void
           CallSubSV(name)
               SV *    name
               CODE:
               PUSHMARK(SP) ;
               perl_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, perl_call_sv gives you much greater
       flexibility in how you can specify the Perl subroutine.



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


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

           static SV * rememberSub ;

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

           void
           CallSavedSub1()
               CODE:
               PUSHMARK(SP) ;
               perl_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 ...




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


       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() ;

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

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

           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()
               CODE:
               PUSHMARK(SP) ;
               perl_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,



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


       keepSub, is overwritten with the new value using SvSetSV.

       Using perl_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 perl_call_argv which will call
       PrintList.

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

           static void
           call_PrintList()
           {
               dSP ;

               perl_call_argv("PrintList", G_DISCARD, words) ;
           }

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

       Using perl_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" ;
               }








29/Apr/1999            perl 5.005, patch 03                    23





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


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

       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

       -    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;

               perl_call_method(method, G_DISCARD) ;









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


           void
           call_PrintID(class, method)
               char *  class
               char *  method
               CODE:
               PUSHMARK(SP);
               XPUSHs(sv_2mortal(newSVpv(class, 0))) ;
               PUTBACK;

               perl_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
       virtual methods, the method name is not passed via the
       stack - it is used as the first parameter to
       perl_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







29/Apr/1999            perl 5.005, patch 03                    25





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


       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
       perl_call_* function or values returned via the stack)
       have been freed by one of these methods

       -    specifying the G_DISCARD flag with perl_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
       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).

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

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

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

           perl --> XSUB --> external library

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






29/Apr/1999            perl 5.005, patch 03                    26





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


           perl --> XSUB --> external library
                             ...
                             error occurs
                             ...
                             external library --> perl_call --> perl
                                                                 |
           perl <-- XSUB <-- external library <-- perl_call <----+

       After processing of the error using perl_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 --> perl_call --> perl
                                                              |
                             event handler <-- perl_call <----+
                             ...
                             event handler --> perl_call --> perl
                                                              |
                             event handler <-- perl_call <----+
                             ...
                             event handler --> perl_call --> perl
                                                              |
                             event handler <-- perl_call <----+

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

           event handler --> perl_call --> perl

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

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



29/Apr/1999            perl 5.005, patch 03                    27





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


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

       Strategies for storing Callback Context Information

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

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

           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 */
               perl_call_sv(callback, G_DISCARD) ;
           }







29/Apr/1999            perl 5.005, patch 03                    28





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


           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) ;

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

           # Register the sub pcb1
           register_fatal(\&pcb1) ;

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

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

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

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

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

           asynch_read(fh, ProcessRead)

       This may expect the C ProcessRead function of this form





29/Apr/1999            perl 5.005, patch 03                    29





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


           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) ;

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

       and asynch_read_if could look like this

           static void
           asynch_read_if(fh, buffer)
           int fh ;
           char *      buffer ;
           {
               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 */
               perl_call_sv(*sv, G_DISCARD) ;
           }



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


       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 ;
           {
               ...
           }

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

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

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




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           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 */
               perl_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) ;
           }





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           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 ;

               asynch_close(fh) ;



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       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 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 the perlxs manpage 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.




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       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 = perl_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))) ;

               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
            necessary 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.




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


       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, perl_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 = perl_eval_pv("sub { print 'You will not find me cluttering any namespace!' }", TRUE);

        ...

        perl_call_sv(cvrv, G_VOID|G_NOARGS);

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

SEE ALSO
       the perlxs manpage, the perlguts manpage, the perlembed
       manpage

AUTHOR
       Paul Marquess <pmarquess@bfsec.bt.co.uk>

       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
















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