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NAME    [Toc]    [Back]

       perlcall - Perl calling conventions from C

DESCRIPTION    [Toc]    [Back]

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

       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

            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 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    [Toc]    [Back]

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

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

           I32 call_sv(SV* sv, I32 flags) ;
           I32 call_pv(char *subname, I32 flags) ;
           I32 call_method(char *methname, I32 flags) ;
           I32 call_argv(char *subname, I32 flags, register  char
**argv) ;
       The key function is call_sv.  All the other functions are
       fairly simple wrappers which make it easier to call Perl
       subroutines in special cases. At the end of the day they
       will all call call_sv to invoke the Perl subroutine.

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

       Each of the functions will now be discussed in turn.

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

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

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

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

       All the functions return an integer. This is a count of
       the number of items returned by the Perl subroutine. The
       actual items returned by the subroutine are stored on the
       Perl stack.
       As a general rule you should always check the return value
       from these functions.  Even if you are expecting only a
       particular number of values to be returned from the Perl
       subroutine, there is nothing to stop someone from doing
       something unexpected--don't say you haven't been warned.

FLAG VALUES    [Toc]    [Back]

       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    [Toc]    [Back]

       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 wan-
            tarray the result will be the undefined value).

       2.   It ensures that nothing is actually returned from the

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

       G_SCALAR    [Toc]    [Back]

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

       This flag has 2 effects:

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

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

       If 0, then you have specified the G_DISCARD flag.

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

       G_ARRAY    [Toc]    [Back]

       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 subroutine
 will be accessible when control returns from the
            call_* function.

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

       If 0, then you have specified the G_DISCARD flag.

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

       G_DISCARD    [Toc]    [Back]

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

       If you do not set this flag then it is very important that
       you make sure that any temporaries (i.e., parameters
       passed to the Perl subroutine and values returned from the
       subroutine) are disposed of yourself.  The section Return-
       ing 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

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

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

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

           sub fred
             { print "@_0  }

           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    [Toc]    [Back]

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

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

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

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

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

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

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

       See Using G_EVAL for details on using G_EVAL.

       G_KEEPERR    [Toc]    [Back]

       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 "(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 a list context, "G_SCALAR" if in a scalar context,
 or "G_VOID" if in a void context (i.e. the return
       value will not be used).  An older version of this macro
       is called "GIMME"; in a void context it returns "G_SCALAR"
       instead of "G_VOID".  An example of using the "GIMME_V"
       macro is shown in section Using GIMME_V.

KNOWN PROBLEMS    [Toc]    [Back]

       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 program
 will immediately terminate.

            For example, say you want to call this Perl sub

                sub fred
                    eval { die "Fatal Error" ; }
                    print "Trapped error: $@0
                        if $@ ;

            via this XSUB

                    PUSHMARK(SP) ;
                    call_pv("fred", G_DISCARD|G_NOARGS) ;
                    fprintf(stderr, "back in Call_fred0) ;
            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
            call_* as shown below

                    PUSHMARK(SP) ;
                    call_pv("fred", G_EVAL|G_DISCARD|G_NOARGS) ;
                    fprintf(stderr, "back in Call_fred0) ;

EXAMPLES    [Toc]    [Back]

       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 $<0 ;

       and here is a C function to call it

           static void
               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
            time control returns from call_pv.

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

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

       Passing Parameters    [Toc]    [Back]

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

       So the Perl subroutine would look like this

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

       The C function required to call LeftString would look like

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

               ENTER ;
               SAVETMPS ;
               PUSHMARK(SP) ;
               XPUSHs(sv_2mortal(newSVpv(a, 0)));
               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 initializes
 a local copy of the Perl stack pointer.

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

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

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

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

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

            See the section Using Perl to dispose of temporaries
            for details of an alternative to using these  macros.

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

       Returning a Scalar    [Toc]    [Back]

       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 ;

               PUSHMARK(SP) ;
               PUTBACK ;

               count = call_pv("Adder", G_SCALAR);

               SPAGAIN ;

               if (count != 1)
                   croak("Big trouble0) ;

               printf ("The sum of %d and %d is %d0, 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

       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
            call_pv call.

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

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

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

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

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

                POPs        SV
                POPp        pointer
                POPn        double
                POPi        integer
                POPl        long

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

       Returning a list of values    [Toc]    [Back]

       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 ;

               PUSHMARK(SP) ;
               PUTBACK ;

               count = call_pv("AddSubtract", G_ARRAY);

               SPAGAIN ;

               if (count != 2)
                   croak("Big trouble0) ;

               printf ("%d - %d = %d0, a, b, POPi) ;
               printf ("%d + %d = %d0, 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


       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

       Returning a list in a scalar context    [Toc]    [Back]

       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 ;

               PUSHMARK(SP) ;
               PUTBACK ;

               count = call_pv("AddSubtract", G_SCALAR);

               SPAGAIN ;

               printf ("Items Returned = %d0, count) ;

               for (i = 1 ; i <= count ; ++i)
                   printf ("Value %d = %d0, i, POPi) ;

               PUTBACK ;
               FREETMPS ;
               LEAVE ;

       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, AddSub-
       tract actually made it back to call_AddSubScalar.

       Returning Data from Perl via the parameter list    [Toc]    [Back]

       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 ;

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

               PUSHMARK(SP) ;
               PUTBACK ;

               count = call_pv("Inc", G_DISCARD);

               if (count != 0)
                   croak   ("call_Inc:  expected  0  values  from
'Inc', got %d0,
                          count) ;

               printf ("%d + 1 = %d0, a, SvIV(sva)) ;
               printf ("%d + 1 = %d0, b, SvIV(svb)) ;

               FREETMPS ;
               LEAVE ;

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

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

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

               die "death can be fatal0 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 ;

               PUSHMARK(SP) ;
               PUTBACK ;

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

               SPAGAIN ;

               /* Check the eval first */
               if (SvTRUE(ERRSV))
                   STRLEN n_a;
                   printf ("Uh oh - %s0, SvPV(ERRSV, n_a)) ;
                   POPs ;
                   if (count != 1)
                      croak("call_Subtract: wanted 1  value  from
'Subtract', got %d0,
                               count) ;

                   printf ("%d - %d = %d0, 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


       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 - %s0, SvPV(ERRSV, n_a)) ;
                    POPs ;

            is the direct equivalent of this bit of Perl

                print "Uh oh - $@0 if $@ ;

            "PL_errgv" is a perl global of type "GV *" that
            points to the symbol table entry containing the
            error.  "ERRSV" therefore refers to the C equivalent
            of $@.

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

       Using G_KEEPERR    [Toc]    [Back]

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

       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",

       will preserve the error and restore reliable error handling.

       Using call_sv    [Toc]    [Back]

       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

       Consider the Perl code below

           sub fred
               print "Hello there0 ;

           CallSubPV("fred") ;

       Here is a snippet of XSUB which defines CallSubPV.
               char *  name
               PUSHMARK(SP) ;
               call_pv(name, G_DISCARD|G_NOARGS) ;

       That is fine as far as it goes. The thing is, the Perl
       subroutine can be specified as only a string.  For Perl 4
       this was adequate, but Perl 5 allows references to subroutines
 and anonymous subroutines.  This is where call_sv is

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

               SV *    name
               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 there0 } ) ;

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

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

           static SV * rememberSub ;

               SV *    name
               rememberSub = name ;
               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 there0 } ) ;
           CallSavedSub1() ;

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

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

       for each of the "CallSavedSub1" lines.

       Similarly, with this code

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

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

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

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

       A similar but more subtle problem is illustrated with this
           $ref = fred ;
           SaveSub1($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

       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 ;

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

               PUSHMARK(SP) ;
               call_sv(keepSub, G_DISCARD|G_NOARGS) ;

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

       Using call_argv    [Toc]    [Back]

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

           sub PrintList
               my(@list) = @_ ;

               foreach (@list) { print "$_0 }
       and here is an example of call_argv which will call Print-

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

           static void
               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    [Toc]    [Back]

       Consider the following Perl code

               package Mine ;

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

               sub Display
                   my ($self, $index) = @_ ;
                   print "$index: $$self[$index]0 ;

               sub PrintID
                   my($class) = @_ ;
                   print "This is Class $class version 1.00 ;

       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

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

       o    the name of the method.

       o    any other parameters specific to the method.

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

           call_Method(ref, method, index)
               SV *    ref
               char *  method
               int             index
               XPUSHs(sv_2mortal(newSViv(index))) ;

               call_method(method, G_DISCARD) ;

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

               call_method(method, G_DISCARD) ;

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

           $a = new Mine ('red', 'green', 'blue') ;
           call_Method($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 call_method.
       Using GIMME_V

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

               I32 gimme = GIMME_V;
               if (gimme == G_VOID)
                   printf ("Context is Void0) ;
               else if (gimme == G_SCALAR)
                   printf ("Context is Scalar0) ;
                   printf ("Context is Array0) ;

       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    [Toc]    [Back]

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

       o    specifying the G_DISCARD flag with call_*.

       o    explicitly disposed of using the "ENTER"/"SAVETMPS" -
            "FREETMPS"/"LEAVE" pairing.

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

           ENTER ;
           SAVETMPS ;
           FREETMPS ;
           LEAVE ;

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

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

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

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

       In the second example, an event driven program, the flow
       of control will be more like this
           perl --> XSUB --> event handler
                             event handler --> call_* --> perl
                             event handler <-- call_* <----+
                             event handler --> call_* --> perl
                             event handler <-- call_* <----+
                             event handler --> call_* --> perl
                             event handler <-- call_* <----+

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

           event handler --> call_* --> perl

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

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

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

       Strategies for storing Callback Context Information    [Toc]    [Back]

       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
               printf ("Fatal Error0) ;
               exit(1) ;

       Now change that to call a Perl subroutine instead

           static SV * callback = (SV*)NULL;

           static void
               dSP ;

               PUSHMARK(SP) ;

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

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

               /* register the callback with the external library
               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...0 ;
       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

       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

           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

           static HV * Mapping = (HV*)NULL ;

           asynch_read(fh, callback)
               int     fh
               SV *    callback
               /* If the hash doesn't already exist, create it */
               if (Mapping == (HV*)NULL)
                   Mapping = newHV() ;
               /* Save the fh -> callback mapping */
               hv_store(Mapping,     (char*)&fh,      sizeof(fh),
newSVsv(callback), 0) ;

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

       and "asynch_read_if" could look like this

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

               /* Get the callback associated with fh */
               sv  =   hv_fetch(Mapping, (char*)&fh , sizeof(fh),
               if (sv == (SV**)NULL)
                   croak("Internal error...0) ;

               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".

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

               /* 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) ;


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