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PERLCALL(1)							   PERLCALL(1)

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 your 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
       perlapi 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

	   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
       actually 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 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_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.

       The value returned by the perl_call_* function indicates how may 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 behaviour.

       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 may items
       have been returned by the Perl subroutine.

       If 0, the 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 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
       explicitly dispose of these temporaries 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 of events occurs, the process will
       terminate immediately.  If though, 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 of using G_EVAL.

       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 macro. This will return G_SCALAR if you
       have been called in a scalar context and G_ARRAY if in an array
       context. An example of using the GIMME macro is shown in section Using
       GIMME.

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

		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 later), 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.  Hopefully 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.

       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 actually
	    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

	   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 ;

	       PUSHMARK(sp) ;
	       XPUSHs(sv_2mortal(newSVpv(a, 0)));
	       XPUSHs(sv_2mortal(newSViv(b)));
	       PUTBACK ;

	       perl_call_pv("LeftString", G_DISCARD);
	   }

       Here are a few notes on the C function call_LeftString.

       1.   Parameters are passed to the Perl subroutine using the Perl stack.
	    This is the purpose of the code beginning with the line dSP and
	    ending with the line PUTBACK.

       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 explicitly use the dSP macro - 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 automatically count the number of
	    parameters you are pushing. 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. Since 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 the section on XSUB'S and the Argument Stack in
	    the perlguts manpage for details on how the XPUSH macros work.

       6.   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,  which takes 2 integer parameters and
       simply returns their sum.

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

       Since 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 ;

	       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.   Because we are interested in what is returned from Adder we cannot
	    specify G_DISCARD. This means that we will have to tidy up the
	    Perl stack and dispose of any temporary values ourselves. 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, plus it will also dump the mortal SV's 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.

       3.   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 re-allocated
	    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 your local copy using SPAGAIN whenever you
	    make use of the perl_call_* functions or any other Perl internal
	    function.

       4.   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 ever happen.

       5.   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

       6.   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 ;
	   }

       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 ;
	   }

       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 returned from the subroutine, Adder 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 ;

	       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 ;
	       SV * sv ;

	       ENTER ;
	       SAVETMPS;

	       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 */
	       sv = GvSV(gv_fetchpv("@", TRUE, SVt_PV));
	       if (SvTRUE(sv))
	       {
		   printf ("Uh oh - %s\n", SvPV(sv, na)) ;
		   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

		sv = GvSV(gv_fetchpv("@", TRUE, SVt_PV));
		if (SvTRUE(sv))
		{
		    printf ("Uh oh - %s\n", SvPVx(sv, na)) ;
		    POPs ;
		}

	    is the direct equivalent of this bit of Perl

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

       3.   Note that the stack is popped using POPs in the block where
	    SvTRUE(sv) 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. Since we want
	    the program to continue after detecting this error, it is
	    essential that the stack is tidied up by removing the undef.

       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 only as 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) ;

       Since 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.

       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 to just 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
       dependant 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. Since 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) ;

       In order 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 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" ;
	       }

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

       It just implements 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, simply prints out 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) ;

	   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

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

	   void
	   PrintContext()
	       CODE:
	       if (GIMME == G_SCALAR)
		   printf ("Context is Scalar\n") ;
	       else
		   printf ("Context is Array\n") ;

       and here is some Perl to test it

	   $a = PrintContext ;
	   @a = PrintContext ;

       The output from that will be

	   Context is Scalar
	   Context is Array

       Using Perl to dispose of temporaries

       In the examples given to date, any temporaries created in the callback
       (i.e. parameters passed on the stack to the 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

	   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 the 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 actually 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 explicitly dispose 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) ;
	   }

	   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 1 callback
       registered at any time. An example could be an error handler like the
       code sketched out above. Remember though, repeated calls to
       register_fatal will replace the previously registered callback function
       with the new one.

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

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

	   asynch_read(fh, ProcessRead)

       This may expect the C ProcessRead function of this form

	   void
	   ProcessRead(fh, buffer)
	   int fh ;
	   char *      buffer ;
	   {
		...
	   }

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

	   static HV * Mapping = (HV*)NULL ;

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

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

	       /* 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) ;
	   }

       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 pre-define a
       series of C functions to act as the interface to Perl, thus

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

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

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

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

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

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

	       /* Call the Perl sub */
	       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) ;
	   }

	   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) ;

       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 re-
       compiling.  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 perlapi 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.

       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 - 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 - stack_base) + 1 ;

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

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

SEE ALSO
       the perlapi 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 and Larry Wall.

DATE
       Version 1.1, 17th May 1995

3rd Berkeley Distribution					   PERLCALL(1)
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