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PERLCALL(1)	       Perl Programmers	Reference Guide		   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

       o    An Error Handler

	    You	have created an	XSUB interface to an application's C API.

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

       o    An Event-Driven Program

	    The	classic	example	of where callbacks are used is when writing an
	    event driven program, such as for an X11 application.  In this
	    case you register functions	to be called whenever specific events
	    occur, e.g., a mouse button	is pressed, the	cursor moves into a
	    window or a	menu item is selected.

       Although	the techniques described here are applicable when embedding
       Perl in a C program, this is not	the primary goal of this document.
       There are other details that must be considered and are specific	to
       embedding Perl. For details on embedding	Perl in	C refer	to perlembed.

       Before you launch yourself head first into the rest of this document,
       it would	be a good idea to have read the	following two
       documents--perlxs and perlguts.

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

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

	   I32 call_sv(SV* sv, I32 flags);
	   I32 call_pv(char *subname, I32 flags);
	   I32 call_method(char	*methname, I32 flags);
	   I32 call_argv(char *subname,	I32 flags, 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 VALUES".

       Each of the functions will now be discussed in turn.

       call_sv
	    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.

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

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

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

       All the functions return	an integer. This is a count of the number of
       items returned by the Perl subroutine. The actual items returned	by the
       subroutine are stored on	the Perl stack.

       As a general rule you should always check the return value from these
       functions.  Even	if you are expecting only a particular number of
       values to be returned from the Perl subroutine, there is	nothing	to
       stop someone from doing something unexpected--don't say you haven't
       been warned.

FLAG VALUES
       The "flags" parameter in	all the	call_* functions is one	of "G_VOID",
       "G_SCALAR", or "G_ARRAY", which indicate	the call context, OR'ed
       together	with a bit mask	of any combination of the other	G_* symbols
       defined below.

   G_VOID
       Calls the Perl subroutine in a void context.

       This flag has 2 effects:

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

       2.   It ensures that nothing is actually	returned from the subroutine.

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

   G_SCALAR
       Calls the Perl subroutine in a scalar context.  This is the default
       context flag setting for	all the	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 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 Scalar Context"
       shows an	example	of this	behavior.

   G_ARRAY
       Calls the Perl subroutine in a list context.

       As with G_SCALAR, this flag has 2 effects:

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

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

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

       If 0, then you have specified the G_DISCARD flag.

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

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

       If you do not set this flag then	it is very important that you make
       sure that any temporaries (i.e.,	parameters passed to the Perl
       subroutine and values returned from the subroutine) are disposed	of
       yourself.  The section "Returning a Scalar" gives details of how	to
       dispose of these	temporaries explicitly and the section "Using Perl to
       Dispose of Temporaries" discusses the specific circumstances where you
       can ignore the problem and let Perl deal	with it	for you.

   G_NOARGS
       Whenever	a Perl subroutine is called using one of the 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 "@_\n"  }

	   sub joe
	     { &fred }

	   &joe(1,2,3);

       This will print

	   1 2 3

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

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

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

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

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

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

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

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

       See "Using G_EVAL" for details on using G_EVAL.

   G_KEEPERR
       Using the G_EVAL	flag described above will always set $@: clearing it
       if there	was no error, and setting it to	describe the error if there
       was an error in the called code.	 This is what you want if your
       intention is to handle possible errors, but sometimes you just want to
       trap errors and stop them interfering with the rest of the program.

       This scenario will mostly be applicable to code that is meant to	be
       called from within destructors, asynchronous callbacks, and signal
       handlers.  In such situations, where the	code being called has little
       relation	to the surrounding dynamic context, the	main program needs to
       be insulated from errors	in the called code, even if they can't be
       handled intelligently.  It may also be useful to	do this	with code for
       "__DIE__" or "__WARN__" hooks, and "tie"	functions.

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

       When G_KEEPERR is used, any error in the	called code will terminate the
       call as usual, and the error will not propagate beyond the call (as
       usual for G_EVAL), but it will not go into $@.  Instead the error will
       be converted into a warning, prefixed with the string "\t(in cleanup)".
       This can	be disabled using "no warnings 'misc'".	 If there is no	error,
       $@ will not be cleared.

       Note that the G_KEEPERR flag does not propagate into inner evals; these
       may still set $@.

       The G_KEEPERR flag was introduced in Perl version 5.002.

       See "Using G_KEEPERR" for an example of a situation that	warrants the
       use of this flag.

   Determining the Context
       As mentioned above, you can determine the context of the	currently
       executing subroutine in Perl with wantarray.  The equivalent test can
       be made in C by using the "GIMME_V" macro, which	returns	"G_ARRAY" if
       you have	been called in a list context, "G_SCALAR" if in	a scalar
       context,	or "G_VOID" if in a void context (i.e.,	the return value will
       not be used).  An older version of this macro is	called "GIMME";	in a
       void context it returns "G_SCALAR" instead of "G_VOID".	An example of
       using the "GIMME_V" macro is shown in section "Using GIMME_V".

EXAMPLES
       Enough of the definition	talk! Let's have a few examples.

       Perl provides many macros to assist in accessing	the Perl stack.
       Wherever	possible, these	macros should always be	used when interfacing
       to Perl internals.  We hope this	should make the	code less vulnerable
       to any changes made to Perl in the future.

       Another point worth noting is that in the first series of examples I
       have made use of	only the call_pv function.  This has been done to keep
       the code	simpler	and ease you into the topic.  Wherever possible, if
       the choice is between using call_pv and call_sv,	you should always try
       to use call_sv.	See "Using call_sv" for	details.

   No Parameters, Nothing Returned
       This first trivial example will call a Perl subroutine, PrintUID, to
       print out the UID of the	process.

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

       and here	is a C function	to call	it

	   static void
	   call_PrintUID()
	   {
	       dSP;

	       PUSHMARK(SP);
	       call_pv("PrintUID", G_DISCARD|G_NOARGS);
	   }

       Simple, eh?

       A few points to note about this example:

       1.   Ignore "dSP" and "PUSHMARK(SP)" for	now. They will be discussed in
	    the	next example.

       2.   We aren't passing any parameters to	PrintUID so G_NOARGS can be
	    specified.

       3.   We aren't interested in anything returned from PrintUID, so
	    G_DISCARD is specified. Even if PrintUID was changed to return
	    some value(s), having specified G_DISCARD will mean	that they will
	    be wiped by	the time control returns from call_pv.

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

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

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

       So the Perl subroutine would look like this:

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

       The C function required to call LeftString would	look like this:

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

	       ENTER;
	       SAVETMPS;

	       PUSHMARK(SP);
	       EXTEND(SP, 2);
	       PUSHs(sv_2mortal(newSVpv(a, 0)));
	       PUSHs(sv_2mortal(newSViv(b)));
	       PUTBACK;

	       call_pv("LeftString", G_DISCARD);

	       FREETMPS;
	       LEAVE;
	   }

       Here are	a few notes on the C function call_LeftString.

       1.   Parameters are passed to the Perl subroutine using the Perl	stack.
	    This is the	purpose	of the code beginning with the line "dSP" and
	    ending with	the line "PUTBACK".  The "dSP" declares	a local	copy
	    of the stack pointer.  This	local copy should always be accessed
	    as "SP".

       2.   If you are going to	put something onto the Perl stack, you need to
	    know where to put it. This is the purpose of the macro "dSP"--it
	    declares and initializes a local copy of the Perl stack pointer.

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

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

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

	    The	"PUSHMARK" macro tells Perl to make a mental note of the
	    current stack pointer. Even	if you aren't passing any parameters
	    (like the example shown in the section "No Parameters, Nothing
	    Returned") you must	still call the "PUSHMARK" macro	before you can
	    call any of	the call_* functions--Perl still needs to know that
	    there are no parameters.

	    The	"PUTBACK" macro	sets the global	copy of	the stack pointer to
	    be the same	as our local copy. If we didn't	do this, call_pv
	    wouldn't know where	the two	parameters we pushed were--remember
	    that up to now all the stack pointer manipulation we have done is
	    with our local copy, not the global	copy.

       4.   Next, we come to EXTEND and	PUSHs. This is where the parameters
	    actually get pushed	onto the stack.	In this	case we	are pushing a
	    string and an integer.

	    Alternatively you can use the XPUSHs() macro, which	combines a
	    "EXTEND(SP,	1)" and	"PUSHs()".  This is less efficient if you're
	    pushing multiple values.

	    See	"XSUBs and the Argument	Stack" in perlguts for details on how
	    the	PUSH 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
	    destroyed.

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

	    See	the section "Using Perl	to Dispose of Temporaries" for details
	    of an alternative to using these macros.

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

   Returning a Scalar
       Now for an example of dealing with the items returned from a Perl
       subroutine.

       Here is a Perl subroutine, Adder, that takes 2 integer parameters and
       simply returns their sum.

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

       Because we are now concerned with the return value from Adder, the C
       function	required to call it is now a bit more complex.

	   static void
	   call_Adder(a, b)
	   int a;
	   int b;
	   {
	       dSP;
	       int count;

	       ENTER;
	       SAVETMPS;

	       PUSHMARK(SP);
	       EXTEND(SP, 2);
	       PUSHs(sv_2mortal(newSViv(a)));
	       PUSHs(sv_2mortal(newSViv(b)));
	       PUTBACK;

	       count = call_pv("Adder",	G_SCALAR);

	       SPAGAIN;

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

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

	       PUTBACK;
	       FREETMPS;
	       LEAVE;
	   }

       Points to note this time	are

       1.   The	only flag specified this time was G_SCALAR. That means that
	    the	@_ array will be created and that the value returned by	Adder
	    will still exist after the call to call_pv.

       2.   The	purpose	of the macro "SPAGAIN" is to refresh the local copy of
	    the	stack pointer. This is necessary because it is possible	that
	    the	memory allocated to the	Perl stack has been reallocated	during
	    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 (PV)
		POPpbytex   pointer to bytes (PV)
		POPn	    double (NV)
		POPi	    integer (IV)
		POPu	    unsigned integer (UV)
		POPl	    long
		POPul	    unsigned long

	    Since these	macros have side-effects don't use them	as arguments
	    to macros that may evaluate	their argument several times, for
	    example:

	      /* Bad idea, don't do this */
	      STRLEN len;
	      const char *s = SvPV(POPs, len);

	    Instead, use a temporary:

	      STRLEN len;
	      SV *sv = POPs;
	      const char *s = SvPV(sv, len);

	    or a macro that guarantees it will evaluate	its arguments only
	    once:

	      STRLEN len;
	      const char *s = SvPVx(POPs, len);

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

   Returning a List of Values
       Now, let's extend the previous example to return	both the sum of	the
       parameters and the difference.

       Here is the Perl	subroutine

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

       and this	is the C function

	   static void
	   call_AddSubtract(a, b)
	   int a;
	   int b;
	   {
	       dSP;
	       int count;

	       ENTER;
	       SAVETMPS;

	       PUSHMARK(SP);
	       EXTEND(SP, 2);
	       PUSHs(sv_2mortal(newSViv(a)));
	       PUSHs(sv_2mortal(newSViv(b)));
	       PUTBACK;

	       count = call_pv("AddSubtract", G_ARRAY);

	       SPAGAIN;

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

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

	       PUTBACK;
	       FREETMPS;
	       LEAVE;
	   }

       If call_AddSubtract is called like this

	   call_AddSubtract(7, 4);

       then here is the	output

	   7 - 4 = 3
	   7 + 4 = 11

       Notes

       1.   We wanted list context, so G_ARRAY was used.

       2.   Not	surprisingly "POPi" is used twice this time because we were
	    retrieving 2 values	from the stack.	The important thing to note is
	    that when using the	"POP*" macros they come	off the	stack in
	    reverse order.

   Returning a List in 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);
	       EXTEND(SP, 2);
	       PUSHs(sv_2mortal(newSViv(a)));
	       PUSHs(sv_2mortal(newSViv(b)));
	       PUTBACK;

	       count = 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 is returned	from the subroutine. AddSubtract actually made it back
       to call_AddSubScalar.

   Returning Data from Perl via	the Parameter List
       It is also possible to return values directly via the parameter
       list--whether it	is actually desirable to do it is another matter
       entirely.

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

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

       and here	is a C function	to call	it.

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

	       ENTER;
	       SAVETMPS;

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

	       PUSHMARK(SP);
	       EXTEND(SP, 2);
	       PUSHs(sva);
	       PUSHs(svb);
	       PUTBACK;

	       count = 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 call_pv it is necessary to make a	note of	their
       addresses--thus the two variables "sva" and "svb".

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

   Using G_EVAL
       Now an example using G_EVAL. Below is a Perl subroutine which computes
       the difference of its 2 parameters. If this would result	in a negative
       result, the subroutine calls die.

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

	       die "death can be fatal\n" if $a	< $b;

	       $a - $b;
	   }

       and some	C to call it

	static void
	call_Subtract(a, b)
	int a;
	int b;
	{
	    dSP;
	    int	count;
	    SV *err_tmp;

	    ENTER;
	    SAVETMPS;

	    PUSHMARK(SP);
	    EXTEND(SP, 2);
	    PUSHs(sv_2mortal(newSViv(a)));
	    PUSHs(sv_2mortal(newSViv(b)));
	    PUTBACK;

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

	    SPAGAIN;

	    /* Check the eval first */
	    err_tmp = ERRSV;
	    if (SvTRUE(err_tmp))
	    {
		printf ("Uh oh - %s\n",	SvPV_nolen(err_tmp));
		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

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

	    is the direct equivalent of	this bit of Perl

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

	    "PL_errgv" is a perl global	of type	"GV *" that points to the
	    symbol table entry containing the error.  "ERRSV" therefore	refers
	    to the C equivalent	of $@.	We use a local temporary, "err_tmp",
	    since "ERRSV" is a macro that calls	a function, and
	    "SvTRUE(ERRSV)" would end up calling that function multiple	times.

       3.   Note that the stack	is popped using	"POPs" in the block where
	    "SvTRUE(err_tmp)" 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 be tidied up	by removing the	undef.

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

	   package Foo;
	   sub new { bless {}, $_[0] }
	   sub Subtract	{
	       my($a,$b) = @_;
	       die "death can be fatal"	if $a <	$b;
	       $a - $b;
	   }
	   sub DESTROY { call_Subtract(5, 4); }
	   sub foo { die "foo dies"; }

	   package main;
	   {
	       my $foo = Foo->new;
	       eval { $foo->foo	};
	   }
	   print "Saw: $@" if $@;	      #	should be, but isn't

       This example will fail to recognize that	an error occurred inside the
       "eval {}".  Here's why: the call_Subtract code got executed while perl
       was cleaning up temporaries when	exiting	the outer braced block,	and
       because call_Subtract is	implemented with call_pv using the G_EVAL
       flag, it	promptly reset $@.  This results in the	failure	of the
       outermost test for $@, and thereby the failure of the error trap.

       Appending the G_KEEPERR flag, so	that the call_pv call in call_Subtract
       reads:

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

       will preserve the error and restore reliable error handling.

   Using call_sv
       In all the previous examples I have 'hard-wired'	the name of the	Perl
       subroutine to be	called from C.	Most of	the time though, it is more
       convenient to be	able to	specify	the name of the	Perl subroutine	from
       within the Perl script, and you'll want to use call_sv.

       Consider	the Perl code below

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

	   CallSubPV("fred");

       Here is a snippet of XSUB which defines CallSubPV.

	   void
	   CallSubPV(name)
	       char *  name
	       CODE:
	       PUSHMARK(SP);
	       call_pv(name, G_DISCARD|G_NOARGS);

       That is fine as far as it goes. The thing is, the Perl subroutine can
       be specified as only a string, however, Perl allows references to
       subroutines and anonymous subroutines.  This is where call_sv is
       useful.

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

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

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

	   CallSubSV("fred");
	   CallSubSV(\&fred);
	   $ref	= \&fred;
	   CallSubSV($ref);
	   CallSubSV( sub { print "Hello there\n" } );

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

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

	   static SV * rememberSub;

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

	   void
	   CallSavedSub1()
	       CODE:
	       PUSHMARK(SP);
	       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 has been executed,
       the SV*s	which corresponded to the parameters will no longer exist.
       Expect an error message from Perl of the	form

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

       for each	of the "CallSavedSub1" lines.

       Similarly, with this code

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

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

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

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

       A similar but more subtle problem is illustrated	with this code:

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

       This time whenever "CallSavedSub1" gets 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.

	   /* this isn't thread-safe */
	   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);
	       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".

       Note: using a static or global variable to store	the SV isn't thread-
       safe.  You can either use the "MY_CXT" mechanism	documented in "Safely
       Storing Static Data in XS" in perlxs which is fast, or store the	values
       in perl global variables, using get_sv(), which is much slower.

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

	   sub PrintList
	   {
	       my(@list) = @_;

	       foreach (@list) { print "$_\n" }
	   }

       And here	is an example of call_argv which will call PrintList.

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

	   static void
	   call_PrintList()
	   {
	       call_argv("PrintList", G_DISCARD, words);
	   }

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

   Using call_method
       Consider	the following Perl code:

	   {
	       package Mine;

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

	       sub Display
	       {
		   my ($self, $index) =	@_;
		   print "$index: $$self[$index]\n";
	       }

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

       It implements just a very simple	class to manage	an array.  Apart from
       the constructor,	"new", it declares methods, one	static and one
       virtual.	The static method, "PrintID", prints out simply	the class name
       and a version number. The virtual method, "Display", prints out a
       single element of the array.  Here is an	all-Perl example of using it.

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

       will print

	   1: green
	   This	is Class Mine version 1.0

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

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

       o    The	name of	the method

       o    Any	other parameters specific to the method

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

	   void
	   call_Method(ref, method, index)
	       SV *    ref
	       char *  method
	       int	       index
	       CODE:
	       PUSHMARK(SP);
	       EXTEND(SP, 2);
	       PUSHs(ref);
	       PUSHs(sv_2mortal(newSViv(index)));
	       PUTBACK;

	       call_method(method, G_DISCARD);

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

	       call_method(method, G_DISCARD);

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

	   $a =	Mine->new('red', 'green', 'blue');
	   call_Method($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.

	   void
	   PrintContext()
	       CODE:
	       U8 gimme	= GIMME_V;
	       if (gimme == G_VOID)
		   printf ("Context is Void\n");
	       else if (gimme == G_SCALAR)
		   printf ("Context is Scalar\n");
	       else
		   printf ("Context is Array\n");

       And here	is some	Perl to	test it.

	   PrintContext;
	   $a =	PrintContext;
	   @a =	PrintContext;

       The output from that will be

	   Context is Void
	   Context is Scalar
	   Context is Array

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

       o    Specifying the G_DISCARD flag with call_*

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

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

	   register_fatal(cb1);

       The single parameter "cb1" is a pointer to a function, so you must have
       defined "cb1" in	your code, say something like this

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

       Now change that to call a Perl subroutine instead

	   static SV * callback	= (SV*)NULL;

	   static void
	   cb1()
	   {
	       dSP;

	       PUSHMARK(SP);

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

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

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

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

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

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

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

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

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

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

	   asynch_read(fh, ProcessRead)

       This may	expect the C ProcessRead function of this form

	   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);
	       EXTEND(SP, 2);
	       PUSHs(sv_2mortal(newSViv(fh)));
	       PUSHs(sv_2mortal(newSVpv(buffer,	0)));
	       PUTBACK;

	       /* Call the Perl	sub */
	       call_sv(*sv, G_DISCARD);
	   }

       For completeness, here is "asynch_close".  This shows how to remove the
       entry from the hash "Mapping".

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

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

       So the Perl interface would look	like this

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

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

	   asynch_close($fh);

       The mapping between the C callback and Perl is stored in	the global
       hash "Mapping" this time. Using a hash has the distinct advantage that
       it allows an unlimited number of	callbacks to be	registered.

       What if the interface provided by the C callback	doesn't	contain	a
       parameter which allows the file handle to Perl subroutine mapping?  Say
       in the asynchronous i/o package,	the callback function gets passed only
       the "buffer" parameter like this

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

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

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

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

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

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

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

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

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

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

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

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

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

	   void
	   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
       recompiling.  None the less, as long as the number of functions is
       chosen with some	care, it is still a workable solution and in some
       cases is	the only one available.

       To summarize, here are a	number of possible methods for you to consider
       for storing the mapping between C and the Perl callback

       1. Ignore the problem - Allow only 1 callback
	    For	a lot of situations, like interfacing to an error handler,
	    this may be	a perfectly adequate solution.

       2. Create a sequence of callbacks - hard	wired limit
	    If it is impossible	to tell	from the parameters passed back	from
	    the	C callback what	the context is,	then you may need to create a
	    sequence of	C callback interface functions,	and store pointers to
	    each in an array.

       3. Use a	parameter to map to the	Perl callback
	    A hash is an ideal mechanism to store the mapping between C	and
	    Perl.

   Alternate Stack Manipulation
       Although	I have made use	of only	the "POP*" macros to access values
       returned	from Perl subroutines, it is also possible to bypass these
       macros and read the stack using the "ST"	macro (See perlxs for a	full
       description of the "ST" macro).

       Most of the time	the "POP*" macros should be adequate; the main problem
       with them is that they force you	to process the returned	values in
       sequence. This may not be the most suitable way to process the values
       in some cases. What we want is to be able to access the stack in	a
       random order. The "ST" macro as used when coding	an XSUB	is ideal for
       this purpose.

       The code	below is the example given in the section "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);
	       EXTEND(SP, 2);
	       PUSHs(sv_2mortal(newSViv(a)));
	       PUSHs(sv_2mortal(newSViv(b)));
	       PUTBACK;

	       count = call_pv("AddSubtract", G_ARRAY);

	       SPAGAIN;
	       SP -= count;
	       ax = (SP	- PL_stack_base) + 1;

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

	       printf ("%d + %d	= %d\n", a, b, SvIV(ST(0)));
	       printf ("%d - %d	= %d\n", a, b, SvIV(ST(1)));

	       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	us.

       2.   The	code

		    SPAGAIN;
		    SP -= count;
		    ax = (SP - PL_stack_base) +	1;

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

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

   Creating and	Calling	an Anonymous Subroutine	in C
       As we've	already	shown, "call_sv" can be	used to	invoke an anonymous
       subroutine.  However, our example showed	a Perl script invoking an XSUB
       to perform this operation.  Let's see how it can	be done	inside our C
       code:

	...

	SV *cvrv
	   = eval_pv("sub {
		       print 'You will not find	me cluttering any namespace!'
		      }", TRUE);

	...

	call_sv(cvrv, G_VOID|G_NOARGS);

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

LIGHTWEIGHT CALLBACKS
       Sometimes you need to invoke the	same subroutine	repeatedly.  This
       usually happens with a function that acts on a list of values, such as
       Perl's built-in sort(). You can pass a comparison function to sort(),
       which will then be invoked for every pair of values that	needs to be
       compared. The first() and reduce() functions from List::Util follow a
       similar pattern.

       In this case it is possible to speed up the routine (often quite
       substantially) by using the lightweight callback	API.  The idea is that
       the calling context only	needs to be created and	destroyed once,	and
       the sub can be called arbitrarily many times in between.

       It is usual to pass parameters using global variables (typically	$_ for
       one parameter, or $a and	$b for two parameters) rather than via @_. (It
       is possible to use the @_ mechanism if you know what you're doing,
       though there is as yet no supported API for it. It's also inherently
       slower.)

       The pattern of macro calls is like this:

	   dMULTICALL;		       /* Declare local	variables */
	   U8 gimme = G_SCALAR;	       /* context of the call: G_SCALAR,
					* G_ARRAY, or G_VOID */

	   PUSH_MULTICALL(cv);	       /* Set up the context for calling cv,
					  and set local	vars appropriately */

	   /* loop */ {
	       /* set the value(s) af your parameter variables */
	       MULTICALL;	       /* Make the actual call */
	   } /*	end of loop */

	   POP_MULTICALL;	       /* Tear down the	calling	context	*/

       For some	concrete examples, see the implementation of the first() and
       reduce()	functions of List::Util	1.18. There you	will also find a
       header file that	emulates the multicall API on older versions of	perl.

SEE ALSO
       perlxs, perlguts, perlembed

AUTHOR
       Paul Marquess

       Special thanks to the following people who assisted in the creation of
       the document.

       Jeff Okamoto, Tim Bunce,	Nick Gianniotis, Steve Kelem, Gurusamy Sarathy
       and Larry Wall.

DATE
       Last updated for	perl 5.23.1.

perl v5.34.0			  2020-11-18			   PERLCALL(1)

NAME | DESCRIPTION | THE CALL_ FUNCTIONS | FLAG VALUES | EXAMPLES | LIGHTWEIGHT CALLBACKS | SEE ALSO | AUTHOR | DATE

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