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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

       ·    An Error Handler

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

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

       ·    An Event Driven Program

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

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

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

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

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

	   I32 call_sv(SV* sv, I32 flags);
	   I32 call_pv(char *subname, I32 flags);
	   I32 call_method(char *methname, I32 flags);
	   I32 call_argv(char *subname, I32 flags, register char **argv);

       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 a bit mask which
       can consist of any combination of the symbols defined below, OR'ed
       together.

   G_VOID
       Calls the Perl subroutine in a void context.

       This flag has 2 effects:

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

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

       The value returned by the 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 a scalar context shows an example of
       this behavior.

   G_ARRAY
       Calls the Perl subroutine in a list context.

       As with G_SCALAR, this flag has 2 effects:

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

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

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

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

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

       See Using G_EVAL for details on using G_EVAL.

   G_KEEPERR
       You may have noticed that using the G_EVAL flag described above will
       always clear the $@ variable and set it to a string describing the
       error iff there was an error in the called code.	 This unqualified
       resetting of $@ can be problematic in the reliable identification of
       errors using the "eval {}" mechanism, because the possibility exists
       that perl will call other code (end of block processing code, for
       example) between the time the error causes $@ to be set within "eval
       {}", and the subsequent statement which checks for the value of $@ gets
       executed in the user's script.

       This scenario will mostly be applicable to code that is meant to be
       called from within destructors, asynchronous callbacks, signal
       handlers, "__DIE__" or "__WARN__" hooks, and "tie" functions.  In such
       situations, you will not want to clear $@ at all, but simply to append
       any new errors to any existing value of $@.

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

       When G_KEEPERR is used, any errors in the called code will be prefixed
       with the string "\t(in cleanup)", and appended to the current value of
       $@.  an error will not be appended if that same error string is already
       at the end of $@.

       In addition, a warning is generated using the appended string. This can
       be disabled using "no warnings 'misc'".

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

	       call_pv("LeftString", G_DISCARD);

	       FREETMPS;
	       LEAVE;
	   }

       Here are a few notes on the C function call_LeftString.

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

       2.   If you are going to put something onto the Perl stack, you need to
	    know where to put it. This is the purpose of the macro "dSP"--it
	    declares and 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 XPUSHs. This is where the parameters actually get
	    pushed onto the stack. In this case we are pushing a string and an
	    integer.

	    See "XSUBs and the Argument Stack" in perlguts for details on how
	    the XPUSH macros work.

       5.   Because we created temporary values (by means of sv_2mortal()
	    calls) we will have to tidy up the Perl stack and dispose of
	    mortal SVs.

	    This is the purpose of

		ENTER;
		SAVETMPS;

	    at the start of the function, and

		FREETMPS;
		LEAVE;

	    at the end. The "ENTER"/"SAVETMPS" pair creates a boundary for any
	    temporaries we create.  This means that the temporaries we get rid
	    of will be limited to those which were created after these calls.

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

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

	       count = call_pv("Adder", G_SCALAR);

	       SPAGAIN;

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

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

	       PUTBACK;
	       FREETMPS;
	       LEAVE;
	   }

       Points to note this time are

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

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

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

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

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

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

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

		POPs	    SV
		POPp	    pointer
		POPn	    double
		POPi	    integer
		POPl	    long

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

   Returning a list of values
       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 = 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 a scalar context
       Say the Perl subroutine in the previous section was called in a scalar
       context, like this

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

	       ENTER;
	       SAVETMPS;

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

	       count = call_pv("AddSubtract", G_SCALAR);

	       SPAGAIN;

	       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);
	       XPUSHs(sva);
	       XPUSHs(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;

	       ENTER;
	       SAVETMPS;

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

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

	       SPAGAIN;

	       /* Check the eval first */
	       if (SvTRUE(ERRSV))
	       {
		   printf ("Uh oh - %s\n", SvPV_nolen(ERRSV));
		   POPs;
	       }
	       else
	       {
		   if (count != 1)
		      croak("call_Subtract: wanted 1 value from 'Subtract', got %d\n",
			       count);

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

	       PUTBACK;
	       FREETMPS;
	       LEAVE;
	   }

       If call_Subtract is called thus

	   call_Subtract(4, 5)

       the following will be printed

	   Uh oh - death can be fatal

       Notes

       1.   We want to be able to catch the die so we have used the G_EVAL
	    flag.  Not specifying this flag would mean that the program would
	    terminate immediately at the die statement in the subroutine
	    Subtract.

       2.   The code

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

	    is the direct equivalent of this bit of Perl

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

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

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

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

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

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

       This example will fail to recognize that an error occurred inside the
       "eval {}".  Here's why: the call_Subtract code got executed while perl
       was cleaning up temporaries when exiting the eval block, and because
       call_Subtract is implemented with 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.

       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.  For Perl 4 this was adequate, but Perl
       5 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 have been executed,
       the SV*s which corresponded to the parameters will no longer exist.
       Expect an error message from Perl of the form

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

       for each of the "CallSavedSub1" lines.

       Similarly, with this code

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

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

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

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

       A similar but more subtle problem is illustrated with this 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);
	       call_sv(keepSub, G_DISCARD|G_NOARGS);

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

   Using call_argv
       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()
	   {
	       dSP;

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

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

   Using call_method
       Consider the following Perl code

	   {
	       package Mine;

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

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

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

       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

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

       ·    the name of the method.

       ·    any other parameters specific to the method.

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

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

	       call_method(method, G_DISCARD);

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

	       call_method(method, G_DISCARD);

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

	   $a = Mine->new('red', 'green', 'blue');
	   call_Method($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:
	       I32 gimme = GIMME_V;
	       if (gimme == G_VOID)
		   printf ("Context is Void\n");
	       else if (gimme == G_SCALAR)
		   printf ("Context is Scalar\n");
	       else
		   printf ("Context is Array\n");

       and here is some Perl to test it

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

       The output from that will be

	   Context is Void
	   Context is Scalar
	   Context is Array

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

       ·    specifying the G_DISCARD flag with call_*.

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

       There is another method which can be used, namely letting Perl do it
       for you automatically whenever it regains 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);
	       XPUSHs(sv_2mortal(newSViv(fh)));
	       XPUSHs(sv_2mortal(newSVpv(buffer, 0)));
	       PUTBACK;

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

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

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

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

       So the Perl interface would look like this

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

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

	   asynch_close($fh);

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

       What if the interface provided by the C callback doesn't contain a
       parameter which allows the file handle to Perl subroutine mapping?  Say
       in the asynchronous i/o package, the callback function gets passed only
       the "buffer" 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);
	       XPUSHs(sv_2mortal(newSViv(a)));
	       XPUSHs(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 you.

       2.   The code

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

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

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

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

	...

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

	...

	call_sv(cvrv, G_VOID|G_NOARGS);

       "eval_pv" is used to compile the anonymous subroutine, which will be
       the return value as well (read more about "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 */
	   I32 gimme = G_SCALAR;       /* context of the call: G_SCALAR,
					* G_LIST, 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
       Version 1.3, 14th Apr 1997

perl v5.10.1			  2009-04-12			   PERLCALL(1)
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