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+=head1 NAME
+
+perlcall - Perl calling conventions from C
+
+=head1 DESCRIPTION
+
+The purpose of this document is to show you how to call Perl subroutines
+directly from C, i.e., how to write I<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
+
+=over 5
+
+=item * 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.
+
+=item * 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.
+
+=back
+
+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
+L<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 -
+L<perlxs> and L<perlguts>.
+
+=head1 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 I<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 I<call_sv>
+to invoke the Perl subroutine.
+
+All the I<call_*> functions have a C<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 L<FLAG VALUES>.
+
+Each of the functions will now be discussed in turn.
+
+=over 5
+
+=item call_sv
+
+I<call_sv> takes two parameters, the first, C<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, I<Using call_sv>, shows how you can make
+use of I<call_sv>.
+
+=item call_pv
+
+The function, I<call_pv>, is similar to I<call_sv> except it
+expects its first parameter to be a C char* which identifies the Perl
+subroutine you want to call, e.g., C<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., C<"pkg::fred">.
+
+=item call_method
+
+The function I<call_method> is used to call a method from a Perl
+class. The parameter C<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 L<perlobj> for more information on
+static and virtual methods and L<Using call_method> for an example
+of using I<call_method>.
+
+=item call_argv
+
+I<call_argv> calls the Perl subroutine specified by the C string
+stored in the C<subname> parameter. It also takes the usual C<flags>
+parameter. The final parameter, C<argv>, consists of a NULL terminated
+list of C strings to be passed as parameters to the Perl subroutine.
+See I<Using call_argv>.
+
+=back
+
+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 I<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.
+
+=head1 FLAG VALUES
+
+The C<flags> parameter in all the I<call_*> functions is a bit mask
+which can consist of any combination of the symbols defined below,
+OR'ed together.
+
+
+=head2 G_VOID
+
+Calls the Perl subroutine in a void context.
+
+This flag has 2 effects:
+
+=over 5
+
+=item 1.
+
+It indicates to the subroutine being called that it is executing in
+a void context (if it executes I<wantarray> the result will be the
+undefined value).
+
+=item 2.
+
+It ensures that nothing is actually returned from the subroutine.
+
+=back
+
+The value returned by the I<call_*> function indicates how many
+items have been returned by the Perl subroutine - in this case it will
+be 0.
+
+
+=head2 G_SCALAR
+
+Calls the Perl subroutine in a scalar context. This is the default
+context flag setting for all the I<call_*> functions.
+
+This flag has 2 effects:
+
+=over 5
+
+=item 1.
+
+It indicates to the subroutine being called that it is executing in a
+scalar context (if it executes I<wantarray> the result will be false).
+
+=item 2.
+
+It ensures that only a scalar is actually returned from the subroutine.
+The subroutine can, of course, ignore the I<wantarray> and return a
+list anyway. If so, then only the last element of the list will be
+returned.
+
+=back
+
+The value returned by the I<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 I<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
+I<call_*> function. The section I<Returning a list in a scalar
+context> shows an example of this behavior.
+
+
+=head2 G_ARRAY
+
+Calls the Perl subroutine in a list context.
+
+As with G_SCALAR, this flag has 2 effects:
+
+=over 5
+
+=item 1.
+
+It indicates to the subroutine being called that it is executing in a
+list context (if it executes I<wantarray> the result will be true).
+
+
+=item 2.
+
+It ensures that all items returned from the subroutine will be
+accessible when control returns from the I<call_*> function.
+
+=back
+
+The value returned by the I<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 I<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.
+
+=head2 G_DISCARD
+
+By default, the I<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 I<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 I<Returning a Scalar> gives details of how to
+dispose of these temporaries explicitly and the section I<Using Perl to
+dispose of temporaries> discusses the specific circumstances where you
+can ignore the problem and let Perl deal with it for you.
+
+=head2 G_NOARGS
+
+Whenever a Perl subroutine is called using one of the I<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 C<@_> 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 C<@_> array from a previous Perl subroutine. This will
+occur when the code that is executing the I<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 C<fred> accesses the C<@_> array which
+belongs to C<joe>.
+
+
+=head2 G_EVAL
+
+It is possible for the Perl subroutine you are calling to terminate
+abnormally, e.g., by calling I<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 I<eval { }>
+around the subroutine call.
+
+Whenever control returns from the I<call_*> function you need to
+check the C<$@> variable as you would in a normal Perl script.
+
+The value returned from the I<call_*> function is dependent on
+what other flags have been specified and whether an error has
+occurred. Here are all the different cases that can occur:
+
+=over 5
+
+=item *
+
+If the I<call_*> function returns normally, then the value
+returned is as specified in the previous sections.
+
+=item *
+
+If G_DISCARD is specified, the return value will always be 0.
+
+=item *
+
+If G_ARRAY is specified I<and> an error has occurred, the return value
+will always be 0.
+
+=item *
+
+If G_SCALAR is specified I<and> an error has occurred, the return value
+will be 1 and the value on the top of the stack will be I<undef>. This
+means that if you have already detected the error by checking C<$@> and
+you want the program to continue, you must remember to pop the I<undef>
+from the stack.
+
+=back
+
+See I<Using G_EVAL> for details on using G_EVAL.
+
+=head2 G_KEEPERR
+
+You may have noticed that using the G_EVAL flag described above will
+B<always> clear the C<$@> variable and set it to a string describing
+the error iff there was an error in the called code. This unqualified
+resetting of C<$@> can be problematic in the reliable identification of
+errors using the C<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 C<$@> to be set within
+C<eval {}>, and the subsequent statement which checks for the value of
+C<$@> 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, C<__DIE__> or C<__WARN__> hooks, and C<tie> functions. In
+such situations, you will not want to clear C<$@> at all, but simply to
+append any new errors to any existing value of C<$@>.
+
+The G_KEEPERR flag is meant to be used in conjunction with G_EVAL in
+I<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 C<$@>. an error will not be appended if that same error string is
+already at the end of C<$@>.
+
+In addition, a warning is generated using the appended string. This can be
+disabled using C<no warnings 'misc'>.
+
+The G_KEEPERR flag was introduced in Perl version 5.002.
+
+See I<Using G_KEEPERR> for an example of a situation that warrants the
+use of this flag.
+
+=head2 Determining the Context
+
+As mentioned above, you can determine the context of the currently
+executing subroutine in Perl with I<wantarray>. The equivalent test
+can be made in C by using the C<GIMME_V> macro, which returns
+C<G_ARRAY> if you have been called in a list context, C<G_SCALAR> if
+in a scalar context, or C<G_VOID> if in a void context (i.e. the
+return value will not be used). An older version of this macro is
+called C<GIMME>; in a void context it returns C<G_SCALAR> instead of
+C<G_VOID>. An example of using the C<GIMME_V> macro is shown in
+section I<Using GIMME_V>.
+
+=head1 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 I<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 I<call_pv> and
+I<call_sv>, you should always try to use I<call_sv>. See
+I<Using call_sv> for details.
+
+=head2 No Parameters, Nothing returned
+
+This first trivial example will call a Perl subroutine, I<PrintUID>, to
+print out the UID of the process.
+
+ sub PrintUID
+ {
+ print "UID is $<\n";
+ }
+
+and here is a C function to call it
+
+ static void
+ call_PrintUID()
+ {
+ dSP;
+
+ PUSHMARK(SP);
+ call_pv("PrintUID", G_DISCARD|G_NOARGS);
+ }
+
+Simple, eh.
+
+A few points to note about this example.
+
+=over 5
+
+=item 1.
+
+Ignore C<dSP> and C<PUSHMARK(SP)> for now. They will be discussed in
+the next example.
+
+=item 2.
+
+We aren't passing any parameters to I<PrintUID> so G_NOARGS can be
+specified.
+
+=item 3.
+
+We aren't interested in anything returned from I<PrintUID>, so
+G_DISCARD is specified. Even if I<PrintUID> was changed to
+return some value(s), having specified G_DISCARD will mean that they
+will be wiped by the time control returns from I<call_pv>.
+
+=item 4.
+
+As I<call_pv> is being used, the Perl subroutine is specified as a
+C string. In this case the subroutine name has been 'hard-wired' into the
+code.
+
+=item 5.
+
+Because we specified G_DISCARD, it is not necessary to check the value
+returned from I<call_pv>. It will always be 0.
+
+=back
+
+=head2 Passing Parameters
+
+Now let's make a slightly more complex example. This time we want to
+call a Perl subroutine, C<LeftString>, which will take 2 parameters--a
+string ($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 I<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 I<call_LeftString>.
+
+=over 5
+
+=item 1.
+
+Parameters are passed to the Perl subroutine using the Perl stack.
+This is the purpose of the code beginning with the line C<dSP> and
+ending with the line C<PUTBACK>. The C<dSP> declares a local copy
+of the stack pointer. This local copy should B<always> be accessed
+as C<SP>.
+
+=item 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 C<dSP>--it declares
+and initializes a I<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 C<dSP> macro explicitly--it will be declared for you
+automatically.
+
+=item 3.
+
+Any parameters to be pushed onto the stack should be bracketed by the
+C<PUSHMARK> and C<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 C<@_> array for the
+subroutine, it knows how big to make it.
+
+The C<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 I<No Parameters, Nothing returned>) you
+must still call the C<PUSHMARK> macro before you can call any of the
+I<call_*> functions--Perl still needs to know that there are no
+parameters.
+
+The C<PUTBACK> macro sets the global copy of the stack pointer to be
+the same as our local copy. If we didn't do this I<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, I<not> the global copy.
+
+=item 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 L<perlguts/"XSUBs and the Argument Stack"> for details
+on how the XPUSH macros work.
+
+=item 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 C<ENTER>/C<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 C<FREETMPS>/C<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 C<ENTER>/C<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 C<{> and C<}> in Perl
+to limit the scope of local variables.
+
+See the section I<Using Perl to dispose of temporaries> for details of
+an alternative to using these macros.
+
+=item 6.
+
+Finally, I<LeftString> can now be called via the I<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.
+
+=back
+
+=head2 Returning a Scalar
+
+Now for an example of dealing with the items returned from a Perl
+subroutine.
+
+Here is a Perl subroutine, I<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 I<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
+
+=over 5
+
+=item 1.
+
+The only flag specified this time was G_SCALAR. That means the C<@_>
+array will be created and that the value returned by I<Adder> will
+still exist after the call to I<call_pv>.
+
+=item 2.
+
+The purpose of the macro C<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
+I<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 I<call_*> functions or any other Perl internal function.
+
+=item 3.
+
+Although only a single value was expected to be returned from I<Adder>,
+it is still good practice to check the return code from I<call_pv>
+anyway.
+
+Expecting a single value is not quite the same as knowing that there
+will be one. If someone modified I<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
+I<really> don't want to happen ever.
+
+=item 4.
+
+The C<POPi> macro is used here to pop the return value from the stack.
+In this case we wanted an integer, so C<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
+
+=item 5.
+
+The final C<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 C<POPi> it updated only our
+local copy of the stack pointer. Remember, C<PUTBACK> sets the global
+stack pointer to be the same as our local copy.
+
+=back
+
+
+=head2 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 I<call_AddSubtract> is called like this
+
+ call_AddSubtract(7, 4);
+
+then here is the output
+
+ 7 - 4 = 3
+ 7 + 4 = 11
+
+Notes
+
+=over 5
+
+=item 1.
+
+We wanted list context, so G_ARRAY was used.
+
+=item 2.
+
+Not surprisingly C<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 C<POP*> macros they come off the stack in I<reverse>
+order.
+
+=back
+
+=head2 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 I<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
+I<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, I<AddSubtract> actually made it back to
+I<call_AddSubScalar>.
+
+
+=head2 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, I<Inc>, below takes 2 parameters and increments
+each directly.
+
+ sub Inc
+ {
+ ++ $_[0];
+ ++ $_[1];
+ }
+
+and here is a C function to call it.
+
+ static void
+ call_Inc(a, b)
+ int a;
+ int b;
+ {
+ dSP;
+ int count;
+ SV * sva;
+ SV * svb;
+
+ ENTER;
+ SAVETMPS;
+
+ sva = sv_2mortal(newSViv(a));
+ svb = sv_2mortal(newSViv(b));
+
+ PUSHMARK(SP);
+ XPUSHs(sva);
+ XPUSHs(svb);
+ PUTBACK;
+
+ count = call_pv("Inc", G_DISCARD);
+
+ if (count != 0)
+ croak ("call_Inc: expected 0 values from 'Inc', got %d\n",
+ count);
+
+ printf ("%d + 1 = %d\n", a, SvIV(sva));
+ printf ("%d + 1 = %d\n", b, SvIV(svb));
+
+ FREETMPS;
+ LEAVE;
+ }
+
+To be able to access the two parameters that were pushed onto the stack
+after they return from I<call_pv> it is necessary to make a note
+of their addresses--thus the two variables C<sva> and C<svb>.
+
+The reason this is necessary is that the area of the Perl stack which
+held them will very likely have been overwritten by something else by
+the time control returns from I<call_pv>.
+
+
+
+
+=head2 Using G_EVAL
+
+Now an example using G_EVAL. Below is a Perl subroutine which computes
+the difference of its 2 parameters. If this would result in a negative
+result, the subroutine calls I<die>.
+
+ sub Subtract
+ {
+ my ($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 I<call_Subtract> is called thus
+
+ call_Subtract(4, 5)
+
+the following will be printed
+
+ Uh oh - death can be fatal
+
+Notes
+
+=over 5
+
+=item 1.
+
+We want to be able to catch the I<die> so we have used the G_EVAL
+flag. Not specifying this flag would mean that the program would
+terminate immediately at the I<die> statement in the subroutine
+I<Subtract>.
+
+=item 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 $@;
+
+C<PL_errgv> is a perl global of type C<GV *> that points to the
+symbol table entry containing the error. C<ERRSV> therefore
+refers to the C equivalent of C<$@>.
+
+=item 3.
+
+Note that the stack is popped using C<POPs> in the block where
+C<SvTRUE(ERRSV)> is true. This is necessary because whenever a
+I<call_*> function invoked with G_EVAL|G_SCALAR returns an error,
+the top of the stack holds the value I<undef>. Because we want the
+program to continue after detecting this error, it is essential that
+the stack is tidied up by removing the I<undef>.
+
+=back
+
+
+=head2 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 $@; # should be, but isn't
+
+This example will fail to recognize that an error occurred inside the
+C<eval {}>. Here's why: the call_Subtract code got executed while perl
+was cleaning up temporaries when exiting the eval block, and because
+call_Subtract is implemented with I<call_pv> using the G_EVAL
+flag, it promptly reset C<$@>. This results in the failure of the
+outermost test for C<$@>, and thereby the failure of the error trap.
+
+Appending the G_KEEPERR flag, so that the I<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.
+
+=head2 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 I<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 I<call_sv> is useful.
+
+The code below for I<CallSubSV> is identical to I<CallSubPV> except
+that the C<name> parameter is now defined as an SV* and we use
+I<call_sv> instead of I<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 I<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, I<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 (C<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 C<rememberSub> in C<CallSavedSub1>, it may or may not still refer
+to the Perl subroutine that was recorded in C<SaveSub1>. This is
+particularly true for these cases
+
+ SaveSub1(\&fred);
+ CallSavedSub1();
+
+ SaveSub1( sub { print "Hello there\n" } );
+ CallSavedSub1();
+
+By the time each of the C<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 C<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 $ref may have referred to the subroutine C<fred>
+whenever the call to C<SaveSub1> was made but by the time
+C<CallSavedSub1> gets called it now holds the number C<47>. Because we
+saved only a pointer to the original SV in C<SaveSub1>, any changes to
+$ref will be tracked by the pointer C<rememberSub>. This means that
+whenever C<CallSavedSub1> gets called, it will attempt to execute the
+code which is referenced by the SV* C<rememberSub>. In this case
+though, it now refers to the integer C<47>, so expect Perl to complain
+loudly.
+
+A similar but more subtle problem is illustrated with this code
+
+ $ref = \&fred;
+ SaveSub1($ref);
+ $ref = \&joe;
+ CallSavedSub1();
+
+This time whenever C<CallSavedSub1> get called it will execute the Perl
+subroutine C<joe> (assuming it exists) rather than C<fred> as was
+originally requested in the call to C<SaveSub1>.
+
+To get around these problems it is necessary to take a full copy of the
+SV. The code below shows C<SaveSub2> modified to do that
+
+ static SV * keepSub = (SV*)NULL;
+
+ void
+ SaveSub2(name)
+ SV * name
+ CODE:
+ /* Take a copy of the callback */
+ if (keepSub == (SV*)NULL)
+ /* First time, so create a new SV */
+ keepSub = newSVsv(name);
+ else
+ /* Been here before, so overwrite */
+ SvSetSV(keepSub, name);
+
+ void
+ CallSavedSub2()
+ CODE:
+ PUSHMARK(SP);
+ call_sv(keepSub, G_DISCARD|G_NOARGS);
+
+To avoid creating a new SV every time C<SaveSub2> is called,
+the function first checks to see if it has been called before. If not,
+then space for a new SV is allocated and the reference to the Perl
+subroutine, C<name> is copied to the variable C<keepSub> in one
+operation using C<newSVsv>. Thereafter, whenever C<SaveSub2> is called
+the existing SV, C<keepSub>, is overwritten with the new value using
+C<SvSetSV>.
+
+=head2 Using call_argv
+
+Here is a Perl subroutine which prints whatever parameters are passed
+to it.
+
+ sub PrintList
+ {
+ my(@list) = @_;
+
+ foreach (@list) { print "$_\n" }
+ }
+
+and here is an example of I<call_argv> which will call
+I<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 C<PUSHMARK> in this instance.
+This is because I<call_argv> will do it for you.
+
+=head2 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, C<new>, it declares methods, one static and one
+virtual. The static method, C<PrintID>, prints out simply the class
+name and a version number. The virtual method, C<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
+
+=over 5
+
+=item *
+
+a reference to the object for a virtual method or the name of the class
+for a static method.
+
+=item *
+
+the name of the method.
+
+=item *
+
+any other parameters specific to the method.
+
+=back
+
+Here is a simple XSUB which illustrates the mechanics of calling both
+the C<PrintID> and C<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 C<PrintID> and C<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 I<call_method>.
+
+=head2 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
+
+=head2 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 I<call_*> function or
+values returned via the stack) have been freed by one of these methods
+
+=over 5
+
+=item *
+
+specifying the G_DISCARD flag with I<call_*>.
+
+=item *
+
+explicitly disposed of using the C<ENTER>/C<SAVETMPS> -
+C<FREETMPS>/C<LEAVE> pairing.
+
+=back
+
+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 I<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 I<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.
+
+
+=head2 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 C<register_fatal> which registers the C function
+to get called when a fatal error occurs.
+
+ register_fatal(cb1);
+
+The single parameter C<cb1> is a pointer to a function, so you must
+have defined C<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 C<register_fatal> and the callback it
+registers, C<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 C<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
+C<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 C<asynch_read> which associates a C
+function C<ProcessRead> with a file handle C<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 I<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 C<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 C<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 C<asynch_close>. This shows how to remove
+the entry from the hash C<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 C<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 C<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 C<fn1>, C<fn2>, and C<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 C<Pcb> to
+access the C<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
+
+=over 5
+
+=item 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.
+
+=item 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.
+
+=item 3. Use a parameter to map to the Perl callback
+
+A hash is an ideal mechanism to store the mapping between C and Perl.
+
+=back
+
+
+=head2 Alternate Stack Manipulation
+
+
+Although I have made use of only the C<POP*> macros to access values
+returned from Perl subroutines, it is also possible to bypass these
+macros and read the stack using the C<ST> macro (See L<perlxs> for a
+full description of the C<ST> macro).
+
+Most of the time the C<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 C<ST> macro as used when coding an XSUB is ideal
+for this purpose.
+
+The code below is the example given in the section I<Returning a list
+of values> recoded to use C<ST> instead of C<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
+
+=over 5
+
+=item 1.
+
+Notice that it was necessary to define the variable C<ax>. This is
+because the C<ST> macro expects it to exist. If we were in an XSUB it
+would not be necessary to define C<ax> as it is already defined for
+you.
+
+=item 2.
+
+The code
+
+ SPAGAIN;
+ SP -= count;
+ ax = (SP - PL_stack_base) + 1;
+
+sets the stack up so that we can use the C<ST> macro.
+
+=item 3.
+
+Unlike the original coding of this example, the returned
+values are not accessed in reverse order. So C<ST(0)> refers to the
+first value returned by the Perl subroutine and C<ST(count-1)>
+refers to the last.
+
+=back
+
+=head2 Creating and calling an anonymous subroutine in C
+
+As we've already shown, C<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);
+
+C<eval_pv> is used to compile the anonymous subroutine, which
+will be the return value as well (read more about C<eval_pv> in
+L<perlapi/eval_pv>). Once this code reference is in hand, it
+can be mixed in with all the previous examples we've shown.
+
+=head1 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 L<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.
+
+=head1 SEE ALSO
+
+L<perlxs>, L<perlguts>, L<perlembed>
+
+=head1 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.
+
+=head1 DATE
+
+Version 1.3, 14th Apr 1997
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