Chapter 8

To call this subroutine from C, I'd have to do the following:

dSP;                                             // declare SP

PUSHMARK(SP);                                    // setup for call

call_pv("Hello::say_hello", G_NOARGS|G_DISCARD); // call say_hello with no args

                                                 // and no return value

The dSP macro is necessary to declare the SP variable used by the PUSHMARK macro. 

These two macros set up the correct context for a call to call_pv(). The first 

argument is a string giving the name of the subroutine. The second specifies 

options for the call. In this case, combining  G_NOARGS and G_DISCARD with | specifies 

a call with no arguments and no return value.

Example 2: One Parameter, No Return Value

A slightly more complicated call is one that passes a parameter. For example, let's 

say I modified say_hello to take an argument:

sub say_hello {

   my $who = shift;

   print "Hello, $who.\n";

}

This code would be required to call it from C:

dSP;                                    // declare SP

ENTER; SAVETMPS;                        // start a new scope

PUSHMARK(SP) ;                          // prepare to push args

XPUSHs(sv_2mortal(newSVpv("Human",0))); // push a single parameter onto the

                                        // argument stack

PUTBACK;                                // done pushing arguments

call_pv("Hello::say_hello", G_DISCARD); // make the call

FREETMPS; LEAVE;                        // end the scope   freeing mortal

                                        // variables

There are a few new things to notice here. First, this code creates a new scope using 

the ENTER, SAVETMPS, FREETMPS, and LEAVE macros you saw back in the  Memory 

Management  section. This is done to provide for mortal variables created for use 

as parameters as well as those created by the Perl code to be called. Second, a single 

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