Programming Perl in C

argument is pushed onto the argument stack using the XPUSHs macro. Next, the 

PUTBACK macro is used to mark that the last of the arguments has been pushed onto 

the stack. Finally, call_pv() is used, just as in the first example, but this time only 

G_DISCARD is used as an option.

Example 3: Variable Parameters, One Return Value

For a slightly more realistic example, let's call the Data::Counter::count function 

defined a couple of chapters back. If you remember, this subroutine takes a 

variable number of arguments and returns the number of arguments it received:

sub count { return scalar @_; }

To call this subroutine, I need the following code:

dSP;                                    // declare SP

SV *return                              // declare an SV for the return value

ENTER; SAVETMPS;                        // start a new scope

PUSHMARK(SP) ;                          // prepare to push args

XPUSHs(sv_2mortal(newSVpv("one",0)));   // push three parameters onto the stack

XPUSHs(sv_2mortal(newSVpv("two",0)));

XPUSHs(sv_2mortal(newSVpv("three",0)));

PUTBACK;                                // done pushing arguments

call_pv("Data::Counter::count", G_SCALAR); // make the call

SPAGAIN;                                // refresh SP   it might have changed

return = POPs;                          // get return value off the stack

printf("count: %d\n", SvIV(return));    // print out the return as an integer

FREETMPS; LEAVE;                        // end the scope   freeing mortal

                                        // variables

This code is similar to the last example with a couple additions. First, I passed 

G_SCALAR as an option to call_pv(), indicating that I expect to get a single argument 

back. After the call, a new macro is used: SPAGAIN. This macro refreshes SP in case it 

changed during the call. Finally, the return value from the subroutine is popped off 

the stack with POPs. The  s  in POPs refers to the fact that it pops a scalar off the stack.

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