Chapter 8

NOTE The return value from a Perl subroutine obtained by POPs is a 

mortal variable. This means you must handle it before you end the 

scope with FREETMPS and LEAVE. After that point the variable will have 

been freed by the garbage collector and will no longer be accessible.

Example 4: Variable Parameters, Variable Return Values

In order to demonstrate multiple value returns, let's suppose that count() were 

modified to take a list of array refs as a parameter and returns a list of counts for 

each array:

sub count { map { scalar @$_ } @_ }

Now count() would be called like this from Perl:

@counts = Data::Counter::count([1,2,3],[4,5,6],[7,8,9]); # returns (3,3,3)

Here's what the equivalent call would look like from C, assuming I already 

have the preceding three arrays in the variables av1, av2, and av3.

dSP;                                    // declare SP

int num;                                // declare an int for the return count

int i;                                  // declare a loop counter

ENTER; SAVETMPS;                        // start a new scope

PUSHMARK(SP) ;                          // prepare to push args

XPUSHs(sv_2mortal(newRV_inc((SV*)av1))); // push three arrays onto the stack

XPUSHs(sv_2mortal(newRV_inc((SV*)av2))); // by reference

XPUSHs(sv_2mortal(newRV_inc((SV*)av3)));

PUTBACK;                                // done pushing arguments

num = call_pv("Data::Counter::count", G_ARRAY); // make the call

SPAGAIN;                                // refresh SP   it might have changed

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