Programming Perl in C

Explicitly Freeing Memory

A simple way to make sure you don't leak memory is to explicitly decrement the ref 

erence counts of the variables you create. This is done using the SvREFCNT_dec macro:

SvREFCNT_dec(sv); // decrement sv s refcount

Perl's garbage collector works by freeing an object the moment its reference count 

reaches zero. After an SvREFCNT_dec that causes an object's refcount to reach zero, 

the object is no longer valid calls that attempt operate on it will usually yield 

crashes or other unpleasant behavior.

Using SvREFCNT and SvREFCNT_dec, you can write a function to unconditionally 

free any Perl object:

void free_it_now (SV *sv) {

   while(SvREFCNT(sv)) SvREFCNT_dec(sv);

}

But you shouldn't need to do something like this very often; in fact, if you do you 

should stop and consider what's wrong with the way you're managing the refer 

ence counts on your variables.

Implicitly Freeing Memory

The Perl API provides a way for you to hook into Perl's automatic garbage collection 

from C. The way this is done is by marking an SV, AV, or HV as mortal. Marking an 

object mortal is simply a way of deferring an SvREFCNT_dec until the end of the 

current scope. Here's an example that marks an SV as mortal using sv2_mortal():

SV *sv = newSVpv("foo",0); // sv contains "foo" and has a refcount of 1

sv_2mortal(sv);            // sv is now mortal.  At the next scope exit

                           // SvREFCNT_dec(sv) will be called and the sv

                           // will be freed.

This can be stated more succinctly, since sv_2mortal() returns the SV* passed 

to it:

SV *sv = sv_2mortal(newSVpv("foo",0)); // creates sv and mortalizes it

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