Chapter 9

Finally, RETVAL is assigned a reference to the hash to be returned:

  // return a reference to the new hash

  RETVAL = newRV_noinc((SV *)hv);

You might have expected to see a call to sv_2mortal() at the end, but XSUB 

return values of SV* are automatically mortalized by xsubpp. As proof, here's the 

relevant slice of the generated MIME.c:

  // return a reference to the new hash

  RETVAL = newRV_noinc((SV *)hv);

#line 52 "MIME.c"

               ST(0) = RETVAL;

      sv_2mortal(ST(0));

The generated code places RETVAL into the return stack and is then mortalized. 

Since the reference to hv was created with newRV_noinc(), the hv has a refcount of 1 

and will live as long RETVAL does. The same is true of the SVs stored in hv they have 

a refcount of 1 and will be freed when hv is freed. The end result is a chain of Perl 

objects with refcounts of 1 anchored by a single mortal reference. When building 

up complex data structures in XS, this is the end result you should be working toward.

Returning Multiple Values in List Context

The new interface for Gnome::MIME::type_data() is a big improvement. Now a user 

who wants to open a file can do something like the following:

$type_data = Gnome::MIME::type_data("text/html");

$program = $type_data >{open};

However, inevitably a less experienced user is going to mix up the hash ref 

erence with a hash and try something like this:

%type_data = Gnome::MIME::type_data("text/html");

$program = $type_data{open};

This will only work if Gnome::MIME::type_data() is smart enough to return a list of 

key/value pairs in list context. Supporting this usage provides a chance to demon 

strate a new technique for building XSUBs: using PPCODE to access the return stack 

directly. The complete listing for the new XSUB is in Listing 9 8.

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