Writing C Modules with XS

# test some simple mime type recognitions

is(Gnome::MIME::file_type("foo.gif"),   image/gif ,  "recognizes .gif");

is(Gnome::MIME::file_type("foo.jpg"),   image/jpeg , "recognizes .jpg");

is(Gnome::MIME::file_type("foo.html"),  text/html ,  "recognizes .html");

Managing Memory Usage

As it turns out, the preceding XSUB does not leak memory. This came as a surprise 

to me I assumed that the gnome_mime_type() function returned a dynamically 

allocated string that I would need to clean up. If you look at the generated code in 

MIME.c (Listing 9 5), you'll see this line at the end of the function:

sv_setpv(TARG, RETVAL); XSprePUSH; PUSHTARG;

This line copies the string pointed to by RETVAL into TARG. TARG is the SV that will 

actually be returned by the subroutine. After that, TARG is pushed onto the stack 

and the function returns. My expectation was that this would result in a leak since 

the pointer stored in RETVAL wouldn't be freed before going out of scope. As it turns 

out, this pointer doesn't need to be freed because it comes from an internal pool 

managed by the Gnome API.

But, for the sake of the example, what would I need to do if the return value from 

gnome_mime_type() did need to be freed? My first draft might have been as follows:

const char *

file_type(filename)

     const gchar * filename

CODE:

     RETVAL = gnome_mime_type(filename);

OUTPUT:

     RETVAL

CLEANUP:

     Safefree(RETVAL);

The CLEANUP block specifies code to be run at the end of the generated function. 

This might work fine, but it's incorrect. The problem is that Gnome and Perl might 

be using different memory allocators. Thus, calling Perl's Safefree() function on 

memory allocated by Gnome is not guaranteed to yield the expected results. Instead, I 

would need to use the same call that Gnome uses, g_free():

CLEANUP:

     g_free(RETVAL);

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