Writing C Modules with XS

  # make sure filename is specified

  croak("Missing required parameter filename.") unless exists $args{filename};

  # call appropriate back end function

  if ($args{read_file} and $args{default_type}) {

    return Gnome::MIME::Backend::type_or_default_of_file($args{filename},

                                                 $args{default_type});

  } elsif ($args{read_file}) {

    return Gnome::MIME::Backend::type_of_file($args{filename});

  } elsif ($args{default_type}) {

    return Gnome::MIME::Backend::type_or_default($args{filename},

                                                 $args{default_type});

  } else {

    return Gnome::MIME::Backend::type($args{filename});

  }

}

This code essentially does the same things as the XS code in the previous section, 

but instead of being in C, it's in good old maintainable, forgiving Perl. As an added 

bonus, instead of translating the calls to croak() in the XS version into die() calls, 

I added a use Carp and used Carp's version of croak(), which will yield much better 

error messages than die() or its C equivalent.

This pattern, a thin layer of XS with a Perl front end, is worthy of emulation. It 

provides a way to write Perl ish interfaces in Perl and get the most out of XS at the 

same time.

Providing Access to Complex Data Structures

Many C interfaces are more complicated than the gnome_mime_type functions 

they manipulate complex data structures, often using C structs and arrays. Pro 

viding a convenient interface to Perl programmers is often a matter of translating 

data from C structs into Perl arrays and hashes, and back again.

It just so happens that the Gnome::MIME module has need of this functionality. 

The Gnome MIME API supplies two functions that access a set of key/value pairs 

associated with each MIME type, gnome_mime_get_keys() and gnome_mime_get_value().

There are keys for the program the user has chosen to open the MIME type (that is, 

an image viewer for image/gif, a text editor for text/plain, and so on) as well as 

other types of metadata.

It would, of course, be possible to provide a Perl interface directly to these 

calls. For example, to print out the available key/value pairs for image/gif, you 

could do the following:

22

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