The past 6 months I've been struggling with a weird bug I had in pVoice. The application itself ran fine, but when the application closed, 99% of the time I got an Access Violation in Wx::Bitmap, which tried to free the memory used by some Bitmap I had obviously created...or so I thought

Yesterday evening I used Data::Dumper to dump the datafile I use for pVoice, which showed me lots of entries like this:
[ 'wassen', 'wassen', '1', 'C:\\Mijn documenten\\PCS\\Verbs\\Gr +ooming\\wash 1.jpg', bless( do{\(my $o = 12451544)}, 'Wx: +:Bitmap' ) ],
I had seen entries like that before and never really worried about it. Indeed, once I tried to save the actual Bitmap in the Storable file, which didn't work, but I didn't care about having that entry in my datafile. I didn't use it, did I?

But yesterday it struck me. Was it just a coincidence that my Access Violations happened in Wx::Bitmap and that I had those lines in my datafile? Obviously not. Removing all references to those Wx::Bitmap objects, and saving the datastructure again solved the problem. So what has been going on here?

My theory is this: the code references are only valid for the time the application runs. When the datastructure is read in another run, the data the Wx::Bitmap entry refers to doesn't exist, but when the application closes, it will still try to free the memory, which is obviously not in use by my application, so an Access Violation is generated.

Moral of the story: never use Storable to save objects that aren't pure Perl, but consist of XS code.

Jouke Visser
Speaking at the 2004 O'Reilly Open Source Convention about pVoice