You have to pass the device class you are looking for in dev_class. In dev_class_mask, you have to set the bits of the device class that should match with the value you have passed to dev_class.
For example, say you are looking for a wrist watch with audio features:
Set dev_class to 0x200704 and
set dev_class_mask to 0xFFFFFF
If you wanted to look for only audio devices:
set dev_class to 0x200000 and
set dev_class_mask to 0xFF0000