int parport_find_class (parport_device_class cls, int from);
This walks through the list of parallel port devices looking for a device whose 'CLS' string matches cls in their IEEE 1284 Device ID.
When a device is found matching those requirements, its device number is returned; if there is no matching device, a negative value is returned.
A new search it initiated by passing NULL as the from argument. If from is not NULL, the search continues from that device.