Can anyone expalin this kind of syntax?Why there a kind of data type or something in front of const?

Question

extern SL_API const SLInterfaceID SL_IID_NULL;

Answer 1

The SL_API, SLInterfaceID (and probably SL_IID_NULL) are not from C standard. It's most likely from a library or extension we don't know. You have to look into the official documents about those.