What does double underscore ( __const) mean in C?

Question

extern int ether_hostton (__const char *__hostname, struct ether_addr *__addr)
 __THROW;

I found the above function definition in /usr/include/neti

Answer 1

The underscore in __const means that this keyword is a compiler extension and using it is not portable (The const keyword was added to C in a later revision, 89 I think). The __THROW is also some kind of extension, I assume that it gets defined to some __attribute__(something) if gcc is used, But I'm not sure on that and too lazy to check. The __addr can mean anything the programmer wanted it to mean, It's just a name.