What does double underscore ( __const) mean in C?

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

I found the above function definition in /usr/include/netinet/ether.h on a Linux box.

Can someone explain what the double underscores mean in front of const (keyword), addr (identifier) and at last __THROW.

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