Default DSB sockets:

      int getsockopt (
          int         s,
          int         level,
          int         optname,
          void       *optval,
          int        *optlen
          );

    UNIX 03 Only (X/Open Sockets):

      int getsockopt (
          int                    s,
          int                    level,
          int                    optname,
          void       *__restrict optval,
          socklen_t  *__restrict optlen
          );