FD_NFDBITS
long fds_bits[__howmany(FD_SETSIZE, FD_NFDBITS)];
#define FD_SET(__n, __p) ((__p)->fds_bits[(__n)/FD_NFDBITS] |= \
(1ul << ((__n) % FD_NFDBITS)))
#define FD_CLR(__n, __p) ((__p)->fds_bits[(__n)/FD_NFDBITS] &= \
~(1ul << ((__n) % FD_NFDBITS)))
#define FD_ISSET(__n, __p) (((__p)->fds_bits[(__n)/FD_NFDBITS] & \
(1ul << ((__n) % FD_NFDBITS))) != 0l)