NFDBITS 123 include/sys/types.h fd_mask fds_bits[howmany(FD_SETSIZE, NFDBITS)]; NFDBITS 126 include/sys/types.h #define FD_SET(n, p) ((p)->fds_bits[(n)/NFDBITS] |= (1 << ((n) % NFDBITS))) NFDBITS 127 include/sys/types.h #define FD_CLR(n, p) ((p)->fds_bits[(n)/NFDBITS] &= ~(1 << ((n) % NFDBITS))) NFDBITS 128 include/sys/types.h #define FD_ISSET(n, p) ((p)->fds_bits[(n)/NFDBITS] & (1 << ((n) % NFDBITS)))