__fdset_idx
#define FD_CLR(n, p) ((p)->__fds_bits[__fdset_idx(p, n)] &= ~__fdset_mask(n))
(((p)->__fds_bits[__fdset_idx(p, n)] & __fdset_mask(n)) != 0)
#define FD_SET(n, p) ((p)->__fds_bits[__fdset_idx(p, n)] |= __fdset_mask(n))