__NFDSHIFT
const int idx = fd >> __NFDSHIFT;
((p)->fds_bits[(unsigned)(n) >> __NFDSHIFT] |= (1U << ((n) & __NFDMASK)))
((p)->fds_bits[(unsigned)(n) >> __NFDSHIFT] &= ~(1U << ((n) & __NFDMASK)))
((p)->fds_bits[(unsigned)(n) >> __NFDSHIFT] & (1U << ((n) & __NFDMASK)))