__NFDBITS
bytes = howmany(nd, __NFDBITS) * sizeof(__fd_mask);
__fd_mask fds_bits[__howmany(FD_SETSIZE, __NFDBITS)];
#define _fdset_mask(n) ((__fd_mask)1 << ((unsigned int)(n) % __NFDBITS))
((p)->fds_bits[(unsigned int)(n)/__NFDBITS] |= _fdset_mask(n))
((p)->fds_bits[(unsigned int)(n)/__NFDBITS] &= ~_fdset_mask(n))
((p)->fds_bits[(unsigned int)(n)/__NFDBITS] & _fdset_mask(n))
#define NFDBITS __NFDBITS