__fd_mask
bytes = howmany(nd, __NFDBITS) * sizeof(__fd_mask);
#define __NFDBITS ((unsigned int)sizeof(__fd_mask) * __NBBY) /* bits per mask */
__fd_mask fds_bits[__howmany(FD_SETSIZE, __NFDBITS)];
#define _fdset_mask(n) ((__fd_mask)1 << ((unsigned int)(n) % __NFDBITS))
#define fd_mask __fd_mask