FD_SETIDXMASK
unsigned long fds[(FD_SETSIZE + FD_SETBITMASK) / FD_SETIDXMASK];
__set->fds[__fd / FD_SETIDXMASK] &= \
__set->fds[__fd / FD_SETIDXMASK] |= \
__r = !!(__set->fds[__fd / FD_SETIDXMASK] & \
int __size = (FD_SETSIZE+FD_SETBITMASK) / FD_SETIDXMASK;\