_FD_BITSINDEX
#define FD_SET(fd, set) ((set)->bits[_FD_BITSINDEX(fd)] |= _FD_BIT(fd))
#define FD_CLR(fd, set) ((set)->bits[_FD_BITSINDEX(fd)] &= ~_FD_BIT(fd))
#define FD_ISSET(fd, set) ((set)->bits[_FD_BITSINDEX(fd)] & _FD_BIT(fd))