fd_mask
#define NFDBITS (sizeof(fd_mask) * 8) /* bits per mask */
fd_mask bits[((FD_SETSIZE) + ((NFDBITS) - 1)) / (NFDBITS)];
tmp = howmany(maxfd+1, NFDBITS) * sizeof(fd_mask);
if ((fdsp = malloc(nfds * sizeof(fd_mask))) == NULL)
memset(fdsp, 0, nfds * sizeof(fd_mask));
fdsn = howmany(sock + 1, NFDBITS) * sizeof(fd_mask);
uint32 bytes = HOWMANY(count, NFDBITS) * sizeof(fd_mask);
memset(set, 0, HOWMANY(numFDs, NFDBITS) * sizeof(fd_mask));
uint32 bytes = HOWMANY(numFDs, NFDBITS) * sizeof(fd_mask);