__NFDBITS
nwords = nbits / __NFDBITS;
fb = howmany(nf, __NFDBITS); /* how many fd_masks */
db = howmany(nd, __NFDBITS);
off = howmany(nf, __NFDBITS) * sizeof(__fd_mask);
nd = db * __NFDBITS;
#define __NFDMASK (__NFDBITS - 1)
#define __NFD_LEN(a) (((a) + (__NFDBITS - 1)) / __NFDBITS)
#define NFDBITS __NFDBITS