N2RNG_FAILED
#define n2rng_setfailed(n2rng) ((n2rng)->n_flags |= N2RNG_FAILED)
#define n2rng_clrfailed(n2rng) ((n2rng)->n_flags &= ~N2RNG_FAILED)
#define n2rng_isfailed(n2rng) ((n2rng)->n_flags & N2RNG_FAILED)