Function get_rand_u64

zic.c:1485–1542

Return a random uint_fast64_t. */

}

/* Return a random uint_fast64_t.  */
static uint_fast64_t
get_rand_u64(void)
{
#if HAVE_GETRANDOM
  static uint_fast64_t entropy_buffer[max(1, 256 / sizeof(uint_fast64_t))];
  static int nwords;
  if (!nwords) {
    ssize_t s;
    for (;; check_for_signal()) {
      s = getrandom(entropy_buffer, sizeof entropy_buffer, 0);
      if (! (s < 0 && errno == EINTR))
	break;
    }

    nwords = s < 0 ? -1 : s / sizeof *entropy_buffer;
  }
  if (0 < nwords)
    return entropy_buffer[--nwords];
#endif

  /* getrandom didn't work, so fall back on portable code that is
     not the best because the seed isn't cryptographically random and
     'rand' might not be cryptographically secure.  */
  {
    static bool initialized;
    if (!initialized) {
      srand(time(NULL));
      initialized = true;
    }
  }

  /* Return a random number if rand() yields a random number and in
     the typical case where RAND_MAX is one less than a power of two.
     In other cases this code yields a sort-of-random number.  */
  {
    uint_fast64_t rand_max = RAND_MAX,
      nrand = rand_max < UINT_FAST64_MAX ? rand_max + 1 : 0,
      rmod = INT_MAX < UINT_FAST64_MAX ? 0 : UINT_FAST64_MAX / nrand + 1,
      r = 0, rmax = 0;

    for (;; check_for_signal()) {
      uint_fast64_t rmax1 = rmax;
      if (rmod) {
	/* Avoid signed integer overflow on theoretical platforms
	   where uint_fast64_t promotes to int.  */
	rmax1 %= rmod;
	r %= rmod;
      }
      rmax1 = nrand * rmax1 + rand_max;
      r = nrand * r + rand();
      rmax = rmax < rmax1 ? rmax1 : UINT_FAST64_MAX;
      if (UINT_FAST64_MAX <= rmax)
	break;
    }

    return r;
  }