diplodocus.org Git - nmh/blob - sbr/m_rand.c

   1 /*
   2  * m_rand -- provides pseudorandom bytes
   3  *
   4  * This code is Copyright (c) 2012, by the authors of nmh.  See the
   5  * COPYRIGHT file in the root directory of the nmh distribution for
   6  * complete copyright information.
   7  */
   8
   9 #include <stdlib.h>  /* for abs(), srand(), rand() */
  10 #include <stdio.h>   /* for fopen(), fread(), fclose() */
  11 #include <unistd.h>  /* for getpid() */
  12 #include <time.h>    /* for time() */
  13
  14 static int seeded = 0;
  15
  16
  17 int
  18 m_rand (unsigned char *buf, size_t n) {
  19   if (! seeded) {
  20     FILE *devurandom;
  21     unsigned int seed;
  22
  23     if ((devurandom = fopen ("/dev/urandom", "r"))) {
  24       if (fread (&seed, sizeof (seed), 1, devurandom) == 1) seeded = 1;
  25       fclose (devurandom);
  26     }
  27
  28     if (! seeded) {
  29       /* This seed calculation is from Helmut G. Katzgraber, "Random
  30          Numbers in Scientific Computing:  An Introduction",
  31          arXiv:1005.4117v1 [physics.comp-ph], 22 May 2010, p. 19.
  32          time() and getpid() shouldn't fail on POSIX platforms. */
  33       seed = abs ((int) ((time (0) * 181 * ((getpid ()-83) * 359)) % 104729));
  34       seeded = 1;
  35     }
  36
  37     srand (seed);
  38   }
  39
  40   while (n > 0) {
  41     int rnd = rand ();
  42     unsigned char *rndp = (unsigned char *) &rnd;
  43     unsigned int i;
  44
  45     for (i = 0; i < sizeof rnd  &&  n > 0;  ++i, --n) {
  46       *buf++ = *rndp++;
  47     }
  48   }
  49
  50   return 0;
  51 }