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33 * Posix rand_r function added May 1999 by Wes Peters <wes@softweyr.com>.
35 * $FreeBSD: src/lib/libc/stdlib/rand.c,v 1.15 2001/03/05 11:33:57 ache Exp $
36 * $DragonFly: src/lib/libc/stdlib/rand.c,v 1.4 2005/04/28 13:47:43 joerg Exp $
39 #include "namespace.h"
40 #include <sys/time.h> /* for sranddev() */
41 #include <fcntl.h> /* for sranddev() */
43 #include <unistd.h> /* for sranddev() */
44 #include "un-namespace.h"
53 do_rand(unsigned long *ctx)
55 #ifdef USE_WEAK_SEEDING
57 * Historic implementation compatibility.
58 * The random sequences do not vary much with the seed,
59 * even with overflowing.
61 return ((*ctx = *ctx * 1103515245 + 12345) % ((u_long)RAND_MAX + 1));
62 #else /* !USE_WEAK_SEEDING */
64 * Compute x = (7^5 * x) mod (2^31 - 1)
65 * wihout overflowing 31 bits:
66 * (2^31 - 1) = 127773 * (7^5) + 2836
67 * From "Random number generators: good ones are hard to find",
68 * Park and Miller, Communications of the ACM, vol. 31, no. 10,
69 * October 1988, p. 1195.
73 /* Can't be initialized with 0, so use another value. */
78 x = 16807 * lo - 2836 * hi;
81 return ((*ctx = x) % ((u_long)RAND_MAX + 1));
82 #endif /* !USE_WEAK_SEEDING */
87 rand_r(unsigned int *ctx)
89 u_long val = (u_long) *ctx;
90 int r = do_rand(&val);
92 *ctx = (unsigned int) val;
97 static u_long next = 1;
102 return (do_rand(&next));
116 * Many programs choose the seed value in a totally predictable manner.
117 * This often causes problems. We seed the generator using the much more
118 * secure random(4) interface.
120 * FreeBSD chooses to use /dev/random for their seed. However, because
121 * of possible problems surrounding /dev/random, we use /dev/urandom for now,
122 * which is guaranteed not to block and will produce a "good enough" seed
123 * for us for the time being.
131 fd = _open("/dev/urandom", O_RDONLY, 0);
133 if (_read(fd, (void *) &next, sizeof(next)) == sizeof(next))
142 gettimeofday(&tv, NULL);
143 srand((getpid() << 16) ^ tv.tv_sec ^ tv.tv_usec ^ junk);
155 printf("seeding rand with 0x19610910: \n");
158 printf("generating three pseudo-random numbers:\n");
159 for (i = 0; i < 3; i++)
161 printf("next random number = %d\n", rand());
164 printf("generating the same sequence with rand_r:\n");
166 for (i = 0; i < 3; i++)
168 printf("next random number = %d\n", rand_r(&myseed));