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29 * Posix rand_r function added May 1999 by Wes Peters <wes@softweyr.com>.
32 #if defined(LIBC_SCCS) && !defined(lint)
33 static char sccsid[] = "@(#)rand.c 8.1 (Berkeley) 6/14/93";
34 #endif /* LIBC_SCCS and not lint */
35 #include <sys/cdefs.h>
36 __FBSDID("$FreeBSD$");
38 #include "namespace.h"
39 #include <sys/param.h>
40 #include <sys/sysctl.h>
42 #include "un-namespace.h"
49 do_rand(unsigned long *ctx)
52 * Compute x = (7^5 * x) mod (2^31 - 1)
53 * without overflowing 31 bits:
54 * (2^31 - 1) = 127773 * (7^5) + 2836
55 * From "Random number generators: good ones are hard to find",
56 * Park and Miller, Communications of the ACM, vol. 31, no. 10,
57 * October 1988, p. 1195.
61 /* Transform to [1, 0x7ffffffe] range. */
62 x = (*ctx % 0x7ffffffe) + 1;
65 x = 16807 * lo - 2836 * hi;
68 /* Transform to [0, 0x7ffffffd] range. */
88 static u_long next = 1;
93 return (do_rand(&next));
106 * Many programs choose the seed value in a totally predictable manner.
107 * This often causes problems. We seed the generator using pseudo-random
108 * data from the kernel.
120 sysctl(mib, 2, (void *)&next, &len, NULL, 0);
131 printf("seeding rand with 0x19610910: \n");
134 printf("generating three pseudo-random numbers:\n");
135 for (i = 0; i < 3; i++)
137 printf("next random number = %d\n", rand());
140 printf("generating the same sequence with rand_r:\n");
142 for (i = 0; i < 3; i++)
144 printf("next random number = %d\n", rand_r(&myseed));