rand.c revision 252648 
1/*-
2 * Copyright (c) 1990, 1993
3 *	The Regents of the University of California.  All rights reserved.
4 *
5 * Redistribution and use in source and binary forms, with or without
6 * modification, are permitted provided that the following conditions
7 * are met:
8 * 1. Redistributions of source code must retain the above copyright
9 *    notice, this list of conditions and the following disclaimer.
10 * 2. Redistributions in binary form must reproduce the above copyright
11 *    notice, this list of conditions and the following disclaimer in the
12 *    documentation and/or other materials provided with the distribution.
13 * 3. Neither the name of the University nor the names of its contributors
14 *    may be used to endorse or promote products derived from this software
15 *    without specific prior written permission.
16 *
17 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
18 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
19 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
20 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
21 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
22 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
23 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
24 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
25 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
26 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
27 * SUCH DAMAGE.
28 *
29 * Posix rand_r function added May 1999 by Wes Peters <wes@softweyr.com>.
30 */
31
32#if defined(LIBC_SCCS) && !defined(lint)
33static 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: head/lib/libc/stdlib/rand.c 252648 2013-07-03 23:27:04Z ache $");
37
38#include "namespace.h"
39#include <sys/param.h>
40#include <sys/sysctl.h>
41#include <sys/types.h>
42#include <stdlib.h>
43#include "un-namespace.h"
44
45#ifdef TEST
46#include <stdio.h>
47#endif /* TEST */
48
49static int
50do_rand(unsigned long *ctx)
51{
52#ifdef  USE_WEAK_SEEDING
53/*
54 * Historic implementation compatibility.
55 * The random sequences do not vary much with the seed,
56 * even with overflowing.
57 */
58	return ((*ctx = *ctx * 1103515245 + 12345) % ((u_long)RAND_MAX + 1));
59#else   /* !USE_WEAK_SEEDING */
60/*
61 * Compute x = (7^5 * x) mod (2^31 - 1)
62 * without overflowing 31 bits:
63 *      (2^31 - 1) = 127773 * (7^5) + 2836
64 * From "Random number generators: good ones are hard to find",
65 * Park and Miller, Communications of the ACM, vol. 31, no. 10,
66 * October 1988, p. 1195.
67 */
68	long hi, lo, x;
69
70	/* Must be in [1, 0x7ffffffe] range at this point. */
71	hi = *ctx / 127773;
72	lo = *ctx % 127773;
73	x = 16807 * lo - 2836 * hi;
74	if (x < 0)
75		x += 0x7fffffff;
76	*ctx = x;
77	/* Transform to [0, 0x7ffffffd] range. */
78	return (x - 1);
79#endif  /* !USE_WEAK_SEEDING */
80}
81
82
83int
84rand_r(unsigned int *ctx)
85{
86	u_long val = (u_long) *ctx;
87#ifndef USE_WEAK_SEEDING
88	/* Transform to [1, 0x7ffffffe] range. */
89	val = (val % 0x7ffffffe) + 1;
90#endif
91	int r = do_rand(&val);
92
93#ifdef  USE_WEAK_SEEDING
94	*ctx = (unsigned int)val;
95#else
96	*ctx = (unsigned int)(val - 1);
97#endif
98	return (r);
99}
100
101
102static u_long next = 1;
103
104int
105rand()
106{
107	return (do_rand(&next));
108}
109
110void
111srand(seed)
112u_int seed;
113{
114	next = seed;
115#ifndef USE_WEAK_SEEDING
116	/* Transform to [1, 0x7ffffffe] range. */
117	next = (next % 0x7ffffffe) + 1;
118#endif
119}
120
121
122/*
123 * sranddev:
124 *
125 * Many programs choose the seed value in a totally predictable manner.
126 * This often causes problems.  We seed the generator using pseudo-random
127 * data from the kernel.
128 */
129void
130sranddev()
131{
132	int mib[2];
133	size_t len;
134
135	len = sizeof(next);
136
137	mib[0] = CTL_KERN;
138	mib[1] = KERN_ARND;
139	sysctl(mib, 2, (void *)&next, &len, NULL, 0);
140#ifndef USE_WEAK_SEEDING
141	/* Transform to [1, 0x7ffffffe] range. */
142	next = (next % 0x7ffffffe) + 1;
143#endif
144}
145
146
147#ifdef TEST
148
149main()
150{
151    int i;
152    unsigned myseed;
153
154    printf("seeding rand with 0x19610910: \n");
155    srand(0x19610910);
156
157    printf("generating three pseudo-random numbers:\n");
158    for (i = 0; i < 3; i++)
159    {
160	printf("next random number = %d\n", rand());
161    }
162
163    printf("generating the same sequence with rand_r:\n");
164    myseed = 0x19610910;
165    for (i = 0; i < 3; i++)
166    {
167	printf("next random number = %d\n", rand_r(&myseed));
168    }
169
170    return 0;
171}
172
173#endif /* TEST */
174
175