lib/libc/stdlib/rand.c

   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  * @(#)rand.c   8.1 (Berkeley) 6/14/93
  32  * $FreeBSD: src/lib/libc/stdlib/rand.c,v 1.17 2007/12/11 20:39:32 ache Exp $
  33  */
  34
  35 #include "namespace.h"
  36 #include <sys/time.h>          /* for sranddev() */
  37 #include <sys/types.h>
  38 #include <sys/sysctl.h>
  39 #include <fcntl.h>             /* for sranddev() */
  40 #include <stdlib.h>
  41 #include <unistd.h>            /* for sranddev() */
  42 #include "un-namespace.h"
  43
  44 #ifdef TEST
  45 #include <stdio.h>
  46 #endif /* TEST */
  47
  48 /*
  49  * Compute x = (7^5 * x) mod (2^31 - 1)
  50  * without overflowing 31 bits:
  51  *      (2^31 - 1) = 127773 * (7^5) + 2836
  52  * From "Random number generators: good ones are hard to find",
  53  * Park and Miller, Communications of the ACM, vol. 31, no. 10,
  54  * October 1988, p. 1195.
  55  */
  56 static int
  57 do_rand(unsigned long *ctx)
  58 {
  59         long hi, lo, x;
  60
  61         /* Can't be initialized with 0, so use another value. */
  62         if (*ctx == 0)
  63                 *ctx = 123459876;
  64         hi = *ctx / 127773;
  65         lo = *ctx % 127773;
  66         x = 16807 * lo - 2836 * hi;
  67         if (x < 0)
  68                 x += 0x7fffffff;
  69         return ((*ctx = x) % ((u_long)RAND_MAX + 1));
  70 }
  71
  72
  73 int
  74 rand_r(unsigned int *ctx)
  75 {
  76         u_long val = (u_long) *ctx;
  77         int r = do_rand(&val);
  78
  79         *ctx = (unsigned int) val;
  80         return (r);
  81 }
  82
  83
  84 static u_long next = 1;
  85
  86 int
  87 rand(void)
  88 {
  89         return (do_rand(&next));
  90 }
  91
  92 void
  93 srand(u_int seed)
  94 {
  95         next = seed;
  96 }
  97
  98
  99 /*
 100  * sranddev:
 101  *
 102  * Many programs choose the seed value in a totally predictable manner.
 103  * This often causes problems.  We seed the generator using the much more
 104  * secure random(4) interface.
 105  */
 106 void
 107 sranddev(void)
 108 {
 109         int fd, done;
 110
 111         done = 0;
 112         fd = _open("/dev/random", O_RDONLY, 0);
 113         if (fd >= 0) {
 114                 if (_read(fd, (void *) &next, sizeof(next)) == sizeof(next))
 115                         done = 1;
 116                 _close(fd);
 117         }
 118
 119         if (!done) {
 120                 size_t len = sizeof(next);
 121
 122                 if (sysctlbyname("kern.random", &next, &len, NULL, 0) == 0)
 123                         done = 1;
 124         }
 125
 126         if (!done) {
 127                 struct timeval tv;
 128
 129                 gettimeofday(&tv, NULL);
 130                 srand((getpid() << 16) ^ tv.tv_sec ^ tv.tv_usec);
 131         }
 132 }
 133
 134
 135 #ifdef TEST
 136
 137 int
 138 main(int argc __unused, char *argv[] __unused)
 139 {
 140     int i;
 141     unsigned myseed;
 142
 143     printf("seeding rand with 0x19610910: \n");
 144     srand(0x19610910);
 145
 146     printf("generating three pseudo-random numbers:\n");
 147     for (i = 0; i < 3; i++)
 148     {
 149         printf("next random number = %d\n", rand());
 150     }
 151
 152     printf("generating the same sequence with rand_r:\n");
 153     myseed = 0x19610910;
 154     for (i = 0; i < 3; i++)
 155     {
 156         printf("next random number = %d\n", rand_r(&myseed));
 157     }
 158
 159     return 0;
 160 }
 161
 162 #endif /* TEST */
 163