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  * 4. 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  * $DragonFly: src/lib/libc/stdlib/rand.c,v 1.6 2005/11/20 14:58:40 swildner Exp $
  34  */
  35
  36 #include "namespace.h"
  37 #include <sys/time.h>          /* for sranddev() */
  38 #include <sys/types.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 static int
  49 do_rand(unsigned long *ctx)
  50 {
  51 #ifdef  USE_WEAK_SEEDING
  52 /*
  53  * Historic implementation compatibility.
  54  * The random sequences do not vary much with the seed,
  55  * even with overflowing.
  56  */
  57         return ((*ctx = *ctx * 1103515245 + 12345) % ((u_long)RAND_MAX + 1));
  58 #else   /* !USE_WEAK_SEEDING */
  59 /*
  60  * Compute x = (7^5 * x) mod (2^31 - 1)
  61  * without overflowing 31 bits:
  62  *      (2^31 - 1) = 127773 * (7^5) + 2836
  63  * From "Random number generators: good ones are hard to find",
  64  * Park and Miller, Communications of the ACM, vol. 31, no. 10,
  65  * October 1988, p. 1195.
  66  */
  67         long hi, lo, x;
  68
  69         /* Can't be initialized with 0, so use another value. */
  70         if (*ctx == 0)
  71                 *ctx = 123459876;
  72         hi = *ctx / 127773;
  73         lo = *ctx % 127773;
  74         x = 16807 * lo - 2836 * hi;
  75         if (x < 0)
  76                 x += 0x7fffffff;
  77         return ((*ctx = x) % ((u_long)RAND_MAX + 1));
  78 #endif  /* !USE_WEAK_SEEDING */
  79 }
  80
  81
  82 int
  83 rand_r(unsigned int *ctx)
  84 {
  85         u_long val = (u_long) *ctx;
  86         int r = do_rand(&val);
  87
  88         *ctx = (unsigned int) val;
  89         return (r);
  90 }
  91
  92
  93 static u_long next = 1;
  94
  95 int
  96 rand(void)
  97 {
  98         return (do_rand(&next));
  99 }
 100
 101 void
 102 srand(u_int seed)
 103 {
 104         next = seed;
 105 }
 106
 107
 108 /*
 109  * sranddev:
 110  *
 111  * Many programs choose the seed value in a totally predictable manner.
 112  * This often causes problems.  We seed the generator using the much more
 113  * secure random(4) interface.
 114  */
 115 void
 116 sranddev(void)
 117 {
 118         int fd, done;
 119
 120         done = 0;
 121         fd = _open("/dev/random", O_RDONLY, 0);
 122         if (fd >= 0) {
 123                 if (_read(fd, (void *) &next, sizeof(next)) == sizeof(next))
 124                         done = 1;
 125                 _close(fd);
 126         }
 127
 128         if (!done) {
 129                 struct timeval tv;
 130                 unsigned long junk;     /* XXX left uninitialized on purpose */
 131
 132                 gettimeofday(&tv, NULL);
 133                 srand((getpid() << 16) ^ tv.tv_sec ^ tv.tv_usec ^ junk);
 134         }
 135 }
 136
 137
 138 #ifdef TEST
 139
 140 int
 141 main(int argc __unused, char *argv[] __unused)
 142 {
 143     int i;
 144     unsigned myseed;
 145
 146     printf("seeding rand with 0x19610910: \n");
 147     srand(0x19610910);
 148
 149     printf("generating three pseudo-random numbers:\n");
 150     for (i = 0; i < 3; i++)
 151     {
 152         printf("next random number = %d\n", rand());
 153     }
 154
 155     printf("generating the same sequence with rand_r:\n");
 156     myseed = 0x19610910;
 157     for (i = 0; i < 3; i++)
 158     {
 159         printf("next random number = %d\n", rand_r(&myseed));
 160     }
 161
 162     return 0;
 163 }
 164
 165 #endif /* TEST */
 166