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. All advertising materials mentioning features or use of this software
  14  *    must display the following acknowledgement:
  15  *      This product includes software developed by the University of
  16  *      California, Berkeley and its contributors.
  17  * 4. Neither the name of the University nor the names of its contributors
  18  *    may be used to endorse or promote products derived from this software
  19  *    without specific prior written permission.
  20  *
  21  * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
  22  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  23  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  24  * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
  25  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  26  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  27  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  28  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  29  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  30  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  31  * SUCH DAMAGE.
  32  *
  33  * Posix rand_r function added May 1999 by Wes Peters <wes@softweyr.com>.
  34  *
  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 1.6 2005/11/20 14:58:40 swildner Exp $
  37  */
  38
  39 #include "namespace.h"
  40 #include <sys/time.h>          /* for sranddev() */
  41 #include <fcntl.h>             /* for sranddev() */
  42 #include <stdlib.h>
  43 #include <unistd.h>            /* for sranddev() */
  44 #include "un-namespace.h"
  45
  46 #ifdef TEST
  47 #include <stdio.h>
  48 #endif /* TEST */
  49
  50 void    sranddev(void);
  51
  52 static int
  53 do_rand(unsigned long *ctx)
  54 {
  55 #ifdef  USE_WEAK_SEEDING
  56 /*
  57  * Historic implementation compatibility.
  58  * The random sequences do not vary much with the seed,
  59  * even with overflowing.
  60  */
  61         return ((*ctx = *ctx * 1103515245 + 12345) % ((u_long)RAND_MAX + 1));
  62 #else   /* !USE_WEAK_SEEDING */
  63 /*
  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.
  70  */
  71         long hi, lo, x;
  72
  73         /* Can't be initialized with 0, so use another value. */
  74         if (*ctx == 0)
  75                 *ctx = 123459876;
  76         hi = *ctx / 127773;
  77         lo = *ctx % 127773;
  78         x = 16807 * lo - 2836 * hi;
  79         if (x < 0)
  80                 x += 0x7fffffff;
  81         return ((*ctx = x) % ((u_long)RAND_MAX + 1));
  82 #endif  /* !USE_WEAK_SEEDING */
  83 }
  84
  85
  86 int
  87 rand_r(unsigned int *ctx)
  88 {
  89         u_long val = (u_long) *ctx;
  90         int r = do_rand(&val);
  91
  92         *ctx = (unsigned int) val;
  93         return (r);
  94 }
  95
  96
  97 static u_long next = 1;
  98
  99 int
 100 rand(void)
 101 {
 102         return (do_rand(&next));
 103 }
 104
 105 void
 106 srand(u_int seed)
 107 {
 108         next = seed;
 109 }
 110
 111
 112 /*
 113  * sranddev:
 114  *
 115  * Many programs choose the seed value in a totally predictable manner.
 116  * This often causes problems.  We seed the generator using the much more
 117  * secure random(4) interface.
 118  *
 119  * FreeBSD chooses to use /dev/random for their seed.  However, because
 120  * of possible problems surrounding /dev/random, we use /dev/urandom for now,
 121  * which is guaranteed not to block and will produce a "good enough" seed
 122  * for us for the time being.
 123  */
 124 void
 125 sranddev(void)
 126 {
 127         int fd, done;
 128
 129         done = 0;
 130         fd = _open("/dev/urandom", O_RDONLY, 0);
 131         if (fd >= 0) {
 132                 if (_read(fd, (void *) &next, sizeof(next)) == sizeof(next))
 133                         done = 1;
 134                 _close(fd);
 135         }
 136
 137         if (!done) {
 138                 struct timeval tv;
 139                 unsigned long junk;     /* XXX left uninitialized on purpose */
 140
 141                 gettimeofday(&tv, NULL);
 142                 srand((getpid() << 16) ^ tv.tv_sec ^ tv.tv_usec ^ junk);
 143         }
 144 }
 145
 146
 147 #ifdef TEST
 148
 149 int
 150 main(int argc __unused, char *argv[] __unused)
 151 {
 152     int i;
 153     unsigned myseed;
 154
 155     printf("seeding rand with 0x19610910: \n");
 156     srand(0x19610910);
 157
 158     printf("generating three pseudo-random numbers:\n");
 159     for (i = 0; i < 3; i++)
 160     {
 161         printf("next random number = %d\n", rand());
 162     }
 163
 164     printf("generating the same sequence with rand_r:\n");
 165     myseed = 0x19610910;
 166     for (i = 0; i < 3; i++)
 167     {
 168         printf("next random number = %d\n", rand_r(&myseed));
 169     }
 170
 171     return 0;
 172 }
 173
 174 #endif /* TEST */
 175