The Design and Implementation of the FreeBSD Operating System, Second Edition
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FreeBSD/Linux Kernel Cross Reference
sys/libkern/random.c

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    1 /*-
    2  * Copyright (c) 1992, 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  *      @(#)random.c    8.1 (Berkeley) 6/10/93
   30  */
   31 
   32 #include <sys/cdefs.h>
   33 __FBSDID("$FreeBSD: src/sys/libkern/random.c,v 1.13.24.1 2008/10/02 02:57:24 kensmith Exp $");
   34 
   35 #include <sys/libkern.h>
   36 
   37 #define NSHUFF 50       /* to drop some "seed -> 1st value" linearity */
   38 
   39 static u_long randseed = 937186357; /* after srandom(1), NSHUFF counted */
   40 
   41 void
   42 srandom(seed)
   43         u_long seed;
   44 {
   45         int i;
   46 
   47         randseed = seed;
   48         for (i = 0; i < NSHUFF; i++)
   49                 (void)random();
   50 }
   51 
   52 /*
   53  * Pseudo-random number generator for randomizing the profiling clock,
   54  * and whatever else we might use it for.  The result is uniform on
   55  * [0, 2^31 - 1].
   56  */
   57 u_long
   58 random()
   59 {
   60         register long x, hi, lo, t;
   61 
   62         /*
   63          * Compute x[n + 1] = (7^5 * x[n]) mod (2^31 - 1).
   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         /* Can't be initialized with 0, so use another value. */
   69         if ((x = randseed) == 0)
   70                 x = 123459876;
   71         hi = x / 127773;
   72         lo = x % 127773;
   73         t = 16807 * lo - 2836 * hi;
   74         if (t < 0)
   75                 t += 0x7fffffff;
   76         randseed = t;
   77         return (t);
   78 }

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