The Design and Implementation of the FreeBSD Operating System, Second Edition
Now available: 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  * 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  *      @(#)random.c    8.1 (Berkeley) 6/10/93
   30  * $FreeBSD: src/sys/libkern/random.c,v 1.7 1999/08/28 00:46:36 peter Exp $
   31  * $DragonFly: src/sys/libkern/random.c,v 1.6 2006/09/03 18:52:29 dillon Exp $
   32  */
   33 
   34 #include <sys/libkern.h>
   35 
   36 static u_long randseed = 1;
   37 
   38 void
   39 skrandom(u_long seed)
   40 {
   41         randseed = seed;
   42 }
   43 
   44 /*
   45  * Pseudo-random number generator for randomizing the profiling clock,
   46  * and whatever else we might use it for.  The result is uniform on
   47  * [0, 2^31 - 1].
   48  */
   49 u_long
   50 krandom(void)
   51 {
   52         long x, hi, lo, t;
   53 
   54         /*
   55          * Compute x[n + 1] = (7^5 * x[n]) mod (2^31 - 1).
   56          * From "Random number generators: good ones are hard to find",
   57          * Park and Miller, Communications of the ACM, vol. 31, no. 10,
   58          * October 1988, p. 1195.
   59          */
   60         x = randseed;
   61         hi = x / 127773;
   62         lo = x % 127773;
   63         t = 16807 * lo - 2836 * hi;
   64         if (t <= 0)
   65                 t += 0x7fffffff;
   66         randseed = t;
   67         return (t);
   68 }

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