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. 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  *      @(#)random.c    8.1 (Berkeley) 6/10/93
   34  */
   35 
   36 #include <sys/cdefs.h>
   37 __FBSDID("$FreeBSD: releng/5.2/sys/libkern/random.c 118732 2003-08-10 17:56:58Z ache $");
   38 
   39 #include <sys/libkern.h>
   40 
   41 #define NSHUFF 50       /* to drop some "seed -> 1st value" linearity */
   42 
   43 static u_long randseed = 937186357; /* after srandom(1), NSHUFF counted */
   44 
   45 void
   46 srandom(seed)
   47         u_long seed;
   48 {
   49         int i;
   50 
   51         randseed = seed;
   52         for (i = 0; i < NSHUFF; i++)
   53                 (void)random();
   54 }
   55 
   56 /*
   57  * Pseudo-random number generator for randomizing the profiling clock,
   58  * and whatever else we might use it for.  The result is uniform on
   59  * [0, 2^31 - 1].
   60  */
   61 u_long
   62 random()
   63 {
   64         register long x, hi, lo, t;
   65 
   66         /*
   67          * Compute x[n + 1] = (7^5 * x[n]) mod (2^31 - 1).
   68          * From "Random number generators: good ones are hard to find",
   69          * Park and Miller, Communications of the ACM, vol. 31, no. 10,
   70          * October 1988, p. 1195.
   71          */
   72         /* Can't be initialized with 0, so use another value. */
   73         if ((x = randseed) == 0)
   74                 x = 123459876;
   75         hi = x / 127773;
   76         lo = x % 127773;
   77         t = 16807 * lo - 2836 * hi;
   78         if (t < 0)
   79                 t += 0x7fffffff;
   80         randseed = t;
   81         return (t);
   82 }

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