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  * $FreeBSD: releng/5.0/sys/libkern/random.c 50477 1999-08-28 01:08:13Z peter $
   35  */
   36 
   37 #include <sys/libkern.h>
   38 
   39 static u_long randseed = 1;
   40 
   41 void
   42 srandom(seed)
   43         u_long seed;
   44 {
   45         randseed = seed;
   46 }
   47 
   48 /*
   49  * Pseudo-random number generator for randomizing the profiling clock,
   50  * and whatever else we might use it for.  The result is uniform on
   51  * [0, 2^31 - 1].
   52  */
   53 u_long
   54 random()
   55 {
   56         register long x, hi, lo, t;
   57 
   58         /*
   59          * Compute x[n + 1] = (7^5 * x[n]) mod (2^31 - 1).
   60          * From "Random number generators: good ones are hard to find",
   61          * Park and Miller, Communications of the ACM, vol. 31, no. 10,
   62          * October 1988, p. 1195.
   63          */
   64         x = randseed;
   65         hi = x / 127773;
   66         lo = x % 127773;
   67         t = 16807 * lo - 2836 * hi;
   68         if (t <= 0)
   69                 t += 0x7fffffff;
   70         randseed = t;
   71         return (t);
   72 }

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