FreeBSD/Linux Kernel Cross Reference
sys/libkern/random.c

     /*-
      * Copyright (c) 1992, 1993
      *      The Regents of the University of California.  All rights reserved.
      *
      * Redistribution and use in source and binary forms, with or without
      * modification, are permitted provided that the following conditions
      * are met:
      * 1. Redistributions of source code must retain the above copyright
      *    notice, this list of conditions and the following disclaimer.
     * 2. Redistributions in binary form must reproduce the above copyright
     *    notice, this list of conditions and the following disclaimer in the
     *    documentation and/or other materials provided with the distribution.
     * 3. All advertising materials mentioning features or use of this software
     *    must display the following acknowledgement:
     *      This product includes software developed by the University of
     *      California, Berkeley and its contributors.
     * 4. Neither the name of the University nor the names of its contributors
     *    may be used to endorse or promote products derived from this software
     *    without specific prior written permission.
     *
     * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
     * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
     * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
     * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
     * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
     * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
     * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
     * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
     * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
     * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
     * SUCH DAMAGE.
     *
     *      @(#)random.c    8.1 (Berkeley) 6/10/93
     */
    
    #include <sys/cdefs.h>
    __FBSDID("$FreeBSD: releng/5.2/sys/libkern/random.c 118732 2003-08-10 17:56:58Z ache $");
    
    #include <sys/libkern.h>
    
    #define NSHUFF 50       /* to drop some "seed -> 1st value" linearity */
    
    static u_long randseed = 937186357; /* after srandom(1), NSHUFF counted */
    
    void
    srandom(seed)
            u_long seed;
    {
            int i;
    
            randseed = seed;
            for (i = 0; i < NSHUFF; i++)
                    (void)random();
    }
    
    /*
     * Pseudo-random number generator for randomizing the profiling clock,
     * and whatever else we might use it for.  The result is uniform on
     * [0, 2^31 - 1].
     */
    u_long
    random()
    {
            register long x, hi, lo, t;
    
            /*
             * Compute x[n + 1] = (7^5 * x[n]) mod (2^31 - 1).
             * From "Random number generators: good ones are hard to find",
             * Park and Miller, Communications of the ACM, vol. 31, no. 10,
             * October 1988, p. 1195.
             */
            /* Can't be initialized with 0, so use another value. */
            if ((x = randseed) == 0)
                    x = 123459876;
            hi = x / 127773;
            lo = x % 127773;
            t = 16807 * lo - 2836 * hi;
            if (t < 0)
                    t += 0x7fffffff;
            randseed = t;
            return (t);
    }

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