Read Peter Bloomfield's answer here: https://arduino.stackexchange.com/a/1482/7727
Then read the following:
For everyone's reference, Arduino's WMath.cpp is found here.
Documentation for AVR-Libc's random() function is here.
Source code for AVR-Libc is found here, as linked-to from here.
I downloaded "avr-libc-2.0.0.tar.bz2.sig 08-Feb-2016" and extracted it. Like Peter said, the source code for "random" is in "avr-libc-2.0.0\libc\stdlib\random.c"
To save you the hassle, here's the source code, with their included copyright and license statement right at the top of it.
* Copyright (c) 1990, 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. 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.
* Posix rand_r function added May 1999 by Wes Peters <firstname.lastname@example.org>.
* $Id: random.c 1944 2009-04-01 23:12:20Z arcanum $
static char sccsid = "@(#)rand.c 8.1 (Berkeley) 6/14/93";
do_random(unsigned long *ctx)
* Compute x = (7^5 * x) mod (2^31 - 1)
* wihout overflowing 31 bits:
* (2^31 - 1) = 127773 * (7^5) + 2836
* 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.
long hi, lo, x;
x = *ctx;
/* Can't be initialized with 0, so use another value. */
if (x == 0)
x = 123459876L;
hi = x / 127773L;
lo = x % 127773L;
x = 16807L * lo - 2836L * hi;
if (x < 0)
x += 0x7fffffffL;
return ((*ctx = x) % ((unsigned long)RANDOM_MAX + 1));
random_r(unsigned long *ctx)
static unsigned long next = 1;
srandom(unsigned long seed)
next = seed;