Formula / calculation of the function random() / randomSeed()

Question

In which file can I find the calculation that is called with random()? If it is not too much could you also post the content in your answer? I am using an Arduino Uno and its standard IDE.

I found this in the "WMath.cpp" but that is not the final calculation.

void randomSeed(unsigned int seed)
{
 if (seed != 0) {
 srandom(seed);
 }
}
long random(long howbig)
{
 if (howbig == 0) {
 return 0;
 }
 return random() % howbig;
}
long random(long howsmall, long howbig)
{
 if (howsmall >= howbig) {
 return howsmall;
 }
 long diff = howbig - howsmall;
 return random(diff) + howsmall;
}

Answer 1

The underlying random() function isn't part of the Arduino code. It's actually part of avr-libc. The Arduino IDE only comes with pre-compiled object code for it (as far as I know).

It's open source though. You can get information about it here:

• http://www.nongnu.org/avr-libc

If you look down the page a short way, you'll find the "Source code and documentation" link.

The random() function is under libc/stdlib/random.c. I would paste the source code here, but technically I'd have to include a whole copyright notice with it.

• kimliv: That are 76 lines with copyright, so why not post it?
• Peter Bloomfield: @kimliv - To be honest, I'd feel discourteous posting a significant piece of someone else's library code without their permission.
• kimliv: But with copyright it is ok an the writer of the library was not the inventor of the algorithm I guess. (What is the main job for me)

Answer 2

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 <[email protected]>.
 *
 * $Id: random.c 1944 2009年04月01日 23:12:20Z arcanum $
 */
/*
 * From:
static char sccsid[] = "@(#)rand.c 8.1 (Berkeley) 6/14/93";
*/
#include <stdlib.h>
#include "sectionname.h"
ATTRIBUTE_CLIB_SECTION
static long
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));
}
ATTRIBUTE_CLIB_SECTION
long
random_r(unsigned long *ctx)
{
 return do_random(ctx);
}
static unsigned long next = 1;
ATTRIBUTE_CLIB_SECTION
long
random(void)
{
 return do_random(&next);
}
ATTRIBUTE_CLIB_SECTION
void
srandom(unsigned long seed)
{
 next = seed;
}