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

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;
}
``````

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:

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.

• That are 76 lines with copyright, so why not post it? May 9 '14 at 21:22
• @kimliv - To be honest, I'd feel discourteous posting a significant piece of someone else's library code without their permission. May 9 '14 at 22:21
• 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) May 9 '14 at 22:28

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 <wes@softweyr.com>.
*
* \$Id: random.c 1944 2009-04-01 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;
}
``````