What common algorithms are used for C's rand()?

Question

I understand that the C specification does not give any specification about the specific implementation of rand(). What different algorithms are commonly used on different major platforms? How do they differ?

Answer 1

See this article: http://en.wikipedia.org/wiki/List_of_random_number_generators

This is the source code of glibc's rand():

/* Reentrant random function from POSIX.1c.
   Copyright (C) 1996, 1999, 2009 Free Software Foundation, Inc.
   This file is part of the GNU C Library.
   Contributed by Ulrich Drepper <[email protected]>, 1996.

   The GNU C Library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public
   License as published by the Free Software Foundation; either
   version 2.1 of the License, or (at your option) any later version.

   The GNU C Library is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public
   License along with the GNU C Library; if not, write to the Free
   Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
   02111-1307 USA.  */

#include <stdlib.h>


/* This algorithm is mentioned in the ISO C standard, here extended
   for 32 bits.  */
int
rand_r (unsigned int *seed)
{
  unsigned int next = *seed;
  int result;

  next *= 1103515245;
  next += 12345;
  result = (unsigned int) (next / 65536) % 2048;

  next *= 1103515245;
  next += 12345;
  result <<= 10;
  result ^= (unsigned int) (next / 65536) % 1024;

  next *= 1103515245;
  next += 12345;
  result <<= 10;
  result ^= (unsigned int) (next / 65536) % 1024;

  *seed = next;

  return result;
}

Source: https://sourceware.org/git/?p=glibc.git;a=blob_plain;f=stdlib/rand_r.c;hb=HEAD

As you can see, it's simply multiply with an addition and a shift. The values are carefully chosen to make sure that you get no repeat of the output for RAND_MAX iterations.

Note that this is an old implementation which has been replaced by a more complex algorithm: https://sourceware.org/git/?p=glibc.git;a=blob_plain;f=stdlib/random_r.c;hb=HEAD

If the link if broken, Google for "glibc rand_r"