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

I once wrote a report on CRNGs for a course in Discrete Mathematics. For it, I disassembled rand() in msvcrt.dll:

 msvcrt.dll:77C271D8 mov     ecx, [eax+14h] msvcrt.dll:77C271DB imul    ecx, 343FDh msvcrt.dll:77C271E1 add     ecx, 269EC3h msvcrt.dll:77C271E7 mov     [eax+14h], ecx msvcrt.dll:77C271EA mov     eax, ecx msvcrt.dll:77C271EC shr     eax, 10h msvcrt.dll:77C271EF and     eax, 7FFFh 

So it's a LCG something like (untested)...

 int ms_rand(int& seed) {   seed = seed*0x343fd+0x269EC3;  // a=214013, b=2531011   return (seed >> 0x10) & 0x7FFF; }

Answer 2

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"