How can I find the exact rand() used in a C library?

Question

As a part of my coursework I need to find and re-code a rand() random number generator which outputs the same numbers as the original. The starting sequence is 1804289383 846930886 1681692777 1714636915 1957747793 424238335 719885386 1649760492 596516649 1189641421 1025202362 and can be generated at http://ideone.com/H7tsSI

#include <stdlib.h>     /* rand */
#include <iostream>
using namespace std;

int main ()
{
    for (int i = 0 ; i< 10 ; i++) {
        cout << rand() << " ";
    }
    cout << rand();

    return 0;
}

My issue is that I can't find the original source of this generator, and I don't know how I can figure out the way the generator works from the full sequence of the generator, which is 100 numbers long. Could someone help me either find the original generator or teach me how I can find a generator from its sequence? Thanks!

Answer 1

Depending on your specific compiler you may have the source code available. On Visual Studio 12.0, for example, the rand() source code is:

int __cdecl rand (
        void
        )
{
        _ptiddata ptd = _getptd();

        return( ((ptd->_holdrand = ptd->_holdrand * 214013L
            + 2531011L) >> 16) & 0x7fff );
}

If your compiler does not include the source code for its C library, you could try using a disassembler to piece together what its version of the rand() function does. In general, most of them would be along the same lines of the above code: access a state variable that was the result of the last call to rand() (or the seed if it's the first call), perform a permutation on it, and then write that back to the state variable.

Answer 2

You can find the source library implementation used by GNU at http://www.gnu.org/software/libc/ If you're familiar with GIT, you can use the GIT source code management like :

$> git clone git://sourceware.org/git/glibc.git

rand() function essentially calling different functions - first __random which in turn calls __random_r . Click on the function names to refer to the source repository at version 2.15

For more details, refer to answers here - gcc implementation of rand()