Question

I’m developing some library in C that can be used by various user applications.

The library should be completely “transparent” – a user application can init it and finalize,
and it’s not supposed to see any change in the running application.

The problem is – I’m using C srand()/rand() functions in the library initialization,
which means that the library does affect user’s application – if a user generates random numbers, they will be affected by the fact that rand() was already called.

So, can anyone point to some simple non-GPL alternative to rand() random number generator in C?

It doesn’t have to be really strong – I’n not doing any crypto with the numbers.
I was thinking to write some small and really simple generator (something like take time and XOR with something and do something with some prime number and bla bla bla), but I was wondering if someone has a pointer to a more decent generator.

Answer 1

It generates the next number by keeping some state and modifying the state every time you call the function. Such a function is called a pseudorandom number generator. An old method of creating a PRNG is the linear congruential generator, which is easy enough:

static int rand_state;
int rand(void)
{
    rand_state = (rand_state * 1103515245 + 12345) & 0x7fffffff;
    return rand_state;
}

As you can see, this method allows you to predict the next number in the series if you know the previous number. There are more sophisticated methods.

Various types of pseudorandom number generators have been designed for specific purposes. There are secure PRNGs which are slow but hard to predict even if you know how they work, and there are big PRNGs like Mersenne Twister which have nice distribution properties and are therefore useful for writing Monte Carlo simulations.

As a rule of thumb, a linear congruential generator is good enough for writing a game (how much damage does the monster deal) but not good enough for writing a simulation. There is a colorful history of researchers who have chosen poor PRNGs for their programs; the results of their simulations are suspect as a result.