Seeding rand() for random global variables

Question

I'm trying to use C's rand() to initialize random inputs for a pseudo-random number generator. Due to constraints of a PRNG testing library I'm using, my generator function cannot accept any parameters, so it seems like I'll need to store the initial values as global variables.

Ideally, I would seed the generator with srand(time(NULL)), but this throws the "initializer element is not a compile-time constant" error when I try to do it globally.

What's the most straightforward way to do this? So far I've come up with passing the global variables into a function and doing the work there, like this:

unsigned int* w;
unsigned int* x;
unsigned int* y;
unsigned int* z;

void seed (unsigned int* first, unsigned int* second, unsigned int* third, unsigned int* fourth)
{
    srand((unsigned int) time(NULL));
    unsigned int a = rand();
    unsigned int b = rand();
    unsigned int c = rand();
    unsigned int d = rand();

    first =  &a;
    second = &b;
    third = &c;
    fourth = &d;
}

However, when I try to access my values in main, I get a EXC_BAD_ACCESS error in Xcode:

int main (void)
{
    seed(w, x, y, z);
    printf("%i", *w);     // throws error
...
}

... which I'm guessing has something to do with scope and the memory being freed before I want it to be. Don't have a ton of experience with C, but is this the right approach? If so how can I fix this error?

Thanks!

How random do these numbers have to be? `rand()` is notorously awful. — tadman, Sep 07 '17 at 04:46
The program execution begins from `main()` and from there possibly to other functions. I think if you try to use a function like `srand()` before `main()` begins, it will give an error. In other words, only a value known at compile time would do. — J...S, Sep 07 '17 at 04:49
@tadman Not so random, as remember, these are only values I'm passing in as initial values to my prng. So I'm sort of randomizing my seeds, if you will. I want to see how well the battery of randomness tests turn out when I do it this way, rather than hardcoding in my own inputs. — Davigor, Sep 07 '17 at 04:51
@Davigor I'm saying `rand()` in general is really, really bad, even when you try and use it "properly". This `srand(time(NULL))` thing is known to be hazardous. If this needs to be *unpredictably* random you're not going to succeed, these sequences can be easily predicted. If you need this to be actually random then you need a better PRNG than this. — tadman, Sep 07 '17 at 04:53
Note [`srand()` — why call it only once?](http://stackoverflow.com/questions/7343833/srand-why-call-it-only-once/) — Jonathan Leffler, Sep 07 '17 at 05:11

tadman · Accepted Answer · 2017-09-07T04:55:09.320

You're assigning pointers to values that exist only on the stack, not pushing things back like you think you are. Once that stack goes out of scope you're into dangerous territory.

Here's how it should be written:

void seed (unsigned int* a, unsigned int* b, unsigned int* c, unsigned int* d)
{
    srand((unsigned int) time(NULL));
    *a = rand();
    *b = rand();
    *c = rand();
    *d = rand();
}

int main() {
   // Note: These can be global, they're just put here for convenience
   // Remember, global variables are bad and you want to avoid them.
   unsigned int a, b, c, d;
   seed(&a, &b, &c, &d);

   // ...
}

score 1 · Answer 2 · answered Sep 07 '17 at 04:54

The scope of variable a, b, c, d are within the seed function, so accessing those references outside the function will result unexpected results.
Either you need to pass address of variable to fill or allocate memory for each number in seed function.

Try below snippet

unsigned int w;
unsigned int x;
unsigned int y;
unsigned int z;

void seed (unsigned int* first, unsigned int* second, unsigned int* third, unsigned int* fourth)
{
    srand((unsigned int) time(NULL));

    *first =  rand();
    *second = rand();
    *third = rand();
    *fourth = rand();
}


int main (void)
{
    seed(&w, &x, &y, &z);
    printf("%i", w);
}

score 0 · Answer 3 · answered Sep 07 '17 at 04:49

0

EXC_BAD_ACCESS is because you are assigning address of local variables from seed() to your pointers

These variable and their addresses will be invalid when seed() returns

answered Sep 07 '17 at 04:49

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Seeding rand() for random global variables

3 Answers3