Why is a function call, rather than variable addresses, used to detect stack growth direction?

Question

I read different responses to the question of detecting stack growth detection and I understand that, in modern architectures, stack might grow randomly, might be created off heap, and so on.

However, in this classic interview question, I want to understand why people use a function call rather than comparing 2 local variables in the same function. I think there must be some particular reason for doing this but, not being a C/low level developer [Java :)], I am simply guessing.

Here is the code I tried:

void sub (int *a)  {
    int b;
    int c;
    printf ("a:%d\n", a);
    printf ("b:%d\n", &b);
    printf ("c:%d\n", &c);
    if (&b > a) {
        printf ("Stack grows up.\n");
    } else {
        printf ("Stack grows down.\n");
    }
}

int main (void) {
    int a;
    int b;
    sub (&a);
    printf ("\nHere we go again!!\n");
    if (&b > &a)  {
        printf ("Stack grows up.\n");
    } else  {
        printf ("Stack grows down.\n");
    }
    return 0;
}

I also found this article which tries to optimize the solution which I don't understand either: http://www.devx.com/tips/Tip/37412

P.S: From different responses to this and other threads, it seems like the question itself is flawed/irrelevant, as an interview question it probably re-enforces incorrect assumptions unless someone researches the answer !

Thanks!

Answer 1

You can't fully control the order the compiler chooses to allocate the local variables in. You can however reasonably control the functions that will be called, and in what order.