Question

I want to understand exactly where the global variables are stored in my program. On the stack? On the heap? Somewhere else?

So for that I wrote this small code:

int global_vector[1000000];
int main () {
    global_vector[0] = 1; // just to avoid a compilation warning
    while(true); // to give me time to check the amount of RAM used by my program
    return 0;
}

No matter how large I make global_vector, the program only uses a really tiny amount of RAM. I do not understand the reason for this. Could someone please explain?

Answer 1

This is completely implementation-dependent, but typically global variables are stored in a special memory segment that is separate from the stack and the heap. This memory could be allocated as a fixed-size buffer inside of the executable itself, or in a segment that is given to the program at startup by the operating system.

The reason that you’re not seeing the memory usage go up probably has to do with how virtual memory is handled by the OS. As an optimization, the operating system won’t actually give any memory to the program for that giant array unless you actually use it. Try changing your program to for-loop over the entire contents of the array and see if that causes the RAM usage to go up. (It’s also possible that the optimizer in your compiler is eliminating the giant array, since it’s almost completely unused. Putting a loop to read/write all the values might also force the compiler to keep it).

Hope this helps!