Question

I am just about finished reading K&R, and that is all the C that I know. All my compilation is done from Windows command line using MinGW, and I have no knowledge of advanced debugging methods (hence the “ghetto debug” comment in my 2nd program below).

I am trying to make a few small test programs to help me better understand how memory allocation works. These first couple programs do not use malloc or free, I just wanted to see how memory is allocated and de-allocated for standard arrays local to a function. The idea is that I watch my running processes RAM usage to see if it corresponds with what I understand. For this first program below, it does work as I expected. The alloc_one_meg() function allocates and initializes 250,000 4-byte integers, but that MB is de-allocated as soon as the function returns. So if I call that function 1000000 times in a row, I should never see my RAM usage go much above 1MB. And, it works.

#include <stdio.h>
#include <stdlib.h>

void alloc_one_meg() {
    int megabyte[250000];
    int i;
    for (i=0; i<250000; i++) {
        megabyte[i] = rand();
    }
}

main()
{
    int i;
    for (i=0; i<1000000; i++) {
        alloc_one_meg();
    }
}

For this second program below, the idea was to not allow the function to exit, to have 1000 copies of the same function running at once, which I accomplished with recursion. My theory was that the program would consume 1GB of RAM before it de-allocated it all after the recursion finished. However, it doesn’t get past the 2nd loop through the recursion (see my ghetto debug comment). The program crashes with a pretty non-informative (to me) message (a Windows pop-up saying ____.exe has encountered a problem). Usually I can always get to the bottom of things with my ghetto debug method… but it’s not working here. I’m stumped. What is the problem with this code? Thanks!

#include <stdio.h>
#include <stdlib.h>

int j=0;

void alloc_one_meg() {
    int megabyte[250000];
    int i;
    for (i=0; i<250000; i++) {
        megabyte[i] = rand();
    }
    j++;
    printf("Loop %d\n", j); // ghetto debug
    if (j<1000) {
        alloc_one_meg();
    }
}

main()
{
    alloc_one_meg();
}

Followup question posted here.

Answer 1

You’re running into a stack overflow.

Local automatic storage variables (such as megabyte) are allocated on the stack, which has limited amount of space. malloc allocates on the heap, which allows much larger allocations.

You can read more here:

http://en.wikipedia.org/wiki/Stack_overflow

(I should note that the C language does not specify where memory is allocated – stack and heap are implementation details)