Why for loop doesn't work for 2D array with an overflow but still works for 1D array

Question

The for loop to store values to two 1D arrays can store and print values even with an overflow but why a program to store values to 2D array using for loop doesn't run when there is an overflow?

The following program runs:

#include <stdio.h>

int main()
{
    int i, a[5], b[6];

    for (i = 0; i <= 6; i++)
    {
        a[i] = i;
        b[i] = i;
    }

        for (i = 0; i <= 6; i++)
    {
        printf(" %d", a[i], b[i]);
    }
    
    return 0;
}

But why the following program doesn't run?

#include <stdio.h>

int main()
{
    int i, j, a[5][5];

    for (i = 0; i <= 5; i++)
    {
        for (j = 0; j <= 5; j++)
        {
            a[i][j] = j;
        }
    }


    for (i = 0; i <= 5; i++)
    {
        for (j = 0; j <= 5; j++)
        {
            printf(" %d", a[i][j]);
        }
        printf("\n");
    }

    return 0;
}

Answer 1

C allocation places static or stack variable allocations next to each other. So when you overrun a you are accessing b’s allocation. If you reversed the declaration order of a and b then an overrun of a would corrupt the stack.

note: you should really use a better c compiler. Your examples often use uninitiated variables. Most modern c compilers mark the first page of memory as inaccessible, so a runtime error is thrown when accessing memory location zero.