c - Why doesn't this code cause a segfault?

Question

Why doesn't this segfault?

#include <stdio.h>
int main()
{
    int i;
    int arr[] = {1, 2, 3, 4};

    for(i=0;i<8;i++)
    {
        arr[i] = i;
        printf(" %d", arr[i]);
    }

    printf("
");

    return 0;
}

But it does when I replace 8 with 9 in the for loop.

Note: I am trying it on 32 bit crunchbang linux

Answer 1

Technically speaking, this program results in undefined behavior, meaning that there are absolutely no guarantees whatsoever about what this program is allowed to do. It could in principle format your hard drive, email nasty messages to all of your friends, set your computer on fire, or become sentient and enslave humanity.

In this case, the undefined behavior when n = 8 happens to not do anything bad, while the undefined behavior when n = 9 causes a segfault. Both are totally permissible behaviors for the program, but aren't at all guaranteed to be portable.

Hope this helps!