Array boundary checking in C

Hi All

prog 1:

Code:

int main(void)
{
    int i;
    int arr[3];
    for(i=0;i<4;i++)
        scanf("%d",&arr[i]);
    for(i=0;i<4;i++)
        printf(" %d",arr[i]);
}

INPUT: 1,2,3,4
OUTPUT: 1,2,3,4

prog 2:

Code:

int i;
int arr[3];
int main(void)
{
    for(i=0;i<4;i++)
        scanf("%d",&arr[i]);
    for(i=0;i<4;i++)
        printf(" %d",arr[i]);
}

INPUT: 1,2,3,4
OUTPUT: 1,2,3,3

As C never checks for boundary, we can insert values in an
array even its boundary is crossed, but why do I get different output
if I declare int arr[3] and int i within & out side of main.

Why dint I get segmentation fault?
please comment

When you overstep the boundary of your array, your program will do one of two things:

1) Corrupt other data that your program has access to.
2) Attempt to corrupt data that your program does not have access to.

If the second option occurs, then your program will hopefully suffer a segmentation fault and die. On some operating systems, this action is allowed, leading to all kinds of unintentional behavior.

Since your program does not crash, and you're hopefully on a system that does protect memory, the chances are that you are corrupting other memory that your program is allowed to overwrite. In the first example, i and arr are variables that are allocated on the stack. In the second example, i and arr are "global variables". That could simply just mean they are found in the .data segment of your program.

This means you are most likely writing into very different locations in your program's allocated memory in both examples. Either way, it's wrong to do (as I'm sure you already know).

http://www.howstuffworks.com/c23.htm

This article provides a scenario about overstepping the bounds of two arrays and what bugs occur from doing so.

thanks for your valuable comment....