Question

Possible Duplicate:
Array index out of bound in C
Can a local variable's memory be accessed outside its scope?
C no out of bounds error

I am trying out this code snippet,

#include <stdio.h>

main(){
int a[2],i;
a[5] = 12;
for(i=0;i<10;i++){
    printf("%d\n", a[i]);
}
return 0;
}

It gives me output :

1053988144
32767
0
3
0
12
-1267323827
32716
0
0

Why a[5] is accessible ? Shouldn’t it through RunTime Error?

Answer 1

int a[2]; means “allocate memory that is 2 * sizeof(int)“

a[5] is syntactic sugar for *(a + 5), which point to an area of memory a + (5 * sizeof(int)). So 3 * sizeof(int) past the end of your array. Where is that? Who knows?

Some languages do bound checking, and I have heard of some C compilers that can do it as well, but most do not. Why not do bound checking? Performance. And performance is a major reason for using C in the first place. But that’s OK, because C programmers are good programmers and never go beyond the bounds of an array. (hopefully)