Why no error is thrown in compilation or running

Question

int main()
{
    int a[2] = {1,2};
    a[2] = 3;
    printf("\n\n%d  %d  %d\n\n",a[0],a[1],a[2]);
    return 0;
}

I get output as 1 2 3

Why no error is thrown at run time or compile time?

What language? This could actually work in a few different languages ... — Daniel Williams, Aug 16 '13 at 04:47
Since no one has mentioned that it's undefined behaviour, I'll do that. — chris, Aug 16 '13 at 04:59

score 13 · Answer 1 · answered Aug 16 '13 at 04:48

13

Have you heard about all the security problems caused by buffer overruns? They exist because C doesn't have any automatic array bounds checking. It's the programmer's responsibility to ensure that they don't address outside the array limit, the compiler doesn't check for it.

answered Aug 16 '13 at 04:48

Barmar

741,623
53
500
612

C doesn't *require* automatic array bounds checking. Attempting to access an array outside its bounds has *undefined behavior*. A C compiler could legally implement bounds checking, but most don't – Keith Thompson Aug 16 '13 at 05:12
And obviously his doesn't, which explains what he sees. – Barmar Aug 16 '13 at 05:15
@Barmar, I think it's also important to note that its technically undefined behaviour as another variable/constant (from a future stack frame or previous stack frame) could be constantly writing and reading to/from a subset of that block of memory -- meaning he should not always expect that result. – Jacob Pollack Aug 16 '13 at 05:17

score 0 · Answer 2 · answered Aug 16 '13 at 05:16

0

Just make sure you don't address anything out of the boundary since, C doesn't check array bounds.

answered Aug 16 '13 at 05:16

user2687481

1,283
3
10
8

score 0 · Answer 3 · answered Aug 16 '13 at 06:04

int a[2] is allocated as an automatic variable on the stack. In Windows the stack is initially allocated as 1MB for a process. So, what has really happened is that when the code assigned a[2] = 3; an area outside of the stack frame for main() was updated.

More often than not this causes a problem, such as a segmentation fault, but in simple programs stuff like this sometimes still works.

An interesting test would be to call a sub-routine that also defines and sets some automatic variables and after it returns print the value of a[2] to see if it got overwritten by the stack frame for the sub-routine? If you do this, please let me know the results!

Why no error is thrown in compilation or running

3 Answers3