Exceeding array bound in C -- Why does this NOT crash?

Question

I have this piece of code, and it runs perfectly fine, and I don't why:

int main(){
   int len = 10;
   char arr[len];
   arr[150] = 'x';
}

Seriously, try it! It works (at least on my machine)! It doesn't, however, work if I try to change elements at indices that are too large, for instance index 20,000. So the compiler apparently isn't smart enough to just ignore that one line.

So how is this possible? I'm really confused here...

---

Okay, thanks for all the answers!

So I can use this to write into memory consumed by other variables on the stack, like so:

#include <stdio.h>
main(){
   char b[4] = "man";
   char a[10];
   a[10] = 'c';
   puts(b);
}

Outputs "can". That's a really bad thing to do.

Okay, thanks.

Answer 1

C compilers generally do not generate code to check array bounds, for the sake of efficiency. Out-of-bounds array accesses result in "undefined behavior", and one possible outcome is that "it works". It's not guaranteed to cause a crash or other diagnostic, but if you're on an operating system with virtual memory support, and your array index points to a virtual memory location that hasn't yet been mapped to physical memory, your program is more likely to crash.