Question: Here *len* is at A[10] and *i* is at A[11]. Is there a way to catch these errors??
I tried compiling with gcc -Wall -W but no warnings are displayed.

```
int main()
{
  int A[10];
  int i, len;
  len = sizeof(A) / sizeof(0[A]);
  printf("Len = %d\n",len);
  for(i = 0; i < len; ++i){
    A[i] = i*19%7;
  }
  A[i] = 5;
  A[i + 1] = 6;
  printf("Len = %d i = %d\n",len,i);
  return 0;
}

```

**Output :**
Len = 10
Len = 5 i = 6

Answer: 
You are accessing memory outside the bounds of the array; in C, there is no bounds checking done on array indices.

Accessing memory beyond the end of the array technically results in *undefined behavior*. This means that there are no guarantees about what happens when you do it. In your example, you end up overwriting the memory occupied by another variable. However, undefined behavior can also cause your application to crash, or worse.

> 
> Is there a way to catch these errors?
> 
> 
> 

The compiler can catch some errors like this, but not many. It is often impossible to catch this sort of error at compile-time and report a warning.

Static analysis tools can catch other instances of this sort of error and are usually built to report warnings about code that is likely to cause this sort of error.
The array allocation of A is adjacent in memory to i and len. Remember that when you address via an array, it's exactly like using pointers, and you're walking off the end of the array, bumping into the other stuff you put there.

C by default does not do bounds checking. You're expected to be careful as a programmer; in exchange you get speed and size benefits.

Usually external tools, like lint, will catch the problems via static code analysis. Some libraries, depending on compiler vendor, will add additional padding or memory protection to detect when you've walked off the end.

Lots of interesting, dangerous, and non-portable things reside in memory at "random spots." Most of the house keeping for heap memory allocations occur in memory locations before the one the compiler gives you.

The general rule is that if you didn't allocate or request it, don't mess with it.