Why do compilers not warn about out-of-bounds static array indices?

Question

A colleague of mine recently got bitten badly by writing out of bounds to a static array on the stack (he added an element to it without increasing the array size). Shouldn

Answer 1

You're right, the behavior is undefined. C99 pointers must point within or just one element beyond declared or heap-allocated data structures.

I've never been able to figure out how the gcc people decide when to warn. I was shocked to learn that -Wall by itself will not warn of uninitialized variables; at minimum you need -O, and even then the warning is sometimes omitted.

I conjecture that because unbounded arrays are so common in C, the compiler probably doesn't have a way in its expression trees to represent an array that has a size known at compile time. So although the information is present at the declaration, I conjecture that at the use it is already lost.

I second the recommendation of valgrind. If you are programming in C, you should run valgrind on every program, all the time until you can no longer take the performance hit.