(Why) is using an uninitialized variable undefined behavior?

Question

If I have:

unsigned int x;
x -= x;

it\'s clear that x should be zero after this expression, but everywhere I look, th

Answer 1

Yes, it's undefined. The code can crash. C says the behavior is undefined because there's no specific reason to make an exception to the general rule. The advantage is the same advantage as all other cases of undefined behavior -- the compiler doesn't have to output special code to make this work.

Clearly, the compiler could simply use whatever garbage value it deemed "handy" inside the variable, and it would work as intended... what's wrong with that approach?

Why do you think that doesn't happen? That's exactly the approach taken. The compiler isn't required to make it work, but it is not required to make it fail.