(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, the program might crash. There might, for example, be trap representations (specific bit patterns which cannot be handled) which might cause a CPU interrupt, which unhandled could crash the program.

(6.2.6.1 on a late C11 draft says) Certain object representations need not represent a value of the object type. If the stored value of an object has such a representation and is read by an lvalue expression that does not have character type, the behavior is undefined. If such a representation is produced by a side effect that modifies all or any part of the object by an lvalue expression that does not have character type, the behavior is undefined.50) Such a representation is called a trap representation.

(This explanation only applies on platforms where unsigned int can have trap representations, which is rare on real world systems; see comments for details and referrals to alternate and perhaps more common causes which lead to the standard's current wording.)