What is undefined behavior, and why is “it works on my machine” not enough?

Undefined behavior means the language imposes no requirements on the program’s behavior for that operation. Examples include accessing beyond an array, dereferencing an invalid pointer, and signed integer overflow in ordinary C and C++ arithmetic. A program may appear correct under one build and fail after optimization or a compiler change. Use clear ownership, bounds checks, compiler wa ings, and suitable sanitizers to find problems, but do not assume tests prove their absence. Unsigned overflow follows different defined rules and should not be confused with signed overflow.