Undefined Behavior

Undefined behavior refers to code whose outcome the C standard places no constraints on, meaning a compiler is free to produce any result, including seemingly correct output, a crash, or subtle corruption that only appears under certain conditions. Common causes include signed integer overflow, reading uninitialized memory, out-of-bounds array access, and dereferencing invalid pointers. Because undefined behavior can appear to work correctly during testing and then fail later or on a different compiler, it is one of the most important sources of hard-to-diagnose bugs in C.