Undefined behavior in C is a reading error

Because there seems to be some confusion in this thread: - "Implementation-defined behavior" means that the C standard specifies the allowable behaviors that a C implementation must choose from, and the implementation must document its particular choice. - "Unspecified behavior" means that the C standard places no particular restrictions on the behavior, but a C implementation must pick a behavior and document its choice. - "Undefined behavior" means that C implementations are allowed to assume that the respective…

TFA is misunderstanding. As he cites from the C standard, “Undefined behavior gives the implementor license not to catch certain program errors that are difficult to diagnose.” Since it’s difficult (and even, in the general case of runtime conditions, impossible at compile time) to diagnose, the implementor (compiler writer) has two choices: (a) assume that the undefined behavior doesn’t occur, and implement optimizations under that assumption, or (b) nevertheless implement a defined behavior for it, which in many…

Unfortunately you have to choose what you want your C compiler to be. It can be little more than a macro assembler that preserves "the obvious" behavior of undefined constructs on the target platform. Most compilers will do this by default (-O0). Or you can opt into using an optimizing compiler (-O2). But when optimization passes ignore the possibility of undefined behavior with unpredictable results (as explicitly allowed by the standard), you get, well, unpredictable results. Nowadays compilers even offer…