UNDEFINED BEHAVIOUR

In computer programming, undefined behavior (UB) is the result of executing computer code whose behavior is not prescribed by the language specification to which the code adheres, for the current state of the program. This happens when the translator of the source code makes certain assumptions, but these assumptions are not satisfied during execution.

The behavior of some programming languages—most famously C and C—is undefined in some cases.In the standards for these languages the semantics of certain operations is described as undefined. These cases typically represent unambiguous bugs in the code, for example indexing an array outside of its bounds. An implementation is allowed to assume that such operations never occur in correct standard-conforming program code. In the case of C/C, the compiler is allowed to give a compile-time diagnostic in these cases, but is not required to: the implementation will be considered correct whatever it does in such cases, analogous to don't-care terms in digital logic. It is the responsibility of the programmer to write code that never invokes undefined behavior, although compiler implementations are allowed to issue diagnostics when this happens. This assumption can make various program transformations valid or simplify their proof of correctness, giving flexibility to the implementation. As a result, the compiler can often make more optimizations. It also allows more compile-time checks by both compilers and static program analysis.

In the C community, undefined behavior may be humorously referred to as "nasal demons", after a comp.std.c post that explained undefined behavior as allowing the compiler to do anything it chooses, even "to make demons fly out of your nose". Under some circumstances there can be specific restrictions on undefined behavior.