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 (e.g. memory). 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. An implementation is allowed to assume that such operations never occur in standard-conforming program code; the implementation will be considered correct whatever it does in such cases, analogous to don't-care terms in digital logic. 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 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.

Undefined behavior is often unpredictable and a frequent cause of software bugs. 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. For example, the instruction set specifications of a CPU might leave the behavior of some forms of an instruction undefined, but if the CPU supports memory protection then the specification will probably include a blanket rule stating that no user-accessible instruction may cause a hole in the operating system's security; so an actual CPU would be permitted to corrupt user registers in response to such an instruction, but would not be allowed to, for example, switch into supervisor mode.

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