Noreturn

Noreturn refers to a property of a function indicating that it does not return to the caller. When such a function is invoked, control flow ends at the function, typically by terminating the program, throwing an exception in languages that use exceptions, or transferring control elsewhere in a non-returning manner. The concept is important for both correctness and optimization, as compilers can assume that code following a noreturn call is unreachable.

In different languages and environments, noreturn is expressed with several syntaxes. In C11 and later, a function can be declared as _Noreturn, for example: _Noreturn void fatal(void); In GCC and Clang extensions for C, the attribute __attribute__((noreturn)) is used, such as: void fatal(void) __attribute__((noreturn)); In C++11 and later, the standard name [[noreturn]] can be applied to a function: [[noreturn]] void fatal(); In practice, many standard library routines that terminate a program (such as exit or abort) are noreturn.

Notes and caveats include that noreturn is a promise by the programmer or standard library implementer. If a noreturn function unexpectedly returns, behavior is undefined in languages where the contract is not fulfilled. Noreturn also interacts with compiler diagnostics: compilers may warn if control can flow off the end of a noreturn function or if a non-noreturn function appears not to return where required.

Common uses include terminating a program path after reporting an unrecoverable error, ending a thread, or signaling a non-returning error path. The concept differs from, but is related to, exceptions and to languages that use a distinct never type (as in some languages) to represent non-returning computations.