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15.6 Non-returning Functions

A "non-returning" function is a function which cannot return normally. It can transfer control only through longjmp(), throw (in C++), or similar mechanisms. The most prominent function of this class is the abort function. Non-returning functions are declared with a void return type.

It helps the compiler’s ability to emit sensible warnings, following data-flow analysis, to declare which functions are non-returning.

To decorate function declarations and function definitions, you can use the _Noreturn keyword. No modules are needed, as Gnulib arranges for <config.h> to define _Noreturn to an appropriate replacement on platforms lacking it.

Gnulib has two modules that support such a declaration:

• The ‘noreturn’ module. It provides a way to put this declaration at function declarations, at function definitions, and in function pointer types. The identifiers to use are:
  • - _GL_NORETURN_FUNC for use in function declarations and function definitions.
  • - _GL_NORETURN_FUNCPTR for use on function pointers.

  The include file is <noreturn.h>.

• The ‘stdnoreturn’ module. This can improve readability by letting you use noreturn instead of _Noreturn; unfortunately, noreturn is a no-op on some platforms even though _Noreturn works on them. The include file is <stdnoreturn.h>.

Which of the two modules to use? If the non-returning functions you have to declare are unlikely to be accessed through function pointers, you should use module stdnoreturn; otherwise the module noreturn provides for better data-flow analysis and thus for better warnings.

For a detailed description of the stdnoreturn module, see stdnoreturn.h.