_Noreturn keyword and noreturn macro (C11)

The _Noreturn keyword was introduced in C11. It tells the compiler that the function it's applied to doesn't return to the caller. The compiler knows that the code following a call to a _Noreturn function is unreachable. An example of a function that doesn't return is abort. If there's a possibility for control flow to return to the caller, the function must not have the _Noreturn attribute.

The keyword is typically used through the convenience macro, noreturn, provided in <stdnoreturn.h>, which maps to the _Noreturn keyword.

The primary benefits for using _Noreturn (or the equivalent noreturn) are making the intention of the function clear in the code for future readers, and detecting unintentionally unreachable code.

A function marked noreturn shouldn't include a return type because it doesn't return a value to the caller. It should be void.

Example using noreturn macro and _Noreturn keyword

The following example demonstrates the _Noreturn keyword and the equivalent noreturn macro.

IntelliSense may generate a spurious error, E0065, if you use the macro noreturn that you can ignore. It doesn't prevent you from running the sample.

// Compile with Warning Level4 (/W4) and /std:c11
#include <stdio.h>
#include <stdlib.h>
#include <stdnoreturn.h>

noreturn void fatal_error(void)
{
    exit(3);
}

_Noreturn void not_coming_back(void)
{
    puts("There's no coming back");
    fatal_error();
    return; // warning C4645 - function declared with noreturn has a return statement
}

void done(void)
{
    puts("We'll never get here");
}

int main(void)
{
    not_coming_back();
    done(); // warning c4702 - unreachable code

    return 0;
}

Requirements

See also

/std (Specify language standard version)
/W4 (Specify warning level)
C4702 warning
__declspec(noreturn)