Why does “noreturn” function return?

The function specifiers in C are a hint to the compiler, the degree of acceptance is implementation defined.

First of all, _Noreturn function specifier (or, noreturn, using <stdnoreturn.h>) is a hint to the compiler about a theoretical promise made by the programmer that this function will never return. Based on this promise, compiler can make certain decisions, perform some optimizations for the code generation.

IIRC, if a function specified with noreturn function specifier eventually returns to its caller, either

• by using and explicit return statement
• by reaching end of function body

the behaviour is undefined. You MUST NOT return from the function.

To make it clear, using noreturn function specifier does not stop a function form returning to its caller. It is a promise made by the programmer to the compiler to allow it some more degree of freedom to generate optimized code.

Now, in case, you made a promise earlier and later, choose to violate this, the result is UB. Compilers are encouraged, but not required, to produce warnings when a _Noreturn function appears to be capable of returning to its caller.

According to chapter §6.7.4, C11, Paragraph 8

A function declared with a _Noreturn function specifier shall not return to its caller.

and, the paragraph 12, (Note the comments!!)

EXAMPLE 2
_Noreturn void f () {
abort(); // ok
}
_Noreturn void g (int i) { // causes undefined behavior if i <= 0
if (i > 0) abort();
}

For C++, the behaviour is quite similar. Quoting from chapter §7.6.4, C++14, paragraph 2 (emphasis mine)

If a function f is called where f was previously declared with the noreturn attribute and f eventually
returns, the behavior is undefined. [ Note: The function may terminate by throwing an exception. —end
note ]

[ Note: Implementations are encouraged to issue a warning if a function marked [[noreturn]] might
return. —end note ]

3 [ Example:

[[ noreturn ]] void f() {
throw "error"; // OK
}
[[ noreturn ]] void q(int i) { // behavior is undefined if called with an argument <= 0
if (i > 0)
throw "positive";
}

—end example ]