Type-qualifiers on function return type

Question

Given the following C source code:

const int foo(void)
{
    return 42;
}

gcc compiles without errors, but with -Wextra or -Wignored-qualifiers, the following warning appears:

warning: type qualifiers ignored on function return type

I understand that there's good reason in C++ to distinguish between const functions and non-const functions, e.g. in the context of operator overloading.

In plain C however, I fail to see why gcc doesn't emit an error, or more concisely, why the standard allows const functions.

Why is it allowed to use type qualifiers on function return types?

Answer 1

Consider:

#include <stdio.h>

const char* f()
{
    return "hello";
}
int main()
{
    const char* c = f();

    *(c + 1) = 'a';

    return 0;
}

If const were not permitted on the return value then the code would compile (and cause undefined behaviour at runtime).

const is useful when a function returns a pointer to something unmodifiable.

Answer 2

It's irrelevant if the value returned from the function is qualified as const.

You cannot change the value returned even if it wasn't qualified.

foo() = -42; /* impossible to change the returned value */

So using const is redundant (and normally omitted).