Emulating GCC's __builtin_unreachable?

Question

I get a whole lot of warnings about switches that only partially covers the range of an enumeration switched over. Therefor, I would like to have a "default" for all those switches and put __builtin_unreachable (GCC builtin) in that case, so that the compiler know that case is not reachable.

However, I came to know that GCC4.3 does not support that builtin yet. Is there any good way to emulate that functionality? I thought about dereferencing a null pointer instead, but that may have other undesirable effects/warnings and such. Do you have any better idea?

Answer 1

You can call an inline function declared _Noreturn to mark anything after that call as unreachable. The compiler is allowed to throw out any code after such a function. If the function itself is static (and does return), the compiler will usually also inline the function. Here is an example:

static _Noreturn void unreachable() {
    return; /* intentional */
}

/* ... */

foo();
bar(); /* should better not return */
unreachable();
baz(); /* compiler will know this is not reachable */

Notice that you invoke undefined behavior if a function marked _Noreturn indeed returns. Be sure that said function will never be called.