How to hint to GCC that a line should be unreachable?

Question

It's common for compilers to provide a switch to warn when code is unreachable. I've also seen macros for some libraries, that provide assertions for unreachable code.

Is there a hint, such as through a pragma, or builtin that I can pass to GCC (or any other compilers for that matter), that will warn or error during compilation if it's determined that a line expected to be unreachable can actually be reached?

Here's an example:

    if (!conf->devpath) {
        conf->devpath = arg;
        return 0;
    } // pass other opts into fuse
    else {
        return 1;
    }
    UNREACHABLE_LINE();

The value of this is in detecting, after changes in conditions above the expected unreachable line, that the line is in fact reachable.

Answer 1

If your compiler does not have the warning that you need, it can be complemented with a static analyzer. The kind of analyzer I am talking about would have its own annotation language and/or recognize C assert, and use these for hints of properties that should be true at specific points of the execution. If there isn't a specific annotation for unreachable statements, you could probably use assert (false);.

I am not personally familiar with them but Klokwork and CodeSonar are two famous analyzers. Goanna is a third one.

Answer 2

gcc 4.5 supports the __builtin_unreachable() compiler inline, combining this with -Wunreachable-code might do what you want, but will probably cause spurious warnings