Is it valid to check if a function is defined in C?

Question

void f();
int main(int argc, char** argv)
{
    if (f)
    {
        // other code
    }
}

With VS2017, the linker complaint about unsolved external symbol, while it works with GCC. According to C99 spec, is it valid? Or it's implementation detail?

Answer 1

C standard requires that every symbol should be defined exactly once in a correct program, but does not require any diagnostic if the rule is not observed. So if you declare a function that is never defined in any compilation unit any use of that function is beyond C specification.

The gcc compiler is known to have plenty of extensions, some of which are also accepted by clang. If you know that you will only use gcc, you can use them, if you want to write portable programs you should not.