Warn when using parameter marked as __unused

Question

Using the -Wunused-parameter flag, you can enforce __unused for unused parameters, as a compiler optimization. The following code causes two warnings:

#include &         


        

Answer 1

Another way to skin this cat is to remove (or comment out) the name of the parameter.

int main ( int argc, char ** /* argv */ ) {
    printf("hello world. there are %d args\n", argc);
    return 0;
}

Now the compiler won't warn about argv being unused, and you can't use it, because it has no name.

Answer 2

The __unused attribute is intended to prevent complaints when arguments to functions/methods or functions/methods are unused, not to enforce their lack of use.

The term used in the GCC manual is:

This attribute, attached to a function, means that the function is meant to be possibly unused

And for variables:

This attribute, attached to a variable, means that the variable is meant to be possibly unused.

The most common use is for development against an interface - e.g. callbacks, where you may be forced to accept several parameters but don't make use of all of them.

I use it a bit when I'm doing test driven development - my initial routine takes some parameters and does nothing, so all the parameters take __attribute__((unused)). As I develop it I make use of the parameters. At the end of development I remove them from the method, and see what shakes out.