I'd suggest adding a line below the statement:
(void)err;

When I write new code, I generally enable as many checks and diagnostics as possible on the assumption that it will improve the quality of the code, and make it more maintainable and readable. I find the aforementioned method a good way of saying "yes, I know this is unused" in e.g. main(). Wrapping it in a macro and optionally using __attribute__((unused)) might be even better.

(I'm a measly CS student, so I wouldn't know whether it actually works out there in the Real World ;))