As a general note (also regarding your other reviews), using EXIT_SUCCESS and EXIT_FAILURE instead of 0 and 1 is a matter of taste, but whichever option you choose should be applied consistently. If the program you're working on has a large number of instances of xo_err(1, ...) or xo_errx(1, ...), you'll have to decide whether to change them all to xo_err(EXIT_FAILURE, ...) or xo_errx(EXIT_FAILURE, ...) or whether to stick to 1 and 0 to reduce the magnitude of the change.