The new code use rc (an int) with a %s format string:

    rc = pthread_create(&main_thr, NULL, &apple_main, &args);
    if (rc != 0) {
        JLI_ReportErrorMessageSys("Could not create main thread, return code: %s\n", rc);
        exit(1);
    }

The compiler should be able to catch this if JLI_ReportErrorMessageSys was declared with something like __attribute__ ((format (printf, 1, 2))).

I didn't compile this code or test it, this bug report is based on visual code inspection only. I assume pthread_detach cannot actually fail on MacOS, so I think this is P5.