typedef int (*handler_t)(blah_t *, splat_t *, int);

// REQ_DO_THIS, REQ_DO_THAT, REQ_DEFENESTRATE are all small integers

static const handler_t handlers[] = {
        [REQ_DO_THIS] = handle_do_this,
        [REQ_DO_THAT] = handle_do_that,
        [REQ_DEFENESTRATE] = handle_defenestrate,
	...
};

...

        if (req < 0 || req >= array_count(handlers) || !handlers[req]) {
                return -1;
        } else {
                return handlers[req](...);
        }