kvprintf
retval = kvprintf(cfmt, NULL, NULL, 10, ap);
retval = kvprintf(cfmt, &snprint_func, &arg, 10, ap);
retval = kvprintf(cfmt, &snprint_func, &arg, 10, ap);
retval = kvprintf(cfmt, NULL, (void *)buf, 10, ap);
retval = kvprintf(cfmt, &snprint_func, &arg, 10, ap);
static int kvprintf(char const *, kvprintf_fn_t *, void *, int, va_list);
retval = kvprintf(fmt, putchar_wrapper, NULL, 10, ap);
kvprintf(fmt, putchar_wrapper, NULL, 10, ap);
retval = kvprintf(cfmt, NULL, (void *)buf, 10, ap);