-int ustream_printf(struct ustream *s, const char *format, ...);
-int ustream_vprintf(struct ustream *s, const char *format, va_list arg);
+int ustream_printf(struct ustream *s, const char *format, ...)
+ __attribute__ ((format (printf, 2, 3)));
+int ustream_vprintf(struct ustream *s, const char *format, va_list arg)
+ __attribute__ ((format (printf, 2, 0)));