+#if __GNUC__ >= 7
+#define _mul_overflow(a, b, res) __builtin_mul_overflow (a, b, res)
+#else
+/* Assumes unsigned values. Careful! Args evaluated multiple times. */
+#define _mul_overflow(a, b, res) \
+ ((*res) = (a), (*res) *= (b), (b) != 0 && (*res) / (b) != (a))
+#endif
+