You implement it by using unsigned arithmetic, then checking that the sign bit flipped. See https://git.yzena.com/Yzena/Yc/src/branch/master/include/yc/arith.h#

You implement it by using unsigned arithmetic, then checking that the sign bit flipped. See https://git.yzena.com/Yzena/Yc/src/branch/master/include/yc/arith.h#L305-L315 https://git.yzena.com/Yzena/Yc/src/branch/master/include/yc/... . It's a bit more subtle than that, but that is completely devoid of Undefined, Unspecified, and Implementation-Defined Behavior.