Apart from fact that it is not an integer overflow, what you can see in system headers is not what you can do in regular program’s headers. System headers are part of toolchain and can rely on specifics of platform and actively use these to provide compliant values for you. When you’re on a different platform, a different set of definitions is provided (though most cross-platform standard library implementations try to abstract specifics deep into their internals and builtins, since it is useful for stdlib writers too).