You are right, it appears to be C++-specific and looks like a typo in the link. C++98, 17.4.3.1.2 Global names - Each name that contains a double underscore (

You are right, it appears to be C++-specific and looks like a typo in the link. C++98, 17.4.3.1.2 Global names - Each name that contains a double underscore (__) [...] is reserved to the implementation for any use.