[mmoll]

Afaik, underscore-underscore anywhere in a name is reserved in C++ only. C only reserves names starting with underscore-underscore. And yes, you can get away with using these identifiers, but you could almost never be certain that you did.

[moloch-hai]

Microsoft could be absolutely, utterly, undeniably certain it got away with using names reserved to the implementation in its implementation. Arguably some headers are not part of the implementation, because they don't come with MSVS. But that is a matter of packaging, which the Standard does not cover.

And, only C++ is of any interest, here. Microsoft never gave a damn about C, and any name reserved in C is also reserved in C++.