nope, all good, ty

No, not 100% - it needs to be in the MSDN/PlatformSDK documentation. Stuff in a public .h file ought to serve as a code contract, but it doesn't - especially not with some of the older C stuff

Well that is the point actually. Undocumented stuff is not in any document that we can rely upon. That is why we call it undocumented. You take your life in your hands when you use undocumented things. Sure, they work, usually. But, as you have discovered in the first post, they can also easily break later.

The existence of the segment registers is documented in the Intel/AMD manuals. But the runtime contents of the segment registers is not documented by MS, so you have to guess and pray your code will still work after the next service pack installation.

I can't completely agree: there is function-like macro GetCurrentFiber(), published in MSDN/PSDK, that uses undocumented NtCurrentTeb() and NT_TIB, particularly .FiberData field. Moreover, NtCurrentTeb() uses fixed offset in fs segment, 0x18, to retrieve .Self field contents, quite a surprise. Therefore any program that uses GetCurrentFiber() expects Windows to retain at least two fields at their offsets in NT_TIB.