This is interesting. So basically, my understanding is that the Posix subsystem is just a user mode portion of Windows that translates API calls to calls into Executive services through ntdll.dll<p>Now this means that if you know the ntdll.dll interface then this remains stable. So if you want to develop your own environmental subsystem it should be possible. After all, the Posix subsystem was purchased by Microsoft!<p>What I&#x27;m <i>really</i> interested in is what the ReactOS guys are doing. Have they implemented the same layering? If they have implemented ntdll.dll, then this could actually mean that they could technically get in ahead and do what Microsoft are doing right now.<p>For that matter, it starts to make me wonder whether someone could do what Microsoft have done but in reverse on Linux! In other words, implement a translation layer that translate ntdll.dll function calls to corresponding kernel syscalls, then work backwards implementing each of the user mode subsystems. Maybe WINE has done this already?