Decus cpp
An almost-ANSI C preprocessor by Martin Minow. It is shipped with X11R5 (contrib/util/cpp) because some systems don't have a working cpp.
It runs on VMS (Vax C, Decus C), RSX-11M, RSTS/E, P/OS, RT11, A/UX and Apollo Domain/IX 9.6 and is highly portable.