C with a module system

Question

In the history of the C language, have any experiments been done towards adding a module system?

I know about the C++ proposal which didn't make the latest standard. I'm thinking of something more along the lines of particular implementations which added a module system as a non-standard feature.

By "module system" I mean something which at the very least allowed the user to write one file instead of a .h/.c pair.

Answer 1

D programming language is a C++-like language with a module system. I'm not aware of any module system designed for plain C.