This is what you call a prototype. Normally, you’ll create a header file with the prototype. You can include that header in both A.c and B.c. The problem is when you change the function in A.c: ...

As I'm spending some time back in the land of low-level C, I'm trying to fully get my head around best-practices for header files and code organization, so I thought I'd toss out some questions ...

Basically once you've put a using namespace std; in your header, and someone has included your header, there's no way to undo the using. This can reduce your options when a name conflict occurs.