Global Variables In Libraries

I have a project in which a variable is defined in a static library as "extern SCREEN g_Screen;" (where SCREEN is "enum SCREEN {FULL,WINDOWED};"). The library is included in a dynamic library that uses it and, through inclusion of the dynamic library header, in the test application. For some reason when I set g_Screen in the application the library doesn't see the change (and vice versa).

Am I missing something? I read something somewhere about hooks but I wasn't too sure that it was what I needed. I very much need the library and application to be able to control/read the same instance of the variable without having to pass the variable's value as a parameter between the library and application.

I suppose that the header file contains the DECLARATION
extern SCREEN g_screen;
as per definition, the extern keyword marks a declaration.
Then in some cpp file of the library there ought to be a
SCREEN g_screen;
(the definition) and if a change to g_screen in your main app doesn't affect the library then something's seriously wrong.