Constant across Files - newbie in C !!

Hi,
I'm new in C programming. I've one problem, I have to deploy some kind of flags in several functions in different files. This flag can be a Int value. I've to check the flag value in one file, suppose test.c, but the flag is changed in a different file.

What suggestions, aproach do you have?

Thanks in advanced!

http://www.google.com/search?q=c+extern

extern is a C keyword that just tells the compiler that variable that succeeds it will be declared in another file, to be linked in later.