"I was thinking that putting my definition in a header file would make it as a global variable."

No.
You must not put the definition in the header; you must put only a declaration in the header.

The definition must be located in a 'C' source file (and, if you include the header in that file, you will ensure that the Compiler can inform you if the declaration doesn't match the definition)

As I said in my previous post, I've covered this before, so try that 'Search' that I suggested.