3.3.1.4 How Can I Use A Variable Defined In Another Source File?

Provided the variable defined in the other source file is not static (see Static Variables) or auto (see Auto Variable Allocation and Access), adding a declaration for that variable into the current file will allow you to access it. A declaration consists of the keyword extern in addition to the type and the name of the variable, as specified in its definition, for example,

extern int systemStatus;

This is part of the C language. Your favorite C textbook will give you more information.

The position of the declaration in the current file determines the scope of the variable. That is, if you place the declaration inside a function, it will limit the scope of the variable to that function. If you place it outside of a function, it allows access to the variable in all functions for the remainder of the current file.

Often, declarations are placed in header files and then they are #included into the C source code (see Pragma Directives).