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 specified static or auto, then adding a declaration (as opposed to a definition) for that variable into the current file will allow you to access it. A declaration consists of the keyword extern in addition to the correct type and the exact name of the variable specified in its definition, e.g.,

extern int systemStatus;  // declare systemStatus for use here

This storage-class specifier is part of the C language and your favorite C textbook will provide more information on its usage.

The position of the declaration in the current file determines the scope of the variable. That is, if you place the extern declaration inside a function, it will limit the scope of the variable to within 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.

Declarations are often placed in header files and then #included into the C source code (see 5.14.1 Preprocessor Directives).