extern and Linkage

Linkage describes whether a name can be shared between different source files.

A name with external linkage refers to the same entity across the whole program. A name with internal linkage is private to its own file. Understanding linkage is key to multi-file programs.

Suppose two files need access to one global counter. You cannot simply define it in both files; that creates two separate variables and a duplicate-symbol link error.

Instead, you define it once and declare it everywhere else with extern.