The keyword extern is used in declarations of objects and functions.

On a function, extern emphasizes that the function has external linkage. However, it is usually redundant, as this is the default anyway. One occasion when it is not redundant is to provide a definition of an in-line function.

On an object declaration that doesn't include an initializer, extern ensures that the construct is purely a declaration, and not a tentative definition, which is essential if the object has incomplete type. On an object declaration inside a block, it ensures that the declaration is for a static object with external linkage.