Question

AFAIK, any declaration of a variable or a function in file scope has external linkage by default. static mean “it has internal linkage”, extern — “it maybe defined elsewhere”, not “it has external linkage”.

If so, why we need extern keyword? In other words, what is difference between int foo; and extern int foo; (file scope)?

Answer 1

The extern keyword is used primarily for variable declarations. When you forward-declare a function, the keyword is optional.

The keyword lets the compiler distinguish a forward declaration of a global variable from a definition of a variable:

extern double xyz; // Declares xyz without defining it

If you keep this declaration by itself and then use xyz in your code, you would trigger an “undefined symbol” error during the linking phase.

double xyz; // Declares and defines xyz

If you keep this declaration in a header file and use it from several C/C++ files, you would trigger a “multiple definitions” error during the linking phase.

The solution is to use extern in the header, and not use extern in exactly one C or C++ file.