In global scope, identifiers for the following kinds of entities declared without the static storage class specifier have external linkage:

• An object
• A function

If an identifier in C is declared with the extern keyword and if a previous declaration of an object or function with the same identifier is visible, the identifier has the same linkage as the first declaration. For example, a variable or function that is first declared with the keyword static and later declared with the keyword extern has internal linkage. However, a variable or function that has no linkage and was later declared with a linkage specifier will have the linkage that was expressly specified.

In namespace scope, the identifiers for the following kinds of entities have external linkage:

• A reference or an object that does not have internal linkage
• A function that does not have internal linkage
• A named class or enumeration
• An unnamed class or enumeration defined in a typedef declaration
• An enumerator of an enumeration that has external linkage
• A template, unless it is a function template with internal linkage
• A namespace, unless it is declared in an unnamed namespace

If the identifier for a class has external linkage, then, in the implementation of that class, the identifiers for the following will also have external linkage:

• A member function.
• A static data member.
• A class of class scope.
• An enumeration of class scope.