An executable program is usually created by compiling several independent translation units, then linking the resulting object files with preexisting libraries. A term translation unit refers to a source code file together with any included files, but without the source lines omitted by conditional preprocessor directives. A problem arises when the same identifier is declared in different scopes (for example, in different files), or declared more than once in the same scope.

The linkage is a process that allows each instance of an identifier to be associated correctly with one particular object or function. All identifiers have one of two linkage attributes, closely related to their scope: external linkage or internal linkage. These attributes are determined by the placement and format of your declarations, together with an explicit (or implicit by default) use of the storage class specifier static or extern.

Each instance of a particular identifier with external linkage represents the same object or function throughout the entire set of files and libraries making up the program. Each instance of a particular identifier with internal linkage represents the same object or function within one file only.

Linkage Rules

Local names have internal linkage; the same identifier can be used in different files to signify different objects. Global names have external linkage; identifier signifies the same object throughout all program files.

If the same identifier appears with both internal and external linkage within the same file, the identifier will have internal linkage.

Internal Linkage Rules

1. names having file scope, explicitly declared as static, have internal linkage
2. names having file scope, explicitly declared as const and not explicitly declared as extern, have internal linkage
3. typedef names have internal linkage
4. enumeration constants have internal linkage

External Linkage Rules

1. names having file scope, that do not comply to any of previously stated internal linkage rules, have external linkage

The storage class specifiers auto and register cannot appear in an external declaration. No more than one external definition can be given for each identifier in a translation unit declared with internal linkage. An external definition is an external declaration that defines an object or a function and also allocates a storage. If an identifier declared with external linkage is used in an expression (other than as part of the operand of sizeof), then exactly one external definition of that identifier must be somewhere in the entire program.

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