TCPL/A.10.2_External_Declarations

A.10.2 External Declarations

External declarations specify the characteristics of objects, functions and other identifiers. The term external refers to their location outside functions, and is not directly connected with the extern keyword; the storage class for an externally-declared object may be left empty, or it may be specified as extern or static.

Several external declarations for the same identifier may exist within the same translation unit if they agree in type and linkage, and if there is at most one definition for the identifier.

Two declarations for an object or function are deemed to agree in type under the rule discussed in Par.A.8.10. In addition, if the declarations differ because one type is an incomplete structure, union, or enumeration type (Par.A.8.3) and the other is the corresponding completed type with the same tag, the types are taken to agree. Moreover, if one type is an incomplete array type (Par.A.8.6.2) and the other is a completed array type, the types, if otherwise identical, are also taken to agree. Finally, if one type specifies an old-style function, and the other an otherwise identical new-style function, with parameter declarations, the types are taken to agree.

If the first external declarator for a function or object includes the static specifier, the identifier has internal linkage; otherwise it has external linkage. Linkage is discussed in Par.11.2.

An external declaration for an object is a definition if it has an initializer. An external object declaration that does not have an initializer, and does not contain the extern specifier, is a tentative definition. If a definition for an object appears in a translation unit, any tentative definitions are treated merely as redundant declarations. If no definition for the object appears in the translation unit, all its tentative definitions become a single definition with initializer 0.

Each object must have exactly one definition. For objects with internal linkage, this rule applies separately to each translation unit, because internally-linked objects are unique to a translation unit. For objects with external linkage, it applies to the entire program.

• Although the one-definition rule is formulated somewhat differently in the first edition of this book, it is in effect identical to the one stated here. Some implementations relax it by generalizing the notion of tentative definition. In the alternate formulation, which is usual in UNIX systems and recognized as a common extension by the Standard, all the tentative definitions for an externally linked object, throughout all the translation units of the program, are considered together instead of in each translation unit separately. If a definition occurs somewhere in the program, then the tentative definitions become merely declarations, but if no definition appears, then all its tentative definitions become a definition with initializer 0.