6.2.2 Linkages of identifiers

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417 An identifier declared in different scopes or in the same scope more than once can be made to refer to the same object or function by a process called linkage.²¹⁾

418 There are three kinds of linkage: external, internal, and none.

419 In the set of translation units and libraries that constitutes an entire program, each declaration of a particular identifier with external linkage denotes the same object or function.

420 Within one translation unit, each declaration of an identifier with internal linkage denotes the same object or function.

421 Each declaration of an identifier with no linkage denotes a unique entity.

422 If the declaration of a file scope identifier for an object or a function contains the storage-class specifier static, the identifier has internal linkage.²²⁾

423 For an identifier declared with the storage-class specifier extern in a scope in which a prior declaration of that identifier is visible,²³⁾ if the prior declaration specifies internal or external linkage, the linkage of the identifier at the later declaration is the same as the linkage specified at the prior declaration.

424 21) There is no linkage between different identifiers.

425 22) A function declaration can contain the storage-class specifier static only if it is at file scope; see 6.7.1.

426 If no prior declaration is visible, or if the prior declaration specifies no linkage, then the identifier has external linkage.

427 If the declaration of an identifier for a function has no storage-class specifier, its linkage is determined exactly as if it were declared with the storage-class specifier extern.

428 If the declaration of an identifier for an object has file scope and no storage-class specifier, its linkage is external.

429 The following identifiers have no linkage:

430 an identifier declared to be anything other than an object or a function;

431 an identifier declared to be a function parameter;

432 a block scope identifier for an object declared without the storage-class specifier extern.

433 If, within a translation unit, the same identifier appears with both internal and external linkage, the behavior is undefined.

434 Forward references: declarations (6.7), expressions (6.5), external definitions (6.9), statements (6.8).