An identifier declared in different scopes or multiple times within the same scope can be made to refer to the same object or function by linkage. An identifier can be classified as externally linked, internally linked, or not-linked. These three kinds of linkage have the following characteristics:
- External linkage. An identifier
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- whose declaration contains the storage-class specifier
extern, where no prior declaration of that identifier is visible. - for a function whose declaration contains no storage-class specifier.
- for an object with file scope whose declaration contains no storage-class specifier.
An identifier that is classified as internally linked includes identifiers whose declaration contains the storage-class specifier static.
An identifier that is classified as not-linked include:
- An identifier declared to be anything other than an object or a function.
- An identifier declared to be a function parameter.
- A block scope identifier for an object declared without the storage-class specifier
extern.
If a prior declaration is visible and has no linkage, the latter declaration is externally linked. If a prior declaration is visible and has either internal or external linkage, the latter declaration is classified with the same linkage as the prior declaration.
Use of an identifier (within one translational unit) classified as both internally and externally linked causes undefined behavior. A translational unit includes the source file together with its headers, and all source files included via the preprocessing directive #include.
According to C99:
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- with external linkage represents the same object or function throughout the entire program, that is, in all compilation units and libraries belonging to the program. The identifier is available to the linker. When a second declaration of the same identifier with external linkage occurs, the linker associates the identifier with the same object or function.
- Internal linkage. An identifier with internal linkage represents the same object or function within a given translation unit. The linker has no information about identifiers with internal linkage. Consequently, these identifiers are internal to the translation unit.
- No linkage. If an identifier has no linkage, then any further declaration using the identifier declares something new, such as a new variable or a new type.
According to C99:
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. Within one translation unit, each declaration of an identifier with internal linkage denotes the same object or function. Each declaration of an identifier with no linkage denotes a unique entity.
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.For an identifier declared with the storage-class specifier extern in a scope in which a prior declaration of that identifier is visible, 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. If no prior declaration is visible, or if the prior declaration specifies no linkage, then the identifier has external linkage.
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. If the declaration of an identifier for an object has file scope and no storage-class specifier, its linkage is external.The following identifiers have no linkage: an identifier declared to be anything other than an object or a function; an identifier declared to be a function parameter; a block scope identifier for an object declared without the storage-class specifier extern.If, within a translation unit, the same identifier appears with both internal and external linkage, the behavior is undefined
Use of an identifier (within one translational unit) classified as both internally and externally linked causes undefined behavior. A translational unit includes the source file together with its headers, and all source files included via the preprocessing directive #include.
Non-Compliant Code Example
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