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A.11.1 Lexical Scope
Identifiers fall into several name spaces that do not interfere with one another; the same identifier may be used for different purposes, even in the same scope, if the uses are in different name spaces. These classes are: objects, functions, typedef names, and enum constants; labels; tags of structures or unions, and enumerations; and members of each structure or union individually.
- These rules differ in several ways from those described in the first edition of this manual. Labels did not previously have their own name space; tags of structures and unions each had a separate space, and in some implementations enumerations tags did as well; putting different kinds of tags into the same space is a new restriction. The most important departure from the first edition is that each structure or union creates a separate name space for its members, so that the same name may appear in several different structures. This rule has been common practice for several years.
The lexical scope of an object or function identifier in an external declaration begins at the end of its declarator and persists to the end of the translation unit in which it appears. The scope of a parameter of a function definition begins at the start of the block defining the function, and persists through the function; the scope of a parameter in a function declaration ends at the end of the declarator. The scope of an identifier declared at the head of a block begins at the end of its declarator, and persists to the end of the block. The scope of a label is the whole of the function in which it appears. The scope of a structure, union, or enumeration tag, or an enumeration constant, begins at its appearance in a type specifier, and persists to the end of a translation unit (for declarations at the external level) or to the end of the block (for declarations within a function).
If an identifier is explicitly declared at the head of a block, including the block constituting a function, any declaration of the identifier outside the block is suspended until the end of the block.