6.4.2.1
General

    1  Syntax
        identifier:
            identifier-nondigit
            identifier identifier-nondigit
            identifier digit
        identifier-nondigit:
            nondigit
            universal-character-name
            other implementation-defined characters
       nondigit: one of
            _ a b c d e f g h i j k l m
            n o p q r s t u v w x y z
            A B C D E F G H I J K L M
            N O P Q R S T U V W X Y Z
        digit: one of
            0 1 2 3 4 5 6 7 8 9

Semantics

    2    An identifier is a sequence of nondigit characters (including
         the underscore _, the lowercase and uppercase Latin letters,
         and other characters) and digits, which designates one or more
         entities as described in 6.2.1. Lowercase and uppercase
         letters are distinct. There is no specific limit on the
         maximum length of an identifier.

    3    Each universal character name in an identifier shall designate
         a character whose encoding in ISO/IEC 10646 falls into one
         of the ranges specified in annex D.52) The initial character
         shall not be a universal character name designating a digit.
         An implementation may allow multibyte characters that are not
         part of the required source character set to appear in
         identifiers; which characters and their correspondence to
         universal character names is implementation defined.
    4    When preprocessing tokens are c<onverted to tokens during
         translation phase 7, if a preprocessing token could be
         converted to either a keyword or an identifier, it is
         converted to a keyword.



7.1.3 Reserved identifiers

    1    Each header declares or defines all identifiers listed in
         its associated subclause, and optionally declares or defines
         identifiers listed in its associated future library directions
         subclause and identifiers which are always reserved either
         for any use or for use as file scope identifiers.
            - All identifiers that begin with an underscore and
                 either an uppercase letter or another underscore
                 are always reserved for any use.
            - All identifiers that begin with an underscore are
                 always reserved for use as identifiers with file
                 scope in both the ordinary and tag name spaces.
            - Each macro name in any of the following subclauses
                 (including the future library directions) is
                 reserved for use as specified if any of its
                 associated headers is included; unless explicitly
                 stated otherwise (see 7.1.4).
            - All identifiers with external linkage in any of the
                 following subclauses (including the future library
                 directions) are always reserved for use as
                 identifiers with external linkage.143)
            - Each identifier with file scope listed in any of the
                 following subclauses (including the future library
                 directions) is reserved for use as macro and as an
                 identifier with file scope in the same name space
                 if any of its associated headers is included.

    2    No other identifiers are reserved.  If the program declares or
         defines an identifier in a context in which it is reserved (other
         than as allowed by 7.1.4), or defines a reserved identifier as a
         macro name, the behavior is undefined.

    3    If the program removes (with #undef) any macro definition of an
         identifier in the first group listed above, the behavior is
         undefined.