1.2 Identifiers

An identifier is a sequence of characters that represents a name for the following:

• Variable

• Function

• Label

• Type definition

• Structure, enumeration, or union tag

• Structure, enumeration, or union member

• Enumeration constant

• Macro

• Macro parameter

The following rules apply to identifiers:

• Identifiers consist of a sequence of one or more uppercase or lowercase alphabetic characters, the digits 0 to 9, the dollar sign ($), and the underscore character (_).

  Using the $ character provokes a warning from the compiler in strict ANSI mode.

• Character case is significant in identifiers; for example, the identifier Test1 is different from the identifier test1 .

• Identifiers cannot begin with a digit.

• Do not begin identifiers with an underscore; the ANSI C standard reserves these identifiers for internal names.

• Keywords are not identifiers (Section 1.4 lists the C keywords).

• Using the names of library functions for identifiers is bad practice (Chapter 9 lists the C library function names). A function with the same name as a library function will supersede the library function. This may be the desired outcome, but program maintenance can be confusing.

• In general, identifiers are separated by white space, punctuators, or operators. For example, the following code fragment has four identifiers:

  struct employee { int number; char sex; } emp;
  

  The identifiers are: employee , number , sex , and emp . (struct , int , and char are keywords).

An identifier without external linkage has at most 32,767 significant characters. An identifier with external linkage has 1023 significant characters on Digital UNIX systems and 31 significant characters for OpenVMS platforms. (Section 2.8 describes linkage in more detail.) Case is not significant in external identifiers on OpenVMS systems.

Identifiers that differ within their significant characters are different identifiers. If two or more identifiers differ in nonsignificant characters only, they are treated as the same identifier.