Identifier

An identifier is an arbitrarily long sequence of digits, underscores, lowercase and uppercase Latin letters, and Unicode characters specified using \u and \U escape notation (since C99). A valid identifier must begin with a non-digit character (Latin letter, underscore, or Unicode non-digit character (since C99)). Identifiers are case-sensitive (lowercase and uppercase letters are distinct).

char *\U0001f431 = "cat"; // supported
char *🐱 = "cat"; // implementation-defined (e.g. works with Clang, but not GCC)

Identifiers can denote the following types of entities:

• objects
• functions
• tags (struct, union, or enumerations)
• structure or union members
• enumeration constants
• typedef names
• label names
• macro names
• macro parameter names

Every identifier other than macro name or macro parameter name has scope, belongs to a name space, and may have linkage. The same identifier can denote different entities at different points in the program, or may denote different entities at the same point if the entities are in different name spaces.

Reserved identifiers

The following identifiers are reserved and may not be declared in a program (doing so invokes undefined behavior):

All other identifiers are available, with no fear of unexpected collisions when moving programs from one compiler and library to another.

Note: in C++, identifiers with a double underscore anywhere are reserved everywhere; in C, only the ones that begin with a double underscore are reserved.

Translation limits

Even though there is no specific limit on the length of identifiers, early compilers had limits on the number of significant initial characters in identifiers and the linkers imposed stricter limits on the names with external linkage. C requires that at least the following limits are supported by any standard-compliant implementation:

References