### Lexical Elements - Identifiers

Identifiers consist of an arbitrary number of letters or digits. They provide names for the following language elements:

- Functions
- Data objects
- Labels
- Tags
- Parameters
- Macros
- Typedefs
- Structure and union members.

There is no limit for the number of characters in an identifier.

However, the linkage editor limits the number of significant characters in external identifiers and truncates them after 4095 characters.

The linker on the Intel platform limits the number of significant characters in external identifiers and truncates them after 255 characters.

The compiler distinguishes between uppercase and lowercase letters in identifiers. For example, `PROFIT` and `profit` represent different data objects.

The $ character can only be accepted on AIX unless, at the extended and compatible language levels, the identifiers are C++ identifiers.

While the underscore character ( _ ) is not really considered a letter, it is accepted to be part of an identifier. In identifiers can begin with _ or $, but you should avoid using these characters at the beginning of identifiers ansi mode, identifiers can begin with _ but not with $. In extended mode, because they are reserved for internal system names.

The -qdollar compiler option lets you use the $ character in identifiers.

Identifiers used by library functions that begin with two underscores or an underscore followed by a capital letter, are reserved in all contexts.

Although the names of system calls and library functions are not reserved words if you do not include the appropriate headers, avoid using them as identifiers. Duplication of a predefined name can lead to confusion for the maintainers of your code and can cause errors at link time or run time. If you include a library in a program, be aware of the function names in that library to avoid name duplications.

You should always include the appropriate headers when using standard library functions.

Lexical Elements - Tokens
Lexical Elements of C - Comments
Lexical Elements - Constants