What is user identifier in C language?

User identifier is: an identifier defined by the user according to his or her needs (a symbol used to identify a certain entity, which has different meanings in different application environments); generally used to assign variables, functions, Arrays, etc. are named to establish the relationship between name and use.

Identifier refers to a symbol used to identify an entity and has different meanings in different application environments. In computer programming languages, identifiers are names used by users when programming. They are used to name variables, constants, functions, statement blocks, etc., to establish a relationship between name and use. Identifiers usually consist of letters, numbers, and other characters.

User identifiers are: identifiers defined by users according to their own needs. They are generally used to name variables, functions, arrays, etc., to establish a relationship between names and uses.

If the user identifier is the same as the keyword, an error will occur during compilation; if it is the same as the predefined identifier, no error will occur during compilation, but the original meaning of the predefined identifier will be lost, or an error will occur during compilation. The result is incorrect, so predefined identifiers are generally not used as user identifiers.

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Predefined identifiers are identifiers predefined by the system in C language, such as system class library names, system constant names, and system function names. Predefined identifiers have the characteristics of clear meaning, such as the function "format output" (full English name plus abbreviation: printf), "format input" (full English name plus abbreviation: scanf), sin, isalnum, etc.

Predefined identifiers can be used as user identifiers, but this will lose the original meaning specified by the system, and improper use will cause program errors.

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