What are the symbols that are already in the C language with special meanings and cannot be used for

The so-called "words and symbols with special meanings that are already in the C language" refer to the "reserved words" of the C language, such as for, while, if, etc., and cannot be used for other purposes of the user.

The C language stipulates that identifiers can only consist of three characters: letters, numbers, and underscores, and that the first character must be a letter or underscore.

Listed below are valid identifiers that can be used as variable names:

A acds su 1 2 class The identifiers listed below are invalid and cannot be used as variable names:

12a %acds #su 1_2 a>b

To put it simply!

Can only consist of character numbers underscored! Must start with a character and an underscore (no case sensitive).

You can't define a good keyword for the system.

(1) The first character must be a letter (case-insensitive) or an underscore ( ).

2) followed by letters (caseless), underscores ( ) or numbers;

3) There is a difference between uppercase and lowercase letters in the identifier. For example, the variables sum, sum, and sum represent three different variables;

4) It cannot have the same name as a reserved identifier (i.e., keyword) that has been predefined by the C compilation system and has a special purpose. For example, identifiers cannot be named float, auto, break, case, this, try, for, while, int, char, short, unsigned, etc.

As an identifier, the following rules must be met:

1.All identifiers must begin with a letter (a z, a z) or an underscore ( );

2.The rest of the identifier may be composed of letters, underscores, or numbers (0 9);

3.Uppercase and lowercase letters represent different meanings, i.e., different identifiers;

4.The identifier is valid only for the first 32 characters;

5.Keywords cannot be used for identifiers.

Legal identifier: Consists of letters, digits, and underscores and must begin with a letter or underscore.

Invalid identifiers: Keywords cannot be used, identifiers cannot be the same as library functions, numbers cannot be prefaced, and special characters cannot be used.

Hope it helps!!

Identifiers in C are made up of letters, numbers, and underscores.

The identifier cannot begin with a letter, and the identifier cannot be the same as a keyword in C (e.g., int, double).

Here's the correct identifier:

a，_a，a1，_a1

The following is the wrong identifier:

1a，float，a#

1. The identifier consists of a-z, a-z, 0-9 and (underscore).

2. Numbers cannot be used as identifiers, and in principle, they do not need to be used as the beginning.

3. The identifier cannot be the same as the C language keyword.