What are the requirements for legal identifiers in C language?

The requirements for legal identifiers in c language are: 1. Identifiers can only consist of letters (A~Z, a~z), numbers (0~9) and underscores (_); 2. First The characters must be letters or underscores, not numbers; 3. The uppercase and lowercase letters in the identifier are different and represent different meanings; 4. The identifier cannot be a keyword.

The operating environment of this tutorial: windows7 system, c99 version, Dell G3 computer.

Identifier

When defining variables, we use names such as a, abc, mn123. They are all given by the programmers themselves. Generally Being able to express the role of a variable is called an identifier.

Identifiers are names given by programmers themselves. In addition to variable names, function names, macro names, structure names, etc. will also be discussed later. They are all identifiers. However, the name cannot be chosen casually and must abide by the regulations; C language stipulates that identifiers can only consist of letters (A~Z, a~z), numbers (0~9) and underscores (_), and the first A character must be a letter or underscore, not a number.

The rules for judging legal identifiers in c language are as follows:

• can only consist of letters (A~Z, a~z) and numbers (0~9) and underscore (_) to form
  

• The first character must be a letter (not case-sensitive) or an underscore (_); followed by a letter (not case-sensitive) or an underscore (_) or numbers;

• There is a difference between uppercase and lowercase letters in the identifier. For example, the variables Sum, sUm, and suM represent three different variables;

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

The following are legal Identifier:

a, x,  x3, BOOK_1, sum5

The following is an illegal identifier:

• 3s cannot start with a number

• s*T appears Illegal characters *

• -3x Cannot start with a minus sign (-)

• bowy-1 Illegal character minus sign (-)## appears

#You must also pay attention to the following points when using identifiers:

Although the C language does not limit the length of identifiers, it is affected by different compilers. restrictions, but also limited by the operating system. For example, a certain compiler stipulates that the first 128 bits of an identifier are valid. When the first 128 bits of two identifiers are the same, they are considered to be the same identifier.

In identifiers, there is a difference between upper and lower case. For example, BOOK and book are two different identifiers.

Although identifiers can be defined at will by programmers, identifiers are symbols used to identify a certain quantity. Therefore, the naming should have corresponding meanings as much as possible to facilitate reading and understanding, and be "just as the name implies".

Note:

The length of the identifier is determined by the compilation system on the machine. The general limit is 8 characters (Note: The 8-character length limit is the C89 standard. The C99 standard has been extended in length, in fact most industry standards are longer). Identifiers are case-sensitive, that is, they are strictly case-sensitive. Generally, lowercase is used for variable names, and uppercase is used for naming symbolic constants

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