What types of identifiers are there in C language?

There are three types of C language identifiers: 1. Keywords, which are strings with specific meanings specified by the C language, often also called reserved words; 2. Predefined identifiers, which are predefined by the system Identifiers, such as function names, macro definitions and type aliases in function libraries; 3. User-defined identifiers are identifiers defined by users according to their needs and are generally used to name variables, functions, arrays, etc. If the user identifier is the same as a keyword, an error will occur during compilation; if it is the same as a predefined identifier, no error will occur during compilation, but the original meaning of the predefined identifier is lost.

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

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 the relationship between name and use. Identifiers usually consist of letters, numbers, and other characters.

Types of c language identifiers

C language identifiers include keywords, user identifiers, and predefined identifiers. The C language stipulates that identifiers can only be composed of letters, numbers, and underscores, and the first character must be a letter or an underscore, not a number.

1. Keywords

Keywords are strings with specific meanings specified by the C language. They are also usually called reserved words, such as int, char, long, float, unsigned, etc. The identifier we define cannot be the same as the keyword, otherwise an error will occur.

You can also understand keywords as identifiers with special meanings. They have been used by the system and we can no longer use them.

There are 32 keywords specified in the standard C language:

Keywords	Description
auto	Declare automatic variables
short	Declare short integer variables or functions
int	Declare an integer variable or function
long	Declare a long integer variable or function
float	Declare a floating-point variable or function
double	Declare a double-precision variable or function
char	Declare character variables or functions
struct	Declare structure variables or functions
union	Declaration of shared data types
enum	Declaration of enumeration types
typedef	Used to alias the data type
const	Declare read-only variables
unsigned	Declare unsigned type variables or functions
signed	Declare signed type variables or functions
extern	Declared variables are declared in other files
register	Declared register variables
static	Declare static variables
volatile	Indicates that variables can be changed implicitly during program execution
void	Declare a function with no return value or parameters, declare an untyped pointer
if	Conditional statement
else	Conditional statement negation branch (used with if)
switch	For switch statement
case	Switch statement branch
for	A loop statement
do	The loop body of the loop statement
while	The loop condition of the loop statement
goto	Unconditional jump statement
continue	End the current loop and start the next cycle
break	Jump out of the current loop
default	The "other" branch in the switch statement
sizeof	Calculate data type length
return	Subprogram return statement (can be with or without parameters) loop condition

2. Predefined identifiers

Identifiers predefined by the system in C language, such as function names, macro definitions and type aliases (typedef) in function libraries.

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. wait.

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.

Note: The function name in the C language function library can become a user-defined variable name! ! !

3. User-defined identifier

User-defined identifier according to needs. Generally used to name variables, functions, arrays, etc. If the user identifier is the same as a keyword, an error will occur during compilation; if it is the same as a predefined identifier, no error will occur during compilation, but the original meaning of the predefined identifier will be lost or the result will be wrong, so predefined identifiers Generally not used as a user identifier.

The C language stipulates that user-defined identifiers can be a string composed of letters, numbers, and underscores (A～Z, a～z, 0~9, _), and the first character must are letters or underscores.

(1) Identifier can only start with English uppercase and lowercase letters, and underscore (_) , and cannot start with any other characters or numbers .

For example, the following is an incorrect example:

int %a;//错误，标识符不能以符号开头
int 8C[8];//错误，不能以数字开头

(2) The identifier can only contain English uppercase and lowercase letters, underscores and Arabic numerals , but cannot start with an Arabic numeral .

int a8;//正确
int _adf589S_5;//正确

(3)IdentifierCase-sensitive

In C language, uppercase letters and lowercase letters are considered different characters, Therefore, the identifiers expressed by ABC and abc are different (Acd and acd, caDd and cadd, _8Fc9 and _8fc9 are all different identifiers).

(4)Identifier cannot be a keyword.

int float;//错误，float是关键字。
int Float;//正确

(5)ANSI standard stipulates that the identifier can be of any length , but the external name must be uniquely distinguishable by at least the first 6 characters, and no case sensitive. This is because some compilers (such as MS C for the IBM PC) only recognize the first 6 characters.

In addition to these hard rules, it is best to name identifiers with specific meanings to facilitate observation, reading, and maintenance.

//定义某长方体的长度、宽度和高度
int a;//高度
int b;//宽度
int c;//高度

can be changed to the following definition:

int c_Long;
int c_Width;
int c_Height;

Related recommendations: "C Video Tutorial"

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