[Basic knowledge of C language]

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Preface

提示：此篇博客为C语言基础常识：

Hello everyone, I am Xiao Ming. The article updated today and the entire blog content are the common sense knowledge of C language summarized by Xiao Ming. You can understand more and lay a good foundation for learning C language.

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提示：以下是本篇文章正文内容，下面案例可供参考

1. What is a programming language?

Before learning a programming language, you must first understandProgramming languageThis concept:

• When we were very young, our parents taught us to speak and communicate with others. Although there are many languages, such as Chinese, English, French, Korean, etc., and the vocabulary and formats used in communication are different, they can achieve the same purpose. . The language used for communication between people is called " natural language ".

• Similarly, we can also communicate with computers through "language" and let computers do things for us. The language for communication between humans and computers is calledProgramming language. Programming languages ​​also have fixed formats and vocabulary. There are currently thousands of known programming languages. Commonly used languages ​​include C language, C++, Java, C#, Python, JavaScript, Go language, etc.

The development of programming languages ​​has probably gone through the following stages:

Among them, not long after the C language appeared, the computer industry began to explode. Computer hardware became smaller and cheaper, gradually entering government agencies and ordinary households. C language became the main force in programming, operating systems, commonly used software, Hardware drivers, core algorithms, databases, mini-games, etc. are all developed using C language.

2. Introduction to C language

1. Introduction to C language

• The C language is a general-purpose, high-level language originally designed by Dennis Rich at Bell Laboratories for the development of the UNIX operating system. The C language was first implemented in 1972 on the DEC PDP-11 computer.
• C/C++, Java and Python are the most commonly used programming languages ​​in recent years, accounting for almost half of the programming languages!

2.C program structure

Conversion of C language program:
1. After writing the code, save it as a file with the suffix .c
2. Compile the .c program into the target program .obj through the compiler
3. Finally, the linker combines multiple modules and libraries Together generate the executable file .exe

3. The first C program example

#include <stdio.h>
int main()
{
    
    
	//第一个C程序实例
   printf("Hello, World! \n");
   return 0;
}

1. The first line of the program #include <stdio.h> is a preprocessor directive that tells the C compiler to include stdio.hthe file before actually compiling it.

2. The next line int main()is the main function, and the program execution starts from here.

3. The next line is used in pairs with the seventh line. Curly braces can be used to gather the statements in the function into a code block. The code between the two curly braces is the function to be implemented by the function ｛ . ｝

4. The next line // is a comment, which will be ignored by the compiler, and the comment content of the program is placed here.

5. The next line printf(...)is a function that comes with the C language, and its function is to print a message on the screen "Hello, World!".

6. The next line return 0;terminates the main() function and returns a value of 0.

3. Basic syntax of C language

1.Header file

#include<stdio.h>

This is the first line of the program. Its function is to import the entire contents of the stdio.h file at the location of this line in the code, introduce the header file using the command, and #includeput the file name < >in, #include and < > there can be a space between Or not. The files in the following angle brackets <>can also be other header files, which will be covered later.

2. Main function

int main()

This line of code declares a main function. main函数It is a function that comes with C language. The function that comes with C language is calledLibrary Functions. In addition to library functions, we can also write our own functions to expand the functionality of the program.

Characteristics of custom functions and main functions:
1. A C program consists of one or more functions
2. A C program has one and only one main function main
3. Program execution mainstarts from the main function and endsmain in the main function

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The function is a C language programbasic unit
An obvious feature of functions is that they must be used with parentheses（ ）

3. Parentheses, braces and semicolons

int main()
{
    
    
	//一个简单的例子
	/*一个简单的例子*/
}

1. main is the name of the function, and the brackets （ ）indicate that this is a function definition.
2. The code between the curly braces { }is the function to be implemented, and only curly braces { } can play this role. Neither parentheses ( ) nor square brackets [ ] will work. Curly braces can also be used to enclose functions in statements are grouped into a code block.

In C programs, the semicolon ;is the statement terminator. That is, each statement must end with a semicolon. It indicates the end of a logical entity.

Parentheses, curly braces, and semicolons all 必需exist and cannot be discarded.

4. Comments

//单行注释

/*
多行注释
多行注释
多行注释
*/

The comment formats of C language are single-line comments// and multi-line comments/* */ .

• //Single line comments can be made using
• /* */Multi-line comments can be used, /* and */ must be used in pairs, and multi-line comments are 不可以nested

effect: 单行注释later和多行注释middle的所有内容都会被编译器忽略掉，不产生编译代码，所以可以在一些容易忘记和混淆的代码后面加以备注。

5.Identifier

In layman's terms, an identifier is a name. The name used to identify variables, functions, macro names, and structure names.

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Naming rules for identifiers:
1. Can only consist of letters [AZ or az ], numbers [0~9], underscores [ _ ], and the first character 不能isnumber
2. Identifiers in C language are case -sensitive
3. Cannot be usedKeywords

demonstration:

correct	Incorrect
Sum	@day
sum	3sum
_above	a>b
student_name	M.D.John

Among them:
1.correctThis column: the first row Sum and the second row sum are different identifiers, because identifiers are case-sensitive, and _above and student_name are identifiers that follow naming rules.
2.mistakeThis column: the first row cannot use the "@" symbol, the second row cannot start with a number, the third row cannot use the ">" symbol, and the fourth row cannot use the "." symbol.

6.Keywords

In C language, in order to define variables , express statement functions and preprocess some files , some characters with special meaning must also be used . This isKeywords, keywordcannotAs a constant name, variable name, or other identifier name.

• Control statement 12keywords

data type keyword	illustrate
short	Declare short integer variable
int	Declare integer variable
long	Declare long integer variable
float	Declare floating point variables
double	Declare a double-precision floating-point variable
char	Declare character variables
struct	Declare structure type
union	Declare a union type
enum	Declare enumeration type
signed	Declare signed type variables
unsigned	Declare unsigned type variables
void	Declare a function with no return value or parameters, declare an untyped pointer

• Data type 12keywords

control statement keyword	illustrate
if	Conditional statements
else	Negative branch of conditional statement (used with if)
switch	for multi-branch statements
case	branch of multi-branch statement
default	Multi-branch statement default
for	a loop statement
do	The loop body of the loop statement
while	Loop condition of loop statement
goto	unconditional jump statement
continue	End the current cycle and start the next cycle
break	Break out of current loop
return	Subprogram return statement (can take parameters or not)

• Storage type keywords 4and other 4keywords

storage types and other types	illustrate
auto	Declare automatic variables
extern	expand its scope
register	Declare register variables
static	Declare static variables (variables with immortal properties)
volatile	Specifies that variables can be changed implicitly during program execution
sizeof	Calculate the data type or variable length (i.e. the number of bytes occupied)
typedef	Used to alias data types
const	Declare read-only variables

Summarize

Okay, after reading this, you have read the entire content of this blog. The content is not much and it is very basic. After understanding the content of this chapter, everyone will be basically familiar with some of the knowledge required to learn C language.
[Final and final]: This blog is about the basic knowledge of C language. It is not only Xiao Ming’s own notes, but also the knowledge points that Xiao Ming thinks are shared with everyone. If there are any errors or deficiencies in the content, you are welcome to point them out. Your feedback is Xiao Ming’s. motivation! ! !