Full analysis of storage types in C language learning

• The format of defining variables in C language:

	存储类型 数据类型 变量名;

• C language storage type:
• The common ones are const、static、extern、auto、register、volatile;
• For the const keyword , see my original blog for details, so I won’t repeat it here.链接：

https://blog.csdn.net/qq_41878292/article/details/132284761?spm=1001.2014.3001.5502

• Storage in the static storage area:
• 1. data 段: 初始化stored 全局变量和静态变量;
• 2. bss 段: 未初始化stored 全局变量和静态变量;
• Among them, the bss segment will be blocked by the system before the program is executed 自动清0;
• static keyword:
• static is often used to modify variables;
• The role of static:
• Extend the lifetime of a local variable:
• static修饰Only , so that the original end 静态局部变量of the nearest curly braces, after the static modification, ;执行初始化1次局部变量的生命周期延长至整个程序结束
• Limit scope:
• 1. Static modified 全局变量, can only 本源文件be accessed in, even if it extern外部声明is 不可以;
• 2. Static modified 函数, can only 本源文件be called in;
• Variables modified by static:
• It is stored in 全局数据区, 静态变量区including 全局静态变量and 局部静态变量, are 全局数据区allocating memory, when it is initialized, 自动初始化为 0;
• pay attention:
• 1. If the static modification 局部变量is present 主函数执行之前, it has already been allocated;
• 2. In the function function, the local variable modified by static is 只是一个声明also called when it is called multiple times 不会重新赋值;
• 3. The local variables modified by static are still local variables, 作用域还是离的最近的花括号;
• Test code:

#include <stdio.h>

void func1(){
    
    

    int x = 1;
    static int y1 = 5;
    x++;
    y1++;

    printf("x = %d\n",x);
    printf("y1 = %d\n",y1);
   

}

void func2(){
    
    

    static int y = 1;
    int x1 = 5;
   
    y++;
    x1++;


    printf("y = %d\n",y);
    printf("x1 = %d\n",x1);
   

}

int x1 = 999;
static int y1 = 666;

int main(int argc, char const *argv[])
{
    
    
    int x2;
    static int y2;

    printf("x1 = %d\n",x1);
    printf("y1 = %d\n",y1);

    puts("--------------------------------");

    printf("x2 = %d\n",x2);
    printf("y2 = %d\n",y2);

    puts("--------------------------------");

    func1();
    puts("********************************");
    func1();
    puts("********************************");
    func1();

    puts("--------------------------------");

    func2();
    puts("********************************");
    func2();
    puts("********************************");
    func2();

    return 0;
}

• operation result:

	x1 = 999
	y1 = 666
	--------------------------------
	x2 = 0
	y2 = 0
	--------------------------------
	x = 2
	y1 = 6
	********************************
	x = 2
	y1 = 7
	********************************
	x = 2
	y1 = 8
	--------------------------------
	y = 2
	x1 = 6
	********************************
	y = 3
	x1 = 6
	********************************
	y = 4
	x1 = 6

• extern keyword:
• Extern-modified variables or extern-modified functions are defined in other source files, that is, in a source file, if you want to use variables or functions defined in other source files, you need to use the extern keyword declaration;
• Test code:

//代码1
#include <stdio.h>


int a = 4;
int b = 5;

int my_mul(int x,int y){
    
    


    return x * y;   
}

//代码2
#include <stdio.h>

extern int a;
extern int b;

extern int my_mul(int,int);


int main(int argc, char const *argv[])
{
    
    
    printf("my_mul = %d\n",my_mul(a,b));
    
    return 0;
}

• operation result:

my_mul = 20

• auto keyword:
• In addition to 全局变量and static的变量, when defining variables, omit variables of storage type, which are all by default auto修饰;
• register keyword:
• register修饰is 寄存器类型a variable that is executed by the processor 效率是最高的;
• When the CPU fetches data, the priority is: register > cache ( cache) > memory space;
• Since the registers of the central processing unit are set 个数是有限, it is necessary to define all variables as register variables 不现实;
• pay attention:
• The variable modified by the register keyword is yes 不能取地址;
• Summarize:
• In actual development, it is generally not necessary, just understand;
• Test code:

#include<stdio.h>
	
	
int main(int argc, const char *argv[]){
    
    
		
		register int k = 999;
			
		printf("k = %d\n",k); 
		    
		return 0;
	    
	    
}

• operation result:

	k = 999

• volatile keyword:
• The main function is to prevent compiler optimization and require the CPU to fetch data in memory every time;
• Common usage scenarios:
• 1. Multiple threads access the same variable;
• 2. The register is in an interrupted state;