3、 C language storage class

1、auto Storage class

• effect ：auto Storage class is the default storage class for all local variables
• Be careful ： Only local variables can be modified

  {
        
    int a;
    auto int a;// The two are equivalent 
  }
  

2、register Storage class

• effect ： Used to define variables stored in registers . Both dynamic and static variables are stored in memory , When the program needs to be used , Then send the value to the calculator , But if you use a number frequently , In order to improve efficiency , You can put variables in CPU In the register of , It's called register variable
• Be careful ： It cannot be used &, Because it has no memory location
• characteristic ： Because it is stored in registers , Its access speed is particularly fast

  register int a;
  

3、static Storage class

• effect ：
  • local variable ： Throughout the life cycle of the program , Keep the existence of local variables , Instead of creating and destroying when entering and leaving the scope
  • Global variables and functions ： Can be called throughout the file
• Code implementation ：

  #include<stdio.h>
  
  void func1(void);
  
  static int count = 10;// Global variables ,static By default 
  // int count = 10;// Equivalent to the above 
  
  int main(){
        
      while (count--)
      {
        
          func1();
      }
      return 0;
  }
  
  void func1(void){
        
      static int a = 4;// local variable , Exists throughout the program cycle , Will not be destroyed 
      int b = 4;// local variable , Every time you enter the scope , Will be created , Destroyed when leaving 
      b++;
      a++;
      printf("count=%d, a=%d, b=%d\n",count,a,b);
  }
  

• Running results ：

  count=9, a=5, b=5
  count=8, a=6, b=5
  count=7, a=7, b=5
  count=6, a=8, b=5
  count=5, a=9, b=5
  count=4, a=10, b=5
  count=3, a=11, b=5
  count=2, a=12, b=5
  count=1, a=13, b=5
  count=0, a=14, b=5
  

4、extern Storage class

• effect ： References to global variables
• Code implementation ：[ Reference link ]https://blog.csdn.net/qq_43280851/article/details/125424816

5、 Variable type and its scope

• Global variables ： Global variables have global scope , Other files want to use this variable , Just use extern Just refer to this variable
• Static global variables ： When the global variable is static After modification , It becomes a static global variable , Unlike global variables , Static local variables have file scope , That is, the static global variables of a file , It can only work on this file , This also makes it have the same name , But variables that are not in the same file can exist at the same time
• local variable ： Local variables have only local scope , yes auto Storage class , When entering the scope, create , Destroy when exiting the scope
• Static local variables ： Is initialized only once , Throughout the entire function cycle , But unlike global variables , It is only visible to functions that define themselves

5、 summary

• auto Is the default storage class for local variables ,static Is the default storage class for global variables
• When a variable needs to be accessed by all files , You can use global variables
• When a variable needs to be accessed by a single file field , You can use static global variables
• If global variables are accessed only by a single function , You can change this variable into a static local variable
• In the use of static variable , It has no reentry function , Initialize only once