The basis of C language grammar -- learning of local variables and storage categories, global variables and storage categories, and macro definitions

Macro definition

//  Macro definitions are just doubles , You don't need parentheses , Generally, the name is in capital letters 
#define B(c) c*c
main(){
    
int m=5, n=8,k;
k=10*B(m+n); //  yes  10*5+8*5+8=98  instead of 10*(5+8
)*(5+8)
printf("%d",k);
}

local variable

Defining variables may have 3 In this case ：

(1) Define at the beginning of a function ;
(2) Define in a compound statement within a function ;
(3) Define outside the function .

Specific definition ：

 Variables defined within a function are valid only within the scope of the function , In other words, they can only be referenced in this function , These variables cannot be used outside of this function . Variables defined in a compound statement are valid only within the scope of the compound statement , They can only be referenced within this compound statement . These variables cannot be used outside the compound statement , These are called “ local variable ”.

Global variables

Definition ：

 The variables defined outside the function belong to external variables and also become global variables or global variables .
 Global variables can be used by other functions in this document .
 Its valid range is from the beginning of definition to the end of this source file .

Global variables have the same name as local variables

The storage category of the local variable

1.  Automatic variable 
	 use  auto  Keyword declaration .
	 Or not auto, The default is auto store category .
	 The formal parameters and local variables in the function belong to this type . When the function is called , The system automatically allocates storage space to variables , This space is automatically freed at the end of the call .
	int f(int a){
    
		auto int b,c=3;
	}
	
2.  Static local variables 
	 use  static  Keyword declaration .
	 We hope that the local variables in the function will not disappear after the end, but continue to retain the original value , That is, the occupied storage space remains unchanged , On the next call , You can continue to use its value , At this time, you need to use local static variables to declare .

3. Register variables 
	 use  register  Keyword declaration .
	 Generally speaking, static variables 、 Dynamic variables are stored in memory , Use the value of a variable every time , Will be taken out of memory first , Send to calculator , If you need to save , In memory . Too many times ,10000 Secondary cycle ？ This process is too time-consuming , To improve efficiency , OK, these variables are stored in cpu In the register of , Get... Directly from the register when necessary , Because register reading is much faster than memory reading , So these variables are called register variables .
	 Such as ：
	register int a;

Storage category of global variables ：

 Global variables are stored in static storage , Its life cycle is fixed , Exist in the whole program running . But its effect depends on different storage categories . Such as the following situations 
 Declared as an external variable   use  extern  keyword .
 Declared available for inclusion in this document    use  static  keyword 

1.  Expand the scope of external variables inside a file 
2.  Extend the scope of external variables to other files 
3.  Limit the action of external variables to this document