Storage category

Storage class

1. Scope

​ Scope describes the area in the program where identifiers can be accessed . There are mainly ： Block action area 、 Function scope 、 Function prototype scope 、 File scope .

2. link

​ C The variables are 3 Link properties ： External links 、 internal link 、 No links . Where block scope 、 Function scope 、 Function prototype scope variables are all unlinked variables .

3. Storage period

​ Scopes and links describe the visibility of identifiers . A storage period describes the lifetime of an object accessed through these identifiers .C Yes 4 Seed storage period : Static storage period 、 Thread storage period 、 Automatic storage period 、 Dynamically allocate storage periods .

4.5 Storage categories

Storage class	Storage period	Scope	link	Declaration method
Automatically	Automatically	block	nothing	block
register	Automatically	block	nothing	block , Use keywords register
Static external links	static state	file	external	Out of all functions
Static internal links	static state	file	Inside	Out of all functions , Use keywords static
Static no link	static state	block	nothing	block , Use keywords static

// Variables in blocks 
#include <stdio.h>

int main(void)
{
    
    int x = 30;// The original x

    printf("x in outer block :%d at %p\n", x, &x);
    {
    
        int x = 77;
        printf("x in inner block:%d at %p\n", x, &x);
    }
    printf("x in outer block :%d at %p\n", x, &x);
    while (x++ < 33)// original x
    {
    
        int x = 100;// new x, Hide the original x
        x++;
        printf("x in while loop:%d at %p\n", x, &x);
    }
    printf("x in outer block :%d at %p\n", x, &x);

    return 0;
}

Program output :

x in outer block :30 at 000000000061fe08
x in inner block:77 at 000000000061fe0c 
x in outer block :30 at 000000000061fe08
x in while loop:101 at 000000000061fe0c 
x in while loop:101 at 000000000061fe0c 
x in while loop:101 at 000000000061fe0c 
x in outer block :34 at 000000000061fe08

The above program shows the function of variables in different blocks to “{}” As a boundary .

5. Storage category specifier

keyword	explain
typedef	It has nothing to do with any memory , In the vast majority of cases , You cannot use more than one storage category specifier in a declaration , Therefore, you cannot use multiple storage category specifiers as typedef Part of .
auto	Indicates that the variable is an automatic storage period , Can only be used in block scoped variable declarations .
register	Variables for block scope only , Categorize variables into register storage categories , Request the fastest access to the variable , It also protects the address of the variable from being obtained .
static	It indicates that the created object has a static storage period , Creating objects when loading programs , When the program ends, the object disappears . Block scoped static variables have no links ; File scope static variables have internal links .
extern	Indicates that the declared variable is defined elsewhere . If included extern The declaration of has a file scope , Then the referenced variable must have an external link . If included extern The declaration of has a block scope , The referenced variable may have external or internal links , Depends on the defining declaration of variables .