The properties of variables and functions beginning with static specifiers in C language

When to use static As a prefix to variables ？

If used static specifier , Then the first external declaration of the identifier will make the changed identifier have an internal connection , The identifier of the internal link is only in the current .c Source file （transilation Unit, Translation unit ） so , That is, only in the current .c The source file is visible , Other source files are not visible .
Declared global variables , If only in the current .c file （transilation Unit, Translation unit ） Use in , Not elsewhere .c Use... In the document , At this time in .c instead of .h Used in documents static Statement ; If the declared global variables are elsewhere .c Use... In the document , Then declare it as extern Of , And put it on .h In file . except extern The external variables of are declared in the header file , Do not declare any variables in the header file . Otherwise, when expanding the opening file in the source file , There may be multiple variable declarations . Don't define variables in the header file .

Reference resources Stack Overflow：https://stackoverflow.com/questions/1856599/when-to-use-static-keyword-before-global-variables

When to use static As a prefix to a function ？

Declared global functions , If only in the current .c Use... In the document , Don't want external access , The statement is static type .
C The language function definition defaults to global , And the statement is static The function of is only in the current .c It can be seen in the document , other .c The file is not visible ; It can be in other .c Redefine the function with the same name in the file .

example：
static_fun.h

#pragma once
#include <stdio.h>

static void static_fun();

void fun();

void printfun();

static_fun.c

#include "static_fun.h"

static int static_variable = 1;

static void static_fun() {
    
   printf("Static fun call\n");
}

void fun() {
    
   printf("Global fun call\n");
   static_fun();
}

void printfun() {
    
   printf("static variable is: %d\n", static_variable);
}

main.c

#include "static_fun.h"

int main()
{
    
   // static function/variable call test

   static_fun(); // error: static function decleared but not defined

   fun(); // OK: extern function

   printf("Static variable: %d\n", static_variable); // error: static_variable not defined

   printfun(); // OK: access static variable by extern function

   return 0;
}

static_fun() Only in static_fun.c You can see , therefore main.c in static_fun(); The call is treated as a declaration , But the function definition was not found ;
printf static_variable The static variable of is only in static_fun.c You can see , So the variable is undefined ;
You can access static variables or functions indirectly through external functions .

static This feature of keywords is C Languages are often used to design and implement ADT (Abstract Data Type), Such as the list , Abstract data structures such as trees .static The declared functions and variables are only in ADT It's visible inside , External inaccessible .

void func() {
    
   static int val = 1;
   ++val;
   printf("static value: %d\n", val);
}

int main()
{
    
   func();

   func();

   return 0;
}

static value: 2
static value: 3

Two calls contain static Function of , The second definition did not take effect , Nor did static variable reinitialization occur , The program executes to the static variable , Its value is still the result of the last time .