Explanation of static and extern keywords

Background introduction

Global variables

• External variables ： The defined variables can be accessed by this file and other files ( By default , All global variables are external variables , External variables with the same name in different files , All represent the same variable )
• Internal variables ： The defined variables can only be accessed by this file , Can't be accessed by other files ( Internal variables with the same name in different files , They don't influence each other )

Keyword Introduction

• static
  • The effect on variables ： Define an internal variable , Make internal variables with the same name in different files , They don't influence each other ; It is usually used to declare a variable that can only be accessed by this file ;
  • Effect on function ： Define and declare an internal function It is usually used to declare a function that can only be accessed and called by this file ;
• extern
  • The effect on variables ： Declare an external variable , It is usually used to declare a variable that can be accessed by other files ;
  • Effect on function ： Define and declare an external function （ It can be omitted , It is usually used to declare a function that can be called by other files

static Used to modify local variables ：

• Extend the life cycle of local variables ： At the end of the program , Local variables will be destroyed
• Does not change the scope of the local variable

Usage method

1. Usage in functions

===============  aw_data.h  ===============
#include <stdio.h>

// -  Statement alloc_aw_data This function ,  Express  alloc_aw_data  Is an external function ;
extern aw_data * alloc_aw_data(int size);

===============  aw_data.c  ===============
#include "aw_data.h"

// -  Express  alloc_aw_data  Is an external function ;  Just call this function ,  Then you call this function ; 
aw_data * alloc_aw_data(int size){
	// -  Internal implementation 
}

// -   Express  write_bytes  It's an internal function ;  Only when this function is called inside this file ,  It's just write_bytes  Function implementation of ,  If other files call this function ,  And other files have the definition of this function ,  Then there is another one  write_bytes  The implementation of ;
static void write_bytes(aw_data **awdata, const uint8_t *bytes, uint32_t count){
	// -  Internal implementation 
}

2. Application of variables I

===============  Person.h  ===============
#import <Foundation/Foundation.h>

// -  Statement   kCellID  It's an external variable ,  At this point any one contains  Person.h In the file of   visit  kCellID The values are all   @"cell_ID";
extern NSString *const kCellID;

@interface Person : NSObject  @end

===============  Person.m  ===============
#import "Person.h"

// -  Define the external variable and assign a value to it 
NSString *const kCellID= @"cell_ID";

@implementation Person  @end 

2. Application of variables II

#import <Foundation/Foundation.h>
===============  Person.m  ===============
#import "Person.h"

// -  Define the external variable and assign a value to it 
int  age = 10;

@implementation Person  @end 

#import <Foundation/Foundation.h>
===============  Animal.m  ===============
#import "Animal.h"

@implementation Animal 

- (void)eat {
	//  - At this point  extern int  age  Make a statement  int  External variables of type  age,  At this point all access this age All variables are   Access to the  Person.m  The same as defined in age Variable ;   If it is changed to  extern int  age1,  Error will be reported in compilation ,  So there's no problem with robustness .
	extern int  age; 
	NSLog(@"// -  Li Chaoqun  console [log] ...%d", age);
}
 @end