C language keyword extern

One . Preface

Recently I wrote a code , Always appear when compiling ：multiple definition, It's so annoying to think of it , Then enumerate the possible situations , To sum up .

Two . Code

Compile environment ：gcc version 9.4.0 (Ubuntu 9.4.0-1Ubuntu1~20.04)

• a.c

#include <stdio.h>
#include "a.h"
#include "b.h"

int add()
{
    
    printf("%d\n", ++aim);    
}

int main()
{
    
    dec();
    add();
    dec();
    return 0;
}

• a.h

#ifndef _A_H
#define _A_H

int add();

#endif

• b.c

#include <stdio.h>
#include "b.h"

int dec()
{
    
    printf("%d\n", --aim);    
}

• b.h

#ifndef _B_H
#define _B_H

int dec();

#endif

• makefile

TARGET:=./out
obj-y:=a.c b.c
OBJS=$(patsubst %.c, %.o, $(obj-y))
all:clean $(TARGET)
	@./out
$(TARGET):$(OBJS)
	@gcc -o $(TARGET) $(OBJS)
%.o:%.c
	@gcc $(CFLAGS) -c $<
clean:
	@rm -rf $(OBJS) $(TARGET)

3、 ... and . Permutation and combination

hold int aim = 1 and extern int aim Put it under different files 4×3=12 In this case , Then let's try one by one ：

a.c	a.h	b.c	b.h	result
aim=1	extern	×	×	In function 'dec': 'aim' undeclared
aim=1	×	extern	×	Output 0 1 0 You can use √
aim=1	×	×	extern	Output 0 1 0 You can use √
extern	aim=1	×	×	In function 'dec': 'aim' undeclared
×	aim=1	extern	×	Output 0 1 0 You can use √
×	aim=1	×	extern	Output 0 1 0 You can use √
extern	×	aim=1	×	Output 0 1 0 You can use √
×	extern	aim=1	×	Output 0 1 0 You can use √
×	×	aim=1	extern	Output 0 1 0 You can use √
extern	×	×	aim=1	b.o:multiple definition of "aim"; a.o:fisrt defiined here
×	extern	×	aim=1	b.o:multiple definition of "aim"; a.o:fisrt defiined here
×	×	extern	aim=1	b.o:multiple definition of "aim"; a.o:fisrt defiined here

Define only aim=1

a.c	a.h	b.c	b.h	result
aim=1	×	×	×	In function 'dec': 'aim' undeclared
×	aim=1	×	×	In function 'dec': 'aim' undeclared
×	×	aim=1	×	b.o:multiple definition of "aim"; a.o:fisrt defiined here
×	×	×	aim=1	b.o:multiple definition of "aim"; a.o:fisrt defiined here

stay .h Statement int aim, stay .c Instantiation int aim = 1

a.c	a.h	b.c	b.h	result
aim=1	aim;	×	×	In function 'dec': 'aim' undeclared
aim=1	×	aim=1	×	b.o:multiple definition of "aim"; a.o:fisrt defiined here
aim=1	×	×	aim;	Output 0 1 0 You can use √
×	aim;	aim=1	×	Output 0 1 0 You can use √
×	aim;	×	aim;	Output -1 0 -1 Also available

Ideal standardized writing ：

a.c	a.h	b.c	b.h	result
aim=1	aim;	×	aim;	Output 0 1 0 You can use √
aim=1	aim;	×	extern aim;	Output 0 1 0 You can use √
×	aim;	aim=1	aim;	Output 0 1 0 You can use √
×	extern aim;	aim=1	aim;	Output 0 1 0 You can use √

Another special ：

a.c	a.h	b.c	b.h	result
×	int aim;	static int aim=1	×	0 1 -1( there aim Just two variables static The overall role of )
×	int aim;	×	static int aim=1	static declaration of 'aim' follows non-static declaration
×	static int aim;	×	static int aim=1	0 2 -1
static int aim;	×	×	static int aim=1	0 2 -1

Four . Disgusting situation

Considering that header files often reference each other in projects , It will be b.c Add a line of code #include "a.h", Now the two of them quote each other . Then repeat the experiments just now ：

a.c	a.h	b.c	b.h	result
aim=1	extern	×	×	Output 0 1 0 You can use √
aim=1	×	extern	×	Output 0 1 0 You can use √
aim=1	×	×	extern	Output 0 1 0 You can use √
extern	aim=1	×	×	b.o:multiple definition of "aim"; a.o:fisrt defiined here
×	aim=1	extern	×	b.o:multiple definition of "aim"; a.o:fisrt defiined here
×	aim=1	×	extern	b.o:multiple definition of "aim"; a.o:fisrt defiined here
extern	×	aim=1	×	Output 0 1 0 You can use √
×	extern	aim=1	×	Output 0 1 0 You can use √
×	×	aim=1	extern	Output 0 1 0 You can use √
extern	×	×	aim=1	b.o:multiple definition of "aim"; a.o:fisrt defiined here
×	extern	×	aim=1	b.o:multiple definition of "aim"; a.o:fisrt defiined here
×	×	extern	aim=1	b.o:multiple definition of "aim"; a.o:fisrt defiined here

Define only aim=1

a.c	a.h	b.c	b.h	result
aim=1	×	×	×	In function 'dec': 'aim' undeclared
×	aim=1	×	×	b.o:multiple definition of "aim"; a.o:fisrt defiined here
×	×	aim=1	×	In function 'add': 'aim' undeclared
×	×	×	aim=1	b.o:multiple definition of "aim"; a.o:fisrt defiined here

stay .h Statement int aim, stay .c Instantiation int aim = 1

a.c	a.h	b.c	b.h	result
aim=1	aim;	×	×	Output 0 1 0 You can use √
aim=1	×	aim=1	×	b.o:multiple definition of "aim"; a.o:fisrt defiined here
aim=1	×	×	aim;	Output 0 1 0 You can use √
×	aim;	aim=1	×	Output 0 1 0 You can use √
×	aim;	×	aim;	Output -1 0 -1 Also available

Ideal standardized writing ：

a.c	a.h	b.c	b.h	result
aim=1	aim;	×	aim;	Output 0 1 0 You can use √
aim=1	aim;	×	extern aim;	Output 0 1 0 You can use √
×	aim;	aim=1	aim;	Output 0 1 0 You can use √
×	extern aim;	aim=1	aim;	Output 0 1 0 You can use √

5、 ... and . Analysis and usage

Tables are not all cases , But at least we can roughly understand .c Document and .h Scope of the file .
As the most convenient way to use ：

• When a variable needs to be used in other files , Directly in other files .h use extern Just make a statement （ Of course. .c It's fine too ）.
• .h Do not instantiate , A simple statement is enough .
• The same variable , When it .h After declaration , The scope of this variable is in this .c and .h Valid in scope , If by other documents .h After nesting , The scope of this variable extends from this file to this file and other files . If there is instantiation under two files , Then there are two instantiations in the same scope , But this scope has no order , So it exists “ meanwhile ” Assign two values to a variable , So it will remind multiple definition

notes ：

• Instantiation , Refer to int aim = 2;. The assignment is aim = 2; The two are very different , Need to distinguish .

6、 ... and . summary

Declarations can expand the scope , So there can be multiple , Instantiation can only occur once .