How to Understand “extern” Keyword in C (example)

If you know the “extern” keyword, but you are not clear about where and why do we need it. You are in right place.

“extern” can be used to declare variables and functions which are defined in other files. so we can use these variables and functions in this file.

The following case can let you understand why we need “extern” keyword. I used “extern” function as example, the same with variables.

First, In case 1, please let me show you how to invoke a function in other files.

Note: Run the program in Ubuntu 16.04 LTS.

Case 1:

There are four files in case 1: add.h add.c multi.h mult.c main.c.

In add.c

#include <stdio.h>
#include "add.h"
int addNumber(int a, int b) 
{
	return a + b;
}

In add.h

#ifndef __ADD_H__
#define __ADD_H__

int addNumber(int a, int b);

#endif

In multi.c

#include <stdio.h>
#include "multi.h"
#include "add.h"
int multiNumber(int a, int b)
{
	int i = 0, res = 0;
	for (i = 0; i < a; i++) {
		res = addNumber(res, b);
	}
	return res;
}

In multi.h

#ifndef __MULTI_H__
#define __MULTI_H__

int multiNumber(int a, int b);

#endif

In main.c

#include <stdio.h>
#include "multi.h"
#include "add.h"
int main()
{
	int a = 4, b = 5;
	int res = 0;
	res = addNumber(a, b);
	printf("addNumber res = %d\n", res);
	res = multiNumber(a, b);
	printf("multiNumber res = %d\n", res);
	return 0;
}

According to the above code, we know that we can invoke addNumber function only if we add add.h file. because addNumber is exposed in add.h file, right?

NOW!

There is a question: How can we invoke addNumber only in multi.c? How?

“extern” keyword is used now.

Case 2:

Base on Case 1, we only modify the following two places.

1. Remove addNumber function in add.h.
2. Add extern keyword to declare addNumber in multi.c.

In add.h

#ifndef __ADD_H__
#define __ADD_H__

//int addNumber(int a, int b);

#endif

In multi.c

#include <stdio.h>
#include "multi.h"
#include "add.h"

extern int addNumber(int a, int b);

int multiNumber(int a, int b)
{
	int i = 0, res = 0;
	for (i = 0; i < a; i++) {
		res = addNumber(res, b);
	}
	return res;
}

After you do that, you will get the following error info:

Because we didn’t expose addNumber function in add.h, so complier can’t find this function in main.c. But we use extern keyword to declare addNumber in multi.c, so multiNumber function can call addNumber function in multi.c.

Now, We remove res = addNumber(a, b); in main.c, then make and run main, we will get the following result.

multiNumber res = 20

What I learned from these cases:

Using “extern” keyword, we don’t need expose this function to all other files. Only when we want to use it, we use extern keyword to declare it. It is safty.

Hope this post is useful for you to understand “extern” keyword in C.

Have a good day!

If you have any question, please feel free to let me know. I will appreciate it if you give me some feedback!