CODEWITHSUNDEEP
cstaticscopeextern
Pithikos
Pithikos

Reputation: 20300

What's the use of static/extern in source files?

I have a very mixed notion of what happens when I compile many files - mostly when it comes to the visibility of things from one file to an other. From what I read, static limits the scope of a variable or function to the file itself. extern does the opposite. From that I would expect to be able and simply read a global extern from any file. That doesn't work in practice though as seen below.

main.c:

#include <stdio.h>

int main(void){
    printf("%d\n", b); // b is extern global
    return 0;
}

a.c:

static int a = 40;

b.c:

extern int b = 20;

I can't even compile:

> gcc main.c a.c b.c -o test
b.c:1:12: warning: ‘b’ initialized and declared ‘extern’ [enabled by default]
 extern int b = 20;
            ^
main.c: In function ‘main’:
main.c:4:20: error: ‘b’ undeclared (first use in this function)
     printf("%d\n", b); // b is extern global

Upvotes: 2

Views: 188

Answers (3)

Gopi
Gopi

Reputation: 19864

When you have a extern variable the compiler knows that the declaration for this variable is here and definition is somewhere else.

So you can do as shown below

some.h

   extern int b;  /* Declare the variable */

b.c

   int b = 20;  /* Define the variable */

main.c

    #include<some.h>

    int main()
    {
       printf("%d\n",b);  /* Use the variable b */

       return 0;
    }

With this now the file main.c knows the declaration of b since it is there is some.h file.

Upvotes: 0

Ankur
Ankur

Reputation: 3724

You are doing all wrong.When we writing extern int b it means an integer type variable called b has been declared.And we can do this declaration as many times as needed. (remember that declaration can be done any number of times).By externing a variable we can use the variables anywhere in the program provided we know the declaration of them and the variable is defined somewhere.

The correct way is

main.c:

#include <stdio.h>

extern int b;   //declaration of b
 int main(void){
    printf("%d\n", b); // using b 
    return 0;
}

b.c:

int b = 20;   //definition here

and compile as gcc main.c b.c -o test

I have omitted a.c as it was doing nothing in this example. To understand more about externs see this site.Its having very good content http://www.geeksforgeeks.org/understanding-extern-keyword-in-c/

Upvotes: 1

R Sahu
R Sahu

Reputation: 206577

A variable can be used if it is either declared or defined.

In main.c, there is no declaration of b. You can add

extern int b;

in main.c for the compiler to be able to use b. This declares to the compiler that b is defined somewhere else and it is of type int.

In b.c, you can remove the word extern. Unless qualified with static, it is extern by default.

int b = 20;

Even in b.c, you can use a declaration at the top of the file and define it at the bottom.

extern int b;

//
// ... code that uses b
//

int b = 20;

The extern line simply declares the variable. The last line defines the variable.

Upvotes: 0

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