How are extern variables defined?

In your first program, the print statement is printing the value of i based on the extern int i declaration. This is similar to calling a function based on its prototype declaration without seeing its definition. The compiler generates code to retrieve the value in the global variable named i. The symbol is resolved to the correct variable and address at link time.

In your second program, no definition for i is provided, only the extern int i declaration, and the attempt to set its value with i = 20. At link time this fails, since there is no definition, and so the attempt to resolve the reference to the global variable fails. Changing i = 20 to int i = 20 instead creates a local variable named i within the scope of the function main(), and is no longer referencing the globally declared extern int i.

When int i; is used at global scope, it is treated as a declaration and may be treated as a kind of definition. A global variable declared with an initializer, like:

int i = 20;

is always treated as a definition. Only one definition of this type is allowed, even if each is using the same initializer value. However,

int i;

is treated like a declaration. If more than one of these declarations appear, they are all treated as declarations of the same variable. At the same time, if no declaration with an initializer is present, this variable is implicitly defined with an initializer of 0, and the linker will be able to resolve references to it.

Declaration of a variable/function simply declares that the variable/function exists somewhere in the program but the memory is not allocated for them.

Coming to the definition, when we define a variable/function, apart from the role of declaration, it also allocates memory for that variable/function.

So in the 1st program, int i = 31 outside the main() defines the variable i. This variable is then merely declared as an extern variable inside the main(). Hence the program works.

In the 2nd program, without defining the variable i elsewhere, it is directly declared as an extern variable. Hence it gives an error.

In the second program you haven't defined the variable anywhere , not outside the main and instead you have defined it inside main so you are getting that error.

But in the first program you are defining the variable once and that is outside the main so you don't get that error.Program looks outside the main and find its definition.

int i; is a variable definition declaration both for AUTO variables (i.e inside main or any function ) but not for the external variables for extern variables you have to declare it like :

extern int i;

and then define outside main like this :

int i;

The idea behind extern is that you tell the compiler you're going to use a variable that was already defined outside the scope of where it is used.

It doesn't make much sense in your second case as there i is defined as a local variable. If you were to define it as a global, like you do in the first case, it would be fine.

If I can demonstrate the use of extern with a very simple (contrived) case:

main.c:

#include <stdio.h>

extern int a; /* we tell the compiler that a will come from outside the scope of where it is used */

int main()
{
    printf("%d\n", a);

    return 0;
}

source.c:

int a = 3; /* we define a here */

You then compile both .c files like this:

$ gcc main.c source.c

This outputs 3.

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