Extern in multiple files and possible double definition

Question

I was running the following codes compiled together as: gcc A.c B.c -o combined

Program A:

#include<stdio.h>
int a=1;
int b;
int main()
{
extern int a,b;
fun();
printf("%d %d\n",a,b);
}

Program B:

int a;
int b=2;
int fun()
{
printf("%d %d\n",a,b); 
return 0;
}

On running the "combined" program the output was:

1 2
1 2

Now, I've a few doubts about this one:

1. Why isn't the output:

   0 2

   1 0

2. Aren't a and b defined twice?

Please explain these clearly, I've had a lot of problems understanding extern and few of these doubts keep coming from time to time.

Thanks in Advance.

Answer 1

A variable may be declared many times, as long as the declarations are consistent with each other and with the definition. It may be declared in many modules, including the module where it was defined, and even many times in the same module.

An external variable may also be declared inside a function. In this case the extern keyword must be used, otherwise the compiler will consider it a definition of a local variable, which has a different scope, lifetime and initial value. This declaration will only be visible inside the function instead of throughout the function's module.

Now let me repeat again definition of extern which says "external variable is a variable DEFINED outside any function block"(Please read carefully word given in BOLD). So for the Programe A a have definition but b is just declaration so extern will look for its definition of 'b' which is given in Programe B.So print from Programe A is 1 2.Now lets Talk about Programe B which have declaration for a and definition for b so it is priting value of a from programe A and value of b from current file.