What a C static variable in file scope means?

Question

I have three files to demonstrate the use of static variable in file scope. Variable is declared as extern in file2.h, initialized in file2.c. I am declaring another variable with same name in main.c as static to test for static global scope. But I get the error message "main.c|6|error: static declaration of 'var1' follows non-static declaration.

Could someone explain me the usage of static for file scope?

If I do not include file2.h in main.c, I do not get any problem. But what if I need to use some functions of other files in main.c but still want to keep the variable scope to this file only?

main.c

#include <stdio.h>
#include "file2.h"


static int var1;

    int main()
    {
        printf("value of staticVar1 = %d\n",var1);
        func1();
        printf("value of staticVar1 after function call= %d\n",var1);
        return 0;
    }

file2.h

#ifndef _FILE2_H
#define _FILE2_H
#include <stdio.h>

extern int var1;

int func1(void);

#endif // _FILE2_H

file2.c

#include <stdio.h>
#include "file2.h"

int var1=3;

int func1(void)
{
    printf("value of staticVar1 inside the function = %d\n",var1);
    return(0);
}

Answer 1

An object declared at file scope has either external or internal linkage, it cannot have both linkages:

extern int var1;  // declare var1 an int with external linkage
int var1 = 3;     // declare and define var1 with external linkage
static int var1;  // declare and define var1 an int with internal linkage
                  // -> ERROR var1 is redeclared with different linkage

You use static specifier if you want an object with visibility limited to the source file in which you declared it.