a static variable in c

Question

Hi I'm studying for my test in C and i've encountered a question which i can't figure its answer.

A programmer wrote a program to count number of users (Count.h, Count.c):

/******** FILE: Counter.h ***********/
static int counter = 0;
int getUsersNum ();
/******** END OF FILE: Counter.h ****/

/******** FILE: Counter.c ***********/
#include "Counter.h"
int getUsersNum ()
{
    return counter;
}
/******** END OF FILE: Counter.c ****/

And a tester to test it:

/******** FILE: CounterMain.c ***********/
#include "Counter.h"
#include <stdio.h>
int main ()
{
    int i;
    for (i=0;i<5;++i)
    {
        ++counter;
        printf ("Users num:  %d\n", getUsersNum());
    }
    return 0;
}
/******** END OF FILE: CounterMain.c ****/

Suprisingly the output was:

Users num: 0
Users num: 0
Users num: 0
Users num: 0
Users num: 0

I can't see why with this use of static variable the counter does not advance.. why did they get such input?

thank you all!

Answer 1

In C, the scope of a static variable is the source file in which its defined.

Since you're loading this header into 2 separate .c files, each file gets a unique variable. Incrementing "counter" in one file does not affect the "static" variable in the other file.

For details, see this description. In order for the variable to be visible and shared in multiple files, it needs to be declared as extern. Otherwise:

Static global variables: variables declared as static at the top level of a source file (outside any function definitions) are visible throughout that file ("file scope").

Which is the case here.