Is there any wrong about tha static variable? - c

Question

I have a problem about a program. I bet that it has to do with the fact that I use static. Here is my t.h
static int cnt;
void f();
my main.c
#include <stdio.h>
#include "t.h"
void main()
{
cnt=0;
printf("before f : cnt=%d\n",cnt);
f();
printf("after f : cnt=%d\n",cnt);
}
and finally my f.c
#include "t.h"
void f()
{
cnt++;
}
The printf prints cnt=0 both times. How is this possible when I do cnt++? Any ideas?
Thanks in advance

Answer 1

In C, static means "Local to the module"
Take note, that the #include statements just pastes the header file in the including file.
therefore, you are creating two distinct symbols (happens to have the same logical name) in different modules.
f.c cnt is a different cnt then main.c
Note:
static in C has different meaning then its C++ counterpart.
and because C++ is C Compatible, static outside a class have the same meaning as in C
Edit:
In your case, you don't want a static you want a variable, but i guess you had problem with the Linker telling you about "ambiguous symbols".
I would suggest to declare an extern in the header file, and declare the actual variable in a module.
t.h
extern int cnt; // declaration of the variable cnt
main.cpp
#include
#include "t.h"
void main()
{
cnt=0;
printf("before f : cnt=%d\n",cnt);
f();
printf("after f : cnt=%d\n",cnt);
}
t.cpp
#include "t.h"
int cnt = 0; // actual definition of cnt
void f()
{
cnt++;
}

Answer 2

Data should not be defined in the header files.
In your example you will create a separate copy of that static variable in every compilation module which includes this .h file.

Answer 3

Don't define cnt in your header file. Instead, define it in f.c:
#include "t.h"
int cnt = 0;
void f(){
cnt++;
}
Then in main.c, add the following before the beginning of your main function:
extern int cnt;