C language : Static and Extern

Some of points regarding static and extern.

1) By default , global variables are extern

2) Local variables are neither extern or static by default.

3) Defining a local variables as extern, doesn't have any effect.
3.1) defining local variables as extern and assigning value to it results in COMPILE ERROR.

So never have local variables as extern.

4) Local variables as static saves its value between function calls.

Also it is better to have value assigned to static value at time of definition.

5) Define global variable as STATIC only if you want that variable scope as limited to that file.


Both STATIC and EXTERN variable are saved either in BSS if uninitialized and in DATA if initialized.



Regarding to function, compiler treat them as implicitly EXTERN. therefore we don't need to add extern in front of function.

But if you need to limit the scope of function to some one file, then static can be added in front of function.

But then LINK ERROR will come if you are trying to access this function from some other file.

STATIC with function name is mostly used if you have 2 definition of same function, then you can limit the scope of one function by using STATIC.

C Language : Static

So in the last post , we have seen the EXTERN.  In this post we will see STATIC .

We can define STATIC in global variables or local variables.

If we define STATIC in global variables, then the global variables scope will be that file only.
So even if you have extern that variable in some other file , then error will come.

Program 1)

static1.c

#include<stdio.h>

static int a=10;

int main()
{
func()
}

static2.c

extern int a;

void func()
{
printf("%d",a);
}

now if you GCC and compile both files separately then , there will be no COMPILE ERROR.
But on creating object file, LINK ERROR will come. Since a is not visible outside the static1.c

Program 2)

static1.c

#include<stdio.h>

static int a;
a=10;

int main()
{
func()
}

This will give COMPILE ERROR, since non static definition cannot follow the static definition.



Now lets see what happens when static is defined with local variable.

Program 3)

static1.c


#include<stdio.h>

int main()
{

func();
func();
}

int func()
{
static int a;
a=10;
a++;
printf("%d",a);
}

the output of the function will be 11 and 11

Since static variable are assigned value at time of declaration. If we assign value after declaration, then everytime func is called , that value will be put in the function.

Program 4)

static1.c


#include<stdio.h>

int main()
{

func();
func();
}

int func()
{
static int a=10;
a++;
printf("%d",a);
}

this will behave as it should be , output will be 11 and 12.

What will happen if static variable is not initialized.

All static and extern variables will be initialized to zero if not explicitly assigned some value.

C Language : Extern

I marvel at the way we humans are designed or made. We simply can't remember things which we are not in contact with for a certain period of time .

Its like either our memory gets rewritten by new memories or the way of accessing our old memories is not good enough.

I am pretty sure , I would have read about EXTERN a lot of times, but not using it on daily basis and suddenly again we have to go to some book, to check the usage.

Then the second thing is curiosity. I know a certain way of using EXTERN, but then a sudden urge will come to see what if  I have defined it in some other way or some other place and etc etc .

Now One more time and quite possibly not the last time , here again I go about EXTERN.


All the below program snippet use GCC on Ubuntu

To best see how extern works,  compile each .C file separately and  then make executable

Program 1)

extern.c file

#include<stdio.h>
int a=10;

int main()
{
    printf("%d",a);
  func();
}

extern2.c

int a=20;

int func()
{
printf("%d",a);
}

Compile both programs
gcc -c extern.c
gcc -c extern2.c

two object files will be generated  extern.o  and extern2.o

Try to generate executable
gcc -o output extern.o extern2.o

You wll get LINKING ERROR , saying multiple defination of a

Program 2)

Now remove the int a=20;  line in extern2.c  and try again

This time you will see , COMPILE ERROR for extern2.c , since a is undefined.

Program 3)

In extern2.c , add    extern int a;  at the start

  This will work and no error will come , since

1) having extern , tells compiler to check for a at run time .
2) At run time , only one declaration of a is present in extern.c

Program 4)

What if you add  extern int a=2; either in extern.c or extern2.c

There will be no compile error, but extern will not work,  it will be simply as you have defined
int a=2;



What about local variable defined as extern ?

This will not have any effect.
1) If you initialize value at same time as declaring then COMPILE ERROR will come.

else

it will simply treat a as global variable, note that a should be declare as global variable in some file.

ex

Program 5)

extern.c

#include<stdio.h>

int main()
{

extern int a=8;          // COMPILE ERROR


Program 6)

extern.c

#include<stdio.h>

int main()
{

extern int b;
printf("%d",b);
}

extern2.c

int b=10;

int func()
{
printf("%d",b);
}

This will work.

Even

extern.c

#include<stdio.h>

int b=10;

int main()
{

extern int b;

printf("%d",b);
}

will work.

Program 7)

extern.c

#include<stdio.h>

int main()
{

extern int b;
printf("%d",b);
}

extern2.c


int func()
{
int b=10;
printf("%d",b);
}

There will be no COMPILE ERROR, but will be LINK ERROR  since there is no global definition of  a.

If you try the above programs, you will pretty much know the working of extern.