Can we really restrict the scope of a global variable using Static?

Question

I have read somewhere that we can restrict the scope of global variable to a file only by using static keyword before variable name. But, when i tried it practically it comes out to be false:

//1st file - file1.c                   //2nd file - file2.h
#include<file2.h>                      static int a;
main()                                
{
    fun();                                 
}                                                                                 

fun()
{
    printf("%d",a);
}

O/P is 0

Now we do have a global variable a which is declared in file2.h, whose scope is limited to this file only.
Since, we have declared it as static, but still we can access this variable in file1.c. How ??

Are you questioning your own answer?? [link](http://stackoverflow.com/questions/7025934/how-to-declare-a-global-variable-which-is-present-in-only-one-function/7026322#7026322) — cyber_raj, Aug 12 '11 at 10:26

score 2 · Answer 1 · answered Aug 12 '11 at 10:12

2

The scope is not restricted to a file, but to a translation unit. Since you include file2.h into file1.c, this is all one TU -- you might as well have pasted the content of file2.h into the source file.

answered Aug 12 '11 at 10:12

Kerrek SB

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score 2 · Accepted Answer · answered Aug 12 '11 at 10:35

2

In restrict the scope of global variable to a file statement, by file they mean compilation unit, i.e. a .c file. Your file2.h is included by file1.c and they constitute one compilation unit file1.

Move the variable to a second compilation unit e.g. file2.c, and you'll see you can't access it even with extern declaration.

answered Aug 12 '11 at 10:35

hamstergene

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Thats sounds a good idea!!but still in file2.c i have to include the file2.h?? – Amit Singh Tomar Aug 12 '11 at 10:51
Think of #include as of copy-pasting file contents. If you include `file.h` in both `file1.c` and `file2.c`, they both will have a declaration of `a`, but they gonna be two different variables: changing one will not change another. – hamstergene Aug 12 '11 at 11:12
@Amit Singh Tomar: Would not including the file2.h in file2.c end up creating the same scenario that you have now? – Alok Save Aug 12 '11 at 11:12

Alok Save · Answer 3 · 2011-08-12T10:22:34.587

You are including the file file2.h in which you declared the variable as static in the c file which has main, which is as good as writing the declaration in that c file.

When you include a header file which declares a static variable a copy of the variable gets created for each translation unit(c file + included header files) in which the header file is included.

Never declare your static variables in header file.

To test the scenario, You should do this:

//file1.c 

static int i = 10;

//file2.c

extern int i;

int main()
{
    int a = i;
    return 0;   
}

score 0 · Answer 4 · answered Aug 12 '11 at 10:11

0

Scope is defined after C's pre-processor has run. Ie: after all #include-statements have been evaluated and inlined.

answered Aug 12 '11 at 10:11

Patrick B.

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Can we really restrict the scope of a global variable using Static?

4 Answers4