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I recently came across the question like how to access a variable which declared static in file1.c to another file2.c?

Is it possible to access static variable?

My understanding about static keyword in C is,

static is "internal linkage", so they are accessible only from one compilation unit - the one where they were defined. Objects declared with internal linkage are private to single module.

As one of my friend suggest me below solution.

In file1.c

   #include <stdio.h>

   int main()
   {
          int b=foo();
          printf("%d",b);
          return 0;
   }

in file2.c

   static int a=25;

   int foo()
   {
        return a;
   }

compiled by gcc file1.c file2.c -o file

If I do above I can access the variable.

So my questions are:

  1. Does the above program violate static variable rules?

  2. If not, why is this so, and is there any other way to access static variable except including file (#include <…>) not like this.

    How am I able to access a static variable from another file?

    In C, how do I restrict the scope of a global variable to the file in which it's declared?

  3. Correct me if I'm wrong with static variable concept and if any better solutions are available to access static variable?

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vinay hunachyal
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  • You have never used `a` inside file1.c. I think if you try to access it, compiler should throw error. – 0xF1 Jul 29 '14 at 06:24
  • @Don'tYouWorryChild that would be the linker; not the compiler that would puke, but your point of error generation is just as valid regardless. – WhozCraig Jul 29 '14 at 06:26
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    You're not violating anything, nor are you "accessing" `a` outside the translation unit where it is static. You have a function that returns `a` by-value. If you need to modify it outside `file2.c` either return its address via a function or provide a functional get/set api. – WhozCraig Jul 29 '14 at 06:27
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    IDK what "better" is supposed to mean here, but you could write `int *foo() { return &a; }` – M.M Jul 29 '14 at 07:46
  • @Matt McNabb one of the interviewer asked me above question , so i just want to know static variable access is possible or not – vinay hunachyal Jul 29 '14 at 07:49
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    @vinayhunachyal yes it is possible. Mr.32 gave one method and my comment gives another. It's a matter of opinion which one is "better" – M.M Jul 29 '14 at 07:52
  • @WhozCraig : You are right, it just slipped out of my mind that of course linkage will be defined by linker. Thanks anyways for correcting me... :-) – 0xF1 Jul 29 '14 at 08:17
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    Note that `static` hides the name; you can't use that name to access that variable from outside the translation unit (TU) where the static variable is defined. Another variable with the same name may exist in another TU — but that will be a separate variable. But static only hides the name. Pointers can be passed around to make the variable accessible — it is only the name that is completely hidden. But only code within the TU where the static variable is defined can provide access to the variable; ordinary code outside the file can't access the variable by name. – Jonathan Leffler Jul 29 '14 at 13:33
  • similar: https://stackoverflow.com/questions/1973162/access-a-global-static-variable-from-another-file-in-c – Ciro Santilli OurBigBook.com Jan 26 '19 at 11:16

4 Answers4

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1) does the above program violate static variable rules?

No you are not vailoting any rules. Here foo function create copy of value of that static variable and used in other file. Its fine.

2) If not why is this so, and is there any other way to access static variable except including file (#include<>) not like this How am I able to access a static variable from another file?

Static variable are only mean to use in that file only.

You can not use that variable making them extern in other files.

Another dirty hack is to get pointer of that static variable and make that as global pointer and making that as extern in another file you can use that static variable.

file1.c

 #include<stdio.h>
  static int a=25;
  int* ptr = &a;

file2.c

#include<stdio.h>
extern int *ptr;

   int main()
   {
          printf("%d",*ptr);
          return 0;
   }

Correct me if I'm wrong with static variable concept and if any better solutions are available?

  1. A static variable has a lifetime extends across the entire run of the program

  2. If you do not initialize static variable with some value then its default value would be 0.

  3. A static variable has scope limited to its file only. You can not access it by name from a different file.

  4. You have temp1.c and temp2.c both are getting compiled together then also you can have static variable of same name in both files — and they are separate variables.

In C, how do I restrict the scope of a global variable to the file in which it's declared?

By making that global variable as static.

Jonathan Leffler
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Jeegar Patel
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  • is there any better solution to access a static variable in other file? – vinay hunachyal Jul 29 '14 at 07:24
  • i have said static are do not mean to use it in another files. If you want to do such them why you are making that variable as static? just remove static word and access it in another file just adding extern int a. – Jeegar Patel Jul 29 '14 at 07:26
  • still you want to access static variable in another file then Another dirty hack is to get pointer of that static variable and make that as global pointer and making that as extern in another file you can use that static variable. – Jeegar Patel Jul 29 '14 at 07:26
  • @vinayhunachyal I have update code for accessing static from another files. – Jeegar Patel Jul 29 '14 at 07:29
6

What we commonly call a variable in C is actually two things: an object, the memory allocated for the variable interpreted with a certain type, and an identifier, one way to access that object.

There is no problem in accessing a static object or its value from another compilation unit. Your function foo promotes the value to another unit, that is fine, but it could even promote the address of a without problems.

Having internal linkage only concerns the identifer, the name a. This one is only visible inside file2.c.

Jens Gustedt
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1

With the static int a=25; the variable a will have internal linkage; meaning the linker cannot see a anywhere outside of the file2.c TU.

When you're calling foo() in file2.c, you get a copy of a, it's the copy that you print; but this doesn't mean you have access to the actual a defined in file2.c When you need such an access where the same variable is visible across different TUs, you could do this

Defining file

This file both declares and defines the variable; additionally initializes it to 1 too, without which it'll be default initialized to 0.

// (non-static) global variable with external linkage and thus visible across TUs
int var_across = 0;
void use()
{
   var_across = 1;
}

Using file

// just a declaration to tell that it's defined elsewhere; not a definition
extern int var_across;
void use_here()
{
   var_across = 2;
}
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legends2k
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  • with static keyword i need to access – vinay hunachyal Jul 29 '14 at 06:20
  • -1 making extern int a you can not access static int a in file1.c. – Jeegar Patel Jul 29 '14 at 07:04
  • @Mr.32 Please read the answer properly. I said `a` _will have internal linkage and thus the linker cannot see it_ which means the same. Also I'd written removing `static` will make it work with the `extern`. This answer pertains to the [first edit of the question](http://stackoverflow.com/revisions/25009246/1) the OP used `extern int a;` in `file1.c`. – legends2k Jul 29 '14 at 08:54
  • You should update your answer with clarifying "I'd written removing static will make it work with the extern" so i can upvote to you – Jeegar Patel Jul 29 '14 at 09:02
  • @Mr.32 Agreed, point taken. Fixed it now to match the changed question. – legends2k Jul 29 '14 at 09:10
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Assigning address of static variable to pointer will make static variable available to subfiles.

In subfiles we have to use extern keyword to the pointer.

But it is not necessary to do that.

tamlarasn Mohn
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