Is extern keyword in C redundant?

Question

I have found that I could achieve the desired results without using extern (though I do agree that it gives reader some kind of an hint about the variable). In some cases using extern gave undesired results.

xyz.h

int i;

file1.c

#include "xyz.h"
....
i=10;
....

file2.c

#include "xyz.h"
main()
{
printf("i=%d\n",i);
}

Of course, it was a large project, broke it down for simple understanding. With extern keyword, I couldnt get desired results. In fact, I got linker error for the variable i with "extern" approach.

Code with "extern" approach,

file1.c

int i;
main()
{
i=10;
}

file2.c

extern int i;
foo()
{
printf("i=%d\n",i);
}

This gave linker error. I just wanted to know why it worked in the first case and also the practical case where we cannot do it without using the keyword "extern". Thanks.

Answer 1

Formally, your first program is invalid. Defining a variable in header file and then including this header file into multiple translation units will ultimately result in multiple definitions of the same entity with external linkage. This is a constraint violation in C.

6.9 External definitions

5 An external definition is an external declaration that is also a definition of a function (other than an inline definition) or an object. If an identifier declared with external linkage is used in an expression (other than as part of the operand of a sizeof operator whose result is an integer constant), somewhere in the entire program there shall be exactly one external definition for the identifier; otherwise, there shall be no more than one.

The definition of i in your first example is a tentative definition (as it has been mentioned in the comments), but it turns into a regular full fledged external definition of i at the end of each translation unit that includes the header file. So, the "tentativeness" of that definition does not change anything from the "whole program" point of view. It is not really germane to the matter at hand (aside for a little remark below).

What makes your first example to compile without error is a popular compiler extension, which is even mentioned as such in the language standard.

J.5 Common extensions

J.5.11 Multiple external definitions

1 There may be more than one external definition for the identifier of an object, with or without the explicit use of the keyword extern; if the definitions disagree, or more than one is initialized, the behavior is undefined (6.9.2).

(It is quite possible that what originally led to that compiler extension in C is some implementational peculiarities of tentative definition support, but at abstract language level tentative definitions have nothing to do with this.)

Your second program is valid with regard to i (BTW, implicit int is no longer supported in C). I don't see how you could get any linker errors from it.

Answer 2

There are at least 2 cases where extern is meaningful and not "redundant":

1. For objects (not functions) at file scope, it declares the object with external linkage without providing a tentative definition; tentative definitions turn into full definitions at the end of a translation unit, and having the same identifier defined with external linkage in multiple translation units is not permitted.

2. At block scope (in a function), extern allows you to declare and access an object or function with external linkage without bringing the identifier into file scope (so it's not visible outside the block with the declaration). This is useful if the name could conflict with something else at file scope. For example:

   static int a;
   int foo(void)
   {
       return a;
   }
   int bar(void)
   {
       extern int a;
       return a;
   }
   

   Without the extern keyword in bar, int a; would yield a local variable (automatic storage).