Linking against a local `extern` variable

Question

I reckon this has been probably answered before. Please mark as a duplicate if it has!

I'm having trouble understanding what extern means for local variables (or local functions!). I can't seem to see how it is functionally different from a static local variable... For instance:

int foo(void) {
   extern int i;
   return i++;
}

int bar(void) {
   static int i;
   return i++;
}

AFAICT, foo and bar are the same. The difference according to the C99 spec is that i in foo should have external linkage compared to the i in bar. That said, I can't think of any way to exploit that external linkage - since it is a local variable, it isn't accessible outside of foo, let alone outside of the file.

I'm sure I'm missing something here - what is it?

EDIT

Thank you to everyone who pointed out that I'm missing a type on extern and static. You are quite correct. As a frequent answerer on SO, I now understand the frustration of the barrage of down votes that follow this sort of oversight.

Answer 1

int foo() {
   extern i;
   return i++;
}

Here you instrument the interpreter to see that i is declared somewhere in some outside block, and to connect this object with an object i declared somewhere before -- in your case, in file scope.

In the second case you tell the interpreter to store the variable i in some permanent storage that is available all long the execution and it is local to the function.

The algorithm of linkage is defined in ISO 9899 in paragraph 6.2.2--Linkage of identifiers. -- part of it:

For an identifier declared with the storage-class specifier extern in a scope in which a prior declaration of that identifier is visible,23) if the prior declaration specifies internal or external linkage, the linkage of the identifier at the later declaration is the same as the linkage specified at the prior declaration

See also here

External linkage: An identifier with external linkage represents the same object or function throughout the entire program, that is, in all compilation units and libraries belonging to the program. The identifier is available to the linker. When a second declaration of the same identifier with external linkage occurs, the linker associates the identifier with the same object or function.

Answer 2

For starters this function definition and the variable declaration

int foo() {
       ^^^
   extern i;
   ^^^^^^^^^
   return i++;
}

are invalid because there is absent a parameter list and a type specifier for the variable i.

It should be written like

int foo( void ) {
         ^^^^
   extern int i;
   ^^^^^^^^^
   return i++;
}

According to the C Standard (6.2.2 Linkages of identifiers)

4 For an identifier declared with the storage-class specifier extern in a scope in which a prior declaration of that identifier is visible,31) if the prior declaration specifies internal or external linkage, the linkage of the identifier at the later declaration is the same as the linkage specified at the prior declaration. If no prior declaration is visible, or if the prior declaration specifies no linkage, then the identifier has external linkage.

So it is not necessary that the variable i declared in the function has external linkage. For example in this demonstrative program the variable i declared within the function has internal linkage.

#include <stdio.h>

static int i;

int foo( void ) 
{
    extern int i;
    return i++;
}

int main(void) 
{
    printf( "f() = %d\n", foo() );
    printf( "i = %d\n", i );

    return 0;
}

In the function bar that should be defined like

int bar( void ) {
   static int i;
   return i++;
}

the variable i does not have a linkage but it has the static storage duration.

Answer 3

Extern : extern variable are having external linkage ,main usage of extern varaible to link variable in other file so you are saying "since it is a local variable, it isn't accessible outside of foo" which is wrong, check the below code.

    xyz@xyz-PC:~/s_flow/alec$ vi one.c
    #include<stdio.h>
    int foo() {
            extern i;//this is just declaration, not definition
            return i++;
    }
    main()
    {
            int ret = foo();
            printf("ret = %d \n",ret);
    }

Now linker will try to find definition of "i" in other file or below functions , if not found linker will throw error. when you compile above code you will get linker error. So provide the definition in other file as

xyz@xyz-PC:~/s_flow/alec$ vi two.c
i=100;//this is definition of i

Now compile like gcc one.c two.c & check

static : static storage class having internal linkage it means you can't use static variable outside that file(in which it is declared).

     xyz@xyz-PC:~/s_flow/alec$ vi one.c
     main()
     {
          printf("i = %d \n",i);
     }

Now if I am trying to access static variable which is defined in two.c to one.c , it will throw error.

xyz@xyz-PC:~/s_flow/alec$ vi two.c
void bar() {
   static i = 10;
}