An extern variable located in a function?

Question

According to wikipedia:

http://en.wikipedia.org/wiki/External_variable

An external variable may also be declared inside a function.

What is the purpose of an extern variable being declared within a function? Would it have to be static too?

Answer 1

It allows for restricting access to a global to some scope:

int main()
{
    extern int x;
    x = 42;  //OKAY
}

void foo()
{
    x = 42;   //ERROR
}

int x;

Answer 2

The external declaration goes inside a function. It simply means that no other functions can see the variable.

void func()
{
   extern int foo;
   foo ++; 
}


void func2()
{
   foo--;     // ERROR: undeclared variable. 
}

---

In another source file:

int foo;     // Global variable. Used in the other source file, 
             // but only in `func`. 

It is just a way to "isolate" a variable, so it doesn't accidentally get used in places where it isn't supposed to be used.