A variable declared with the extern storage-class specifier is a reference to a variable with the same name defined in another source file. It is used to make the external-level variable definition visible. A variable declared as extern has no storage allocated for itself; it is only a name.

This example illustrates internal- and external-level declarations:

// Source1.c

int i = 1;

// Source2. c

#include <stdio.h>

// Refers to the i that is defined in Source1.c:
extern int i;

void func(void);

int main()
{
    // Prints 1:
    printf_s("%d\n", i);
    func();
    return;
}

void func(void)
{
    // Address of global i assigned to pointer variable:
    static int *external_i = &i;

    // This definition of i hides the global i in Source.c:
    int i = 16;

    // Prints 16, 1:
    printf_s("%d\n%d\n", i, *external_i);
}

In this example, the variable i is defined in Source1.c with an initial value of 1. An extern declaration in Source2.c is makes 'i' visible in that file.

In the func function, the address of the global variable i is used to initialize the static pointer variable external_i. This works because the global variable has static lifetime, meaning its address does not change during program execution. Next, a variable i is defined within the scope of func as a local variable with initial value 16. This definition does not affect the value of the external-level i, which is hidden by the use of its name for the local variable. The value of the global i is now accessible only through the pointer external_i.

Storage-Class Specifiers for Internal-Level Declarations