extern Storage-Class Specifier

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.

Example

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 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.

See also

Storage-Class Specifiers for Internal-Level Declarations