Summary of Lifetime and Visibility

Table 2.1 is a summary of lifetime and visibility characteristics for most identifiers. The first three columns give the attributes that define lifetime and visibility. An identifier with the attributes given by the first three columns has the lifetime and visibility shown in the fourth and fifth columns. However, the table does not cover all possible cases. Refer to Storage Classes in Chapter 3 for more information.

Table 2.1 Summary of Lifetime and Visibility

Attributes: Level	Item	Storage-Class Specifier	Result: Lifetime	Visibility
File scope	Variable definition	static	Global	Remainder of source file in which it occurs
	Variable declaration	extern	Global	Remainder of source file in which it occurs
	Function prototype or definition	static	Global	Single source file
	Function prototype	extern	Global	Remainder of source file
Block scope	Variable declaration	extern	Global	Block
	Variable definition	static	Global	Block
	Variable definition	auto or register	Local	Block

The following example illustrates blocks, nesting, and visibility of variables:

#include <stdio.h>

int i = 1;               /* i defined at external level               */

int main()               /* main function defined at external level   */
{
    printf( "%d\n", i ); /* Prints 1 (value of external level i)      */
    {                            /* Begin first nested block          */
        int i = 2, j = 3;        /* i and j defined at internal level */
        printf( "%d %d\n", i, j );     /* Prints 2, 3                 */
        {                        /* Begin second nested block         */
            int i = 0;           /* i is redefined                    */
            printf( "%d %d\n", i, j ); /* Prints 0, 3                 */
        }                        /* End of second nested block        */
        printf( "%d\n", i );     /* Prints 2 (outer definition        */
                                 /*  restored)                        */
    }                            /* End of first nested block         */
    printf( "%d\n", i );         /* Prints 1 (external level          */
                                 /* definition restored)              */
    return 0;
}

In this example, there are four levels of visibility: the external level and three block levels. The values are printed to the screen as noted in the comments following each statement.