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 the discussion “Storage Classes” for more information.
Table 2.1 Summary of Lifetime and Visibility
| Attributes: | Result:, | |||||
Level |
Item |
Storage-Class Specifier |
Lifetime |
Visibility |
||
| File scope | Variable definition | static | Global | Restricted to remainder of source file in which it occurs | |
| Variable declaration | extern | Global | Remainder of source file | ||
| Function prototype or definition | static | Global | Restricted to 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 program 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\n%d\n", i, j ); /* Prints 2, 3 */
{ /* Begin second nested block */
int i = 0; /* i is redefined: */
printf( "%d\n%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.