Storage-class specifiers affect the storage duration, linkage, or declaration behavior of an object or function. The commonly used C storage-class specifiers are auto, register, static, and extern.

#include <stdio.h>

int global_value = 10;

static int file_value = 20;

int main()
{
    auto int local_value = 30;
    register int counter = 40;

    printf("%d\n", global_value);
    printf("%d\n", file_value);
    printf("%d\n", local_value);
    printf("%d\n", counter);

    return 0;
}

Example Output #

10
20
30
40

Explanation #

auto declares an object with automatic storage duration. For a block-scope variable, automatic storage duration is already the default:

auto int local_value = 30;

register requests that the implementation keep the object in a processor register when appropriate:

register int counter = 40;

The compiler is not required to place the object in a register.

static has different effects depending on where it appears. At file scope, it gives an object or function internal linkage:

static int file_value = 20;

extern declares an object or function that is defined elsewhere or later in the program:

extern int global_value;

Storage-Class Specifiers #

The meaning of static depends on declaration scope, so it is covered separately in the storage-class and linkage section.