Lesson 09·C Programming

Storage Classes

Determining scope, lifetime, memory placement, and initial value of variables

What is a Storage Class?

'Storage' refers to the scope of a variable and the physical memory allocated by the compiler. A storage class defines two essential properties: the lifetime of the variable and its scope of visibility.

Variables can reside in two physical locations: Main Memory (RAM) or high-speed CPU Registers.

1. Automatic Storage Class (auto)

Keyword

auto

Storage Location

Main Memory (Stack)

Initial Value

Garbage (unpredictable)

Scope

Local to the declaring block

Life

Till control remains within the block

C Example

#include <stdio.h>

void main() {
    auto int i = 10;
    {
        auto int i = 20;
        printf("Inner block i: %d\n", i);
    }
    printf("Outer block i: %d\n", i);
}

Program Output

Inner block i: 20
Outer block i: 10

2. Register Storage Class (register)

Variables stored in CPU registers can be accessed substantially faster than those in RAM:

Keyword

Storage Location

CPU Registers

Initial Value

Garbage

Scope

Local to the declaring block

Life

Till control remains within the block

C Example

#include <stdio.h>

void main() {
    register int counter;
    for (counter = 1; counter <= 3; counter++) {
        printf("Counter: %d\n", counter);
    }
}

Program Output

Counter: 1
Counter: 2
Counter: 3

3. Static Storage Class (static)

Static variables preserve their values between multiple function invocations:

Keyword

static

Storage Location

Main Memory (Data Segment)

Initial Value

Zero (0) automatically

Scope

Local to the block where declared

Life

Persists throughout the entire program

C Example

#include <stdio.h>

void incre(void) {
    int avar = 1;         // Automatic variable: re-initialized every call
    static int svar = 1;  // Static variable: initialized once only

    avar++;
    svar++;

    printf("Automatic variable: %d | Static variable: %d\n", avar, svar);
}

void main() {
    int i;
    for (i = 0; i < 3; i++) {
        incre();
    }
}

Program Output

Automatic variable: 2 | Static variable: 2
Automatic variable: 2 | Static variable: 3
Automatic variable: 2 | Static variable: 4

4. External Storage Class (extern)

External variables are declared outside functions and can be referenced globally:

Keyword

extern

Storage Location

Main Memory

Initial Value

Zero (0)

Scope

Global (accessible across program files)

Life

Until the program terminates

C Example

#include <stdio.h>

int i = 20;  // Global variable definition

void show(void) {
    printf("In show(): %d\n", i);
}

void main() {
    printf("In main(): %d\n", i);
    show();
}

Program Output

In main(): 20
In show(): 20