Storage classes in C with examples (auto, extern, register, static)

Storage classes in C with examples (auto, extern, static, register)

When you declare a variable in C programming, you may additionally specify the variable’s lifetime or storage specification. In other words, the storage classes of variables in C specify the scope of variables, memory allocation and default value.

Variable Lifetime: The variable’s lifetime indicates how long it will be active during execution. If you define a variable inside a function, the value of the variable will be preserved until the function is run.

How many storage classes in C? In the C program, we have four separate storage classes.

auto
extern
static
register

Also Read: Basic of C Programming

Table of Contents

auto storage classes in c

The auto storage class in c is the default type of storage. When you use auto to declare a variable, its visibility is confined to the block or function in which it is declared.

When you declare a variable using auto, memory allocation is done for the variable when the function is called, and memory de-allocation id done after the function has completed its execution.

Because every local variable in C is auto by default if you don’t use the auto keyword, variable will be auto variable.


#include <stdio.h>

int main() {
  int intVar;
  char charVar;
  float floatVar;
  printf("Int Value: %d\nChar Value: %c\nFloat Value: %f",intVar, charVar, floatVar); 
  return 0;
}

Output:
Int Value: 0
Char Value: 
Float Value: 0.000000

Fast access is achieved by allocating registers, which is often done for loop counts. 
A global register variable cannot be declared.

extern storage classes in c

When we need to refer to a function or variable that we have defined elsewhere or will implement later, we use extern storage class.

An extern variable is just like a global variable.


#include<stdio.h>

int a = 15;
int main() {
  extern int b;
  printf("The value of a is %d", a);
  printf("\nThe value of b is %d", b);
  return 0;
}
int b = 20;

Output:
The value of a is 15
The value of b is 20

register storage classes in c

When you use register to declare a variable, that value of variable is saved in a register. If you need to access a variable several times and need it to be quick, register is the best way to go.

The time it takes to access a register is substantially shorter than it takes to reach main memory.

Because the CPU has a limited amount of registers, the allocation of variables to the registers cannot be guaranteed.

Example: A loop counter, such variable should be stored in register storage class.


#include <stdio.h>

int main() {
  register int i = 0;

  for (i = 0; i < 5; i++) {
    printf("Value of i is %d\n", i);
  }
  return 0;
}

Output:
Value of i is 0
Value of i is 1
Value of i is 2
Value of i is 3
Value of i is 4

static storage classes in c

The value of a static variable persists until the end of the program if it is declared with a static.


#include<stdio.h>

void showValue();
int main() {
  showValue();
  showValue();
  showValue();
  return 0;
}
void showValue() {
  static int x = 10;
  printf("%d ", x);
  x = x + 10;
}