C - Storage Classes

ü Storage Classes

Features of any variables is defined by its storage class of that variable. Compiler stores the variables and it is defined by the storage class that where the compiler will store the variable, its initial value, scope and lifetime.

Storage classes in C: -

·         auto

·         extern

·         static

·         register

Now, let’s have a look on each of these storage classes closely.

·       auto

Variables which belongs to ‘auto’ storage class are defined within a function or a block. These variables are by default assigned some garbage value and as they are local to the function, these variables are also known as Local Variables. These variables can only be accessed inside that function in which they are declared. While declaring these variables there is no need to put ‘auto’ as these are by default auto.

Example:

#include <stdio.h>

#include<conio.h>

int sum(int n1, int n2)

{

  auto int s;        //declaration of auto (local) variable

  s = n1+n2;

  return s;

}

int main()

{

  int i = 2, j = 3, k;

  k = sum(i, j);

  printf("sum is : %d\n", k);

  return 0;

}

Output:

Here, The variable ‘s’ can only be used inside the function as it is declared inside the function ‘sum’. Also, there is no need to put auto while declaring it.

·     extern

Keyword extern is written before a variable so that compiler can know that this variable was declared somewhere else. Basically, ‘extern’ keyword is written before any variable to tell us that this variable is global variable which is declared in some other program file.

Now, let’s have a look on what actually happens.

We all know what global variable is. ‘Global Variable’ is a declared outside of all the functions. Global variable can be from anywhere in the program and its value can be changed anytime within any function as follows:

#include <stdio.h>

#include <conio.h>

int g;

void print()

{

  g = 10;

  printf("g = %d\n", g);

}

int main()

{

  g = 7;

  printf("g = %d\n", g);

  print();

  return 0;

}

Output:

Here, we can see that ‘g’ is the global variable which is defined outside of every functions. Its value was assigned 7 and in the print function as 10.

When we declare global variable, some space is allocated to it in memory like all other variables. A value can be assigned to it in the same program in which it was declared as we did in the example. But is it possible to use it or assign it a value in any other program file?

It can be done by using extern keyword as shown below.

firstfile.c

int g = 0;

We declared a global variable ‘g’ in the first program file firstfile.c

Now, in a header file firstfile.h declare this variable ‘g’ and after that include it in the other file in which we want to use this variable.

firstfile.h

extern int g;

In order to use global variable ‘g’, in the second program file secondfile.c, we will have to include header file in it by writing #include “firstfile.h”. Here to the variable ‘g’ we assign a value 4 and thus in this program the value of ‘g’ becomes 4.

secondfile.c

#include "firstfile.h"
main(){
  g = 4;
  printf("%d", g);
}

·     static

A variable declared as static once initialized, exists till the end of the program. If a static variable is declared inside a function, it remains into existence till the end of the program and not get destroyed as the function exists (as in auto). If a static variable is declared outside all the functions in a program, it can be used only in the program in which it is declared and is not visible to other program files(as in extern).

Let's see an example of a static variable.

If the variable is declared as ‘static’ is initialized, it exits till the end of the program. A static variable remains in the program and not get destroyed as the function exists, if that variable is declared inside a function.

If a static variable is declared outside of all the functions in a program, it can be used only in the program in which it is not visible to the other program files and in which it is declared.

Now, let’s have a look on an example of a static variable.

#include <stdio.h>

#include<conio.h>

 static int g = 5;

 void fn()

{

               static int i = 0;

               printf("g = %d\t", g--);

               printf("i = %d\n",i++);

 }

int main()

{

  while(g >= 2)

  fn();

  return 0;

}

Output:

Here, static variables are ‘g’ and  'i' in which 'i' is a local variable and ‘g’ is global variable. If before the declaration of 'i' we had not written ‘static’, then every time the function ‘fn()’ would have been called, every time 'i' would have been declared with an initial value of 0 it would also have got destroyed as the function ‘fn()’ would exists.

·     Register

Register let the compiler know that the variable will get stored in a register instead of memory (RAM). A register variable can be accessed faster than a normal variable.  Not all the registers defined as register will get stored in a register as it depends on several restrictions of hardware and implementation.

The address of such variables cannot be accessed since these do not have a memory location as which is clear by the following example:

#include <stdio.h>

#include <conio.h>

int main()

{

    register int n = 20;

    int *ptr;

    ptr = &n;

    printf("address of n : %u", ptr);

    return 0;

}