Storage Classes in C

Storage Classes :

In C all variables have a data type, they also have a storage classes .the following variable storage classes are most relevant to functions :

1. automatic variables

2. external variables

3. static variables

4. register variables

The scope of variable determines over what region of the program a variable is actually available for use

Longevity refers to a period during which a variable retains a given value during execution of a program. The visibility refers to the accessibility of a variable from the memory.

The variables may also be broadly categorized , depending on the place of their declaration ,as internal or external. internal variables are those which are declared with in a particular function ,while external variables are declared outside of any function.

AUTOMATIC VARIABLES :

·         Automatic variables are declared inside a function in which they are to be utilized .

·         They are created when the function is called and destroyed automatically when the function is exited .

·         These are private to a function in which they are declared  because of this property , automatic variables are also referred to be as local or internal variable .

·         A variable declared inside a function without storage classes specifications is, by default, an automatic variable .

Example:

main()

{

int number ;   /* by default number is automatic variable*/

…..

…..

}

We may also use a keyword auto to declare automatic variables

main ()

{

  auto int number;

--

--

}

External variables:

·         These  are both  alive and active through out the entire program are known as external variables.

·         They are also known as global variables.

·         They can be accessed by any function in the program.

·         External variables are declared out side the function.

Example:

   int  number;

  float  length=7.5;

  main()

{

   ---

   ----

}

function1()

{

----

}

External Declaration :

  main()

{

 ----

extern  int  y;   /* external declaration */

  ---

 ----

}

function1()

{

extern   int  y; /* external declaration */

--

}

int  y;  /*  definition*/

·         The External Declaration  of y inside the functions informs the compiler that y is an integer type defined somewhere else in the program.

·         Extern declaration does not allocate storage space for variables. In case of arrays, the definition should include their size as well.

Static  Variables :

·         Static  variables persists until the end of the program. A variable can be declared static using the key word static.

static  int  x;

·         A Static  Variable may be either an internal type or external type depending on the place of declaration.

·         Internal static variables are those which are declared inside the function.

·         The scope of internal static variables extend up to the end of the function in which they are defined. Therefore, they are similar to auto variables.

·         They remain in existence (alive) through out the remainder of the program . It can be used to count the number of calls made to a function.

·         An external static variable is declared outside of all functions and is available to all the functions in that program.

·         Ex:  

void   ex_of_static ()

{

   static int  m=0;

   m++;

   printf(“m in function =%d”, m);

}

        

void main()

            {

               static int n=5;

               printf(“ n in main = %d”, n);

               ex_of_static();

                n++;

                printf(“ n in main = %d”, n);

ex_of_static();

           }

OUTPUT:

  n in main = 5

  m in function = 1

   n in main = 6

  m in function = 2

 

Register variables :

·         We can tell the compiler that a variable should be kept in one of the machine’s registers instead of keeping in the memory where normal variables are stored.

·         Register variables can be accessed  faster than memory variables.

·         Most of the compilers allow only int or char variables to be placed in the register.

·         Ex : register int x;