Storage class

ABOUT STORAGE CLASSES IN C

1. BY, A.Amalajoylet Storage classes

2. STORAGE CLASSES The storage class determines the part of the memory where the variables would be stored. The storage class also determines the initial value of the variable. It is used to define the scope and lifetime of variable . There are two storage locations in computer, 1. CPU Registers 2.Memory. A value stored in CPU register can always be accessed faster than the one that is stored in memory.

3. Four types 1.Automatic storage class. 2.Register storage class. 3.Static storage class. 4.External storage class.

4. Automatic Keywords : Auto Storage : Memory Default initial value : Garbage value Scope : Local/Block scope Life : Exists as long as the control remains in the block

5. Example for Automatic void main() { auto x= 20 ; { auto x = 60 ; printf(" n x: %d ", x ); } printf(" nx: %d ", x ); } OUTPUT : x : 60 x : 20 NOTE : Two variables are declared in different blocks , so they are treated as different variables

6. Static Keywords : Static Storage : Memory Default initial value : 0 Scope : Local to the block in which the variable is defined. Life : Value of the variable persists between different function calls.

7. Difference between auto and static class /*auto class*/ #include < stdio.h > void add(); int  main() {   add();   add();   add();   add();   return 0 ; } void add() {   auto   int   i =1;   printf ("\ n%d",i );   i =i+1 ; OUTPUT:- 1 1 1 1 /* static class*/ #include < stdio.h > void add(); int  main() {   add();   add();   add();   add();    return 0 ; } void add() {    static   int   i =1;   printf ("\ n%d",i );   i =i+1; } OUTPUT:- 1 2 3 4

8. Previous program explanation If the storage class is static, then the statement static int i = 1 is executed only once, irrespective of how many times the same function is called . If the storage class is auto,then the statement auto int i =1 is executed each time where it increments and re-initialize the value of i = 1. The difference between them is that  static  variable do not disappear when the function is no longer active. There value persist. If control comes back to the same function again , the  static  variables have the same values they had last time around.

9. Register Keywords : Register. Storage : CPU register. Default initial value : Garbage value Scope : Local to the block in which the variable is defined. Life : Value of the variable persists till the control remains within the block in which the variable is defined.

10. Why we need Register Variable ? Whenever we declare any variable inside C Program then memory will be randomly allocated at particular memory location. We have to keep track of that memory location. We need to access value at that memory location using ampersand operator/ Address Operator   i.e (&). If we store same variable in the register memory then we can access that memory location directly without using the Address operator. Register variable will be accessed faster than the normal variable thus increasing the operation and program execution. Generally we use register variable as Counter. Note :  It is not applicable for arrays, structures or pointers.

11. External Keywords : Extern. Storage : Memory. Default initial value : 0 Scope : Global Life : Exists as long as variable is running Retains value within the function

12. Example for Extern int num = 75 ; void display(); void main() { extern int num ; printf (" nNum : % d",num ); display(); } void display() { extern int num ; printf (" nNum : % d",num ); } OUTPUT: Num : 75 Num : 75

13. Extern contd … Note : Declaration within the function indicates that the function uses external variable Functions belonging to same source code , does not require declaration (no need to write extern) If variable is defined outside the source code , then declaration using extern keyword is required

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