A storage class represents the visibility and a location of a variable. It tells from what part of code a variable can be accessed.
A storage class is used to describe the following things:
- The variable scope.
- The location where the variable will be stored.
- The initialized value of a variable.
- A lifetime of a variable.
A storage class is used to represent the information about a variable.
There are total four types of standard storage classes. The table below represents the storage classes in 'C'.
Storage class |
Purpose |
auto |
It is a default storage class. |
extern |
It is a global variable. |
static |
It is a local variable which is capable of
returning a value even when control is transferred to the function call. |
register |
It is a variable which is stored inside a
Register. |
1. Auto storage class
The scope of an auto variable is limited with the particular block only. Once the control goes out of the block, the access is destroyed.
A keyword auto is used to define an auto storage class. By default, an auto variable contains a garbage value.
Example, auto int age;
2. Extern storage class
Extern stands for external storage class. Extern storage class is used when there are global functions or variables which are shared between two or more files.
Keyword extern is used to declaring a global variable or function in another file to provide the reference of variable or function which have been already defined in the original file.
The variables defined using an extern keyword are called as global variables. These variables are accessible throughout the program.
Example, extern void display();
Ex:
First File: main.c
#include <stdio.h>
extern i;
main() {
printf("value of the external integer is = %d\n", i);
return 0;}
Second File: original.c
#include <stdio.h>
i=48;
3. Static storage class
The static variables are used within function/ file as local static variables. They can also be used as a global variable
- Static local variable is a local variable that retains and stores
its value between function calls or block and remains visible only to the
function or block in which it is defined.
- Static global variables are global variables visible only to the file in which it is declared.
Global variables are accessible throughout the file whereas static variables are accessible only to the particular part of a code.
The lifespan of a static variable is in the entire program code. A variable which is declared or initialized using static keyword always contains zero as a default value.
Example: static int count = 10;
#include <stdio.h> /* function declaration
*/
void next(void);
static int counter = 7; /* global variable */
main() {
while(counter<10) {
next();
counter++; }
return 0;}
void next( void ) { /* function definition */
static
int iteration = 13; /* local static variable */
iteration ++;
printf("iteration=%d and counter= %d\n", iteration, counter);}
4. Register storage class
Register storage class is used when local variables are to be stored within functions or blocks in CPU registers instead of RAM to have quick access to these variables.
The keyword register is used to declare a register storage class. The variables declared using register storage class has lifespan throughout the program.
It is similar to the auto storage class. The variable is limited to the particular block. The only difference is that the variables declared using register storage class are stored inside CPU registers instead of a memory. Register has faster access than that of the main memory.
Example: register int age;
Principal features of each storage class which
are commonly used in C programming are :
Storage Class Declaration Storage Default Initial Value Scope Lifetime auto Inside a function/block Memory Unpredictable Within the function/block Within the function/block register Inside a function/block CPU Registers Garbage Within the function/block Within the function/block extern Outside all functions Memory Zero Entire the file and other files where the
variable is declared as extern program runtime Static (local) Inside a function/block Memory Zero Within the function/block program runtime Static (global) Outside all functions Memory Zero Global program runtime
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