Storage Classes are associated with variables for describing the features of any variable or function in the C program. These storage classes deal with features such as scope, lifetime and visibility which helps programmers to define a particular variable during the program's runtime. These storage classes are preceded by the data type which they had to modify.

There are four storage classes types in C:
  • auto
  • register
  • static
  • extern

auto Storage Class

auto comes by default with all local variables as its storage class. The keyword auto is used to define this storage class explicitly

int roll; // contains auto by default

is the same as:

auto int roll;    // in addition, we can use auto keyword

The above example has a variable name roll with auto as a storage class. This storage class can only be implemented with the local variables.

register Storage Class

This storage class is implemented for classifying local variables whose value needs to be saved in a register in place of RAM (Random Access Memory). This is implemented when you want your variable the maximum size equivalent to the size of the register. It uses the keyword register.

register int  counter;

Register variables are used when implementing looping in counter variables to make program execution fast. Register variables work faster than variables stored in RAM (primary memory).

for(register int counter=0; counter<=9; counter++)
// loop body

static storage class

This storage class uses static variables that are used popularly for writing programs in C language. Static variables preserve the value of a variable even when the scope limit exceeds. Static storage class has its scope local to the function in which it is defined.

On the other hand, global static variables can be accessed in any part of your program. The default value assigned is '0' by the C compiler. The keyword used to define this storage class is static.

static int var = 6;

extern Storage class

The extern storage class is used to feature a variable to be used from within different blocks of the same program. Mainly, a value is set to that variable which is in a different block or function and can be overwritten or altered from within another block as well. Hence it can be said that an extern variable is a global variable which is assigned with a value that can be accessed and used within the entire program. Moreover, a global variable can be explicitly made an extern variable by implementing the keyword 'extern' preceded the variable name.

Here are some examples of extern:

#include <stdio.h>

int val;
extern void funcExtern();
   val = 10;

Another example:

#include <stdio.h>

extern int val;         // now the variable val can be accessed and used from anywhere

// within the program
void funcExtern() 
   printf("Value is: %d\n", val);