What Is Storage Classes In C?

Storage classes decides the scope of variables (local or global), storage(in register or RAM) where it will be stored, default value(zero or Garbage) and their lifetime (in a bock where it is declared).

Automatic Storage Classes

auto keyword is used to define automatic storage class for variables. automatic storage class is default for all variables, don't need to do that.

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char const *argv[])
{
    int a;  // auto storage class 
    // auto int a; same as above
    printf("The value of %d\n",a);   // default value is garbage
    return 0;
}

Register Storage Class

register keyword is used to define a variable in CPU register instead of RAM. It's default value is garbage. Frequenlty used variables may be define as register storage class. register keyword is maily used for char and int data type only.

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char const *argv[])
{
    register int a;  // register storage class 
    printf("The value of %d\n",a);   // default value is garbage
    return 0;
}

Static Storage Class

static keyword is used to create a variable static storage class that is alive whole life of main program and it's default value is zero.

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char const *argv[])
{
    static int a; // static storage class
    a = 10;
    printf("The value of %d\n", a); // default value is 0
    return 0;
}

External Storage Class

external keyword is used to access external storage class's variables in a function or a block. when we declare a variables outside of main function that will be a external storage class. It's scope is global.

#include <stdio.h>
#include <stdlib.h>
int number = 50; //global variable

int main(int argc, char const *argv[])
{
    int number = 34;
    {
        extern int number;                                        // access external storage class variable
        printf("The value number inside a block is %d \n", number); // global variable
    }
    printf("The value of number is %d \n", number); // local variable
    return 0;
}