What are the different storage classes in C ?

A storage class defines the scope and life time of variables. 
There are four storage classes:

• Auto Storage class
• Register Storage class
• Static Storage class
• Extern Storage class

1.auto - Storage Class
auto storage class is the default for local variables.
    example : 
       {
            int a;
            auto int M;
}
The example above defines two variables with the same storage class. Auto Storage class  only be used within the functions, i.e. local variables.


2.register - Storage Class
register Storage Class is used to define local variables and to stored in a register instead of RAM. 
{
            register int  Miles;
}
Register must be used for variables that need quick access - such as counters.

3.static - Storage Class
static storage class is the default for global variables.

static int Count;
        int Road;

        {
            printf("%d\n", Road);
        }
static variables are 'visible' to all functions in this source file. 
The variable with static storage class will retain its value during various call.

   void func(void);
   
   static count=10; /* Global variable - static storage class*/
   
   main()
   {
     while (count--) 
     {
         func();
     }
   
   }
   
   void func( void )
   {
     static i = 5;
     i++;
     printf("i is %d and count is %d\n", i, count);
   }
   
   This will produce following result
   
   i is 6 and count is 9
   i is 7 and count is 8
   i is 8 and count is 7
   i is 9 and count is 6
   i is 10 and count is 5
   i is 11 and count is 4
   i is 12 and count is 3
   i is 13 and count is 2
   i is 14 and count is 1
   i is 15 and count is 0

4.extern - Storage Class
extern storage class is used to give a reference of a global variable.
'extern' variable cannot be initialized.