This is the default storage class for all the variables. Auto variables can be only accessed within the block/function.
2. extern
the variable is defined elsewhere and not within the same block where it is used, this can be overwritten in a different block as well. So an extern variable is nothing but a global variable.
3. static
initialized only once and exist till the termination of the program, Their scope is local to the function to which they were defined.
4. register
declares same functionality as that of the auto variables, difference is that the compiler tries to store these variables in the register of the microprocessor.
