C: Difference between constant and volatile qualifier?

Question

Also can we declare a variable as "constant volatile" ? if yes then what may be the possible reason to do so?

Answer 1

Constant: Constant means the value of the variable can not be changed in the program.
Volatile: Volatile tells the compiler not to optimize anything that has to do with the volatile variable.

Now coming to the second problem check the following statement

const volatile int T=10;

const qualifier means that the T cannot be modified through code. If you attempt to do so the compiler will provide a diagnostic. Volatile still means that compiler cannot optimize or reorder access to T.

Practical Usage:
Accessing shared memory in read-only mode.
Accessing hardware registers in read-only mode.

Answer 2

constant means values is not change but volatile values is changes in any time.

String str="ramsevak" values in not change this is a constant values of str

but int a=10;
a=a+10;
the values is changes so a is a volatile.