volatile const int meaning in C?

Question

I am reading a book on C. It mentions a concept which does not make sense to me. What I know is that the compiler sometimes optimises code by removing variables which it feels would not affect an expression. So if we specify volatile to a variable, it does not remove this variable from any expression.

So the book introduces something like this:

volatile const int a = 1;

The explaination is only a couple of lines, none of which makes sense to me. Can somebody please explain why would be there a need of such a variable?

PS: I understand the concept of volatile, what I dont understand is the concept of volatile const.

Answer 1

A volatile const variable is one whose value may change due to external influences, and which cannot be written to.

Answer 2

Volatile const means value cannot be changed programmatically but value can be changed indirectly, for eg if the variable is mapped to device register, then value can be modified by the device.