int const x;
volatile
The volatile data-type modifier prevents an object from being
stored in a machine register, forcing it to be allocated in memory.
This data type modifier is useful for declaring data that is to be
accessed asynchronously. A device driver application often uses
volatile data storage. Like const, you can specify the volatile
data-type modifier with any of the storage-class specifiers or
modifiers with the exception of the register storage class.