volatile                Volatile Data Type Modifier

    The keyword volatile is used in declarations to ensure that the
    object it applies to is not subjected to certain optimizations by the
    compiler. Volatile identifies the object as being outside the control
    of this program, meaning that it may be set and tested
    asynchronously.  The volatile keyword warns the compiler not to make
    assumptions concerning the value of the object while evaluation
    expressions, since it could change at any time.  The keyword is
    useful when writing device drivers, interrupt handlers, or other
    tasks that map into shared memory.

      Notes:    The volatile keyword is a recent addition to the language
                and is not universally supported by compilers.