C++ Primer, Fifth Edition

19.8.2. volatile Qualifier

Warning

The precise meaning of volatile is inherently machine dependent and can be understood only by reading the compiler documentation. Programs that use volatile usually must be changed when they are moved to new machines or compilers.

Programs that deal directly with hardware often have data elements whose value is controlled by processes outside the direct control of the program itself. For example, a program might contain a variable updated by the system clock. An object should be declared volatile when its value might be changed in ways outside the control or detection of the program. The volatile keyword ...