Whereas the qualifier const tells the compiler “This never changes” (which allows the compiler to perform extra optimizations), the qualifier volatile tells the compiler “You never know when this will change,” and prevents the compiler from performing any optimizations based on the stability of that variable. Use this keyword when you read some value outside the control of your code, such as a register in a piece of communication hardware. A volatile variable is always read whenever its value is required, even if it was just read the line before.

A special case of some storage being “outside the control of your code” is in a multithreaded program. If you’re watching a particular flag that is modified by another thread or process, that flag should be volatile so the compiler doesn’t make the assumption that it can optimize away multiple reads of the flag.

Note that volatile may have no effect when a compiler is not optimizing, but may prevent critical bugs when you start optimizing the code (which is when the compiler will begin looking for redundant reads).

The const and volatile keywords will be further illuminated in a later chapter.