Volatile is usually applied to a variable when we are declaring it. The main reason behind using volatile keyword is that it is used to prevent optimizations on objects in our source code. Therefore, an object declared as volatile can't be optimized because its value can be easily changed by the code.

When should volatile be used?

A variable should be declared volatile whenever its value could change unexpectedly. In practice, only three types of variables could change: Memory-mapped peripheral registers. Global variables modified by an interrupt service routine.

Why is volatile used?

The volatile keyword is intended to prevent the compiler from applying any optimizations on objects that can change in ways that cannot be determined by the compiler. Objects declared as volatile are omitted from optimization because their values can be changed by code outside the scope of current code at any time.