What is the use of 'volatile' in C?

The use of 'volatile' in C is primarily related to variables that may be altered by external factors outside the control of the program, such as hardware devices or concurrent threads. When a variable is declared as volatile, it informs the compiler that the value of the variable can change at any time, and therefore, the compiler must always fetch the variable's value from memory rather than using a cached copy. This is critical in scenarios where the variable is modified by an interrupt service routine or accessed by multiple threads, ensuring that the most up-to-date value is always used.

In this context, other options represent misconceptions about the 'volatile' keyword. While a variable may change in ways that are outside the direct scope of the code—such as through hardware registers or different threads—options suggesting that it indicates unchangeability, requires ignoring optimizations, or defines constants do not accurately reflect the purpose of 'volatile'. The primary role is to ensure that the program behaves correctly in the presence of such external changes.