Volatile vs. Non-Volatile Variables in C

This page details the differences between volatile and non-volatile variables, with a focus on the volatile keyword in the C programming language.

Volatile Keyword in C

When a variable is declared with the volatile keyword in a C program, it gains special properties compared to regular, non-volatile variables. Specifically, volatile variables are not altered by the compiler during optimization.

In simpler terms, the volatile keyword prevents the compiler from optimizing the variable. This is crucial in situations where the variable’s value can change unexpectedly, outside the normal flow of the program’s execution. For example, this could be due to:

• Hardware interactions (e.g., reading data from a sensor).
• Interrupt handlers.
• Shared memory in a multi-threaded environment.

The volatile keyword is also present in other programming languages, including C++, C#, and Java. It can be declared either before or after the data type:

volatile int employee_ID;
int volatile employee_ID;


Both declarations are equivalent and tell the compiler that `employee_ID` is a volatile variable. The compiler will then ensure that every read and write operation to this variable is performed directly from/to memory, bypassing any potential optimizations that might rely on cached values. This prevents the program from using stale values that it may have stored in a register.

## When to use `volatile`

The primary use case for `volatile` is when a variable's value might be changed by something that's outside the control of the current compilation unit, such as:

- An interrupt routine.
- Another thread.
- Hardware.

Without the `volatile` keyword, the compiler might optimize the code in a way that assumes the variable's value only changes within the program's normal control flow. This could lead to incorrect or unpredictable behavior.