What is the volatile keyword in C ?
volatile tells the compiler that the value of the variable may change at any time, preventing the compiler from optimizing access. 1. Used for hardware registers, signal handlers, or shared variables between threads (but modern C recommends std::atomic). 2. Each access is directly read and write memory instead of cached to registers. 3. It does not provide atomicity or thread safety, and only ensures that the compiler does not optimize read and write. 4. Constantly, the two are sometimes used in combination to represent read-only but externally modifyable variables. 5. It cannot replace mutexes or atomic operations, and excessive use will affect performance.
The volatile
keyword in C is used to tell the compiler that a variable's value can change at any time — even outside the current code flow. This means the compiler should not optimize accesses to that variable, because doing so could lead to incorrect behavior.

You typically see volatile
used when dealing with hardware registers, signal handlers, or variables shared between threads (though for the latter, modern C offers better tools like std::atomic
).

What does volatile do exactly?
When you declare a variable as volatile
, the compiler assumes that any read or write to that variable must actually happen — it can't be cached in a register or reordered for optimization purposes. So every access goes directly to memory.
For example:

volatile int status_flag;
Here, every time status_flag
is accessed, the program will read its actual value from memory instead of assuming what it might be based on previous operations.
This helps prevent bugs in scenarios like:
- Memory-mapped I/O where hardware changes values behind the scenes.
- Variables modified by an interrupt service routine.
- Shared memory in certain low-level concurrency situations (though again, prefer
std::atomic
these days).
When should you use volatile?
Use volatile
when working with:
- Hardware registers – such as those in embedded systems where memory-mapped devices update values independently.
- Memory shared with other threads or processes without using synchronization primitives — though this is tricky and often not sufficient on its own.
- Signal handlers – if a variable is changed inside a signal handler and used elsewhere in the program.
Keep in mind: volatile
does not provide atomicity or thread safety. It only ensures that the compiler doesn't optimize away reads and writes.
So if you're writing multithreaded code, prefer types like std::atomic<T>
over volatile
.
How is volatile different from const?
While const
tells the compiler a variable shouldn't change, volatile
says the opposite — that it might change at any time. Sometimes you'll even see both together:
volatile const int sensor_value;
This would be used for something like a read-only hardware register whose value changes on its own.
Also, note that const volatile
combinations are more common in device drivers or real-time systems where a value is meant to be read-only from the program's perspective but still subject to external updates.
Some gotchas with volatile
- It doesn't replace mutexes or atomics. If two threads modify a
volatile
variable without synchronization, you still get a race condition. - It doesn't stop all optimizations. It prevents caching in registers and some reorderings, but not all concurrency-related issues.
- Misuse can hurt performance. Since the compiler can't optimize access to
volatile
variables, excessive use may slow your code down unnecessarily.
So basically, use volatile
when you need to interact with memory that can be updated asynchronously — but don't expect it to handle synchronization or thread safety for you.
Basically that's it.
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