why volatile is not thread safe?

volatile is not thread-safe on its own because it only guarantees visibility of a variable's value across threads, not atomicity for compound operations.
1. Visibility vs. Atomicity
Visibility: The volatile keyword ensures that when one thread writes to a volatile variable, the change is immediately flushed to main memory and visible to all other threads, preventing them from using a stale, locally-cached value.
Atomicity: An atomic operation is an indivisible unit of work that cannot be interrupted. volatile only guarantees that single reads or single writes are atomic. It does not provide mutual exclusion, meaning multiple threads can still access and attempt to modify the variable simultaneously.
2. The "Read-Modify-Write" Problem
The primary reason volatile is insufficient for general thread safety is the "read-modify-write" problem, which involves a compound operation. For example, the operation count++ looks like a single action but actually involves three distinct steps:
Read the current value of count.
Increment the value (in a CPU register or local cache).
Write the new value back to main memory.
In a multithreaded environment with only volatile, a race condition can occur:
Thread A reads count (e.g., as 10).
Thread B simultaneously reads count (also as 10).
Thread A increments its local copy to 11 and writes it back to main memory.
Thread B increments its local copy to 11 and writes it back to main memory.
The final value is 11, not the expected 12, because the increment operation was not atomic.
3. When volatile is sufficient
volatile is sufficient in specific, restricted scenarios:
Status flags: A common use case is a volatile boolean flag to signal a worker thread to stop running.
Single-writer, multiple-reader scenarios: If only one thread ever writes to a variable, and other threads only read it, volatile is enough to ensure visibility.
For most other multithreading scenarios that require both visibility and atomicity for compound actions, you should use stronger synchronization mechanisms like:
The synchronized keyword (in Java)
std::atomic variables (in C++)
AtomicInteger, AtomicReference, etc. classes (in Java's java.util.concurrent package)
In essence, volatile prevents the compiler from making optimizations that would hide changes from other threads, but it does not manage concurrent access in the way synchronization primitives do.