What is the volatile keyword useful for?

The volatile keyword in computer programming languages such as C, C++, C#, and Java is used to indicate that a variable's value may change between different accesses, even if it does not appear to be modified by the program itself. This keyword is particularly important in the context of multi-threaded programs where multiple threads may access and modify the same variable concurrently.

In C and C++, the volatile keyword is a type qualifier that tells the compiler not to optimize away subsequent reads or writes to a variable, ensuring that every access reflects the most current value. It is commonly used for memory-mapped I/O access, where hardware may change the state of a memory location independently of the program, and for certain uses in signal handlers and between setjmp and longjmp calls in C. However, it is important to note that volatile does not guarantee atomicity or proper synchronization between threads, and its use for thread synchronization is discouraged[1].

In Java and C#, volatile is used to ensure that a variable is read from and written to main memory, and that changes made by one thread are visible to others in a timely manner. This is particularly useful in cases where a variable is accessed by multiple threads without the use of synchronization primitives like synchronized blocks or methods. The volatile keyword in Java guarantees visibility and ordering, but not atomicity. It ensures that any read or write operation on a volatile variable happens directly to the main memory, bypassing any thread-specific caches[3][4].

In Java, the volatile keyword also establishes a "happens-before" relationship, which is a guarantee about memory visibility and the or...