What is the "volatile" keyword used for?

Consider this example:

int i = 5;
System.out.println(i);

The compiler may optimize this to just print 5, like this:

System.out.println(5);

However, if there is another thread which can change i, this is the wrong behaviour. If another thread changes i to be 6, the optimized version will still print 5.

The volatile keyword prevents such optimization and caching, and thus is useful when a variable can be changed by another thread.

For both C# and Java, "volatile" tells the compiler that the value of a variable must never be cached as its value may change outside of the scope of the program itself. The compiler will then avoid any optimisations that may result in problems if the variable changes "outside of its control".

To understand what volatile does to a variable, it's important to understand what happens when the variable is not volatile.

• Variable is Non-volatile

When two threads A & B are accessing a non-volatile variable, each thread will maintain a local copy of the variable in it's local cache. Any changes done by thread A in it's local cache won't be visible to the thread B.

• Variable is volatile

When variables are declared volatile it essentially means that threads should not cache such a variable or in other words threads should not trust the values of these variables unless they are directly read from the main memory.

So, when to make a variable volatile?

When you have a variable which can be accessed by many threads and you want every thread to get the latest updated value of that variable even if the value is updated by any other thread/process/outside of the program.