How to illustrate usage of volatile keyword in C#

Question

I would like to code a little program which visually illustrates the behavior of the 'volatile' keyword. Ideally, it should be a program which performs concurrent access to a non volatile static field and which get incorrect behavior because of that.

Adding the volatile keyword in the same program should fix the problem.

That something I didn't manage to achieve. Even trying several times, enabling optimization, etc., I always get a correct behavior without the 'volatile' keyword.

Do you have any idea about this topic ? Do you know how to simulate such a problem in a simple demo app ? Does it depend on hardware ?

Answer 1

Yes, it's hardware dependent (you are unlikely to see the problem without multiple processors), but it's also implementation dependent. The memory model specifications in the CLR spec permit things which the Microsoft implementation of the CLR do not necessarily do. The best documentation I've seen on the volatile keyword is this blog post by Joe Duffy. Note that he says the MSDN documentation is "highly misleading."

Answer 2

It's not really a matter of a fault happening when the 'volatile' keyword isn't specified, more that an error could happen when it hasn't been specified. Generally you are going to know when this is the case better than the compiler!

The easiest way of thinking about it would be that the compiler could, if it wanted to, inline certain values. By marking the value as volatile, you are telling yourself and the compiler that the value may actually change (even if the compiler doesn't think so). This means the compiler should not in-line values, keep cache or read the value early (in an attempt to optimize).

This behaviour isn't really the same keyword as in C++.

MSDN has a short description here. Here is a perhaps a more in depth post on the subjects of Volatility, Atomicity and Interlocking

Answer 3

It's hard to demonstrate in C#, as the code is abstracted by a virtual machine, thus on one implementation of this machine it work right without volatile, while it might fail on another one.

The Wikipedia has a good example how to demonstrate it in C, though.

The same thing could happen in C# if the JIT compiler decides that the value of the variable cannot change anyway and thus creates machine code that doesn't even check it any longer. If now another thread was changing the value, your first thread might still be caught in the loop.

Another example is Busy Waiting.

Again, this could happen with C# as well, but it strongly depends on the virtual machine and on the JIT compiler (or interpreter, if it has no JIT... in theory, I think MS always uses a JIT compiler and also Mono uses one; but you might be able to disable it manually).

Answer 4

I achieve really working example!

The main idea received from wiki, but with some changes for C#. The wiki article demonstrates this for static field of C++, it is looks like C# allways carefully compile requests to static fields... and i make example with non static one:

If you run this example in RELEASE mode and WITHOUT DEBUGGER (Ctrl+F5) than a line 'while (test.foo != 255)' will be optimized to 'while(true)' and this program never returns. But after adding 'volatile' keyword, you always get 'OK'.

class Test
{
    int foo;

    static void Main()
    {
        var test = new Test();

        new Thread(delegate() { Thread.Sleep(500); test.foo = 255; }).Start();

        while (test.foo != 255) ;
        Console.WriteLine("OK");
    }
}