CODEWITHSUNDEEP
c#multithreadingthread-safety
Kiril
Kiril

Reputation: 40345

Is it safe to use a boolean flag to stop a thread from running in C#?

My main concern is with the boolean flag... is it safe to use it without any synchronization? I've read in several places that it's atomic (including the documentation).

class MyTask
{
    private ManualResetEvent startSignal;
    private CountDownLatch latch;
    private bool running;

    MyTask(CountDownLatch latch)
    {
        running = false;
        this.latch = latch;
        startSignal = new ManualResetEvent(false);
    }

    // A method which runs in a thread
    public void Run()
    {
        startSignal.WaitOne();
        while(running)
        {
            startSignal.WaitOne();
            //... some code
        }
        latch.Signal();
    }

    public void Stop()
    {
        running = false;
        startSignal.Set();
    }

    public void Start()
    {
        running = true;
        startSignal.Set();
    }

    public void Pause()
    {
        startSignal.Reset();
    }

    public void Resume()
    {
        startSignal.Set();
    }
}

Is this a safe way to design a task in this way? Any suggestions, improvements, comments?

Note: I wrote my custom CountDownLatch class in case you're wondering where I'm getting it from.

Update:
Here is my CountDownLatch too: 

public class CountDownLatch 
{
    private volatile int m_remain;
    private EventWaitHandle m_event;

    public CountDownLatch (int count)
    {
        if (count < 0)
            throw new ArgumentOutOfRangeException();
        m_remain = count;
        m_event = new ManualResetEvent(false);
        if (m_remain == 0)
        {
            m_event.Set();
        }
    }

    public void Signal()
    {
        // The last thread to signal also sets the event.
        if (Interlocked.Decrement(ref m_remain) == 0)
            m_event.Set();
    }

    public void Wait()
    {
        m_event.WaitOne();
    }
}

Upvotes: 37

Views: 32811

Answers (4)

akapet
akapet

Reputation: 31

BTW, I just noticed this part of the code:

// A method which runs in a thread
    public void Run()
    {
        startSignal.WaitOne();
        while(running)
        {
            startSignal.WaitOne();
            //... some code
        }
        latch.Signal();
    }

You will need to unblock the worker thread twice using "startSignal.Set()" for the code within the while block to execute.

Is this deliberate?

Upvotes: 0

Michael Burr
Michael Burr

Reputation: 340218

Booleans are atomic in C#, however, if you want to modify it in one thread and read it in another, you will need to mark it volatile at the very least,. Otherwise the reading thread may only actually read it once into a register.

Upvotes: 7

Remus Rusanu
Remus Rusanu

Reputation: 294307

You better mark it volatile though:

The volatile keyword indicates that a field might be modified by multiple concurrently executing threads. Fields that are declared volatile are not subject to compiler optimizations that assume access by a single thread. This ensures that the most up-to-date value is present in the field at all times.

But I would change your loop:

    startSignal.WaitOne();
    while(running)
    {
        //... some code
        startSignal.WaitOne();
    }

As it is in your post the 'some code' might execute when the thread is stopped (ie. when Stop is called) which is unexpected and may be even incorrect.

Upvotes: 49

Kevin Sylvestre
Kevin Sylvestre

Reputation: 38032

Booleans are atomic in C#: http://msdn.microsoft.com/en-us/library/aa691278(VS.71).aspx

Upvotes: 1

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