Question

I want to implement a message queue for 2 threads. Thread #1 will pop the messages in queue and process it. Thread #2 will push the messages into queue.

Here is my code:

 Thread #1 //Pop message and process
 {
    while(true)
    {
        Lock(mutex);
        message = messageQueue.Pop();
        Unlock(mutex);

        if (message == NULL) //the queue is empty
        {
           //assume that the interruption occurs here (*)
            WaitForSingleObject(hWakeUpEvent, INFINITE);
            continue;
        }
        else
        {
            //process message
        }
    }
}

Thread #2 //push  new message in queue and wake up thread #1
{
    Lock(mutex);
    messageQueue.Push(newMessage)
    Unlock(mutex);

    SetEvent(hWakeUpEvent);
}

The problem is there are some cases SetEvent(hWakeUpEvent) will be called before WaitForSingleObject() ( note (*) ), it will be dangerous.

Answer 1

Your code is fine!

There’s no actual problem with timing between SetEvent and WaitForSingleObject: the key issue is that WaitForSingleObject on an event will check the state of the event, and wait until it is triggered. If the event is already triggered, it will return immediately. (In technical terms, it’s level-triggered, not edge-triggered.) This means that it’s fine if SetEvent is called either before or during the call to WaitForSingleObject; WaitForSingleObject will return in either case; either immediately or when SetEvent is called later on.

(BTW, I’m assuming using an Automatic Reset event here. I can’t think of a good reason for using a Manual Reset event; you’d just end up having to call ResetEvent immediately after WaitForSingleObject returns; and there’s a danger that if your forget this, you could end up Waiting for an event you’ve already waited for but forgotten to clear. Additionally,it’s important to Reset before checking the underlying data state, otherwise if SetEvent is called between when the data is processed and Reset() is called, you lose that information. Stick with Automatic Reset, and you avoid all this.)

—

[Edit: I misread the OP’s code as doing a single ‘pop’ on each wake, rather than only waiting on empty, so the comments below refer to code that scenario. The OP’s code is actually equivalent to the second suggested fix below. So the text below is really describing a somewhat common coding error where events are used as through they were semaphores, rather than the OP’s actual code.]

But there is a different problem here [or, there would be if there was only one pop per wait…], and that’s that Win32 Events objects have only two states: unsignaled and signaled, so you can use them only to track binary state, but not to count. If you SetEvent and event that’s already signaled, it remains Signaled, and the information of that extra SetEvent call is lost.

In that case, what could happen is:

• Item is added, SetEvent called, event is now signaled.
• Another item is added, SetEvent is called again, event stays signaled.
• Worker thread calls WaitForSingleObject, which returns, clearing the event,
• only one item is processed,
• worker thread calls WaitForsingleObject, which blocks because the event is unsignaled, even though there’s still an item in the queue.

There’s two ways around this: the classic Comp.Sci way is to use a semaphore instead of an event – semaphores are essentially events that count up all the ‘Set’ calls; you could conversely think of an event as a semaphore with a max count of 1 which ignores any other signals beyond that one.

An alternative way is to continue using events, but when the worker thread wakes up, it can only assume that there may be some items in the queue, and it should attempt to process them all before it returns to waiting – typically by putting the code that pops the item in a loop that pops items and processes them until its empty. The event is now used not to count, but rather to signal “the queue is no longer empty”. (Note that when you do this, you can also get cases where, while processing the queue, you also process an item that was just added and for which SetEvent was called, so that when the worker thread reaches WaitForSingleObject, the thread wakes up but finds the queue is empty as the item has already been processed; this can seem a bit surprising at first, but is actually fine.)

I view these two as mostly equivalent; there’s minor pros and cons to both, but they’re both correct. (Personally I prefer the events approach, since it decouples the concept of “something needing to be done” or “more data is available” from the quantity of that work or data.)