Home/ Questions/Q 4608710
In Process

The Archive Base Latest Questions

Editorial Team
  • 0
Asked: 2026-05-22T00:53:02+00:00

I have an application wherein multiple threads wait on the same event object to

  • 0

I have an application wherein multiple threads wait on the same event object to signal. The problem I am seeing appears to be a type of race condition in that sometimes some threads’ wait states (WaitForMultipleObjects) return as a result of the event signal and other threads’ wait states apparently don’t see the event signal because they don’t return. These events were created using CreateEvent as manual-reset event objects.

My application handles these events such that when an event object is signaled, its “owner” thread is responsible for resetting the event object’s signal state, as shown in the following code snippet. Other threads waiting on the same event do not attempt to reset its signal state.

switch ( dwObjectWaitState = ::WaitForMultipleObjects( i, pHandles, FALSE, INFINITE ) )
{
case WAIT_OBJECT_0 + BAS_MESSAGE_READY_EVT_ID:
    ::ResetEvent( pHandles[BAS_MESSAGE_READY_EVT_ID] );
    /* handles the event */
    break;
}

To put it another way, the problem I am seeing appears to be to what is described in the Remarks section for PulseEvent on the MSDN website:

If the call to PulseEvent occurs
during the time when the thread has
been removed from the wait state, the
thread will not be released because
PulseEvent releases only those threads
that are waiting at the moment it is
called. Therefore, PulseEvent is
unreliable and should not be used by
new applications. Instead, use
condition variables.

If this is what is happening, the only solution I can see is for each thread to register its usage of a given event object with that object’s owner thread, so that the owner thread can determine when it is safe to reset the event object’s signal state.

Is there a better way to do this? Thanks.

Share

Leave an answer

You must login to add an answer.

Need An Account,

1 Answer

  1. Editorial Team

    Use conditional variable as in PulseEvent description. The only problem is that native conditional variable on windows was implemented starting from Vista so older system like XP doesn’t have it. But you can emulate conditional variable using some other synchronization objects (http://www1.cse.wustl.edu/~schmidt/win32-cv-1.html) but I think the easiest way is to use conditional variable from boost library and its notify_all method to wake up all threads (http://www.boost.org/doc/libs/1_41_0/doc/html/thread/synchronization.html#thread.synchronization.condvar_ref)

    Another possibility (but not very beautiful) is to create one event for each thread and when right now you have PulseEvent you can call SetEvent for all of them. For this solution probably auto-reset events would work better.

    • 0