SetEvent() and ResetEvent()?

This is a discussion on SetEvent() and ResetEvent()? within the Windows Programming forums, part of the Platform Specific Boards category; I've never used events directly, so I'm not quite sure what the point of this code is? I see some ...

  1. #1
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    SetEvent() and ResetEvent()?

    I've never used events directly, so I'm not quite sure what the point of this code is? I see some code where they call SetEvent() immediately followed by ResetEvent().
    As usual, there's no comments saying what it's supposed to be doing, so can anyone tell me why someone might want to signal & unsignal an event like that?
    Code:
    hEvent = OpenEvent( EVENT_ALL_ACCESS, TRUE, rt[i].DllResponderEvent );
    BOOL serc = SetEvent( hEvent );
    
    if ( serc == FALSE )
    {
        // Log an error.
    }
    
    ResetEvent( hEvent );
    CloseHandle( hEvent );

  2. #2
    Carnivore ('-'v) Hunter2's Avatar
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    Is this in some sample code? Could be that the Set and Reset lines actually have other code in between them. Alternatively, it could be a really funky way of checking that the Event was created properly (again, followed by some arbitrary lines of code before the Reset). Either way, this entire code section doesn't really do anything except create an event and destroy it.
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    Quote Originally Posted by Hunter2 View Post
    Is this in some sample code? Could be that the Set and Reset lines actually have other code in between them. Alternatively, it could be a really funky way of checking that the Event was created properly (again, followed by some arbitrary lines of code before the Reset). Either way, this entire code section doesn't really do anything except create an event and destroy it.
    Nope, it's actually in production software and it doesn't have any code in between except for some error logging if SetEvent() failed. There's a LOT of things like that in this code that make me scratch my head...

    So basically, you're saying this code has no purpose?
    I'm not gonna mess around with it one way or the other, but I'd like to at least understand the program flow before I start adding my changes...

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    CSharpener vart's Avatar
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    So basically, you're saying this code has no purpose?
    It definitely has a purpose notifying some other thread that this part of the code was reached...

    Event is opened by its name - so it is originally created in some other part of the code...
    Here the event is signeled - so the wating function could exit...
    and when reset back to non-signaled state - so it could be used again
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    Quote Originally Posted by vart View Post
    It definitely has a purpose notifying some other thread that this part of the code was reached...

    Event is opened by its name - so it is originally created in some other part of the code...
    Here the event is signeled - so the wating function could exit...
    and when reset back to non-signaled state - so it could be used again
    But does the call to SetEvent() block until something else happens, or does it return immediately? If it doesn't block, then I don't see how other threads could get notified about the event unless a context switch just happened to occur between the SetEvent() and ResetEvent() calls.

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    Malum in se abachler's Avatar
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    The effect is to release ALL waiting threads, and then reset the event. If you want to release only 1 thread, then use PulseEvent.
    Until you can build a working general purpose reprogrammable computer out of basic components from radio shack, you are not fit to call yourself a programmer in my presence. This is cwhizard, signing off.

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    Quote Originally Posted by abachler View Post
    The effect is to release ALL waiting threads, and then reset the event. If you want to release only 1 thread, then use PulseEvent.
    OK, but does SetEvent() guarantee that all threads will see the event in a signalled state before ResetEvent() sets it back to unsignalled? or do you need to call something like WaitForMultipleObjects() in between to make sure that all the threads heard the signal?

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    Malum in se abachler's Avatar
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    SetEvent will release all waiting threads.
    Until you can build a working general purpose reprogrammable computer out of basic components from radio shack, you are not fit to call yourself a programmer in my presence. This is cwhizard, signing off.

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    C++まいる!Cをこわせ! Elysia's Avatar
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    SetEvent only affects the synchronization object. It does not block. It's used to signal that an event has happened. WaitForSingleObject can be used to wait on a handle created by CreateEvent. As long as the object is non-signaled as ResetEvent, then WaitForSingleObject blocks. When SetEvent signals the object, WaitForSingleObject wakes the thread up.
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    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

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    I'm still a bit confused.
    OK, let me put it this way: Lets say I'm running on single CPU machine, so only 1 thread can run at a time. I'm also running a million threads (just hypothetically of course) that are all waiting for an event.
    Now, if I call:
    Code:
    SetEvent( hEvent );  // Nothing in between these lines.
    ResetEvent( hEvent );
    You're saying that SetEvent() doesn't block, so it will set hEvent to a signalled state, then immediately afterwards ResetEvent() will set hEvent back to an unsignalled event.

    What I'm wondering is, how can all those other threads see that hEvent is signalled BEFORE it gets set back to unsignalled?

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    C++まいる!Cをこわせ! Elysia's Avatar
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    Well, technically, after the event is set, the system would check and release all locks waiting for that event to be set, so all those million threads would wake up and begin to cause mayhem as soon as you set it.
    When resetting it, the threads will fall back into deep slumber as soon as they wait for it to become signaled again.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  12. #12
    Malum in se abachler's Avatar
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    As cutey pie said, SetEvent() is a system call, the system releases all waiting threads before returning. This doesnt mean they all execute immediately though, they are just scheduled for execution.
    Until you can build a working general purpose reprogrammable computer out of basic components from radio shack, you are not fit to call yourself a programmer in my presence. This is cwhizard, signing off.

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    Super Moderator VirtualAce's Avatar
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    Moved to Windows programming.

  14. #14
    Registered User Codeplug's Avatar
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    This is all assuming that the event was created as a manual reset event.

    For manual reset events, here's some things you should keep in mind - say your thread(s) are looping on WaitForSingleObject(), then there is no telling how many loops will execute while the event is signaled (between the Set and Reset calls). Using the same loop construct, it's equally possible for any thread(s) to be in the middle of the loop (not inside a Wait function) during the entire duration the event is signaled - so that when it loops back around and calls a wait function, it blocks. In other words, those thread(s) never knew that the event was ever signaled.

    If it's actually an auto reset event, then the code you posted basically means "release one thread that may be waiting on the event, and just incase there wasn't a thread waiting on the event, reset it".

    gg

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    also, you might have better figuring out luck finding out the reasoning behind that snippet of code by simply going to the source - wherever CreateEvent and WaitForXXX are called. my guess is that the relevent code between was removed while the synchronization code remained (as a placeholder or simply because it was overlooked). who knows though, really.
    Code:
    #include <cmath>
    #include <complex>
    bool euler_flip(bool value)
    {
        return std::pow
        (
            std::complex<float>(std::exp(1.0)), 
            std::complex<float>(0, 1) 
            * std::complex<float>(std::atan(1.0)
            *(1 << (value + 2)))
        ).real() < 0;
    }

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