On Wednesday, December 12, 2012 3:53:28 PM UTC-5, MRAB wrote:
> On 2012-12-12 20:11, Wanderer wrote:
>
> > I have a program that has a main GUI and a camera. In the main GUI, you can
> > manipulate the images taken by the camera. You can also use the menu to
> > check the camera's settings. Images are taken by the camera in a separate
> > thread, so the long exposures don't block the GUI. I block conflicts
> > between the camera snapshot thread and the main thread by setting a flag
> > called self.cameraActive. I check to see if the cameraActive flag is false
> > and set the cameraActive to True just before starting the thread. I
> > generate an event on exiting the thread which sets the cameraActive flag to
> > False. I also check and set and reset the flag in all the menu commands
> > that access the camera. Like this.
>
> >
>
> > def onProperties(self, event):
>
> > """ Display a message window with the camera properties
>
> > event -- The camera properties menu event
>
> > """
>
> > # Update the temperature
>
> > if not self.cameraActive:
>
> > self.cameraActive = True
>
> > self.camera.getTemperature()
>
> > camDict = self.camera.getPropertyDict()
>
> > self.cameraActive = False
>
> > else:
>
> > camDict = {'Error': 'Camera Busy'}
>
> > dictMessage(camDict, 'Camera Properties')
>
> >
>
> > This works but my question is, is there a better way using semaphores,
> > locks or something else to prevent collisions between threads?
>
> >
>
> That suffers from a race condition in that self.cameraActive might be
>
> False when it's checked in the 'if' condition but set to True just
>
> afterwards by the other thread.
>
>
>
> You could try a non-blocking semaphore:
>
>
>
> def __init__(self):
>
> self.cameraActive = Semaphore()
>
>
>
> def onProperties(self, event):
>
> """ Display a message window with the camera properties
>
> event -- The camera properties menu event
>
> """
>
> # Update the temperature
>
> if self.cameraActive.acquire(False): # Non-blocking
>
> # Successfully acquired the semaphore, so the camera wasn't active
>
> self.camera.getTemperature()
>
> camDict = self.camera.getPropertyDict()
>
> self.cameraActive.release()
>
> else:
>
> camDict = {'Error': 'Camera Busy'}
Thanks. Why Non-blocking?
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