On 22 June 2004 21:44, S. Alexander Jacobson wrote:
The GHC documentation on QSem is very sparse. I would like to give a thread exclusive access to a resource.
My *guess* based on the documentation is that I can create an exclusive lock using:
logSem <- newQSem 1
If the maximum value of your QSem is 1, then an MVar will do. That is, an MVar can be used as a simple semaphore, and a QSem is necessary if you need a quantity semaphore (there are > 1 units of the resource).
And then any thread that wants to lock the resource uses:
withLogSem x = do waitQSem logSem; y <- x; signalQSem logSem; return y
as follows:
withLogSem $ rotate curLogPos
import Control.Concurrent, create the semaphore like this: m <- newMVar () and use it like this: withMVar m $ \_ -> ... critical section ... If you are using the semaphore to protect a mutable variable, then you can handily merge the two ideas into an MVar, which behaves like a thread-safe mutable variable. Cheers, Simon