RE: [Haskell-cafe] How to use QSem?
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
Ah, that worked. Thank you. The MVar documentation is also very brief. I would also like to overwrite the contents of an MVar atomically regardless of whether it is full or empty. I am current using. overWrite mvar val = tryTakeMVar mvar >> putMVar mvar val But it is not atomic :-( The lib functions (modifyMVar and tryPutMVar) are atomic but seem to require that you know in advance the MVar's state. Is there a solution to this or do I have to refactor? -Alex- _________________________________________________________________ S. Alexander Jacobson mailto:me@alexjacobson.com tel:917-770-6565 http://alexjacobson.com On Wed, 23 Jun 2004, Simon Marlow wrote:
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
participants (2)
-
S. Alexander Jacobson -
Simon Marlow