[Haskell-cafe] How to use QSem?

Simon Marlow simonmar at microsoft.com
Wed Jun 23 05:56:04 EDT 2004

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.


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