#ifdef __GLASGOW_HASKELL__
#endif
module Control.Concurrent.QSem
(
QSem,
newQSem,
waitQSem,
signalQSem
) where
import Prelude
import Control.Concurrent.MVar
import Control.Exception ( mask_ )
import Data.Typeable
#include "Typeable.h"
newtype QSem = QSem (MVar (Int, [MVar ()])) deriving Eq
INSTANCE_TYPEABLE0(QSem,qSemTc,"QSem")
newQSem :: Int -> IO QSem
newQSem initial =
if initial < 0
then fail "newQSem: Initial quantity must be non-negative"
else do sem <- newMVar (initial, [])
return (QSem sem)
waitQSem :: QSem -> IO ()
waitQSem (QSem sem) = mask_ $ do
(avail,blocked) <- takeMVar sem
if avail > 0 then
let avail' = avail1
in avail' `seq` putMVar sem (avail',[])
else do
b <- newEmptyMVar
putMVar sem (0, blocked++[b])
takeMVar b
signalQSem :: QSem -> IO ()
signalQSem (QSem sem) = mask_ $ do
(avail,blocked) <- takeMVar sem
case blocked of
[] -> let avail' = avail+1
in avail' `seq` putMVar sem (avail',blocked)
(b:blocked') -> do
putMVar sem (0,blocked')
putMVar b ()
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