+{-# LANGUAGE CPP #-}
+
-----------------------------------------------------------------------------
-- |
-- Module : Control.Concurrent.QSem
import Prelude
import Control.Concurrent.MVar
+import Control.Exception ( mask_ )
import Data.Typeable
#include "Typeable.h"
-- |A 'QSem' is a simple quantity semaphore, in which the available
-- \"quantity\" is always dealt with in units of one.
-newtype QSem = QSem (MVar (Int, [MVar ()]))
+newtype QSem = QSem (MVar (Int, [MVar ()])) deriving Eq
INSTANCE_TYPEABLE0(QSem,qSemTc,"QSem")
--- |Build a new 'QSem'
+-- |Build a new 'QSem' with a supplied initial quantity.
+-- The initial quantity must be at least 0.
newQSem :: Int -> IO QSem
-newQSem init = do
- sem <- newMVar (init,[])
- return (QSem sem)
+newQSem initial =
+ if initial < 0
+ then fail "newQSem: Initial quantity must be non-negative"
+ else do sem <- newMVar (initial, [])
+ return (QSem sem)
-- |Wait for a unit to become available
waitQSem :: QSem -> IO ()
-waitQSem (QSem sem) = do
+waitQSem (QSem sem) = mask_ $ do
(avail,blocked) <- takeMVar sem -- gain ex. access
if avail > 0 then
- putMVar sem (avail-1,[])
+ let avail' = avail-1
+ in avail' `seq` putMVar sem (avail',[])
else do
- block <- newEmptyMVar
+ b <- newEmptyMVar
{-
Stuff the reader at the back of the queue,
so as to preserve waiting order. A signalling
The version of waitQSem given in the paper could
lead to starvation.
-}
- putMVar sem (0, blocked++[block])
- takeMVar block
+ putMVar sem (0, blocked++[b])
+ takeMVar b
-- |Signal that a unit of the 'QSem' is available
signalQSem :: QSem -> IO ()
-signalQSem (QSem sem) = do
+signalQSem (QSem sem) = mask_ $ do
(avail,blocked) <- takeMVar sem
case blocked of
- [] -> putMVar sem (avail+1,[])
+ [] -> let avail' = avail+1
+ in avail' `seq` putMVar sem (avail',blocked)
- (block:blocked') -> do
+ (b:blocked') -> do
putMVar sem (0,blocked')
- putMVar block ()
+ putMVar b ()