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|
-----------------------------------------------------------------------------
-- |
-- Module : Control.Concurrent.QSem
-- Copyright : (c) The University of Glasgow 2001
-- License : BSD-style (see the file libraries/base/LICENSE)
--
-- Maintainer : libraries@haskell.org
-- Stability : experimental
-- Portability : non-portable (concurrency)
--
-- Simple quantity semaphores.
--
-----------------------------------------------------------------------------
module Control.Concurrent.QSem
( -- * Simple Quantity Semaphores
QSem, -- abstract
newQSem, -- :: Int -> IO QSem
waitQSem, -- :: QSem -> IO ()
signalQSem -- :: QSem -> IO ()
) where
import Prelude
import Control.Concurrent.MVar
-- General semaphores are also implemented readily in terms of shared
-- @MVar@s, only have to catch the case when the semaphore is tried
-- waited on when it is empty (==0). Implement this in the same way as
-- shared variables are implemented - maintaining a list of @MVar@s
-- representing threads currently waiting. The counter is a shared
-- variable, ensuring the mutual exclusion on its access.
-- |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 ()]))
-- |Build a new 'QSem'
newQSem :: Int -> IO QSem
newQSem init = do
sem <- newMVar (init,[])
return (QSem sem)
-- |Wait for a unit to become available
waitQSem :: QSem -> IO ()
waitQSem (QSem sem) = do
(avail,blocked) <- takeMVar sem -- gain ex. access
if avail > 0 then
putMVar sem (avail-1,[])
else do
block <- newEmptyMVar
{-
Stuff the reader at the back of the queue,
so as to preserve waiting order. A signalling
process then only have to pick the MVar at the
front of the blocked list.
The version of waitQSem given in the paper could
lead to starvation.
-}
putMVar sem (0, blocked++[block])
takeMVar block
-- |Signal that a unit of the 'QSem' is available
signalQSem :: QSem -> IO ()
signalQSem (QSem sem) = do
(avail,blocked) <- takeMVar sem
case blocked of
[] -> putMVar sem (avail+1,[])
(block:blocked') -> do
putMVar sem (0,blocked')
putMVar block ()
|
