1 -----------------------------------------------------------------------------
3 -- Module : Control.Concurrent.QSem
4 -- Copyright : (c) The University of Glasgow 2001
5 -- License : BSD-style (see the file libraries/base/LICENSE)
7 -- Maintainer : libraries@haskell.org
8 -- Stability : experimental
9 -- Portability : non-portable (concurrency)
11 -- Simple quantity semaphores.
13 -----------------------------------------------------------------------------
15 module Control.Concurrent.QSem
16 ( -- * Simple Quantity Semaphores
18 newQSem, -- :: Int -> IO QSem
19 waitQSem, -- :: QSem -> IO ()
20 signalQSem -- :: QSem -> IO ()
24 import Control.Concurrent.MVar
26 -- General semaphores are also implemented readily in terms of shared
27 -- @MVar@s, only have to catch the case when the semaphore is tried
28 -- waited on when it is empty (==0). Implement this in the same way as
29 -- shared variables are implemented - maintaining a list of @MVar@s
30 -- representing threads currently waiting. The counter is a shared
31 -- variable, ensuring the mutual exclusion on its access.
33 -- |A 'QSem' is a simple quantity semaphore, in which the available
34 -- \"quantity\" is always dealt with in units of one.
35 newtype QSem = QSem (MVar (Int, [MVar ()]))
37 -- |Build a new 'QSem'
38 newQSem :: Int -> IO QSem
40 sem <- newMVar (init,[])
43 -- |Wait for a unit to become available
44 waitQSem :: QSem -> IO ()
45 waitQSem (QSem sem) = do
46 (avail,blocked) <- takeMVar sem -- gain ex. access
48 putMVar sem (avail-1,[])
52 Stuff the reader at the back of the queue,
53 so as to preserve waiting order. A signalling
54 process then only have to pick the MVar at the
55 front of the blocked list.
57 The version of waitQSem given in the paper could
60 putMVar sem (0, blocked++[block])
63 -- |Signal that a unit of the 'QSem' is available
64 signalQSem :: QSem -> IO ()
65 signalQSem (QSem sem) = do
66 (avail,blocked) <- takeMVar sem
68 [] -> putMVar sem (avail+1,[])
70 (block:blocked') -> do
71 putMVar sem (0,blocked')