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
29 -- General semaphores are also implemented readily in terms of shared
30 -- @MVar@s, only have to catch the case when the semaphore is tried
31 -- waited on when it is empty (==0). Implement this in the same way as
32 -- shared variables are implemented - maintaining a list of @MVar@s
33 -- representing threads currently waiting. The counter is a shared
34 -- variable, ensuring the mutual exclusion on its access.
36 -- |A 'QSem' is a simple quantity semaphore, in which the available
37 -- \"quantity\" is always dealt with in units of one.
38 newtype QSem = QSem (MVar (Int, [MVar ()]))
40 INSTANCE_TYPEABLE0(QSem,qSemTc,"QSem")
42 -- |Build a new 'QSem' with a supplied initial quantity.
43 -- The initial quantity must be at least 0.
44 newQSem :: Int -> IO QSem
47 then fail "newQSem: Initial quantity must be non-negative"
48 else do sem <- newMVar (initial, [])
51 -- |Wait for a unit to become available
52 waitQSem :: QSem -> IO ()
53 waitQSem (QSem sem) = do
54 (avail,blocked) <- takeMVar sem -- gain ex. access
56 putMVar sem (avail-1,[])
60 Stuff the reader at the back of the queue,
61 so as to preserve waiting order. A signalling
62 process then only have to pick the MVar at the
63 front of the blocked list.
65 The version of waitQSem given in the paper could
68 putMVar sem (0, blocked++[block])
71 -- |Signal that a unit of the 'QSem' is available
72 signalQSem :: QSem -> IO ()
73 signalQSem (QSem sem) = do
74 (avail,blocked) <- takeMVar sem
76 [] -> putMVar sem (avail+1,[])
78 (block:blocked') -> do
79 putMVar sem (0,blocked')