1 % -----------------------------------------------------------------------------
2 % $Id: PrelConc.lhs,v 1.23 2001/02/15 10:02:43 simonmar Exp $
4 % (c) The University of Glasgow, 1994-2000
7 \section[PrelConc]{Module @PrelConc@}
9 Basic concurrency stuff
12 {-# OPTIONS -fno-implicit-prelude #-}
17 -- Forking and suchlike
18 , myThreadId -- :: IO ThreadId
19 , killThread -- :: ThreadId -> IO ()
20 , throwTo -- :: ThreadId -> Exception -> IO ()
21 , par -- :: a -> b -> b
22 , seq -- :: a -> b -> b
26 , threadDelay -- :: Int -> IO ()
27 , threadWaitRead -- :: Int -> IO ()
28 , threadWaitWrite -- :: Int -> IO ()
32 , newMVar -- :: a -> IO (MVar a)
33 , newEmptyMVar -- :: IO (MVar a)
34 , takeMVar -- :: MVar a -> IO a
35 , putMVar -- :: MVar a -> a -> IO ()
36 , tryTakeMVar -- :: MVar a -> IO (Maybe a)
37 , tryPutMVar -- :: MVar a -> a -> IO Bool
38 , isEmptyMVar -- :: MVar a -> IO Bool
44 import PrelErr ( parError, seqError )
45 import PrelIOBase ( IO(..), MVar(..) )
46 import PrelBase ( Int(..) )
47 import PrelException ( Exception(..), AsyncException(..) )
52 %************************************************************************
54 \subsection{@ThreadId@, @par@, and @fork@}
56 %************************************************************************
59 data ThreadId = ThreadId ThreadId#
60 -- ToDo: data ThreadId = ThreadId (Weak ThreadId#)
61 -- But since ThreadId# is unlifted, the Weak type must use open
64 --forkIO has now been hoisted out into the Concurrent library.
66 killThread :: ThreadId -> IO ()
67 killThread (ThreadId id) = IO $ \ s ->
68 case (killThread# id (AsyncException ThreadKilled) s) of s1 -> (# s1, () #)
70 throwTo :: ThreadId -> Exception -> IO ()
71 throwTo (ThreadId id) ex = IO $ \ s ->
72 case (killThread# id ex s) of s1 -> (# s1, () #)
74 myThreadId :: IO ThreadId
75 myThreadId = IO $ \s ->
76 case (myThreadId# s) of (# s1, id #) -> (# s1, ThreadId id #)
80 case (yield# s) of s1 -> (# s1, () #)
82 -- "seq" is defined a bit weirdly (see below)
84 -- The reason for the strange "0# -> parError" case is that
85 -- it fools the compiler into thinking that seq is non-strict in
86 -- its second argument (even if it inlines seq at the call site).
87 -- If it thinks seq is strict in "y", then it often evaluates
88 -- "y" before "x", which is totally wrong.
90 -- Just before converting from Core to STG there's a bit of magic
91 -- that recognises the seq# and eliminates the duff case.
95 seq x y = case (seq# x) of { 0# -> seqError; _ -> y }
99 par x y = case (par# x) of { 0# -> parError; _ -> y }
102 %************************************************************************
104 \subsection[mvars]{M-Structures}
106 %************************************************************************
108 M-Vars are rendezvous points for concurrent threads. They begin
109 empty, and any attempt to read an empty M-Var blocks. When an M-Var
110 is written, a single blocked thread may be freed. Reading an M-Var
111 toggles its state from full back to empty. Therefore, any value
112 written to an M-Var may only be read once. Multiple reads and writes
113 are allowed, but there must be at least one read between any two
117 --Defined in IOBase to avoid cycle: data MVar a = MVar (SynchVar# RealWorld a)
119 newEmptyMVar :: IO (MVar a)
120 newEmptyMVar = IO $ \ s# ->
122 (# s2#, svar# #) -> (# s2#, MVar svar# #)
124 takeMVar :: MVar a -> IO a
125 takeMVar (MVar mvar#) = IO $ \ s# -> takeMVar# mvar# s#
127 putMVar :: MVar a -> a -> IO ()
128 putMVar (MVar mvar#) x = IO $ \ s# ->
129 case putMVar# mvar# x s# of
132 tryPutMVar :: MVar a -> a -> IO Bool
133 tryPutMVar (MVar mvar#) x = IO $ \ s# ->
134 case tryPutMVar# mvar# x s# of
135 (# s, 0# #) -> (# s, False #)
136 (# s, _ #) -> (# s, True #)
138 newMVar :: a -> IO (MVar a)
140 newEmptyMVar >>= \ mvar ->
141 putMVar mvar value >>
144 -- tryTakeMVar is a non-blocking takeMVar
145 tryTakeMVar :: MVar a -> IO (Maybe a)
146 tryTakeMVar (MVar m) = IO $ \ s ->
147 case tryTakeMVar# m s of
148 (# s, 0#, _ #) -> (# s, Nothing #) -- MVar is empty
149 (# s, _, a #) -> (# s, Just a #) -- MVar is full
152 Low-level op. for checking whether an MVar is filled-in or not.
153 Notice that the boolean value returned is just a snapshot of
154 the state of the MVar. By the time you get to react on its result,
155 the MVar may have been filled (or emptied) - so be extremely
156 careful when using this operation.
158 Use tryTakeMVar instead if possible.
160 If you can re-work your abstractions to avoid having to
161 depend on isEmptyMVar, then you're encouraged to do so,
162 i.e., consider yourself warned about the imprecision in
163 general of isEmptyMVar :-)
165 isEmptyMVar :: MVar a -> IO Bool
166 isEmptyMVar (MVar mv#) = IO $ \ s# ->
167 case isEmptyMVar# mv# s# of
168 (# s2#, flg #) -> (# s2#, not (flg ==# 0#) #)
172 %************************************************************************
174 \subsection{Thread waiting}
176 %************************************************************************
178 @threadDelay@ delays rescheduling of a thread until the indicated
179 number of microseconds have elapsed. Generally, the microseconds are
180 counted by the context switch timer, which ticks in virtual time;
181 however, when there are no runnable threads, we don't accumulate any
182 virtual time, so we start ticking in real time. (The granularity is
183 the effective resolution of the context switch timer, so it is
184 affected by the RTS -C option.)
186 @threadWaitRead@ delays rescheduling of a thread until input on the
187 specified file descriptor is available for reading (just like select).
188 @threadWaitWrite@ is similar, but for writing on a file descriptor.
191 threadDelay, threadWaitRead, threadWaitWrite :: Int -> IO ()
193 threadDelay (I# ms) = IO $ \s -> case delay# ms s of s -> (# s, () #)
194 threadWaitRead (I# fd) = IO $ \s -> case waitRead# fd s of s -> (# s, () #)
195 threadWaitWrite (I# fd) = IO $ \s -> case waitWrite# fd s of s -> (# s, () #)