1 -----------------------------------------------------------------------------
3 -- Module : Control.Concurrent
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 -- A common interface to a collection of useful concurrency
14 -----------------------------------------------------------------------------
16 module Control.Concurrent (
17 -- * Concurrent Haskell
21 -- * Basic concurrency operations
24 #ifdef __GLASGOW_HASKELL__
29 #ifdef __GLASGOW_HASKELL__
43 #ifdef __GLASGOW_HASKELL__
45 threadDelay, -- :: Int -> IO ()
46 threadWaitRead, -- :: Int -> IO ()
47 threadWaitWrite, -- :: Int -> IO ()
50 -- * Communication abstractions
52 module Control.Concurrent.MVar,
53 module Control.Concurrent.Chan,
54 module Control.Concurrent.QSem,
55 module Control.Concurrent.QSemN,
56 module Control.Concurrent.SampleVar,
58 -- * Merging of streams
60 mergeIO, -- :: [a] -> [a] -> IO [a]
61 nmergeIO, -- :: [[a]] -> IO [a]
65 #ifdef __GLASGOW_HASKELL__
68 rtsSupportsBoundThreads,
75 -- * GHC's implementation of concurrency
77 -- |This section describes features specific to GHC's
78 -- implementation of Concurrent Haskell.
80 -- ** Haskell threads and Operating System threads
84 -- ** Terminating the program
95 import Control.Exception.Base as Exception
97 #ifdef __GLASGOW_HASKELL__
99 import GHC.Conc ( ThreadId(..), myThreadId, killThread, yield,
100 threadDelay, forkIO, childHandler )
101 import qualified GHC.Conc
102 import GHC.IOBase ( IO(..) )
103 import GHC.IOBase ( unsafeInterleaveIO )
104 import GHC.IOBase ( newIORef, readIORef, writeIORef )
107 import System.Posix.Types ( Fd )
108 import Foreign.StablePtr
109 import Foreign.C.Types ( CInt )
110 import Control.Monad ( when )
112 #ifdef mingw32_HOST_OS
123 import Control.Concurrent.MVar
124 import Control.Concurrent.Chan
125 import Control.Concurrent.QSem
126 import Control.Concurrent.QSemN
127 import Control.Concurrent.SampleVar
135 The concurrency extension for Haskell is described in the paper
137 <http://www.haskell.org/ghc/docs/papers/concurrent-haskell.ps.gz>.
139 Concurrency is \"lightweight\", which means that both thread creation
140 and context switching overheads are extremely low. Scheduling of
141 Haskell threads is done internally in the Haskell runtime system, and
142 doesn't make use of any operating system-supplied thread packages.
144 However, if you want to interact with a foreign library that expects your
145 program to use the operating system-supplied thread package, you can do so
146 by using 'forkOS' instead of 'forkIO'.
148 Haskell threads can communicate via 'MVar's, a kind of synchronised
149 mutable variable (see "Control.Concurrent.MVar"). Several common
150 concurrency abstractions can be built from 'MVar's, and these are
151 provided by the "Control.Concurrent" library.
152 In GHC, threads may also communicate via exceptions.
157 Scheduling may be either pre-emptive or co-operative,
158 depending on the implementation of Concurrent Haskell (see below
159 for information related to specific compilers). In a co-operative
160 system, context switches only occur when you use one of the
161 primitives defined in this module. This means that programs such
165 > main = forkIO (write 'a') >> write 'b'
166 > where write c = putChar c >> write c
168 will print either @aaaaaaaaaaaaaa...@ or @bbbbbbbbbbbb...@,
169 instead of some random interleaving of @a@s and @b@s. In
170 practice, cooperative multitasking is sufficient for writing
171 simple graphical user interfaces.
175 Different Haskell implementations have different characteristics with
176 regard to which operations block /all/ threads.
178 Using GHC without the @-threaded@ option, all foreign calls will block
179 all other Haskell threads in the system, although I\/O operations will
180 not. With the @-threaded@ option, only foreign calls with the @unsafe@
181 attribute will block all other threads.
183 Using Hugs, all I\/O operations and foreign calls will block all other
191 mergeIO :: [a] -> [a] -> IO [a]
192 nmergeIO :: [[a]] -> IO [a]
195 -- The 'mergeIO' and 'nmergeIO' functions fork one thread for each
196 -- input list that concurrently evaluates that list; the results are
197 -- merged into a single output list.
199 -- Note: Hugs does not provide these functions, since they require
200 -- preemptive multitasking.
203 = newEmptyMVar >>= \ tail_node ->
204 newMVar tail_node >>= \ tail_list ->
205 newQSem max_buff_size >>= \ e ->
206 newMVar 2 >>= \ branches_running ->
210 forkIO (suckIO branches_running buff ls) >>
211 forkIO (suckIO branches_running buff rs) >>
212 takeMVar tail_node >>= \ val ->
217 = (MVar (MVar [a]), QSem)
219 suckIO :: MVar Int -> Buffer a -> [a] -> IO ()
221 suckIO branches_running buff@(tail_list,e) vs
223 [] -> takeMVar branches_running >>= \ val ->
225 takeMVar tail_list >>= \ node ->
227 putMVar tail_list node
229 putMVar branches_running (val-1)
232 takeMVar tail_list >>= \ node ->
233 newEmptyMVar >>= \ next_node ->
235 takeMVar next_node >>= \ y ->
237 return y) >>= \ next_node_val ->
238 putMVar node (x:next_node_val) >>
239 putMVar tail_list next_node >>
240 suckIO branches_running buff xs
246 newEmptyMVar >>= \ tail_node ->
247 newMVar tail_node >>= \ tail_list ->
248 newQSem max_buff_size >>= \ e ->
249 newMVar len >>= \ branches_running ->
253 mapIO (\ x -> forkIO (suckIO branches_running buff x)) lss >>
254 takeMVar tail_node >>= \ val ->
258 mapIO f xs = sequence (map f xs)
259 #endif /* __HUGS__ */
261 #ifdef __GLASGOW_HASKELL__
262 -- ---------------------------------------------------------------------------
268 Support for multiple operating system threads and bound threads as described
269 below is currently only available in the GHC runtime system if you use the
270 /-threaded/ option when linking.
272 Other Haskell systems do not currently support multiple operating system threads.
274 A bound thread is a haskell thread that is /bound/ to an operating system
275 thread. While the bound thread is still scheduled by the Haskell run-time
276 system, the operating system thread takes care of all the foreign calls made
279 To a foreign library, the bound thread will look exactly like an ordinary
280 operating system thread created using OS functions like @pthread_create@
283 Bound threads can be created using the 'forkOS' function below. All foreign
284 exported functions are run in a bound thread (bound to the OS thread that
285 called the function). Also, the @main@ action of every Haskell program is
286 run in a bound thread.
288 Why do we need this? Because if a foreign library is called from a thread
289 created using 'forkIO', it won't have access to any /thread-local state/ -
290 state variables that have specific values for each OS thread
291 (see POSIX's @pthread_key_create@ or Win32's @TlsAlloc@). Therefore, some
292 libraries (OpenGL, for example) will not work from a thread created using
293 'forkIO'. They work fine in threads created using 'forkOS' or when called
294 from @main@ or from a @foreign export@.
296 In terms of performance, 'forkOS' (aka bound) threads are much more
297 expensive than 'forkIO' (aka unbound) threads, because a 'forkOS'
298 thread is tied to a particular OS thread, whereas a 'forkIO' thread
299 can be run by any OS thread. Context-switching between a 'forkOS'
300 thread and a 'forkIO' thread is many times more expensive than between
301 two 'forkIO' threads.
303 Note in particular that the main program thread (the thread running
304 @Main.main@) is always a bound thread, so for good concurrency
305 performance you should ensure that the main thread is not doing
306 repeated communication with other threads in the system. Typically
307 this means forking subthreads to do the work using 'forkIO', and
308 waiting for the results in the main thread.
312 -- | 'True' if bound threads are supported.
313 -- If @rtsSupportsBoundThreads@ is 'False', 'isCurrentThreadBound'
314 -- will always return 'False' and both 'forkOS' and 'runInBoundThread' will
316 foreign import ccall rtsSupportsBoundThreads :: Bool
320 Like 'forkIO', this sparks off a new thread to run the 'IO'
321 computation passed as the first argument, and returns the 'ThreadId'
322 of the newly created thread.
324 However, 'forkOS' creates a /bound/ thread, which is necessary if you
325 need to call foreign (non-Haskell) libraries that make use of
326 thread-local state, such as OpenGL (see "Control.Concurrent#boundthreads").
328 Using 'forkOS' instead of 'forkIO' makes no difference at all to the
329 scheduling behaviour of the Haskell runtime system. It is a common
330 misconception that you need to use 'forkOS' instead of 'forkIO' to
331 avoid blocking all the Haskell threads when making a foreign call;
332 this isn't the case. To allow foreign calls to be made without
333 blocking all the Haskell threads (with GHC), it is only necessary to
334 use the @-threaded@ option when linking your program, and to make sure
335 the foreign import is not marked @unsafe@.
338 forkOS :: IO () -> IO ThreadId
340 foreign export ccall forkOS_entry
341 :: StablePtr (IO ()) -> IO ()
343 foreign import ccall "forkOS_entry" forkOS_entry_reimported
344 :: StablePtr (IO ()) -> IO ()
346 forkOS_entry :: StablePtr (IO ()) -> IO ()
347 forkOS_entry stableAction = do
348 action <- deRefStablePtr stableAction
351 foreign import ccall forkOS_createThread
352 :: StablePtr (IO ()) -> IO CInt
354 failNonThreaded :: IO a
355 failNonThreaded = fail $ "RTS doesn't support multiple OS threads "
356 ++"(use ghc -threaded when linking)"
359 | rtsSupportsBoundThreads = do
361 b <- Exception.blocked
363 -- async exceptions are blocked in the child if they are blocked
364 -- in the parent, as for forkIO (see #1048). forkOS_createThread
365 -- creates a thread with exceptions blocked by default.
366 action1 | b = action0
367 | otherwise = unblock action0
369 action_plus = Exception.catch action1 childHandler
371 entry <- newStablePtr (myThreadId >>= putMVar mv >> action_plus)
372 err <- forkOS_createThread entry
373 when (err /= 0) $ fail "Cannot create OS thread."
377 | otherwise = failNonThreaded
379 -- | Returns 'True' if the calling thread is /bound/, that is, if it is
380 -- safe to use foreign libraries that rely on thread-local state from the
382 isCurrentThreadBound :: IO Bool
383 isCurrentThreadBound = IO $ \ s# ->
384 case isCurrentThreadBound# s# of
385 (# s2#, flg #) -> (# s2#, not (flg ==# 0#) #)
389 Run the 'IO' computation passed as the first argument. If the calling thread
390 is not /bound/, a bound thread is created temporarily. @runInBoundThread@
391 doesn't finish until the 'IO' computation finishes.
393 You can wrap a series of foreign function calls that rely on thread-local state
394 with @runInBoundThread@ so that you can use them without knowing whether the
395 current thread is /bound/.
397 runInBoundThread :: IO a -> IO a
399 runInBoundThread action
400 | rtsSupportsBoundThreads = do
401 bound <- isCurrentThreadBound
405 ref <- newIORef undefined
406 let action_plus = Exception.try action >>= writeIORef ref
408 bracket (newStablePtr action_plus)
410 (\cEntry -> forkOS_entry_reimported cEntry >> readIORef ref)
411 case resultOrException of
412 Left exception -> Exception.throw (exception :: SomeException)
413 Right result -> return result
414 | otherwise = failNonThreaded
417 Run the 'IO' computation passed as the first argument. If the calling thread
418 is /bound/, an unbound thread is created temporarily using 'forkIO'.
419 @runInBoundThread@ doesn't finish until the 'IO' computation finishes.
421 Use this function /only/ in the rare case that you have actually observed a
422 performance loss due to the use of bound threads. A program that
423 doesn't need it's main thread to be bound and makes /heavy/ use of concurrency
424 (e.g. a web server), might want to wrap it's @main@ action in
425 @runInUnboundThread@.
427 runInUnboundThread :: IO a -> IO a
429 runInUnboundThread action = do
430 bound <- isCurrentThreadBound
434 forkIO (Exception.try action >>= putMVar mv)
435 takeMVar mv >>= \ei -> case ei of
436 Left exception -> Exception.throw (exception :: SomeException)
437 Right result -> return result
440 #endif /* __GLASGOW_HASKELL__ */
442 #ifdef __GLASGOW_HASKELL__
443 -- ---------------------------------------------------------------------------
444 -- threadWaitRead/threadWaitWrite
446 -- | Block the current thread until data is available to read on the
447 -- given file descriptor (GHC only).
448 threadWaitRead :: Fd -> IO ()
450 #ifdef mingw32_HOST_OS
451 -- we have no IO manager implementing threadWaitRead on Windows.
452 -- fdReady does the right thing, but we have to call it in a
453 -- separate thread, otherwise threadWaitRead won't be interruptible,
454 -- and this only works with -threaded.
455 | threaded = withThread (waitFd fd 0)
456 | otherwise = case fd of
457 0 -> do hWaitForInput stdin (-1); return ()
458 -- hWaitForInput does work properly, but we can only
459 -- do this for stdin since we know its FD.
460 _ -> error "threadWaitRead requires -threaded on Windows, or use System.IO.hWaitForInput"
462 = GHC.Conc.threadWaitRead fd
465 -- | Block the current thread until data can be written to the
466 -- given file descriptor (GHC only).
467 threadWaitWrite :: Fd -> IO ()
469 #ifdef mingw32_HOST_OS
470 | threaded = withThread (waitFd fd 1)
471 | otherwise = error "threadWaitWrite requires -threaded on Windows"
473 = GHC.Conc.threadWaitWrite fd
476 #ifdef mingw32_HOST_OS
477 foreign import ccall unsafe "rtsSupportsBoundThreads" threaded :: Bool
479 withThread :: IO a -> IO a
482 forkIO $ try io >>= putMVar m
486 Left e -> throwIO (e :: IOException)
488 waitFd :: Fd -> CInt -> IO ()
490 throwErrnoIfMinus1 "fdReady" $
491 fdReady (fromIntegral fd) write (fromIntegral iNFINITE) 0
494 iNFINITE = 0xFFFFFFFF :: CInt -- urgh
496 foreign import ccall safe "fdReady"
497 fdReady :: CInt -> CInt -> CInt -> CInt -> IO CInt
500 -- ---------------------------------------------------------------------------
505 #osthreads# In GHC, threads created by 'forkIO' are lightweight threads, and
506 are managed entirely by the GHC runtime. Typically Haskell
507 threads are an order of magnitude or two more efficient (in
508 terms of both time and space) than operating system threads.
510 The downside of having lightweight threads is that only one can
511 run at a time, so if one thread blocks in a foreign call, for
512 example, the other threads cannot continue. The GHC runtime
513 works around this by making use of full OS threads where
514 necessary. When the program is built with the @-threaded@
515 option (to link against the multithreaded version of the
516 runtime), a thread making a @safe@ foreign call will not block
517 the other threads in the system; another OS thread will take
518 over running Haskell threads until the original call returns.
519 The runtime maintains a pool of these /worker/ threads so that
520 multiple Haskell threads can be involved in external calls
523 The "System.IO" library manages multiplexing in its own way. On
524 Windows systems it uses @safe@ foreign calls to ensure that
525 threads doing I\/O operations don't block the whole runtime,
526 whereas on Unix systems all the currently blocked I\/O reqwests
527 are managed by a single thread (the /IO manager thread/) using
530 The runtime will run a Haskell thread using any of the available
531 worker OS threads. If you need control over which particular OS
532 thread is used to run a given Haskell thread, perhaps because
533 you need to call a foreign library that uses OS-thread-local
534 state, then you need bound threads (see "Control.Concurrent#boundthreads").
536 If you don't use the @-threaded@ option, then the runtime does
537 not make use of multiple OS threads. Foreign calls will block
538 all other running Haskell threads until the call returns. The
539 "System.IO" library still does multiplexing, so there can be multiple
540 threads doing I\/O, and this is handled internally by the runtime using
546 In a standalone GHC program, only the main thread is
547 required to terminate in order for the process to terminate.
548 Thus all other forked threads will simply terminate at the same
549 time as the main thread (the terminology for this kind of
550 behaviour is \"daemonic threads\").
552 If you want the program to wait for child threads to
553 finish before exiting, you need to program this yourself. A
554 simple mechanism is to have each child thread write to an
555 'MVar' when it completes, and have the main
556 thread wait on all the 'MVar's before
559 > myForkIO :: IO () -> IO (MVar ())
561 > mvar <- newEmptyMVar
562 > forkIO (io `finally` putMVar mvar ())
565 Note that we use 'finally' from the
566 "Control.Exception" module to make sure that the
567 'MVar' is written to even if the thread dies or
568 is killed for some reason.
570 A better method is to keep a global list of all child
571 threads which we should wait for at the end of the program:
573 > children :: MVar [MVar ()]
574 > children = unsafePerformIO (newMVar [])
576 > waitForChildren :: IO ()
577 > waitForChildren = do
578 > cs <- takeMVar children
582 > putMVar children ms
586 > forkChild :: IO () -> IO ThreadId
588 > mvar <- newEmptyMVar
589 > childs <- takeMVar children
590 > putMVar children (mvar:childs)
591 > forkIO (io `finally` putMVar mvar ())
594 > later waitForChildren $
597 The main thread principle also applies to calls to Haskell from
598 outside, using @foreign export@. When the @foreign export@ed
599 function is invoked, it starts a new main thread, and it returns
600 when this main thread terminates. If the call causes new
601 threads to be forked, they may remain in the system after the
602 @foreign export@ed function has returned.
607 GHC implements pre-emptive multitasking: the execution of
608 threads are interleaved in a random fashion. More specifically,
609 a thread may be pre-empted whenever it allocates some memory,
610 which unfortunately means that tight loops which do no
611 allocation tend to lock out other threads (this only seems to
612 happen with pathological benchmark-style code, however).
614 The rescheduling timer runs on a 20ms granularity by
615 default, but this may be altered using the
616 @-i\<n\>@ RTS option. After a rescheduling
617 \"tick\" the running thread is pre-empted as soon as
621 @aaaa@ @bbbb@ example may not
622 work too well on GHC (see Scheduling, above), due
623 to the locking on a 'System.IO.Handle'. Only one thread
624 may hold the lock on a 'System.IO.Handle' at any one
625 time, so if a reschedule happens while a thread is holding the
626 lock, the other thread won't be able to run. The upshot is that
627 the switch from @aaaa@ to
628 @bbbbb@ happens infrequently. It can be
629 improved by lowering the reschedule tick period. We also have a
630 patch that causes a reschedule whenever a thread waiting on a
631 lock is woken up, but haven't found it to be useful for anything
632 other than this example :-)
634 #endif /* __GLASGOW_HASKELL__ */