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.TopHandler ( reportStackOverflow, reportError )
103 import GHC.IOBase ( IO(..) )
104 import GHC.IOBase ( unsafeInterleaveIO )
105 import GHC.IOBase ( newIORef, readIORef, writeIORef )
108 import System.Posix.Types ( Fd )
109 import Foreign.StablePtr
110 import Foreign.C.Types ( CInt )
111 import Control.Monad ( when )
113 #ifdef mingw32_HOST_OS
124 import Control.Concurrent.MVar
125 import Control.Concurrent.Chan
126 import Control.Concurrent.QSem
127 import Control.Concurrent.QSemN
128 import Control.Concurrent.SampleVar
136 The concurrency extension for Haskell is described in the paper
138 <http://www.haskell.org/ghc/docs/papers/concurrent-haskell.ps.gz>.
140 Concurrency is \"lightweight\", which means that both thread creation
141 and context switching overheads are extremely low. Scheduling of
142 Haskell threads is done internally in the Haskell runtime system, and
143 doesn't make use of any operating system-supplied thread packages.
145 However, if you want to interact with a foreign library that expects your
146 program to use the operating system-supplied thread package, you can do so
147 by using 'forkOS' instead of 'forkIO'.
149 Haskell threads can communicate via 'MVar's, a kind of synchronised
150 mutable variable (see "Control.Concurrent.MVar"). Several common
151 concurrency abstractions can be built from 'MVar's, and these are
152 provided by the "Control.Concurrent" library.
153 In GHC, threads may also communicate via exceptions.
158 Scheduling may be either pre-emptive or co-operative,
159 depending on the implementation of Concurrent Haskell (see below
160 for information related to specific compilers). In a co-operative
161 system, context switches only occur when you use one of the
162 primitives defined in this module. This means that programs such
166 > main = forkIO (write 'a') >> write 'b'
167 > where write c = putChar c >> write c
169 will print either @aaaaaaaaaaaaaa...@ or @bbbbbbbbbbbb...@,
170 instead of some random interleaving of @a@s and @b@s. In
171 practice, cooperative multitasking is sufficient for writing
172 simple graphical user interfaces.
176 Different Haskell implementations have different characteristics with
177 regard to which operations block /all/ threads.
179 Using GHC without the @-threaded@ option, all foreign calls will block
180 all other Haskell threads in the system, although I\/O operations will
181 not. With the @-threaded@ option, only foreign calls with the @unsafe@
182 attribute will block all other threads.
184 Using Hugs, all I\/O operations and foreign calls will block all other
192 mergeIO :: [a] -> [a] -> IO [a]
193 nmergeIO :: [[a]] -> IO [a]
196 -- The 'mergeIO' and 'nmergeIO' functions fork one thread for each
197 -- input list that concurrently evaluates that list; the results are
198 -- merged into a single output list.
200 -- Note: Hugs does not provide these functions, since they require
201 -- preemptive multitasking.
204 = newEmptyMVar >>= \ tail_node ->
205 newMVar tail_node >>= \ tail_list ->
206 newQSem max_buff_size >>= \ e ->
207 newMVar 2 >>= \ branches_running ->
211 forkIO (suckIO branches_running buff ls) >>
212 forkIO (suckIO branches_running buff rs) >>
213 takeMVar tail_node >>= \ val ->
218 = (MVar (MVar [a]), QSem)
220 suckIO :: MVar Int -> Buffer a -> [a] -> IO ()
222 suckIO branches_running buff@(tail_list,e) vs
224 [] -> takeMVar branches_running >>= \ val ->
226 takeMVar tail_list >>= \ node ->
228 putMVar tail_list node
230 putMVar branches_running (val-1)
233 takeMVar tail_list >>= \ node ->
234 newEmptyMVar >>= \ next_node ->
236 takeMVar next_node >>= \ y ->
238 return y) >>= \ next_node_val ->
239 putMVar node (x:next_node_val) >>
240 putMVar tail_list next_node >>
241 suckIO branches_running buff xs
247 newEmptyMVar >>= \ tail_node ->
248 newMVar tail_node >>= \ tail_list ->
249 newQSem max_buff_size >>= \ e ->
250 newMVar len >>= \ branches_running ->
254 mapIO (\ x -> forkIO (suckIO branches_running buff x)) lss >>
255 takeMVar tail_node >>= \ val ->
259 mapIO f xs = sequence (map f xs)
260 #endif /* __HUGS__ */
262 #ifdef __GLASGOW_HASKELL__
263 -- ---------------------------------------------------------------------------
269 Support for multiple operating system threads and bound threads as described
270 below is currently only available in the GHC runtime system if you use the
271 /-threaded/ option when linking.
273 Other Haskell systems do not currently support multiple operating system threads.
275 A bound thread is a haskell thread that is /bound/ to an operating system
276 thread. While the bound thread is still scheduled by the Haskell run-time
277 system, the operating system thread takes care of all the foreign calls made
280 To a foreign library, the bound thread will look exactly like an ordinary
281 operating system thread created using OS functions like @pthread_create@
284 Bound threads can be created using the 'forkOS' function below. All foreign
285 exported functions are run in a bound thread (bound to the OS thread that
286 called the function). Also, the @main@ action of every Haskell program is
287 run in a bound thread.
289 Why do we need this? Because if a foreign library is called from a thread
290 created using 'forkIO', it won't have access to any /thread-local state/ -
291 state variables that have specific values for each OS thread
292 (see POSIX's @pthread_key_create@ or Win32's @TlsAlloc@). Therefore, some
293 libraries (OpenGL, for example) will not work from a thread created using
294 'forkIO'. They work fine in threads created using 'forkOS' or when called
295 from @main@ or from a @foreign export@.
297 In terms of performance, 'forkOS' (aka bound) threads are much more
298 expensive than 'forkIO' (aka unbound) threads, because a 'forkOS'
299 thread is tied to a particular OS thread, whereas a 'forkIO' thread
300 can be run by any OS thread. Context-switching between a 'forkOS'
301 thread and a 'forkIO' thread is many times more expensive than between
302 two 'forkIO' threads.
304 Note in particular that the main program thread (the thread running
305 @Main.main@) is always a bound thread, so for good concurrency
306 performance you should ensure that the main thread is not doing
307 repeated communication with other threads in the system. Typically
308 this means forking subthreads to do the work using 'forkIO', and
309 waiting for the results in the main thread.
313 -- | 'True' if bound threads are supported.
314 -- If @rtsSupportsBoundThreads@ is 'False', 'isCurrentThreadBound'
315 -- will always return 'False' and both 'forkOS' and 'runInBoundThread' will
317 foreign import ccall rtsSupportsBoundThreads :: Bool
321 Like 'forkIO', this sparks off a new thread to run the 'IO'
322 computation passed as the first argument, and returns the 'ThreadId'
323 of the newly created thread.
325 However, 'forkOS' creates a /bound/ thread, which is necessary if you
326 need to call foreign (non-Haskell) libraries that make use of
327 thread-local state, such as OpenGL (see "Control.Concurrent#boundthreads").
329 Using 'forkOS' instead of 'forkIO' makes no difference at all to the
330 scheduling behaviour of the Haskell runtime system. It is a common
331 misconception that you need to use 'forkOS' instead of 'forkIO' to
332 avoid blocking all the Haskell threads when making a foreign call;
333 this isn't the case. To allow foreign calls to be made without
334 blocking all the Haskell threads (with GHC), it is only necessary to
335 use the @-threaded@ option when linking your program, and to make sure
336 the foreign import is not marked @unsafe@.
339 forkOS :: IO () -> IO ThreadId
341 foreign export ccall forkOS_entry
342 :: StablePtr (IO ()) -> IO ()
344 foreign import ccall "forkOS_entry" forkOS_entry_reimported
345 :: 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 = fail $ "RTS doesn't support multiple OS threads "
355 ++"(use ghc -threaded when linking)"
358 | rtsSupportsBoundThreads = do
360 b <- Exception.blocked
362 -- async exceptions are blocked in the child if they are blocked
363 -- in the parent, as for forkIO (see #1048). forkOS_createThread
364 -- creates a thread with exceptions blocked by default.
365 action1 | b = action0
366 | otherwise = unblock action0
368 action_plus = Exception.catch action1 childHandler
370 entry <- newStablePtr (myThreadId >>= putMVar mv >> action_plus)
371 err <- forkOS_createThread entry
372 when (err /= 0) $ fail "Cannot create OS thread."
376 | otherwise = failNonThreaded
378 -- | Returns 'True' if the calling thread is /bound/, that is, if it is
379 -- safe to use foreign libraries that rely on thread-local state from the
381 isCurrentThreadBound :: IO Bool
382 isCurrentThreadBound = IO $ \ s# ->
383 case isCurrentThreadBound# s# of
384 (# s2#, flg #) -> (# s2#, not (flg ==# 0#) #)
388 Run the 'IO' computation passed as the first argument. If the calling thread
389 is not /bound/, a bound thread is created temporarily. @runInBoundThread@
390 doesn't finish until the 'IO' computation finishes.
392 You can wrap a series of foreign function calls that rely on thread-local state
393 with @runInBoundThread@ so that you can use them without knowing whether the
394 current thread is /bound/.
396 runInBoundThread :: IO a -> IO a
398 runInBoundThread action
399 | rtsSupportsBoundThreads = do
400 bound <- isCurrentThreadBound
404 ref <- newIORef undefined
405 let action_plus = Exception.try action >>= writeIORef ref
407 bracket (newStablePtr action_plus)
409 (\cEntry -> forkOS_entry_reimported cEntry >> readIORef ref)
410 case resultOrException of
411 Left exception -> Exception.throw (exception :: SomeException)
412 Right result -> return result
413 | otherwise = failNonThreaded
416 Run the 'IO' computation passed as the first argument. If the calling thread
417 is /bound/, an unbound thread is created temporarily using 'forkIO'.
418 @runInBoundThread@ doesn't finish until the 'IO' computation finishes.
420 Use this function /only/ in the rare case that you have actually observed a
421 performance loss due to the use of bound threads. A program that
422 doesn't need it's main thread to be bound and makes /heavy/ use of concurrency
423 (e.g. a web server), might want to wrap it's @main@ action in
424 @runInUnboundThread@.
426 runInUnboundThread :: IO a -> IO a
428 runInUnboundThread action = do
429 bound <- isCurrentThreadBound
433 forkIO (Exception.try action >>= putMVar mv)
434 takeMVar mv >>= \either -> case either of
435 Left exception -> Exception.throw (exception :: SomeException)
436 Right result -> return result
439 #endif /* __GLASGOW_HASKELL__ */
441 #ifdef __GLASGOW_HASKELL__
442 -- ---------------------------------------------------------------------------
443 -- threadWaitRead/threadWaitWrite
445 -- | Block the current thread until data is available to read on the
446 -- given file descriptor (GHC only).
447 threadWaitRead :: Fd -> IO ()
449 #ifdef mingw32_HOST_OS
450 -- we have no IO manager implementing threadWaitRead on Windows.
451 -- fdReady does the right thing, but we have to call it in a
452 -- separate thread, otherwise threadWaitRead won't be interruptible,
453 -- and this only works with -threaded.
454 | threaded = withThread (waitFd fd 0)
455 | otherwise = case fd of
456 0 -> do hWaitForInput stdin (-1); return ()
457 -- hWaitForInput does work properly, but we can only
458 -- do this for stdin since we know its FD.
459 _ -> error "threadWaitRead requires -threaded on Windows, or use System.IO.hWaitForInput"
461 = GHC.Conc.threadWaitRead fd
464 -- | Block the current thread until data can be written to the
465 -- given file descriptor (GHC only).
466 threadWaitWrite :: Fd -> IO ()
468 #ifdef mingw32_HOST_OS
469 | threaded = withThread (waitFd fd 1)
470 | otherwise = error "threadWaitWrite requires -threaded on Windows"
472 = GHC.Conc.threadWaitWrite fd
475 #ifdef mingw32_HOST_OS
476 foreign import ccall unsafe "rtsSupportsBoundThreads" threaded :: Bool
478 withThread :: IO a -> IO a
481 forkIO $ try io >>= putMVar m
485 Left e -> throwIO (e :: IOException)
487 waitFd :: Fd -> CInt -> IO ()
489 throwErrnoIfMinus1 "fdReady" $
490 fdReady (fromIntegral fd) write (fromIntegral iNFINITE) 0
493 iNFINITE = 0xFFFFFFFF :: CInt -- urgh
495 foreign import ccall safe "fdReady"
496 fdReady :: CInt -> CInt -> CInt -> CInt -> IO CInt
499 -- ---------------------------------------------------------------------------
504 #osthreads# In GHC, threads created by 'forkIO' are lightweight threads, and
505 are managed entirely by the GHC runtime. Typically Haskell
506 threads are an order of magnitude or two more efficient (in
507 terms of both time and space) than operating system threads.
509 The downside of having lightweight threads is that only one can
510 run at a time, so if one thread blocks in a foreign call, for
511 example, the other threads cannot continue. The GHC runtime
512 works around this by making use of full OS threads where
513 necessary. When the program is built with the @-threaded@
514 option (to link against the multithreaded version of the
515 runtime), a thread making a @safe@ foreign call will not block
516 the other threads in the system; another OS thread will take
517 over running Haskell threads until the original call returns.
518 The runtime maintains a pool of these /worker/ threads so that
519 multiple Haskell threads can be involved in external calls
522 The "System.IO" library manages multiplexing in its own way. On
523 Windows systems it uses @safe@ foreign calls to ensure that
524 threads doing I\/O operations don't block the whole runtime,
525 whereas on Unix systems all the currently blocked I\/O reqwests
526 are managed by a single thread (the /IO manager thread/) using
529 The runtime will run a Haskell thread using any of the available
530 worker OS threads. If you need control over which particular OS
531 thread is used to run a given Haskell thread, perhaps because
532 you need to call a foreign library that uses OS-thread-local
533 state, then you need bound threads (see "Control.Concurrent#boundthreads").
535 If you don't use the @-threaded@ option, then the runtime does
536 not make use of multiple OS threads. Foreign calls will block
537 all other running Haskell threads until the call returns. The
538 "System.IO" library still does multiplexing, so there can be multiple
539 threads doing I\/O, and this is handled internally by the runtime using
545 In a standalone GHC program, only the main thread is
546 required to terminate in order for the process to terminate.
547 Thus all other forked threads will simply terminate at the same
548 time as the main thread (the terminology for this kind of
549 behaviour is \"daemonic threads\").
551 If you want the program to wait for child threads to
552 finish before exiting, you need to program this yourself. A
553 simple mechanism is to have each child thread write to an
554 'MVar' when it completes, and have the main
555 thread wait on all the 'MVar's before
558 > myForkIO :: IO () -> IO (MVar ())
560 > mvar <- newEmptyMVar
561 > forkIO (io `finally` putMVar mvar ())
564 Note that we use 'finally' from the
565 "Control.Exception" module to make sure that the
566 'MVar' is written to even if the thread dies or
567 is killed for some reason.
569 A better method is to keep a global list of all child
570 threads which we should wait for at the end of the program:
572 > children :: MVar [MVar ()]
573 > children = unsafePerformIO (newMVar [])
575 > waitForChildren :: IO ()
576 > waitForChildren = do
577 > cs <- takeMVar children
581 > putMVar children ms
585 > forkChild :: IO () -> IO ThreadId
587 > mvar <- newEmptyMVar
588 > childs <- takeMVar children
589 > putMVar children (mvar:childs)
590 > forkIO (io `finally` putMVar mvar ())
593 > later waitForChildren $
596 The main thread principle also applies to calls to Haskell from
597 outside, using @foreign export@. When the @foreign export@ed
598 function is invoked, it starts a new main thread, and it returns
599 when this main thread terminates. If the call causes new
600 threads to be forked, they may remain in the system after the
601 @foreign export@ed function has returned.
606 GHC implements pre-emptive multitasking: the execution of
607 threads are interleaved in a random fashion. More specifically,
608 a thread may be pre-empted whenever it allocates some memory,
609 which unfortunately means that tight loops which do no
610 allocation tend to lock out other threads (this only seems to
611 happen with pathological benchmark-style code, however).
613 The rescheduling timer runs on a 20ms granularity by
614 default, but this may be altered using the
615 @-i\<n\>@ RTS option. After a rescheduling
616 \"tick\" the running thread is pre-empted as soon as
620 @aaaa@ @bbbb@ example may not
621 work too well on GHC (see Scheduling, above), due
622 to the locking on a 'System.IO.Handle'. Only one thread
623 may hold the lock on a 'System.IO.Handle' at any one
624 time, so if a reschedule happens while a thread is holding the
625 lock, the other thread won't be able to run. The upshot is that
626 the switch from @aaaa@ to
627 @bbbbb@ happens infrequently. It can be
628 improved by lowering the reschedule tick period. We also have a
629 patch that causes a reschedule whenever a thread waiting on a
630 lock is woken up, but haven't found it to be useful for anything
631 other than this example :-)
633 #endif /* __GLASGOW_HASKELL__ */