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 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@.
298 -- | 'True' if bound threads are supported.
299 -- If @rtsSupportsBoundThreads@ is 'False', 'isCurrentThreadBound'
300 -- will always return 'False' and both 'forkOS' and 'runInBoundThread' will
302 foreign import ccall rtsSupportsBoundThreads :: Bool
306 Like 'forkIO', this sparks off a new thread to run the 'IO' computation passed as the
307 first argument, and returns the 'ThreadId' of the newly created
310 However, @forkOS@ uses operating system-supplied multithreading support to create
311 a new operating system thread. The new thread is /bound/, which means that
312 all foreign calls made by the 'IO' computation are guaranteed to be executed
313 in this new operating system thread; also, the operating system thread is not
314 used for any other foreign calls.
316 This means that you can use all kinds of foreign libraries from this thread
317 (even those that rely on thread-local state), without the limitations of 'forkIO'.
319 Just to clarify, 'forkOS' is /only/ necessary if you need to associate
320 a Haskell thread with a particular OS thread. It is not necessary if
321 you only need to make non-blocking foreign calls (see
322 "Control.Concurrent#osthreads"). Neither is it necessary if you want
323 to run threads in parallel on a multiprocessor: threads created with
324 'forkIO' will be shared out amongst the running CPUs (using GHC,
325 @-threaded@, and the @+RTS -N@ runtime option).
328 forkOS :: IO () -> IO ThreadId
330 foreign export ccall forkOS_entry
331 :: StablePtr (IO ()) -> IO ()
333 foreign import ccall "forkOS_entry" forkOS_entry_reimported
334 :: StablePtr (IO ()) -> IO ()
336 forkOS_entry stableAction = do
337 action <- deRefStablePtr stableAction
340 foreign import ccall forkOS_createThread
341 :: StablePtr (IO ()) -> IO CInt
343 failNonThreaded = fail $ "RTS doesn't support multiple OS threads "
344 ++"(use ghc -threaded when linking)"
347 | rtsSupportsBoundThreads = do
349 b <- Exception.blocked
351 -- async exceptions are blocked in the child if they are blocked
352 -- in the parent, as for forkIO (see #1048). forkOS_createThread
353 -- creates a thread with exceptions blocked by default.
354 action1 | b = action0
355 | otherwise = unblock action0
357 action_plus = Exception.catch action1 childHandler
359 entry <- newStablePtr (myThreadId >>= putMVar mv >> action_plus)
360 err <- forkOS_createThread entry
361 when (err /= 0) $ fail "Cannot create OS thread."
365 | otherwise = failNonThreaded
367 -- | Returns 'True' if the calling thread is /bound/, that is, if it is
368 -- safe to use foreign libraries that rely on thread-local state from the
370 isCurrentThreadBound :: IO Bool
371 isCurrentThreadBound = IO $ \ s# ->
372 case isCurrentThreadBound# s# of
373 (# s2#, flg #) -> (# s2#, not (flg ==# 0#) #)
377 Run the 'IO' computation passed as the first argument. If the calling thread
378 is not /bound/, a bound thread is created temporarily. @runInBoundThread@
379 doesn't finish until the 'IO' computation finishes.
381 You can wrap a series of foreign function calls that rely on thread-local state
382 with @runInBoundThread@ so that you can use them without knowing whether the
383 current thread is /bound/.
385 runInBoundThread :: IO a -> IO a
387 runInBoundThread action
388 | rtsSupportsBoundThreads = do
389 bound <- isCurrentThreadBound
393 ref <- newIORef undefined
394 let action_plus = Exception.try action >>= writeIORef ref
396 bracket (newStablePtr action_plus)
398 (\cEntry -> forkOS_entry_reimported cEntry >> readIORef ref)
399 case resultOrException of
400 Left exception -> Exception.throw (exception :: SomeException)
401 Right result -> return result
402 | otherwise = failNonThreaded
405 Run the 'IO' computation passed as the first argument. If the calling thread
406 is /bound/, an unbound thread is created temporarily using 'forkIO'.
407 @runInBoundThread@ doesn't finish until the 'IO' computation finishes.
409 Use this function /only/ in the rare case that you have actually observed a
410 performance loss due to the use of bound threads. A program that
411 doesn't need it's main thread to be bound and makes /heavy/ use of concurrency
412 (e.g. a web server), might want to wrap it's @main@ action in
413 @runInUnboundThread@.
415 runInUnboundThread :: IO a -> IO a
417 runInUnboundThread action = do
418 bound <- isCurrentThreadBound
422 forkIO (Exception.try action >>= putMVar mv)
423 takeMVar mv >>= \either -> case either of
424 Left exception -> Exception.throw (exception :: SomeException)
425 Right result -> return result
428 #endif /* __GLASGOW_HASKELL__ */
430 -- ---------------------------------------------------------------------------
431 -- threadWaitRead/threadWaitWrite
433 -- | Block the current thread until data is available to read on the
434 -- given file descriptor (GHC only).
435 threadWaitRead :: Fd -> IO ()
437 #ifdef mingw32_HOST_OS
438 -- we have no IO manager implementing threadWaitRead on Windows.
439 -- fdReady does the right thing, but we have to call it in a
440 -- separate thread, otherwise threadWaitRead won't be interruptible,
441 -- and this only works with -threaded.
442 | threaded = withThread (waitFd fd 0)
443 | otherwise = case fd of
444 0 -> do hWaitForInput stdin (-1); return ()
445 -- hWaitForInput does work properly, but we can only
446 -- do this for stdin since we know its FD.
447 _ -> error "threadWaitRead requires -threaded on Windows, or use System.IO.hWaitForInput"
449 = GHC.Conc.threadWaitRead fd
452 -- | Block the current thread until data can be written to the
453 -- given file descriptor (GHC only).
454 threadWaitWrite :: Fd -> IO ()
456 #ifdef mingw32_HOST_OS
457 | threaded = withThread (waitFd fd 1)
458 | otherwise = error "threadWaitWrite requires -threaded on Windows"
460 = GHC.Conc.threadWaitWrite fd
463 #ifdef mingw32_HOST_OS
464 foreign import ccall unsafe "rtsSupportsBoundThreads" threaded :: Bool
466 withThread :: IO a -> IO a
469 forkIO $ try io >>= putMVar m
475 waitFd :: Fd -> CInt -> IO ()
477 throwErrnoIfMinus1 "fdReady" $
478 fdReady (fromIntegral fd) write (fromIntegral iNFINITE) 0
481 iNFINITE = 0xFFFFFFFF :: CInt -- urgh
483 foreign import ccall safe "fdReady"
484 fdReady :: CInt -> CInt -> CInt -> CInt -> IO CInt
487 -- ---------------------------------------------------------------------------
492 #osthreads# In GHC, threads created by 'forkIO' are lightweight threads, and
493 are managed entirely by the GHC runtime. Typically Haskell
494 threads are an order of magnitude or two more efficient (in
495 terms of both time and space) than operating system threads.
497 The downside of having lightweight threads is that only one can
498 run at a time, so if one thread blocks in a foreign call, for
499 example, the other threads cannot continue. The GHC runtime
500 works around this by making use of full OS threads where
501 necessary. When the program is built with the @-threaded@
502 option (to link against the multithreaded version of the
503 runtime), a thread making a @safe@ foreign call will not block
504 the other threads in the system; another OS thread will take
505 over running Haskell threads until the original call returns.
506 The runtime maintains a pool of these /worker/ threads so that
507 multiple Haskell threads can be involved in external calls
510 The "System.IO" library manages multiplexing in its own way. On
511 Windows systems it uses @safe@ foreign calls to ensure that
512 threads doing I\/O operations don't block the whole runtime,
513 whereas on Unix systems all the currently blocked I\/O reqwests
514 are managed by a single thread (the /IO manager thread/) using
517 The runtime will run a Haskell thread using any of the available
518 worker OS threads. If you need control over which particular OS
519 thread is used to run a given Haskell thread, perhaps because
520 you need to call a foreign library that uses OS-thread-local
521 state, then you need bound threads (see "Control.Concurrent#boundthreads").
523 If you don't use the @-threaded@ option, then the runtime does
524 not make use of multiple OS threads. Foreign calls will block
525 all other running Haskell threads until the call returns. The
526 "System.IO" library still does multiplexing, so there can be multiple
527 threads doing I\/O, and this is handled internally by the runtime using
533 In a standalone GHC program, only the main thread is
534 required to terminate in order for the process to terminate.
535 Thus all other forked threads will simply terminate at the same
536 time as the main thread (the terminology for this kind of
537 behaviour is \"daemonic threads\").
539 If you want the program to wait for child threads to
540 finish before exiting, you need to program this yourself. A
541 simple mechanism is to have each child thread write to an
542 'MVar' when it completes, and have the main
543 thread wait on all the 'MVar's before
546 > myForkIO :: IO () -> IO (MVar ())
548 > mvar <- newEmptyMVar
549 > forkIO (io `finally` putMVar mvar ())
552 Note that we use 'finally' from the
553 "Control.Exception" module to make sure that the
554 'MVar' is written to even if the thread dies or
555 is killed for some reason.
557 A better method is to keep a global list of all child
558 threads which we should wait for at the end of the program:
560 > children :: MVar [MVar ()]
561 > children = unsafePerformIO (newMVar [])
563 > waitForChildren :: IO ()
564 > waitForChildren = do
565 > cs <- takeMVar children
569 > putMVar children ms
573 > forkChild :: IO () -> IO ThreadId
575 > mvar <- newEmptyMVar
576 > childs <- takeMVar children
577 > putMVar children (mvar:childs)
578 > forkIO (io `finally` putMVar mvar ())
581 > later waitForChildren $
584 The main thread principle also applies to calls to Haskell from
585 outside, using @foreign export@. When the @foreign export@ed
586 function is invoked, it starts a new main thread, and it returns
587 when this main thread terminates. If the call causes new
588 threads to be forked, they may remain in the system after the
589 @foreign export@ed function has returned.
594 GHC implements pre-emptive multitasking: the execution of
595 threads are interleaved in a random fashion. More specifically,
596 a thread may be pre-empted whenever it allocates some memory,
597 which unfortunately means that tight loops which do no
598 allocation tend to lock out other threads (this only seems to
599 happen with pathological benchmark-style code, however).
601 The rescheduling timer runs on a 20ms granularity by
602 default, but this may be altered using the
603 @-i\<n\>@ RTS option. After a rescheduling
604 \"tick\" the running thread is pre-empted as soon as
608 @aaaa@ @bbbb@ example may not
609 work too well on GHC (see Scheduling, above), due
610 to the locking on a 'System.IO.Handle'. Only one thread
611 may hold the lock on a 'System.IO.Handle' at any one
612 time, so if a reschedule happens while a thread is holding the
613 lock, the other thread won't be able to run. The upshot is that
614 the switch from @aaaa@ to
615 @bbbbb@ happens infrequently. It can be
616 improved by lowering the reschedule tick period. We also have a
617 patch that causes a reschedule whenever a thread waiting on a
618 lock is woken up, but haven't found it to be useful for anything
619 other than this example :-)