Rewrite of the IO library, including Unicode support
[packages/base.git] / Control / Concurrent.hs
1 {-# OPTIONS_GHC -fno-warn-unused-imports #-}
2 -----------------------------------------------------------------------------
3 -- |
4 -- Module : Control.Concurrent
5 -- Copyright : (c) The University of Glasgow 2001
6 -- License : BSD-style (see the file libraries/base/LICENSE)
7 --
8 -- Maintainer : libraries@haskell.org
9 -- Stability : experimental
10 -- Portability : non-portable (concurrency)
11 --
12 -- A common interface to a collection of useful concurrency
13 -- abstractions.
14 --
15 -----------------------------------------------------------------------------
16
17 module Control.Concurrent (
18 -- * Concurrent Haskell
19
20 -- $conc_intro
21
22 -- * Basic concurrency operations
23
24 ThreadId,
25 #ifdef __GLASGOW_HASKELL__
26 myThreadId,
27 #endif
28
29 forkIO,
30 #ifdef __GLASGOW_HASKELL__
31 killThread,
32 throwTo,
33 #endif
34
35 -- * Scheduling
36
37 -- $conc_scheduling
38 yield, -- :: IO ()
39
40 -- ** Blocking
41
42 -- $blocking
43
44 #ifdef __GLASGOW_HASKELL__
45 -- ** Waiting
46 threadDelay, -- :: Int -> IO ()
47 threadWaitRead, -- :: Int -> IO ()
48 threadWaitWrite, -- :: Int -> IO ()
49 #endif
50
51 -- * Communication abstractions
52
53 module Control.Concurrent.MVar,
54 module Control.Concurrent.Chan,
55 module Control.Concurrent.QSem,
56 module Control.Concurrent.QSemN,
57 module Control.Concurrent.SampleVar,
58
59 -- * Merging of streams
60 #ifndef __HUGS__
61 mergeIO, -- :: [a] -> [a] -> IO [a]
62 nmergeIO, -- :: [[a]] -> IO [a]
63 #endif
64 -- $merge
65
66 #ifdef __GLASGOW_HASKELL__
67 -- * Bound Threads
68 -- $boundthreads
69 rtsSupportsBoundThreads,
70 forkOS,
71 isCurrentThreadBound,
72 runInBoundThread,
73 runInUnboundThread
74 #endif
75
76 -- * GHC's implementation of concurrency
77
78 -- |This section describes features specific to GHC's
79 -- implementation of Concurrent Haskell.
80
81 -- ** Haskell threads and Operating System threads
82
83 -- $osthreads
84
85 -- ** Terminating the program
86
87 -- $termination
88
89 -- ** Pre-emption
90
91 -- $preemption
92 ) where
93
94 import Prelude
95
96 import Control.Exception.Base as Exception
97
98 #ifdef __GLASGOW_HASKELL__
99 import GHC.Exception
100 import GHC.Conc ( ThreadId(..), myThreadId, killThread, yield,
101 threadDelay, forkIO, childHandler )
102 import qualified GHC.Conc
103 import GHC.IO ( IO(..), unsafeInterleaveIO )
104 import GHC.IORef ( newIORef, readIORef, writeIORef )
105 import GHC.Base
106
107 import System.Posix.Types ( Fd )
108 import Foreign.StablePtr
109 import Foreign.C.Types ( CInt )
110 import Control.Monad ( when )
111
112 #ifdef mingw32_HOST_OS
113 import Foreign.C
114 import System.IO
115 #endif
116 #endif
117
118 #ifdef __HUGS__
119 import Hugs.ConcBase
120 #endif
121
122 import Control.Concurrent.MVar
123 import Control.Concurrent.Chan
124 import Control.Concurrent.QSem
125 import Control.Concurrent.QSemN
126 import Control.Concurrent.SampleVar
127
128 #ifdef __HUGS__
129 type ThreadId = ()
130 #endif
131
132 {- $conc_intro
133
134 The concurrency extension for Haskell is described in the paper
135 /Concurrent Haskell/
136 <http://www.haskell.org/ghc/docs/papers/concurrent-haskell.ps.gz>.
137
138 Concurrency is \"lightweight\", which means that both thread creation
139 and context switching overheads are extremely low. Scheduling of
140 Haskell threads is done internally in the Haskell runtime system, and
141 doesn't make use of any operating system-supplied thread packages.
142
143 However, if you want to interact with a foreign library that expects your
144 program to use the operating system-supplied thread package, you can do so
145 by using 'forkOS' instead of 'forkIO'.
146
147 Haskell threads can communicate via 'MVar's, a kind of synchronised
148 mutable variable (see "Control.Concurrent.MVar"). Several common
149 concurrency abstractions can be built from 'MVar's, and these are
150 provided by the "Control.Concurrent" library.
151 In GHC, threads may also communicate via exceptions.
152 -}
153
154 {- $conc_scheduling
155
156 Scheduling may be either pre-emptive or co-operative,
157 depending on the implementation of Concurrent Haskell (see below
158 for information related to specific compilers). In a co-operative
159 system, context switches only occur when you use one of the
160 primitives defined in this module. This means that programs such
161 as:
162
163
164 > main = forkIO (write 'a') >> write 'b'
165 > where write c = putChar c >> write c
166
167 will print either @aaaaaaaaaaaaaa...@ or @bbbbbbbbbbbb...@,
168 instead of some random interleaving of @a@s and @b@s. In
169 practice, cooperative multitasking is sufficient for writing
170 simple graphical user interfaces.
171 -}
172
173 {- $blocking
174 Different Haskell implementations have different characteristics with
175 regard to which operations block /all/ threads.
176
177 Using GHC without the @-threaded@ option, all foreign calls will block
178 all other Haskell threads in the system, although I\/O operations will
179 not. With the @-threaded@ option, only foreign calls with the @unsafe@
180 attribute will block all other threads.
181
182 Using Hugs, all I\/O operations and foreign calls will block all other
183 Haskell threads.
184 -}
185
186 #ifndef __HUGS__
187 max_buff_size :: Int
188 max_buff_size = 1
189
190 mergeIO :: [a] -> [a] -> IO [a]
191 nmergeIO :: [[a]] -> IO [a]
192
193 -- $merge
194 -- The 'mergeIO' and 'nmergeIO' functions fork one thread for each
195 -- input list that concurrently evaluates that list; the results are
196 -- merged into a single output list.
197 --
198 -- Note: Hugs does not provide these functions, since they require
199 -- preemptive multitasking.
200
201 mergeIO ls rs
202 = newEmptyMVar >>= \ tail_node ->
203 newMVar tail_node >>= \ tail_list ->
204 newQSem max_buff_size >>= \ e ->
205 newMVar 2 >>= \ branches_running ->
206 let
207 buff = (tail_list,e)
208 in
209 forkIO (suckIO branches_running buff ls) >>
210 forkIO (suckIO branches_running buff rs) >>
211 takeMVar tail_node >>= \ val ->
212 signalQSem e >>
213 return val
214
215 type Buffer a
216 = (MVar (MVar [a]), QSem)
217
218 suckIO :: MVar Int -> Buffer a -> [a] -> IO ()
219
220 suckIO branches_running buff@(tail_list,e) vs
221 = case vs of
222 [] -> takeMVar branches_running >>= \ val ->
223 if val == 1 then
224 takeMVar tail_list >>= \ node ->
225 putMVar node [] >>
226 putMVar tail_list node
227 else
228 putMVar branches_running (val-1)
229 (x:xs) ->
230 waitQSem e >>
231 takeMVar tail_list >>= \ node ->
232 newEmptyMVar >>= \ next_node ->
233 unsafeInterleaveIO (
234 takeMVar next_node >>= \ y ->
235 signalQSem e >>
236 return y) >>= \ next_node_val ->
237 putMVar node (x:next_node_val) >>
238 putMVar tail_list next_node >>
239 suckIO branches_running buff xs
240
241 nmergeIO lss
242 = let
243 len = length lss
244 in
245 newEmptyMVar >>= \ tail_node ->
246 newMVar tail_node >>= \ tail_list ->
247 newQSem max_buff_size >>= \ e ->
248 newMVar len >>= \ branches_running ->
249 let
250 buff = (tail_list,e)
251 in
252 mapIO (\ x -> forkIO (suckIO branches_running buff x)) lss >>
253 takeMVar tail_node >>= \ val ->
254 signalQSem e >>
255 return val
256 where
257 mapIO f xs = sequence (map f xs)
258 #endif /* __HUGS__ */
259
260 #ifdef __GLASGOW_HASKELL__
261 -- ---------------------------------------------------------------------------
262 -- Bound Threads
263
264 {- $boundthreads
265 #boundthreads#
266
267 Support for multiple operating system threads and bound threads as described
268 below is currently only available in the GHC runtime system if you use the
269 /-threaded/ option when linking.
270
271 Other Haskell systems do not currently support multiple operating system threads.
272
273 A bound thread is a haskell thread that is /bound/ to an operating system
274 thread. While the bound thread is still scheduled by the Haskell run-time
275 system, the operating system thread takes care of all the foreign calls made
276 by the bound thread.
277
278 To a foreign library, the bound thread will look exactly like an ordinary
279 operating system thread created using OS functions like @pthread_create@
280 or @CreateThread@.
281
282 Bound threads can be created using the 'forkOS' function below. All foreign
283 exported functions are run in a bound thread (bound to the OS thread that
284 called the function). Also, the @main@ action of every Haskell program is
285 run in a bound thread.
286
287 Why do we need this? Because if a foreign library is called from a thread
288 created using 'forkIO', it won't have access to any /thread-local state/ -
289 state variables that have specific values for each OS thread
290 (see POSIX's @pthread_key_create@ or Win32's @TlsAlloc@). Therefore, some
291 libraries (OpenGL, for example) will not work from a thread created using
292 'forkIO'. They work fine in threads created using 'forkOS' or when called
293 from @main@ or from a @foreign export@.
294
295 In terms of performance, 'forkOS' (aka bound) threads are much more
296 expensive than 'forkIO' (aka unbound) threads, because a 'forkOS'
297 thread is tied to a particular OS thread, whereas a 'forkIO' thread
298 can be run by any OS thread. Context-switching between a 'forkOS'
299 thread and a 'forkIO' thread is many times more expensive than between
300 two 'forkIO' threads.
301
302 Note in particular that the main program thread (the thread running
303 @Main.main@) is always a bound thread, so for good concurrency
304 performance you should ensure that the main thread is not doing
305 repeated communication with other threads in the system. Typically
306 this means forking subthreads to do the work using 'forkIO', and
307 waiting for the results in the main thread.
308
309 -}
310
311 -- | 'True' if bound threads are supported.
312 -- If @rtsSupportsBoundThreads@ is 'False', 'isCurrentThreadBound'
313 -- will always return 'False' and both 'forkOS' and 'runInBoundThread' will
314 -- fail.
315 foreign import ccall rtsSupportsBoundThreads :: Bool
316
317
318 {- |
319 Like 'forkIO', this sparks off a new thread to run the 'IO'
320 computation passed as the first argument, and returns the 'ThreadId'
321 of the newly created thread.
322
323 However, 'forkOS' creates a /bound/ thread, which is necessary if you
324 need to call foreign (non-Haskell) libraries that make use of
325 thread-local state, such as OpenGL (see "Control.Concurrent#boundthreads").
326
327 Using 'forkOS' instead of 'forkIO' makes no difference at all to the
328 scheduling behaviour of the Haskell runtime system. It is a common
329 misconception that you need to use 'forkOS' instead of 'forkIO' to
330 avoid blocking all the Haskell threads when making a foreign call;
331 this isn't the case. To allow foreign calls to be made without
332 blocking all the Haskell threads (with GHC), it is only necessary to
333 use the @-threaded@ option when linking your program, and to make sure
334 the foreign import is not marked @unsafe@.
335 -}
336
337 forkOS :: IO () -> IO ThreadId
338
339 foreign export ccall forkOS_entry
340 :: StablePtr (IO ()) -> IO ()
341
342 foreign import ccall "forkOS_entry" forkOS_entry_reimported
343 :: StablePtr (IO ()) -> IO ()
344
345 forkOS_entry :: StablePtr (IO ()) -> IO ()
346 forkOS_entry stableAction = do
347 action <- deRefStablePtr stableAction
348 action
349
350 foreign import ccall forkOS_createThread
351 :: StablePtr (IO ()) -> IO CInt
352
353 failNonThreaded :: IO a
354 failNonThreaded = fail $ "RTS doesn't support multiple OS threads "
355 ++"(use ghc -threaded when linking)"
356
357 forkOS action0
358 | rtsSupportsBoundThreads = do
359 mv <- newEmptyMVar
360 b <- Exception.blocked
361 let
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
367
368 action_plus = Exception.catch action1 childHandler
369
370 entry <- newStablePtr (myThreadId >>= putMVar mv >> action_plus)
371 err <- forkOS_createThread entry
372 when (err /= 0) $ fail "Cannot create OS thread."
373 tid <- takeMVar mv
374 freeStablePtr entry
375 return tid
376 | otherwise = failNonThreaded
377
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
380 -- calling thread.
381 isCurrentThreadBound :: IO Bool
382 isCurrentThreadBound = IO $ \ s# ->
383 case isCurrentThreadBound# s# of
384 (# s2#, flg #) -> (# s2#, not (flg ==# 0#) #)
385
386
387 {- |
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.
391
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/.
395 -}
396 runInBoundThread :: IO a -> IO a
397
398 runInBoundThread action
399 | rtsSupportsBoundThreads = do
400 bound <- isCurrentThreadBound
401 if bound
402 then action
403 else do
404 ref <- newIORef undefined
405 let action_plus = Exception.try action >>= writeIORef ref
406 resultOrException <-
407 bracket (newStablePtr action_plus)
408 freeStablePtr
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
414
415 {- |
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.
419
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@.
425 -}
426 runInUnboundThread :: IO a -> IO a
427
428 runInUnboundThread action = do
429 bound <- isCurrentThreadBound
430 if bound
431 then do
432 mv <- newEmptyMVar
433 forkIO (Exception.try action >>= putMVar mv)
434 takeMVar mv >>= \ei -> case ei of
435 Left exception -> Exception.throw (exception :: SomeException)
436 Right result -> return result
437 else action
438
439 #endif /* __GLASGOW_HASKELL__ */
440
441 #ifdef __GLASGOW_HASKELL__
442 -- ---------------------------------------------------------------------------
443 -- threadWaitRead/threadWaitWrite
444
445 -- | Block the current thread until data is available to read on the
446 -- given file descriptor (GHC only).
447 threadWaitRead :: Fd -> IO ()
448 threadWaitRead fd
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"
460 #else
461 = GHC.Conc.threadWaitRead fd
462 #endif
463
464 -- | Block the current thread until data can be written to the
465 -- given file descriptor (GHC only).
466 threadWaitWrite :: Fd -> IO ()
467 threadWaitWrite fd
468 #ifdef mingw32_HOST_OS
469 | threaded = withThread (waitFd fd 1)
470 | otherwise = error "threadWaitWrite requires -threaded on Windows"
471 #else
472 = GHC.Conc.threadWaitWrite fd
473 #endif
474
475 #ifdef mingw32_HOST_OS
476 foreign import ccall unsafe "rtsSupportsBoundThreads" threaded :: Bool
477
478 withThread :: IO a -> IO a
479 withThread io = do
480 m <- newEmptyMVar
481 forkIO $ try io >>= putMVar m
482 x <- takeMVar m
483 case x of
484 Right a -> return a
485 Left e -> throwIO (e :: IOException)
486
487 waitFd :: Fd -> CInt -> IO ()
488 waitFd fd write = do
489 throwErrnoIfMinus1 "fdReady" $
490 fdReady (fromIntegral fd) write (fromIntegral iNFINITE) 0
491 return ()
492
493 iNFINITE :: CInt
494 iNFINITE = 0xFFFFFFFF -- urgh
495
496 foreign import ccall safe "fdReady"
497 fdReady :: CInt -> CInt -> CInt -> CInt -> IO CInt
498 #endif
499
500 -- ---------------------------------------------------------------------------
501 -- More docs
502
503 {- $osthreads
504
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.
509
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
521 simultaneously.
522
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 requests
527 are managed by a single thread (the /IO manager thread/) using
528 @select@.
529
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").
535
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
541 @select@.
542 -}
543
544 {- $termination
545
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\").
551
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
557 exiting:
558
559 > myForkIO :: IO () -> IO (MVar ())
560 > myForkIO io = do
561 > mvar <- newEmptyMVar
562 > forkIO (io `finally` putMVar mvar ())
563 > return mvar
564
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.
569
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:
572
573 > children :: MVar [MVar ()]
574 > children = unsafePerformIO (newMVar [])
575 >
576 > waitForChildren :: IO ()
577 > waitForChildren = do
578 > cs <- takeMVar children
579 > case cs of
580 > [] -> return ()
581 > m:ms -> do
582 > putMVar children ms
583 > takeMVar m
584 > waitForChildren
585 >
586 > forkChild :: IO () -> IO ThreadId
587 > forkChild io = do
588 > mvar <- newEmptyMVar
589 > childs <- takeMVar children
590 > putMVar children (mvar:childs)
591 > forkIO (io `finally` putMVar mvar ())
592 >
593 > main =
594 > later waitForChildren $
595 > ...
596
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.
603 -}
604
605 {- $preemption
606
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).
613
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
618 possible.
619
620 One final note: the
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 :-)
633 -}
634 #endif /* __GLASGOW_HASKELL__ */