    -----------------------------------------------------------------------------
    -- |
    -- Module      :  Control.Concurrent
    -- Copyright   :  (c) The University of Glasgow 2001
    -- License     :  BSD-style (see the file libraries/base/LICENSE)
    --
    -- Maintainer  :  libraries@haskell.org
    -- Stability   :  experimental
    -- Portability :  non-portable (concurrency)
    --
    -- A common interface to a collection of useful concurrency
    -- abstractions.
    --
    -----------------------------------------------------------------------------

    module Control.Concurrent (
            -- * Concurrent Haskell

            -- $conc_intro

            -- * Basic concurrency operations

            ThreadId,
    #ifdef __GLASGOW_HASKELL__
            myThreadId,
    #endif

            forkIO,
    #ifdef __GLASGOW_HASKELL__
            killThread,
            throwTo,
    #endif

            -- * Scheduling

            -- $conc_scheduling
            yield,                  -- :: IO ()

            -- ** Blocking

            -- $blocking

    #ifdef __GLASGOW_HASKELL__
            -- ** Waiting
            threadDelay,            -- :: Int -> IO ()
            threadWaitRead,         -- :: Int -> IO ()
            threadWaitWrite,        -- :: Int -> IO ()
    #endif

            -- * Communication abstractions

            module Control.Concurrent.MVar,
            module Control.Concurrent.Chan,
            module Control.Concurrent.QSem,
            module Control.Concurrent.QSemN,
            module Control.Concurrent.SampleVar,

            -- * Merging of streams
    #ifndef __HUGS__
            mergeIO,                -- :: [a]   -> [a] -> IO [a]
            nmergeIO,               -- :: [[a]] -> IO [a]
    #endif
            -- $merge

    #ifdef __GLASGOW_HASKELL__
            -- * Bound Threads
            -- $boundthreads
            rtsSupportsBoundThreads,
            forkOS,
            isCurrentThreadBound,
            runInBoundThread,
            runInUnboundThread
    #endif

            -- * GHC's implementation of concurrency

            -- |This section describes features specific to GHC's
            -- implementation of Concurrent Haskell.

            -- ** Haskell threads and Operating System threads

            -- $osthreads

            -- ** Terminating the program

            -- $termination

            -- ** Pre-emption

            -- $preemption
        ) where

    import Prelude

    import Control.Exception as Exception

    #ifdef __GLASGOW_HASKELL__
    import GHC.Conc         ( ThreadId(..), myThreadId, killThread, yield,
                              threadDelay, threadWaitRead, threadWaitWrite,
                              forkIO, childHandler )
    import GHC.TopHandler   ( reportStackOverflow, reportError )
    import GHC.IOBase       ( IO(..) )
    import GHC.IOBase       ( unsafeInterleaveIO )
    import GHC.IOBase       ( newIORef, readIORef, writeIORef )
    import GHC.Base

    import Foreign.StablePtr
    import Foreign.C.Types  ( CInt )
    import Control.Monad    ( when )
    #endif

    #ifdef __HUGS__
    import Hugs.ConcBase
    #endif

    import Control.Concurrent.MVar
    import Control.Concurrent.Chan
    import Control.Concurrent.QSem
    import Control.Concurrent.QSemN
    import Control.Concurrent.SampleVar

    #ifdef __HUGS__
    type ThreadId = ()
    #endif

    {- $conc_intro

    The concurrency extension for Haskell is described in the paper
    /Concurrent Haskell/
    <http://www.haskell.org/ghc/docs/papers/concurrent-haskell.ps.gz>.

    Concurrency is \"lightweight\", which means that both thread creation
    and context switching overheads are extremely low.  Scheduling of
    Haskell threads is done internally in the Haskell runtime system, and
    doesn't make use of any operating system-supplied thread packages.

    However, if you want to interact with a foreign library that expects your
    program to use the operating system-supplied thread package, you can do so
    by using 'forkOS' instead of 'forkIO'.

    Haskell threads can communicate via 'MVar's, a kind of synchronised
    mutable variable (see "Control.Concurrent.MVar").  Several common
    concurrency abstractions can be built from 'MVar's, and these are
    provided by the "Control.Concurrent" library.
    In GHC, threads may also communicate via exceptions.
    -}

    {- $conc_scheduling

        Scheduling may be either pre-emptive or co-operative,
        depending on the implementation of Concurrent Haskell (see below
        for information related to specific compilers).  In a co-operative
        system, context switches only occur when you use one of the
        primitives defined in this module.  This means that programs such
        as:

    >   main = forkIO (write 'a') >> write 'b'
    >     where write c = putChar c >> write c

        will print either @aaaaaaaaaaaaaa...@ or @bbbbbbbbbbbb...@,
        instead of some random interleaving of @a@s and @b@s.  In
        practice, cooperative multitasking is sufficient for writing
        simple graphical user interfaces.
    -}

    {- $blocking
    Different Haskell implementations have different characteristics with
    regard to which operations block /all/ threads.

    Using GHC without the @-threaded@ option, all foreign calls will block
    all other Haskell threads in the system, although I\/O operations will
    not.  With the @-threaded@ option, only foreign calls with the @unsafe@
    attribute will block all other threads.

    Using Hugs, all I\/O operations and foreign calls will block all other
    Haskell threads.
    -}

    #ifndef __HUGS__
    max_buff_size :: Int
    max_buff_size = 1

    mergeIO :: [a] -> [a] -> IO [a]
    nmergeIO :: [[a]] -> IO [a]

    -- $merge
    -- The 'mergeIO' and 'nmergeIO' functions fork one thread for each
    -- input list that concurrently evaluates that list; the results are
    -- merged into a single output list.
    --
    -- Note: Hugs does not provide these functions, since they require
    -- preemptive multitasking.

    mergeIO ls rs
     = newEmptyMVar                >>= \ tail_node ->
       newMVar tail_node           >>= \ tail_list ->
       newQSem max_buff_size       >>= \ e ->
       newMVar 2                   >>= \ branches_running ->
       let
        buff = (tail_list,e)
       in
        forkIO (suckIO branches_running buff ls) >>
        forkIO (suckIO branches_running buff rs) >>
        takeMVar tail_node  >>= \ val ->
        signalQSem e        >>
        return val

    type Buffer a
     = (MVar (MVar [a]), QSem)

    suckIO :: MVar Int -> Buffer a -> [a] -> IO ()

    suckIO branches_running buff@(tail_list,e) vs
     = case vs of
            [] -> takeMVar branches_running >>= \ val ->
                  if val == 1 then
                     takeMVar tail_list     >>= \ node ->
                     putMVar node []        >>
                     putMVar tail_list node
                  else
                     putMVar branches_running (val-1)
            (x:xs) ->
                    waitQSem e                       >>
                    takeMVar tail_list               >>= \ node ->
                    newEmptyMVar                     >>= \ next_node ->
                    unsafeInterleaveIO (
                            takeMVar next_node  >>= \ y ->
                            signalQSem e        >>
                            return y)                >>= \ next_node_val ->
                    putMVar node (x:next_node_val)   >>
                    putMVar tail_list next_node      >>
                    suckIO branches_running buff xs

    nmergeIO lss
     = let
        len = length lss
       in
        newEmptyMVar          >>= \ tail_node ->
        newMVar tail_node     >>= \ tail_list ->
        newQSem max_buff_size >>= \ e ->
        newMVar len           >>= \ branches_running ->
        let
         buff = (tail_list,e)
        in
        mapIO (\ x -> forkIO (suckIO branches_running buff x)) lss >>
        takeMVar tail_node  >>= \ val ->
        signalQSem e        >>
        return val
      where
        mapIO f xs = sequence (map f xs)
    #endif /* __HUGS__ */

    #ifdef __GLASGOW_HASKELL__
    -- ---------------------------------------------------------------------------
    -- Bound Threads

    {- $boundthreads
       #boundthreads#

    Support for multiple operating system threads and bound threads as described
    below is currently only available in the GHC runtime system if you use the
    /-threaded/ option when linking.

    Other Haskell systems do not currently support multiple operating system threads.

    A bound thread is a haskell thread that is /bound/ to an operating system
    thread. While the bound thread is still scheduled by the Haskell run-time
    system, the operating system thread takes care of all the foreign calls made
    by the bound thread.

    To a foreign library, the bound thread will look exactly like an ordinary
    operating system thread created using OS functions like @pthread_create@
    or @CreateThread@.

    Bound threads can be created using the 'forkOS' function below. All foreign
    exported functions are run in a bound thread (bound to the OS thread that
    called the function). Also, the @main@ action of every Haskell program is
    run in a bound thread.

    Why do we need this? Because if a foreign library is called from a thread
    created using 'forkIO', it won't have access to any /thread-local state/ -
    state variables that have specific values for each OS thread
    (see POSIX's @pthread_key_create@ or Win32's @TlsAlloc@). Therefore, some
    libraries (OpenGL, for example) will not work from a thread created using
    'forkIO'. They work fine in threads created using 'forkOS' or when called
    from @main@ or from a @foreign export@.
    -}

    -- | 'True' if bound threads are supported.
    -- If @rtsSupportsBoundThreads@ is 'False', 'isCurrentThreadBound'
    -- will always return 'False' and both 'forkOS' and 'runInBoundThread' will
    -- fail.
    foreign import ccall rtsSupportsBoundThreads :: Bool

    {- |
    Like 'forkIO', this sparks off a new thread to run the 'IO' computation passed as the
    first argument, and returns the 'ThreadId' of the newly created
    thread.

    However, @forkOS@ uses operating system-supplied multithreading support to create
    a new operating system thread. The new thread is /bound/, which means that
    all foreign calls made by the 'IO' computation are guaranteed to be executed
    in this new operating system thread; also, the operating system thread is not
    used for any other foreign calls.

    This means that you can use all kinds of foreign libraries from this thread
    (even those that rely on thread-local state), without the limitations of 'forkIO'.

    Just to clarify, 'forkOS' is /only/ necessary if you need to associate
    a Haskell thread with a particular OS thread.  It is not necessary if
    you only need to make non-blocking foreign calls (see
    "Control.Concurrent#osthreads").  Neither is it necessary if you want
    to run threads in parallel on a multiprocessor: threads created with
    'forkIO' will be shared out amongst the running CPUs (using GHC,
    @-threaded@, and the @+RTS -N@ runtime option).

    -}
    forkOS :: IO () -> IO ThreadId

    foreign export ccall forkOS_entry
        :: StablePtr (IO ()) -> IO ()

    foreign import ccall "forkOS_entry" forkOS_entry_reimported
        :: StablePtr (IO ()) -> IO ()

    forkOS_entry stableAction = do
            action <- deRefStablePtr stableAction
            action

    foreign import ccall forkOS_createThread
        :: StablePtr (IO ()) -> IO CInt

    failNonThreaded = fail $ "RTS doesn't support multiple OS threads "
                           ++"(use ghc -threaded when linking)"

    forkOS action
        | rtsSupportsBoundThreads = do
            mv <- newEmptyMVar
            let action_plus = Exception.catch action childHandler
            entry <- newStablePtr (myThreadId >>= putMVar mv >> action_plus)
            err <- forkOS_createThread entry
            when (err /= 0) $ fail "Cannot create OS thread."
            tid <- takeMVar mv
            freeStablePtr entry
            return tid
        | otherwise = failNonThreaded

    -- | Returns 'True' if the calling thread is /bound/, that is, if it is
    -- safe to use foreign libraries that rely on thread-local state from the
    -- calling thread.
    isCurrentThreadBound :: IO Bool
    isCurrentThreadBound = IO $ \ s# ->
        case isCurrentThreadBound# s# of
            (# s2#, flg #) -> (# s2#, not (flg ==# 0#) #)

    {- |
    Run the 'IO' computation passed as the first argument. If the calling thread
    is not /bound/, a bound thread is created temporarily. @runInBoundThread@
    doesn't finish until the 'IO' computation finishes.

    You can wrap a series of foreign function calls that rely on thread-local state
    with @runInBoundThread@ so that you can use them without knowing whether the
    current thread is /bound/.
    -}
    runInBoundThread :: IO a -> IO a

    runInBoundThread action
        | rtsSupportsBoundThreads = do
            bound <- isCurrentThreadBound
            if bound
                then action
                else do
                    ref <- newIORef undefined
                    let action_plus = Exception.try action >>= writeIORef ref
                    resultOrException <-
                        bracket (newStablePtr action_plus)
                                freeStablePtr
                                (\cEntry -> forkOS_entry_reimported cEntry >> readIORef ref)
                    case resultOrException of
                        Left exception -> Exception.throw exception
                        Right result -> return result
        | otherwise = failNonThreaded

    {- |
    Run the 'IO' computation passed as the first argument. If the calling thread
    is /bound/, an unbound thread is created temporarily using 'forkIO'.
    @runInBoundThread@ doesn't finish until the 'IO' computation finishes.

    Use this function /only/ in the rare case that you have actually observed a
    performance loss due to the use of bound threads. A program that
    doesn't need it's main thread to be bound and makes /heavy/ use of concurrency
    (e.g. a web server), might want to wrap it's @main@ action in
    @runInUnboundThread@.
    -}
    runInUnboundThread :: IO a -> IO a

    runInUnboundThread action = do
        bound <- isCurrentThreadBound
        if bound
            then do
                mv <- newEmptyMVar
                forkIO (Exception.try action >>= putMVar mv)
                takeMVar mv >>= \either -> case either of
                    Left exception -> Exception.throw exception
                    Right result -> return result
            else action

    #endif /* __GLASGOW_HASKELL__ */

    -- ---------------------------------------------------------------------------
    -- More docs

    {- $osthreads

          #osthreads# In GHC, threads created by 'forkIO' are lightweight threads, and
          are managed entirely by the GHC runtime.  Typically Haskell
          threads are an order of magnitude or two more efficient (in
          terms of both time and space) than operating system threads.

          The downside of having lightweight threads is that only one can
          run at a time, so if one thread blocks in a foreign call, for
          example, the other threads cannot continue.  The GHC runtime
          works around this by making use of full OS threads where
          necessary.  When the program is built with the @-threaded@
          option (to link against the multithreaded version of the
          runtime), a thread making a @safe@ foreign call will not block
          the other threads in the system; another OS thread will take
          over running Haskell threads until the original call returns.
          The runtime maintains a pool of these /worker/ threads so that
          multiple Haskell threads can be involved in external calls
          simultaneously.

          The "System.IO" library manages multiplexing in its own way.  On
          Windows systems it uses @safe@ foreign calls to ensure that
          threads doing I\/O operations don't block the whole runtime,
          whereas on Unix systems all the currently blocked I\/O reqwests
          are managed by a single thread (the /IO manager thread/) using
          @select@.

          The runtime will run a Haskell thread using any of the available
          worker OS threads.  If you need control over which particular OS
          thread is used to run a given Haskell thread, perhaps because
          you need to call a foreign library that uses OS-thread-local
          state, then you need bound threads (see "Control.Concurrent#boundthreads").

          If you don't use the @-threaded@ option, then the runtime does
          not make use of multiple OS threads.  Foreign calls will block
          all other running Haskell threads until the call returns.  The
          "System.IO" library still does multiplexing, so there can be multiple
          threads doing I\/O, and this is handled internally by the runtime using
          @select@.
    -}

    {- $termination

          In a standalone GHC program, only the main thread is
          required to terminate in order for the process to terminate.
          Thus all other forked threads will simply terminate at the same
          time as the main thread (the terminology for this kind of
          behaviour is \"daemonic threads\").

          If you want the program to wait for child threads to
          finish before exiting, you need to program this yourself.  A
          simple mechanism is to have each child thread write to an
          'MVar' when it completes, and have the main
          thread wait on all the 'MVar's before
          exiting:

    >   myForkIO :: IO () -> IO (MVar ())
    >   myForkIO io = do
    >     mvar <- newEmptyMVar
    >     forkIO (io `finally` putMVar mvar ())
    >     return mvar

          Note that we use 'finally' from the
          "Control.Exception" module to make sure that the
          'MVar' is written to even if the thread dies or
          is killed for some reason.

          A better method is to keep a global list of all child
          threads which we should wait for at the end of the program:

    >    children :: MVar [MVar ()]
    >    children = unsafePerformIO (newMVar [])
    >
    >    waitForChildren :: IO ()
    >    waitForChildren = do
    >      cs <- takeMVar children
    >      case cs of
    >        []   -> return ()
    >        m:ms -> do
    >           putMVar children ms
    >           takeMVar m
    >           waitForChildren
    >
    >    forkChild :: IO () -> IO ThreadId
    >    forkChild io = do
    >        mvar <- newEmptyMVar
    >        childs <- takeMVar children
    >        putMVar children (mvar:childs)
    >        forkIO (io `finally` putMVar mvar ())
    >
    >     main =
    >       later waitForChildren $
    >       ...

          The main thread principle also applies to calls to Haskell from
          outside, using @foreign export@.  When the @foreign export@ed
          function is invoked, it starts a new main thread, and it returns
          when this main thread terminates.  If the call causes new
          threads to be forked, they may remain in the system after the
          @foreign export@ed function has returned.
    -}

    {- $preemption

          GHC implements pre-emptive multitasking: the execution of
          threads are interleaved in a random fashion.  More specifically,
          a thread may be pre-empted whenever it allocates some memory,
          which unfortunately means that tight loops which do no
          allocation tend to lock out other threads (this only seems to
          happen with pathological benchmark-style code, however).

          The rescheduling timer runs on a 20ms granularity by
          default, but this may be altered using the
          @-i\<n\>@ RTS option.  After a rescheduling
          \"tick\" the running thread is pre-empted as soon as
          possible.

          One final note: the
          @aaaa@ @bbbb@ example may not
          work too well on GHC (see Scheduling, above), due
          to the locking on a 'System.IO.Handle'.  Only one thread
          may hold the lock on a 'System.IO.Handle' at any one
          time, so if a reschedule happens while a thread is holding the
          lock, the other thread won't be able to run.  The upshot is that
          the switch from @aaaa@ to
          @bbbbb@ happens infrequently.  It can be
          improved by lowering the reschedule tick period.  We also have a
          patch that causes a reschedule whenever a thread waiting on a
          lock is woken up, but haven't found it to be useful for anything
          other than this example :-)
    -}