    {-# LANGUAGE Trustworthy #-}
    {-# LANGUAGE CPP
               , MagicHash
               , UnboxedTuples
               , ScopedTypeVariables
               , RankNTypes
      #-}
    {-# OPTIONS_GHC -Wno-deprecations #-}
    -- kludge for the Control.Concurrent.QSem, Control.Concurrent.QSemN
    -- and Control.Concurrent.SampleVar imports.

    -----------------------------------------------------------------------------
    -- |
    -- Module      :  Control.Concurrent
    -- Copyright   :  (c) The University of Glasgow 2001
    -- License     :  BSD-style (see the file libraries/base/LICENSE)
    --
    -- Maintainer  :  libraries@haskell.org
    -- Stability   :  stable
    -- 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,
            myThreadId,

            forkIO,
            forkFinally,
            forkIOWithUnmask,
            killThread,
            throwTo,

            -- ** Threads with affinity
            forkOn,
            forkOnWithUnmask,
            getNumCapabilities,
            setNumCapabilities,
            threadCapability,

            -- * Scheduling

            -- $conc_scheduling
            yield,

            -- ** Blocking

            -- $blocking

            -- ** Waiting
            threadDelay,
            threadWaitRead,
            threadWaitWrite,
            threadWaitReadSTM,
            threadWaitWriteSTM,

            -- * Communication abstractions

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

            -- * Bound Threads
            -- $boundthreads
            rtsSupportsBoundThreads,
            forkOS,
            forkOSWithUnmask,
            isCurrentThreadBound,
            runInBoundThread,
            runInUnboundThread,

            -- * Weak references to ThreadIds
            mkWeakThreadId,

            -- * 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

            -- ** Deadlock

            -- $deadlock

        ) where

    -- JavaScript platform doesn't support bound threads
    #if !defined(javascript_HOST_ARCH)
    #define SUPPORT_BOUND_THREADS
    #endif

    import Control.Exception.Base as Exception

    import GHC.Conc hiding (threadWaitRead, threadWaitWrite,
                            threadWaitReadSTM, threadWaitWriteSTM)

    #if defined(SUPPORT_BOUND_THREADS)
    import GHC.IO           ( unsafeUnmask, catchException )
    import GHC.IORef        ( newIORef, readIORef, writeIORef )
    import GHC.Base
    import Foreign.StablePtr
    import Foreign.C.Types
    #endif

    import System.Posix.Types ( Fd )

    #if defined(mingw32_HOST_OS)
    import Foreign.C
    import System.IO
    import Data.Functor ( void )
    import Data.Int ( Int64 )
    #else
    import qualified GHC.Conc
    #endif

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

    {- $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.

    -}

    -- | Fork a thread and call the supplied function when the thread is about
    -- to terminate, with an exception or a returned value.  The function is
    -- called with asynchronous exceptions masked.
    --
    -- > forkFinally action and_then =
    -- >   mask $ \restore ->
    -- >     forkIO $ try (restore action) >>= and_then
    --
    -- This function is useful for informing the parent when a child
    -- terminates, for example.
    --
    -- @since 4.6.0.0
    forkFinally :: IO a -> (Either SomeException a -> IO ()) -> IO ThreadId
    forkFinally :: forall a. IO a -> (Either SomeException a -> IO ()) -> IO ThreadId
    forkFinally IO a
    action Either SomeException a -> IO ()
    and_then =
      ((forall a. IO a -> IO a) -> IO ThreadId) -> IO ThreadId
    forall b. ((forall a. IO a -> IO a) -> IO b) -> IO b
    mask (((forall a. IO a -> IO a) -> IO ThreadId) -> IO ThreadId)
    -> ((forall a. IO a -> IO a) -> IO ThreadId) -> IO ThreadId
    forall a b. (a -> b) -> a -> b
    $ \forall a. IO a -> IO a
    restore ->
        IO () -> IO ThreadId
    forkIO (IO () -> IO ThreadId) -> IO () -> IO ThreadId
    forall a b. (a -> b) -> a -> b
    $ IO a -> IO (Either SomeException a)
    forall e a. Exception e => IO a -> IO (Either e a)
    try (IO a -> IO a
    forall a. IO a -> IO a
    restore IO a
    action) IO (Either SomeException a)
    -> (Either SomeException a -> IO ()) -> IO ()
    forall a b. IO a -> (a -> IO b) -> IO b
    forall (m :: * -> *) a b. Monad m => m a -> (a -> m b) -> m b
    >>= Either SomeException a -> IO ()
    and_then

    -- ---------------------------------------------------------------------------
    -- 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@.

    In terms of performance, 'forkOS' (aka bound) threads are much more
    expensive than 'forkIO' (aka unbound) threads, because a 'forkOS'
    thread is tied to a particular OS thread, whereas a 'forkIO' thread
    can be run by any OS thread.  Context-switching between a 'forkOS'
    thread and a 'forkIO' thread is many times more expensive than between
    two 'forkIO' threads.

    Note in particular that the main program thread (the thread running
    @Main.main@) is always a bound thread, so for good concurrency
    performance you should ensure that the main thread is not doing
    repeated communication with other threads in the system.  Typically
    this means forking subthreads to do the work using 'forkIO', and
    waiting for the results in the main thread.

    -}

    #if !defined(SUPPORT_BOUND_THREADS)
    forkOS :: IO () -> IO ThreadId
    forkOS _ = error "forkOS not supported on this architecture"

    forkOSWithUnmask :: ((forall a . IO a -> IO a) -> IO ()) -> IO ThreadId
    forkOSWithUnmask _ = error "forkOS not supported on this architecture"

    isCurrentThreadBound :: IO Bool
    isCurrentThreadBound = pure False

    runInBoundThread :: IO a -> IO a
    runInBoundThread action = action

    runInUnboundThread :: IO a -> IO a
    runInUnboundThread action = action

    rtsSupportsBoundThreads :: Bool
    rtsSupportsBoundThreads = False
    #else

    -- | '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 unsafe 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' creates a /bound/ thread, which is necessary if you
    need to call foreign (non-Haskell) libraries that make use of
    thread-local state, such as OpenGL (see "Control.Concurrent#boundthreads").

    Using 'forkOS' instead of 'forkIO' makes no difference at all to the
    scheduling behaviour of the Haskell runtime system.  It is a common
    misconception that you need to use 'forkOS' instead of 'forkIO' to
    avoid blocking all the Haskell threads when making a foreign call;
    this isn't the case.  To allow foreign calls to be made without
    blocking all the Haskell threads (with GHC), it is only necessary to
    use the @-threaded@ option when linking your program, and to make sure
    the foreign import is not marked @unsafe@.
    -}

    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 :: StablePtr (IO ()) -> IO ()
    forkOS_entry :: StablePtr (IO ()) -> IO ()
    forkOS_entry StablePtr (IO ())
    stableAction = do
            IO ()
    action <- StablePtr (IO ()) -> IO (IO ())
    forall a. StablePtr a -> IO a
    deRefStablePtr StablePtr (IO ())
    stableAction
            IO ()
    action

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

    failNonThreaded :: IO a
    failNonThreaded :: forall a. IO a
    failNonThreaded = String -> IO a
    forall a. String -> IO a
    forall (m :: * -> *) a. MonadFail m => String -> m a
    fail (String -> IO a) -> String -> IO a
    forall a b. (a -> b) -> a -> b
    $ String
    "RTS doesn't support multiple OS threads "
                           String -> String -> String
    forall a. [a] -> [a] -> [a]
    ++String
    "(use ghc -threaded when linking)"

    forkOS :: IO () -> IO ThreadId
    forkOS IO ()
    action0
        | Bool
    rtsSupportsBoundThreads = do
            MVar ThreadId
    mv <- IO (MVar ThreadId)
    forall a. IO (MVar a)
    newEmptyMVar
            MaskingState
    b <- IO MaskingState
    Exception.getMaskingState
            let
                -- async exceptions are masked in the child if they are masked
                -- in the parent, as for forkIO (see #1048). forkOS_createThread
                -- creates a thread with exceptions masked by default.
                action1 :: IO ()
    action1 = case MaskingState
    b of
                            MaskingState
    Unmasked -> IO () -> IO ()
    forall a. IO a -> IO a
    unsafeUnmask IO ()
    action0
                            MaskingState
    MaskedInterruptible -> IO ()
    action0
                            MaskingState
    MaskedUninterruptible -> IO () -> IO ()
    forall a. IO a -> IO a
    uninterruptibleMask_ IO ()
    action0

                action_plus :: IO ()
    action_plus = IO () -> (SomeException -> IO ()) -> IO ()
    forall e a. Exception e => IO a -> (e -> IO a) -> IO a
    catch IO ()
    action1 SomeException -> IO ()
    childHandler

            StablePtr (IO ())
    entry <- IO () -> IO (StablePtr (IO ()))
    forall a. a -> IO (StablePtr a)
    newStablePtr (IO ThreadId
    myThreadId IO ThreadId -> (ThreadId -> IO ()) -> IO ()
    forall a b. IO a -> (a -> IO b) -> IO b
    forall (m :: * -> *) a b. Monad m => m a -> (a -> m b) -> m b
    >>= MVar ThreadId -> ThreadId -> IO ()
    forall a. MVar a -> a -> IO ()
    putMVar MVar ThreadId
    mv IO () -> IO () -> IO ()
    forall a b. IO a -> IO b -> IO b
    forall (m :: * -> *) a b. Monad m => m a -> m b -> m b
    >> IO ()
    action_plus)
            CInt
    err <- StablePtr (IO ()) -> IO CInt
    forkOS_createThread StablePtr (IO ())
    entry
            Bool -> IO () -> IO ()
    forall (f :: * -> *). Applicative f => Bool -> f () -> f ()
    when (CInt
    err CInt -> CInt -> Bool
    forall a. Eq a => a -> a -> Bool
    /= CInt
    0) (IO () -> IO ()) -> IO () -> IO ()
    forall a b. (a -> b) -> a -> b
    $ String -> IO ()
    forall a. String -> IO a
    forall (m :: * -> *) a. MonadFail m => String -> m a
    fail String
    "Cannot create OS thread."
            ThreadId
    tid <- MVar ThreadId -> IO ThreadId
    forall a. MVar a -> IO a
    takeMVar MVar ThreadId
    mv
            StablePtr (IO ()) -> IO ()
    forall a. StablePtr a -> IO ()
    freeStablePtr StablePtr (IO ())
    entry
            ThreadId -> IO ThreadId
    forall a. a -> IO a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ThreadId
    tid
        | Bool
    otherwise = IO ThreadId
    forall a. IO a
    failNonThreaded

    -- | Like 'forkIOWithUnmask', but the child thread is a bound thread,
    -- as with 'forkOS'.
    forkOSWithUnmask :: ((forall a . IO a -> IO a) -> IO ()) -> IO ThreadId
    forkOSWithUnmask :: ((forall a. IO a -> IO a) -> IO ()) -> IO ThreadId
    forkOSWithUnmask (forall a. IO a -> IO a) -> IO ()
    io = IO () -> IO ThreadId
    forkOS ((forall a. IO a -> IO a) -> IO ()
    io IO a -> IO a
    forall a. IO a -> IO a
    unsafeUnmask)

    -- | 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 Bool
    isCurrentThreadBound = (State# RealWorld -> (# State# RealWorld, Bool #)) -> IO Bool
    forall a. (State# RealWorld -> (# State# RealWorld, a #)) -> IO a
    IO ((State# RealWorld -> (# State# RealWorld, Bool #)) -> IO Bool)
    -> (State# RealWorld -> (# State# RealWorld, Bool #)) -> IO Bool
    forall a b. (a -> b) -> a -> b
    $ \ State# RealWorld
    s# ->
        case State# RealWorld -> (# State# RealWorld, Int# #)
    isCurrentThreadBound# State# RealWorld
    s# of
            (# State# RealWorld
    s2#, Int#
    flg #) -> (# State# RealWorld
    s2#, Int# -> Bool
    isTrue# (Int#
    flg Int# -> Int# -> Int#
    /=# Int#
    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 :: forall a. IO a -> IO a
    runInBoundThread IO a
    action
        | Bool
    rtsSupportsBoundThreads = do
            Bool
    bound <- IO Bool
    isCurrentThreadBound
            if Bool
    bound
                then IO a
    action
                else do
                    IORef (Either SomeException a)
    ref <- Either SomeException a -> IO (IORef (Either SomeException a))
    forall a. a -> IO (IORef a)
    newIORef Either SomeException a
    forall a. HasCallStack => a
    undefined
                    let action_plus :: IO ()
    action_plus = IO a -> IO (Either SomeException a)
    forall e a. Exception e => IO a -> IO (Either e a)
    Exception.try IO a
    action IO (Either SomeException a)
    -> (Either SomeException a -> IO ()) -> IO ()
    forall a b. IO a -> (a -> IO b) -> IO b
    forall (m :: * -> *) a b. Monad m => m a -> (a -> m b) -> m b
    >>= IORef (Either SomeException a) -> Either SomeException a -> IO ()
    forall a. IORef a -> a -> IO ()
    writeIORef IORef (Either SomeException a)
    ref
                    IO (StablePtr (IO ()))
    -> (StablePtr (IO ()) -> IO ())
    -> (StablePtr (IO ()) -> IO (Either SomeException a))
    -> IO (Either SomeException a)
    forall a b c. IO a -> (a -> IO b) -> (a -> IO c) -> IO c
    bracket (IO () -> IO (StablePtr (IO ()))
    forall a. a -> IO (StablePtr a)
    newStablePtr IO ()
    action_plus)
                            StablePtr (IO ()) -> IO ()
    forall a. StablePtr a -> IO ()
    freeStablePtr
                            (\StablePtr (IO ())
    cEntry -> StablePtr (IO ()) -> IO ()
    forkOS_entry_reimported StablePtr (IO ())
    cEntry IO () -> IO (Either SomeException a) -> IO (Either SomeException a)
    forall a b. IO a -> IO b -> IO b
    forall (m :: * -> *) a b. Monad m => m a -> m b -> m b
    >> IORef (Either SomeException a) -> IO (Either SomeException a)
    forall a. IORef a -> IO a
    readIORef IORef (Either SomeException a)
    ref) IO (Either SomeException a)
    -> (Either SomeException a -> IO a) -> IO a
    forall a b. IO a -> (a -> IO b) -> IO b
    forall (m :: * -> *) a b. Monad m => m a -> (a -> m b) -> m b
    >>=
                      Either SomeException a -> IO a
    forall a. Either SomeException a -> IO a
    unsafeResult
        | Bool
    otherwise = IO a
    forall a. IO a
    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 its main thread to be bound and makes /heavy/ use of concurrency
    (e.g. a web server), might want to wrap its @main@ action in
    @runInUnboundThread@.

    Note that exceptions which are thrown to the current thread are thrown in turn
    to the thread that is executing the given computation. This ensures there's
    always a way of killing the forked thread.
    -}
    runInUnboundThread :: IO a -> IO a

    runInUnboundThread :: forall a. IO a -> IO a
    runInUnboundThread IO a
    action = do
      Bool
    bound <- IO Bool
    isCurrentThreadBound
      if Bool
    bound
        then do
          MVar (Either SomeException a)
    mv <- IO (MVar (Either SomeException a))
    forall a. IO (MVar a)
    newEmptyMVar
          ((forall a. IO a -> IO a) -> IO a) -> IO a
    forall b. ((forall a. IO a -> IO a) -> IO b) -> IO b
    mask (((forall a. IO a -> IO a) -> IO a) -> IO a)
    -> ((forall a. IO a -> IO a) -> IO a) -> IO a
    forall a b. (a -> b) -> a -> b
    $ \forall a. IO a -> IO a
    restore -> do
            ThreadId
    tid <- IO () -> IO ThreadId
    forkIO (IO () -> IO ThreadId) -> IO () -> IO ThreadId
    forall a b. (a -> b) -> a -> b
    $ IO a -> IO (Either SomeException a)
    forall e a. Exception e => IO a -> IO (Either e a)
    Exception.try (IO a -> IO a
    forall a. IO a -> IO a
    restore IO a
    action) IO (Either SomeException a)
    -> (Either SomeException a -> IO ()) -> IO ()
    forall a b. IO a -> (a -> IO b) -> IO b
    forall (m :: * -> *) a b. Monad m => m a -> (a -> m b) -> m b
    >>= MVar (Either SomeException a) -> Either SomeException a -> IO ()
    forall a. MVar a -> a -> IO ()
    putMVar MVar (Either SomeException a)
    mv
            let wait :: IO (Either SomeException a)
    wait = MVar (Either SomeException a) -> IO (Either SomeException a)
    forall a. MVar a -> IO a
    takeMVar MVar (Either SomeException a)
    mv IO (Either SomeException a)
    -> (SomeException -> IO (Either SomeException a))
    -> IO (Either SomeException a)
    forall e a. Exception e => IO a -> (e -> IO a) -> IO a
    `catchException` \(SomeException
    e :: SomeException) ->
                         ThreadId -> SomeException -> IO ()
    forall e. Exception e => ThreadId -> e -> IO ()
    Exception.throwTo ThreadId
    tid SomeException
    e IO () -> IO (Either SomeException a) -> IO (Either SomeException a)
    forall a b. IO a -> IO b -> IO b
    forall (m :: * -> *) a b. Monad m => m a -> m b -> m b
    >> IO (Either SomeException a)
    wait
            IO (Either SomeException a)
    wait IO (Either SomeException a)
    -> (Either SomeException a -> IO a) -> IO a
    forall a b. IO a -> (a -> IO b) -> IO b
    forall (m :: * -> *) a b. Monad m => m a -> (a -> m b) -> m b
    >>= Either SomeException a -> IO a
    forall a. Either SomeException a -> IO a
    unsafeResult
        else IO a
    action

    unsafeResult :: Either SomeException a -> IO a
    unsafeResult :: forall a. Either SomeException a -> IO a
    unsafeResult = (SomeException -> IO a)
    -> (a -> IO a) -> Either SomeException a -> IO a
    forall a c b. (a -> c) -> (b -> c) -> Either a b -> c
    either SomeException -> IO a
    forall e a. Exception e => e -> IO a
    Exception.throwIO a -> IO a
    forall a. a -> IO a
    forall (m :: * -> *) a. Monad m => a -> m a
    return

    #endif

    -- ---------------------------------------------------------------------------
    -- threadWaitRead/threadWaitWrite

    -- | Block the current thread until data is available to read on the
    -- given file descriptor (GHC only).
    --
    -- This will throw an 'IOError' if the file descriptor was closed
    -- while this thread was blocked.  To safely close a file descriptor
    -- that has been used with 'threadWaitRead', use
    -- 'GHC.Conc.closeFdWith'.
    threadWaitRead :: Fd -> IO ()
    threadWaitRead :: Fd -> IO ()
    threadWaitRead Fd
    fd
    #if defined(mingw32_HOST_OS)
      -- we have no IO manager implementing threadWaitRead on Windows.
      -- fdReady does the right thing, but we have to call it in a
      -- separate thread, otherwise threadWaitRead won't be interruptible,
      -- and this only works with -threaded.
      | threaded  = withThread (waitFd fd False)
      | otherwise = case fd of
                      0 -> do _ <- hWaitForInput stdin (-1)
                              return ()
                            -- hWaitForInput does work properly, but we can only
                            -- do this for stdin since we know its FD.
                      _ -> errorWithoutStackTrace "threadWaitRead requires -threaded on Windows, or use System.IO.hWaitForInput"
    #else
      = Fd -> IO ()
    GHC.Conc.threadWaitRead Fd
    fd
    #endif

    -- | Block the current thread until data can be written to the
    -- given file descriptor (GHC only).
    --
    -- This will throw an 'IOError' if the file descriptor was closed
    -- while this thread was blocked.  To safely close a file descriptor
    -- that has been used with 'threadWaitWrite', use
    -- 'GHC.Conc.closeFdWith'.
    threadWaitWrite :: Fd -> IO ()
    threadWaitWrite :: Fd -> IO ()
    threadWaitWrite Fd
    fd
    #if defined(mingw32_HOST_OS)
      | threaded  = withThread (waitFd fd True)
      | otherwise = errorWithoutStackTrace "threadWaitWrite requires -threaded on Windows"
    #else
      = Fd -> IO ()
    GHC.Conc.threadWaitWrite Fd
    fd
    #endif

    -- | Returns an STM action that can be used to wait for data
    -- to read from a file descriptor. The second returned value
    -- is an IO action that can be used to deregister interest
    -- in the file descriptor.
    --
    -- @since 4.7.0.0
    threadWaitReadSTM :: Fd -> IO (STM (), IO ())
    threadWaitReadSTM :: Fd -> IO (STM (), IO ())
    threadWaitReadSTM Fd
    fd
    #if defined(mingw32_HOST_OS)
      | threaded = do v <- newTVarIO Nothing
                      mask_ $ void $ forkIO $ do result <- try (waitFd fd False)
                                                 atomically (writeTVar v $ Just result)
                      let waitAction = do result <- readTVar v
                                          case result of
                                            Nothing         -> retry
                                            Just (Right ()) -> return ()
                                            Just (Left e)   -> throwSTM (e :: IOException)
                      let killAction = return ()
                      return (waitAction, killAction)
      | otherwise = errorWithoutStackTrace "threadWaitReadSTM requires -threaded on Windows"
    #else
      = Fd -> IO (STM (), IO ())
    GHC.Conc.threadWaitReadSTM Fd
    fd
    #endif

    -- | Returns an STM action that can be used to wait until data
    -- can be written to a file descriptor. The second returned value
    -- is an IO action that can be used to deregister interest
    -- in the file descriptor.
    --
    -- @since 4.7.0.0
    threadWaitWriteSTM :: Fd -> IO (STM (), IO ())
    threadWaitWriteSTM :: Fd -> IO (STM (), IO ())
    threadWaitWriteSTM Fd
    fd
    #if defined(mingw32_HOST_OS)
      | threaded = do v <- newTVarIO Nothing
                      mask_ $ void $ forkIO $ do result <- try (waitFd fd True)
                                                 atomically (writeTVar v $ Just result)
                      let waitAction = do result <- readTVar v
                                          case result of
                                            Nothing         -> retry
                                            Just (Right ()) -> return ()
                                            Just (Left e)   -> throwSTM (e :: IOException)
                      let killAction = return ()
                      return (waitAction, killAction)
      | otherwise = errorWithoutStackTrace "threadWaitWriteSTM requires -threaded on Windows"
    #else
      = Fd -> IO (STM (), IO ())
    GHC.Conc.threadWaitWriteSTM Fd
    fd
    #endif

    #if defined(mingw32_HOST_OS)
    foreign import ccall unsafe "rtsSupportsBoundThreads" threaded :: Bool

    withThread :: IO a -> IO a
    withThread io = do
      m <- newEmptyMVar
      _ <- mask_ $ forkIO $ try io >>= putMVar m
      x <- takeMVar m
      case x of
        Right a -> return a
        Left e  -> throwIO (e :: IOException)

    waitFd :: Fd -> Bool -> IO ()
    waitFd fd write = do
       throwErrnoIfMinus1_ "fdReady" $
            fdReady (fromIntegral fd) (if write then 1 else 0) (-1) 0

    foreign import ccall safe "fdReady"
      fdReady :: CInt -> CBool -> Int64 -> CBool -> IO CInt
    #endif

    -- ---------------------------------------------------------------------------
    -- 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 requests
          are managed by a single thread (the /IO manager thread/) using
          a mechanism such as @epoll@ or @kqueue@, depending on what is
          provided by the host operating system.

          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
    >     forkFinally io (\_ -> putMVar mvar ())
    >     return mvar

          Note that we use 'forkFinally' 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)
    >        forkFinally io (\_ -> 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 :-)
    -}

    {- $deadlock

    GHC attempts to detect when threads are deadlocked using the garbage
    collector.  A thread that is not reachable (cannot be found by
    following pointers from live objects) must be deadlocked, and in this
    case the thread is sent an exception.  The exception is either
    'BlockedIndefinitelyOnMVar', 'BlockedIndefinitelyOnSTM',
    'NonTermination', or 'Deadlock', depending on the way in which the
    thread is deadlocked.

    Note that this feature is intended for debugging, and should not be
    relied on for the correct operation of your program.  There is no
    guarantee that the garbage collector will be accurate enough to detect
    your deadlock, and no guarantee that the garbage collector will run in
    a timely enough manner.  Basically, the same caveats as for finalizers
    apply to deadlock detection.

    There is a subtle interaction between deadlock detection and
    finalizers (as created by 'Foreign.Concurrent.newForeignPtr' or the
    functions in "System.Mem.Weak"): if a thread is blocked waiting for a
    finalizer to run, then the thread will be considered deadlocked and
    sent an exception.  So preferably don't do this, but if you have no
    alternative then it is possible to prevent the thread from being
    considered deadlocked by making a 'StablePtr' pointing to it.  Don't
    forget to release the 'StablePtr' later with 'freeStablePtr'.
    -}