\begin{code}

    {-# OPTIONS_GHC -XNoImplicitPrelude #-}
    {-# OPTIONS_GHC -fno-warn-missing-signatures #-}
    {-# OPTIONS_HADDOCK not-home #-}
    -----------------------------------------------------------------------------
    -- |
    -- Module      :  GHC.Conc.Sync
    -- Copyright   :  (c) The University of Glasgow, 1994-2002
    -- License     :  see libraries/base/LICENSE
    --
    -- Maintainer  :  cvs-ghc@haskell.org
    -- Stability   :  internal
    -- Portability :  non-portable (GHC extensions)
    --
    -- Basic concurrency stuff.
    --
    -----------------------------------------------------------------------------

    -- No: #hide, because bits of this module are exposed by the stm package.
    -- However, we don't want this module to be the home location for the
    -- bits it exports, we'd rather have Control.Concurrent and the other
    -- higher level modules be the home.  Hence:

    #include "Typeable.h"

    -- #not-home
    module GHC.Conc.Sync
            ( ThreadId(..)

            -- * Forking and suchlike
            , forkIO        -- :: IO a -> IO ThreadId
            , forkIOUnmasked
            , forkOnIO      -- :: Int -> IO a -> IO ThreadId
            , forkOnIOUnmasked
            , numCapabilities -- :: Int
            , numSparks      -- :: IO Int
            , childHandler  -- :: Exception -> IO ()
            , myThreadId    -- :: IO ThreadId
            , killThread    -- :: ThreadId -> IO ()
            , throwTo       -- :: ThreadId -> Exception -> IO ()
            , par           -- :: a -> b -> b
            , pseq          -- :: a -> b -> b
            , runSparks
            , yield         -- :: IO ()
            , labelThread   -- :: ThreadId -> String -> IO ()

            , ThreadStatus(..), BlockReason(..)
            , threadStatus  -- :: ThreadId -> IO ThreadStatus

            -- * TVars
            , STM(..)
            , atomically    -- :: STM a -> IO a
            , retry         -- :: STM a
            , orElse        -- :: STM a -> STM a -> STM a
            , throwSTM      -- :: Exception e => e -> STM a
            , catchSTM      -- :: Exception e => STM a -> (e -> STM a) -> STM a
            , alwaysSucceeds -- :: STM a -> STM ()
            , always        -- :: STM Bool -> STM ()
            , TVar(..)
            , newTVar       -- :: a -> STM (TVar a)
            , newTVarIO     -- :: a -> STM (TVar a)
            , readTVar      -- :: TVar a -> STM a
            , readTVarIO    -- :: TVar a -> IO a
            , writeTVar     -- :: a -> TVar a -> STM ()
            , unsafeIOToSTM -- :: IO a -> STM a

            -- * Miscellaneous
            , withMVar
            , modifyMVar_

            , setUncaughtExceptionHandler      -- :: (Exception -> IO ()) -> IO ()
            , getUncaughtExceptionHandler      -- :: IO (Exception -> IO ())

            , reportError, reportStackOverflow

            , sharedCAF
            ) where

    import Foreign hiding (unsafePerformIO)
    import Foreign.C

    #ifdef mingw32_HOST_OS
    import Data.Typeable
    #endif

    #ifndef mingw32_HOST_OS
    import Data.Dynamic
    #endif
    import Control.Monad
    import Data.Maybe

    import GHC.Base
    import {-# SOURCE #-} GHC.IO.Handle ( hFlush )
    import {-# SOURCE #-} GHC.IO.Handle.FD ( stdout )
    import GHC.IO
    import GHC.IO.Exception
    import GHC.Exception
    import GHC.IORef
    import GHC.MVar
    import GHC.Real         ( fromIntegral )
    import GHC.Pack         ( packCString# )
    import GHC.Show         ( Show(..), showString )

    infixr 0 `par`, `pseq`

\end{code} %************************************************************************ %* * \subsection{@ThreadId@, @par@, and @fork@} %* * %************************************************************************ \begin{code}

    data ThreadId = ThreadId ThreadId# deriving( Typeable )
    -- ToDo: data ThreadId = ThreadId (Weak ThreadId#)
    -- But since ThreadId# is unlifted, the Weak type must use open
    -- type variables.
    {- ^
    A 'ThreadId' is an abstract type representing a handle to a thread.
    'ThreadId' is an instance of 'Eq', 'Ord' and 'Show', where
    the 'Ord' instance implements an arbitrary total ordering over
    'ThreadId's. The 'Show' instance lets you convert an arbitrary-valued
    'ThreadId' to string form; showing a 'ThreadId' value is occasionally
    useful when debugging or diagnosing the behaviour of a concurrent
    program.

    /Note/: in GHC, if you have a 'ThreadId', you essentially have
    a pointer to the thread itself.  This means the thread itself can\'t be
    garbage collected until you drop the 'ThreadId'.
    This misfeature will hopefully be corrected at a later date.

    /Note/: Hugs does not provide any operations on other threads;
    it defines 'ThreadId' as a synonym for ().
    -}

    instance Show ThreadId where
       showsPrec d t =
            showString "ThreadId " .
            showsPrec d (getThreadId (id2TSO t))

    foreign import ccall unsafe "rts_getThreadId" getThreadId :: ThreadId# -> CInt

    id2TSO :: ThreadId -> ThreadId#
    id2TSO (ThreadId t) = t

    foreign import ccall unsafe "cmp_thread" cmp_thread :: ThreadId# -> ThreadId# -> CInt
    -- Returns -1, 0, 1

    cmpThread :: ThreadId -> ThreadId -> Ordering
    cmpThread t1 t2 =
       case cmp_thread (id2TSO t1) (id2TSO t2) of
          -1 -> LT
          0  -> EQ
          _  -> GT -- must be 1

    instance Eq ThreadId where
       t1 == t2 =
          case t1 `cmpThread` t2 of
             EQ -> True
             _  -> False

    instance Ord ThreadId where
       compare = cmpThread

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

    The new thread will be a lightweight thread; if you want to use a foreign
    library that uses thread-local storage, use 'Control.Concurrent.forkOS' instead.

    GHC note: the new thread inherits the /masked/ state of the parent
    (see 'Control.Exception.mask').

    The newly created thread has an exception handler that discards the
    exceptions 'BlockedIndefinitelyOnMVar', 'BlockedIndefinitelyOnSTM', and
    'ThreadKilled', and passes all other exceptions to the uncaught
    exception handler (see 'setUncaughtExceptionHandler').
    -}
    forkIO :: IO () -> IO ThreadId
    forkIO action = IO $ \ s ->
       case (fork# action_plus s) of (# s1, tid #) -> (# s1, ThreadId tid #)
     where
      action_plus = catchException action childHandler

    -- | Like 'forkIO', but the child thread is created with asynchronous exceptions
    -- unmasked (see 'Control.Exception.mask').
    forkIOUnmasked :: IO () -> IO ThreadId
    forkIOUnmasked io = forkIO (unsafeUnmask io)

    {- |
    Like 'forkIO', but lets you specify on which CPU the thread is
    created.  Unlike a `forkIO` thread, a thread created by `forkOnIO`
    will stay on the same CPU for its entire lifetime (`forkIO` threads
    can migrate between CPUs according to the scheduling policy).
    `forkOnIO` is useful for overriding the scheduling policy when you
    know in advance how best to distribute the threads.

    The `Int` argument specifies the CPU number; it is interpreted modulo
    'numCapabilities' (note that it actually specifies a capability number
    rather than a CPU number, but to a first approximation the two are
    equivalent).
    -}
    forkOnIO :: Int -> IO () -> IO ThreadId
    forkOnIO (I# cpu) action = IO $ \ s ->
       case (forkOn# cpu action_plus s) of (# s1, tid #) -> (# s1, ThreadId tid #)
     where
      action_plus = catchException action childHandler

    -- | Like 'forkOnIO', but the child thread is created with
    -- asynchronous exceptions unmasked (see 'Control.Exception.mask').
    forkOnIOUnmasked :: Int -> IO () -> IO ThreadId
    forkOnIOUnmasked cpu io = forkOnIO cpu (unsafeUnmask io)

    -- | the value passed to the @+RTS -N@ flag.  This is the number of
    -- Haskell threads that can run truly simultaneously at any given
    -- time, and is typically set to the number of physical CPU cores on
    -- the machine.
    numCapabilities :: Int
    numCapabilities = unsafePerformIO $  do
                        n <- peek n_capabilities
                        return (fromIntegral n)

    -- | Returns the number of sparks currently in the local spark pool
    numSparks :: IO Int
    numSparks = IO $ \s -> case numSparks# s of (# s', n #) -> (# s', I# n #)

    #if defined(mingw32_HOST_OS) && defined(__PIC__)
    foreign import ccall "_imp__n_capabilities" n_capabilities :: Ptr CInt
    #else
    foreign import ccall "&n_capabilities" n_capabilities :: Ptr CInt
    #endif
    childHandler :: SomeException -> IO ()
    childHandler err = catchException (real_handler err) childHandler

    real_handler :: SomeException -> IO ()
    real_handler se@(SomeException ex) =
      -- ignore thread GC and killThread exceptions:
      case cast ex of
      Just BlockedIndefinitelyOnMVar        -> return ()
      _ -> case cast ex of
           Just BlockedIndefinitelyOnSTM    -> return ()
           _ -> case cast ex of
                Just ThreadKilled           -> return ()
                _ -> case cast ex of
                     -- report all others:
                     Just StackOverflow     -> reportStackOverflow
                     _                      -> reportError se

    {- | 'killThread' raises the 'ThreadKilled' exception in the given
    thread (GHC only).

    > killThread tid = throwTo tid ThreadKilled

    -}
    killThread :: ThreadId -> IO ()
    killThread tid = throwTo tid ThreadKilled

    {- | 'throwTo' raises an arbitrary exception in the target thread (GHC only).

    'throwTo' does not return until the exception has been raised in the
    target thread.
    The calling thread can thus be certain that the target
    thread has received the exception.  This is a useful property to know
    when dealing with race conditions: eg. if there are two threads that
    can kill each other, it is guaranteed that only one of the threads
    will get to kill the other.

    Whatever work the target thread was doing when the exception was
    raised is not lost: the computation is suspended until required by
    another thread.

    If the target thread is currently making a foreign call, then the
    exception will not be raised (and hence 'throwTo' will not return)
    until the call has completed.  This is the case regardless of whether
    the call is inside a 'mask' or not.

    Important note: the behaviour of 'throwTo' differs from that described in
    the paper \"Asynchronous exceptions in Haskell\"
    (<http://research.microsoft.com/~simonpj/Papers/asynch-exns.htm>).
    In the paper, 'throwTo' is non-blocking; but the library implementation adopts
    a more synchronous design in which 'throwTo' does not return until the exception
    is received by the target thread.  The trade-off is discussed in Section 9 of the paper.
    Like any blocking operation, 'throwTo' is therefore interruptible (see Section 5.3 of
    the paper).  Unlike other interruptible operations, however, 'throwTo'
    is /always/ interruptible, even if it does not actually block.

    There is no guarantee that the exception will be delivered promptly,
    although the runtime will endeavour to ensure that arbitrary
    delays don't occur.  In GHC, an exception can only be raised when a
    thread reaches a /safe point/, where a safe point is where memory
    allocation occurs.  Some loops do not perform any memory allocation
    inside the loop and therefore cannot be interrupted by a 'throwTo'.

    Blocked 'throwTo' is fair: if multiple threads are trying to throw an
    exception to the same target thread, they will succeed in FIFO order.

      -}
    throwTo :: Exception e => ThreadId -> e -> IO ()
    throwTo (ThreadId tid) ex = IO $ \ s ->
       case (killThread# tid (toException ex) s) of s1 -> (# s1, () #)

    -- | Returns the 'ThreadId' of the calling thread (GHC only).
    myThreadId :: IO ThreadId
    myThreadId = IO $ \s ->
       case (myThreadId# s) of (# s1, tid #) -> (# s1, ThreadId tid #)

    -- |The 'yield' action allows (forces, in a co-operative multitasking
    -- implementation) a context-switch to any other currently runnable
    -- threads (if any), and is occasionally useful when implementing
    -- concurrency abstractions.
    yield :: IO ()
    yield = IO $ \s ->
       case (yield# s) of s1 -> (# s1, () #)

    {- | 'labelThread' stores a string as identifier for this thread if
    you built a RTS with debugging support. This identifier will be used in
    the debugging output to make distinction of different threads easier
    (otherwise you only have the thread state object\'s address in the heap).

    Other applications like the graphical Concurrent Haskell Debugger
    (<http://www.informatik.uni-kiel.de/~fhu/chd/>) may choose to overload
    'labelThread' for their purposes as well.
    -}

    labelThread :: ThreadId -> String -> IO ()
    labelThread (ThreadId t) str = IO $ \ s ->
       let !ps  = packCString# str
           !adr = byteArrayContents# ps in
         case (labelThread# t adr s) of s1 -> (# s1, () #)

    --      Nota Bene: 'pseq' used to be 'seq'
    --                 but 'seq' is now defined in PrelGHC
    --
    -- "pseq" is defined a bit weirdly (see below)
    --
    -- The reason for the strange "lazy" call is that
    -- it fools the compiler into thinking that pseq  and par are non-strict in
    -- their second argument (even if it inlines pseq at the call site).
    -- If it thinks pseq is strict in "y", then it often evaluates
    -- "y" before "x", which is totally wrong.

    {-# INLINE pseq  #-}
    pseq :: a -> b -> b
    pseq  x y = x `seq` lazy y

    {-# INLINE par  #-}
    par :: a -> b -> b
    par  x y = case (par# x) of { _ -> lazy y }

    -- | Internal function used by the RTS to run sparks.
    runSparks :: IO ()
    runSparks = IO loop
      where loop s = case getSpark# s of
                       (# s', n, p #) ->
                          if n ==# 0# then (# s', () #)
                                      else p `seq` loop s'

    data BlockReason
      = BlockedOnMVar
            -- ^blocked on on 'MVar'
      | BlockedOnBlackHole
            -- ^blocked on a computation in progress by another thread
      | BlockedOnException
            -- ^blocked in 'throwTo'
      | BlockedOnSTM
            -- ^blocked in 'retry' in an STM transaction
      | BlockedOnForeignCall
            -- ^currently in a foreign call
      | BlockedOnOther
            -- ^blocked on some other resource.  Without @-threaded@,
            -- I\/O and 'threadDelay' show up as 'BlockedOnOther', with @-threaded@
            -- they show up as 'BlockedOnMVar'.
      deriving (Eq,Ord,Show)

    -- | The current status of a thread
    data ThreadStatus
      = ThreadRunning
            -- ^the thread is currently runnable or running
      | ThreadFinished
            -- ^the thread has finished
      | ThreadBlocked  BlockReason
            -- ^the thread is blocked on some resource
      | ThreadDied
            -- ^the thread received an uncaught exception
      deriving (Eq,Ord,Show)

    threadStatus :: ThreadId -> IO ThreadStatus
    threadStatus (ThreadId t) = IO $ \s ->
       case threadStatus# t s of
         (# s', stat #) -> (# s', mk_stat (I# stat) #)
       where
            -- NB. keep these in sync with includes/Constants.h
         mk_stat 0  = ThreadRunning
         mk_stat 1  = ThreadBlocked BlockedOnMVar
         mk_stat 2  = ThreadBlocked BlockedOnBlackHole
         mk_stat 3  = ThreadBlocked BlockedOnException
         mk_stat 7  = ThreadBlocked BlockedOnSTM
         mk_stat 11 = ThreadBlocked BlockedOnForeignCall
         mk_stat 12 = ThreadBlocked BlockedOnForeignCall
         mk_stat 16 = ThreadFinished
         mk_stat 17 = ThreadDied
         mk_stat _  = ThreadBlocked BlockedOnOther

\end{code} %************************************************************************ %* * \subsection[stm]{Transactional heap operations} %* * %************************************************************************ TVars are shared memory locations which support atomic memory transactions. \begin{code}

    -- |A monad supporting atomic memory transactions.
    newtype STM a = STM (State# RealWorld -> (# State# RealWorld, a #))

    unSTM :: STM a -> (State# RealWorld -> (# State# RealWorld, a #))
    unSTM (STM a) = a

    INSTANCE_TYPEABLE1(STM,stmTc,"STM")

    instance  Functor STM where
       fmap f x = x >>= (return . f)

    instance  Monad STM  where
        {-# INLINE return #-}
        {-# INLINE (>>)   #-}
        {-# INLINE (>>=)  #-}
        m >> k      = thenSTM m k
        return x    = returnSTM x
        m >>= k     = bindSTM m k

    bindSTM :: STM a -> (a -> STM b) -> STM b
    bindSTM (STM m) k = STM ( \s ->
      case m s of
        (# new_s, a #) -> unSTM (k a) new_s
      )

    thenSTM :: STM a -> STM b -> STM b
    thenSTM (STM m) k = STM ( \s ->
      case m s of
        (# new_s, _ #) -> unSTM k new_s
      )

    returnSTM :: a -> STM a
    returnSTM x = STM (\s -> (# s, x #))

    instance MonadPlus STM where
      mzero = retry
      mplus = orElse

    -- | Unsafely performs IO in the STM monad.  Beware: this is a highly
    -- dangerous thing to do.
    --
    --   * The STM implementation will often run transactions multiple
    --     times, so you need to be prepared for this if your IO has any
    --     side effects.
    --
    --   * The STM implementation will abort transactions that are known to
    --     be invalid and need to be restarted.  This may happen in the middle
    --     of `unsafeIOToSTM`, so make sure you don't acquire any resources
    --     that need releasing (exception handlers are ignored when aborting
    --     the transaction).  That includes doing any IO using Handles, for
    --     example.  Getting this wrong will probably lead to random deadlocks.
    --
    --   * The transaction may have seen an inconsistent view of memory when
    --     the IO runs.  Invariants that you expect to be true throughout
    --     your program may not be true inside a transaction, due to the
    --     way transactions are implemented.  Normally this wouldn't be visible
    --     to the programmer, but using `unsafeIOToSTM` can expose it.
    --
    unsafeIOToSTM :: IO a -> STM a
    unsafeIOToSTM (IO m) = STM m

    -- |Perform a series of STM actions atomically.
    --
    -- You cannot use 'atomically' inside an 'unsafePerformIO' or 'unsafeInterleaveIO'.
    -- Any attempt to do so will result in a runtime error.  (Reason: allowing
    -- this would effectively allow a transaction inside a transaction, depending
    -- on exactly when the thunk is evaluated.)
    --
    -- However, see 'newTVarIO', which can be called inside 'unsafePerformIO',
    -- and which allows top-level TVars to be allocated.

    atomically :: STM a -> IO a
    atomically (STM m) = IO (\s -> (atomically# m) s )

    -- |Retry execution of the current memory transaction because it has seen
    -- values in TVars which mean that it should not continue (e.g. the TVars
    -- represent a shared buffer that is now empty).  The implementation may
    -- block the thread until one of the TVars that it has read from has been
    -- udpated. (GHC only)
    retry :: STM a
    retry = STM $ \s# -> retry# s#

    -- |Compose two alternative STM actions (GHC only).  If the first action
    -- completes without retrying then it forms the result of the orElse.
    -- Otherwise, if the first action retries, then the second action is
    -- tried in its place.  If both actions retry then the orElse as a
    -- whole retries.
    orElse :: STM a -> STM a -> STM a
    orElse (STM m) e = STM $ \s -> catchRetry# m (unSTM e) s

    -- | A variant of 'throw' that can only be used within the 'STM' monad.
    --
    -- Throwing an exception in @STM@ aborts the transaction and propagates the
    -- exception.
    --
    -- Although 'throwSTM' has a type that is an instance of the type of 'throw', the
    -- two functions are subtly different:
    --
    -- > throw e    `seq` x  ===> throw e
    -- > throwSTM e `seq` x  ===> x
    --
    -- The first example will cause the exception @e@ to be raised,
    -- whereas the second one won\'t.  In fact, 'throwSTM' will only cause
    -- an exception to be raised when it is used within the 'STM' monad.
    -- The 'throwSTM' variant should be used in preference to 'throw' to
    -- raise an exception within the 'STM' monad because it guarantees
    -- ordering with respect to other 'STM' operations, whereas 'throw'
    -- does not.
    throwSTM :: Exception e => e -> STM a
    throwSTM e = STM $ raiseIO# (toException e)

    -- |Exception handling within STM actions.
    catchSTM :: Exception e => STM a -> (e -> STM a) -> STM a
    catchSTM (STM m) handler = STM $ catchSTM# m handler'
        where
          handler' e = case fromException e of
                         Just e' -> unSTM (handler e')
                         Nothing -> raiseIO# e

    -- | Low-level primitive on which always and alwaysSucceeds are built.
    -- checkInv differs form these in that (i) the invariant is not
    -- checked when checkInv is called, only at the end of this and
    -- subsequent transcations, (ii) the invariant failure is indicated
    -- by raising an exception.
    checkInv :: STM a -> STM ()
    checkInv (STM m) = STM (\s -> (check# m) s)

    -- | alwaysSucceeds adds a new invariant that must be true when passed
    -- to alwaysSucceeds, at the end of the current transaction, and at
    -- the end of every subsequent transaction.  If it fails at any
    -- of those points then the transaction violating it is aborted
    -- and the exception raised by the invariant is propagated.
    alwaysSucceeds :: STM a -> STM ()
    alwaysSucceeds i = do ( i >> retry ) `orElse` ( return () )
                          checkInv i

    -- | always is a variant of alwaysSucceeds in which the invariant is
    -- expressed as an STM Bool action that must return True.  Returning
    -- False or raising an exception are both treated as invariant failures.
    always :: STM Bool -> STM ()
    always i = alwaysSucceeds ( do v <- i
                                   if (v) then return () else ( error "Transacional invariant violation" ) )

    -- |Shared memory locations that support atomic memory transactions.
    data TVar a = TVar (TVar# RealWorld a)

    INSTANCE_TYPEABLE1(TVar,tvarTc,"TVar")

    instance Eq (TVar a) where
            (TVar tvar1#) == (TVar tvar2#) = sameTVar# tvar1# tvar2#

    -- |Create a new TVar holding a value supplied
    newTVar :: a -> STM (TVar a)
    newTVar val = STM $ \s1# ->
        case newTVar# val s1# of
             (# s2#, tvar# #) -> (# s2#, TVar tvar# #)

    -- |@IO@ version of 'newTVar'.  This is useful for creating top-level
    -- 'TVar's using 'System.IO.Unsafe.unsafePerformIO', because using
    -- 'atomically' inside 'System.IO.Unsafe.unsafePerformIO' isn't
    -- possible.
    newTVarIO :: a -> IO (TVar a)
    newTVarIO val = IO $ \s1# ->
        case newTVar# val s1# of
             (# s2#, tvar# #) -> (# s2#, TVar tvar# #)

    -- |Return the current value stored in a TVar.
    -- This is equivalent to
    --
    -- >  readTVarIO = atomically . readTVar
    --
    -- but works much faster, because it doesn't perform a complete
    -- transaction, it just reads the current value of the 'TVar'.
    readTVarIO :: TVar a -> IO a
    readTVarIO (TVar tvar#) = IO $ \s# -> readTVarIO# tvar# s#

    -- |Return the current value stored in a TVar
    readTVar :: TVar a -> STM a
    readTVar (TVar tvar#) = STM $ \s# -> readTVar# tvar# s#

    -- |Write the supplied value into a TVar
    writeTVar :: TVar a -> a -> STM ()
    writeTVar (TVar tvar#) val = STM $ \s1# ->
        case writeTVar# tvar# val s1# of
             s2# -> (# s2#, () #)

\end{code} MVar utilities \begin{code}

    withMVar :: MVar a -> (a -> IO b) -> IO b
    withMVar m io =
      mask $ \restore -> do
        a <- takeMVar m
        b <- catchAny (restore (io a))
                (\e -> do putMVar m a; throw e)
        putMVar m a
        return b

    modifyMVar_ :: MVar a -> (a -> IO a) -> IO ()
    modifyMVar_ m io =
      mask $ \restore -> do
        a <- takeMVar m
        a' <- catchAny (restore (io a))
                (\e -> do putMVar m a; throw e)
        putMVar m a'
        return ()

\end{code} %************************************************************************ %* * \subsection{Thread waiting} %* * %************************************************************************ \begin{code}

    -- Machinery needed to ensureb that we only have one copy of certain
    -- CAFs in this module even when the base package is present twice, as
    -- it is when base is dynamically loaded into GHCi.  The RTS keeps
    -- track of the single true value of the CAF, so even when the CAFs in
    -- the dynamically-loaded base package are reverted, nothing bad
    -- happens.
    --
    sharedCAF :: a -> (Ptr a -> IO (Ptr a)) -> IO a
    sharedCAF a get_or_set =
       mask_ $ do
         stable_ref <- newStablePtr a
         let ref = castPtr (castStablePtrToPtr stable_ref)
         ref2 <- get_or_set ref
         if ref==ref2
            then return a
            else do freeStablePtr stable_ref
                    deRefStablePtr (castPtrToStablePtr (castPtr ref2))

    reportStackOverflow :: IO ()
    reportStackOverflow = callStackOverflowHook

    reportError :: SomeException -> IO ()
    reportError ex = do
       handler <- getUncaughtExceptionHandler
       handler ex

    -- SUP: Are the hooks allowed to re-enter Haskell land?  If so, remove
    -- the unsafe below.
    foreign import ccall unsafe "stackOverflow"
            callStackOverflowHook :: IO ()

    {-# NOINLINE uncaughtExceptionHandler #-}
    uncaughtExceptionHandler :: IORef (SomeException -> IO ())
    uncaughtExceptionHandler = unsafePerformIO (newIORef defaultHandler)
       where
          defaultHandler :: SomeException -> IO ()
          defaultHandler se@(SomeException ex) = do
             (hFlush stdout) `catchAny` (\ _ -> return ())
             let msg = case cast ex of
                   Just Deadlock -> "no threads to run:  infinite loop or deadlock?"
                   _ -> case cast ex of
                        Just (ErrorCall s) -> s
                        _                  -> showsPrec 0 se ""
             withCString "%s" $ \cfmt ->
              withCString msg $ \cmsg ->
                errorBelch cfmt cmsg

    -- don't use errorBelch() directly, because we cannot call varargs functions
    -- using the FFI.
    foreign import ccall unsafe "HsBase.h errorBelch2"
       errorBelch :: CString -> CString -> IO ()

    setUncaughtExceptionHandler :: (SomeException -> IO ()) -> IO ()
    setUncaughtExceptionHandler = writeIORef uncaughtExceptionHandler

    getUncaughtExceptionHandler :: IO (SomeException -> IO ())
    getUncaughtExceptionHandler = readIORef uncaughtExceptionHandler

\end{code}