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GHC.Conc |

Portability | non-portable (GHC extensions)
Stability | internal
Maintainer | cvs-ghc@haskell.org

Contents

Forking and suchlike
Waiting
MVars
TVars
Miscellaneous

Description
Basic concurrency stuff.
Synopsis

data ThreadId = ThreadId ThreadId#
forkIO :: IO () -> IO ThreadId
forkOnIO :: Int -> IO () -> IO ThreadId
childHandler :: Exception -> IO ()
myThreadId :: IO ThreadId
killThread :: ThreadId -> IO ()
throwTo :: ThreadId -> Exception -> IO ()
par :: a -> b -> b
pseq :: a -> b -> b
yield :: IO ()
labelThread :: ThreadId -> String -> IO ()
threadDelay :: Int -> IO ()
registerDelay :: Int -> IO (TVar Bool)
threadWaitRead :: Fd -> IO ()
threadWaitWrite :: Fd -> IO ()
data MVar a
newMVar :: a -> IO (MVar a)
newEmptyMVar :: IO (MVar a)
takeMVar :: MVar a -> IO a
putMVar :: MVar a -> a -> IO ()
tryTakeMVar :: MVar a -> IO (Maybe a)
tryPutMVar :: MVar a -> a -> IO Bool
isEmptyMVar :: MVar a -> IO Bool
addMVarFinalizer :: MVar a -> IO () -> IO ()
data STM a
atomically :: STM a -> IO a
retry :: STM a
orElse :: STM a -> STM a -> STM a
catchSTM :: STM a -> (Exception -> STM a) -> STM a
data TVar a
newTVar :: a -> STM (TVar a)
newTVarIO :: a -> IO (TVar a)
readTVar :: TVar a -> STM a
writeTVar :: TVar a -> a -> STM ()
unsafeIOToSTM :: IO a -> STM a
asyncRead :: Int -> Int -> Int -> Ptr a -> IO (Int, Int)
asyncWrite :: Int -> Int -> Int -> Ptr a -> IO (Int, Int)
asyncDoProc :: FunPtr (Ptr a -> IO Int) -> Ptr a -> IO Int
asyncReadBA :: Int -> Int -> Int -> Int -> MutableByteArray# RealWorld -> IO (Int, Int)
asyncWriteBA :: Int -> Int -> Int -> Int -> MutableByteArray# RealWorld -> IO (Int, Int)

Documentation
data ThreadId

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 ().

Constructors

ThreadId ThreadId# |

Instances

Data ThreadId
Eq ThreadId
Ord ThreadId
Show ThreadId
Typeable ThreadId

Forking and suchlike
forkIO :: IO () -> IO ThreadId
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. The new thread will be a lightweight thread; if you want to use a foreign library that uses thread-local storage, use forkOS instead.
forkOnIO :: Int -> IO () -> IO ThreadId
childHandler :: Exception -> IO ()
myThreadId :: IO ThreadId
Returns the `ThreadId` of the calling thread (GHC only).
killThread :: ThreadId -> IO ()
`killThread` terminates the given thread (GHC only). Any work already done by the thread isn't lost: the computation is suspended until required by another thread. The memory used by the thread will be garbage collected if it isn't referenced from anywhere. The `killThread` function is defined in terms of `throwTo`:
     killThread tid = throwTo tid (AsyncException ThreadKilled)
throwTo :: ThreadId -> Exception -> IO ()
`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.

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 block or not.

par :: a -> b -> b
pseq :: a -> b -> b
yield :: IO ()
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.
labelThread :: ThreadId -> String -> IO ()
`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.

Waiting
threadDelay :: Int -> IO ()
Suspends the current thread for a given number of microseconds (GHC only). Note that the resolution used by the Haskell runtime system's internal timer is 1/50 second, and `threadDelay` will round its argument up to the nearest multiple of this resolution.

There is no guarantee that the thread will be rescheduled promptly when the delay has expired, but the thread will never continue to run earlier than specified.

registerDelay :: Int -> IO (TVar Bool)
threadWaitRead :: Fd -> IO ()
Block the current thread until data is available to read on the given file descriptor (GHC only).
threadWaitWrite :: Fd -> IO ()
Block the current thread until data can be written to the given file descriptor (GHC only).
MVars
data MVar a

An `MVar` (pronounced "em-var") is a synchronising variable, used for communication between concurrent threads. It can be thought of as a a box, which may be empty or full.
Instances

Typeable1 MVar
Typeable a => Data (MVar a)
Eq (MVar a)

newMVar :: a -> IO (MVar a)
Create an `MVar` which contains the supplied value.
newEmptyMVar :: IO (MVar a)
Create an `MVar` which is initially empty.
takeMVar :: MVar a -> IO a
Return the contents of the `MVar`. If the `MVar` is currently empty, `takeMVar` will wait until it is full. After a `takeMVar`, the `MVar` is left empty. There are two further important properties of `takeMVar`:

- `takeMVar` is single-wakeup. That is, if there are multiple threads blocked in `takeMVar`, and the `MVar` becomes full, only one thread will be woken up. The runtime guarantees that the woken thread completes its `takeMVar` operation.
- When multiple threads are blocked on an `MVar`, they are woken up in FIFO order. This is useful for providing fairness properties of abstractions built using `MVar`s.

putMVar :: MVar a -> a -> IO ()
Put a value into an `MVar`. If the `MVar` is currently full, `putMVar` will wait until it becomes empty. There are two further important properties of `putMVar`:

- `putMVar` is single-wakeup. That is, if there are multiple threads blocked in `putMVar`, and the `MVar` becomes empty, only one thread will be woken up. The runtime guarantees that the woken thread completes its `putMVar` operation.
- When multiple threads are blocked on an `MVar`, they are woken up in FIFO order. This is useful for providing fairness properties of abstractions built using `MVar`s.

tryTakeMVar :: MVar a -> IO (Maybe a)
A non-blocking version of `takeMVar`. The `tryTakeMVar` function returns immediately, with `Nothing` if the `MVar` was empty, or ``Just` a` if the `MVar` was full with contents `a`. After `tryTakeMVar`, the `MVar` is left empty.
tryPutMVar :: MVar a -> a -> IO Bool
A non-blocking version of `putMVar`. The `tryPutMVar` function attempts to put the value `a` into the `MVar`, returning `True` if it was successful, or `False` otherwise.
isEmptyMVar :: MVar a -> IO Bool
Check whether a given `MVar` is empty. Notice that the boolean value returned is just a snapshot of the state of the MVar. By the time you get to react on its result, the MVar may have been filled (or emptied) - so be extremely careful when using this operation. Use `tryTakeMVar` instead if possible.
addMVarFinalizer :: MVar a -> IO () -> IO ()
Add a finalizer to an `MVar` (GHC only). See Foreign.ForeignPtr and System.Mem.Weak for more about finalizers.
TVars
data STM a

A monad supporting atomic memory transactions.
Instances

Functor STM
Monad STM
Typeable1 STM
Typeable a => Data (STM a)

atomically :: STM a -> IO a
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.

retry :: STM a
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)
orElse :: STM a -> STM a -> STM a
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.
catchSTM :: STM a -> (Exception -> STM a) -> STM a
Exception handling within STM actions.
data TVar a

Shared memory locations that support atomic memory transactions.
Instances

Typeable1 TVar
Typeable a => Data (TVar a)
Eq (TVar a)

newTVar :: a -> STM (TVar a)
Create a new TVar holding a value supplied
newTVarIO :: a -> IO (TVar a)
`IO` version of `newTVar`. This is useful for creating top-level `TVar`s using `unsafePerformIO`, because using `atomically` inside `unsafePerformIO` isn't possible.
readTVar :: TVar a -> STM a
Return the current value stored in a TVar
writeTVar :: TVar a -> a -> STM ()
Write the supplied value into a TVar
unsafeIOToSTM :: IO a -> STM a
Unsafely performs IO in the STM monad.
Miscellaneous
asyncRead :: Int -> Int -> Int -> Ptr a -> IO (Int, Int)
asyncWrite :: Int -> Int -> Int -> Ptr a -> IO (Int, Int)
asyncDoProc :: FunPtr (Ptr a -> IO Int) -> Ptr a -> IO Int
asyncReadBA :: Int -> Int -> Int -> Int -> MutableByteArray# RealWorld -> IO (Int, Int)
asyncWriteBA :: Int -> Int -> Int -> Int -> MutableByteArray# RealWorld -> IO (Int, Int)
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