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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
numCapabilities :: Int
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
alwaysSucceeds :: STM a -> STM ()
always :: STM Bool -> STM ()
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
signalHandlerLock :: MVar ()
ensureIOManagerIsRunning :: IO ()

Documentation

data ThreadId | Source

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 | Source

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. GHC note: the new thread inherits the blocked state of the parent (see `block`).

forkOnIO :: Int -> IO () -> IO ThreadId | Source

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).

numCapabilities :: Int | Source

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.

childHandler :: Exception -> IO () | Source

myThreadId :: IO ThreadId | Source

Returns the `ThreadId` of the calling thread (GHC only).

killThread :: ThreadId -> IO () | Source

`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 () | Source

`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.

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 8 of the paper. Like any blocking operation, `throwTo` is therefore interruptible (see Section 4.3 of the paper).

There is currently no guarantee that the exception delivered by `throwTo` will be delivered at the first possible opportunity. In particular, if a thread may unblock and then re-block exceptions (using `unblock` and `block`) without receiving a pending `throwTo`. This is arguably undesirable behaviour.

par :: a -> b -> b | Source

pseq :: a -> b -> b | Source

yield :: IO () | Source

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 () | Source

`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 () | Source

Suspends the current thread for a given number of microseconds (GHC only). 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) | Source

Set the value of returned TVar to True after a given number of microseconds. The caveats associated with threadDelay also apply.

threadWaitRead :: Fd -> IO () | Source

Block the current thread until data is available to read on the given file descriptor (GHC only).

threadWaitWrite :: Fd -> IO () | Source

Block the current thread until data can be written to the given file descriptor (GHC only).
MVars

data MVar a | Source

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) | Source

Create an `MVar` which contains the supplied value.

newEmptyMVar :: IO (MVar a) | Source

Create an `MVar` which is initially empty.

takeMVar :: MVar a -> IO a | Source

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 () | Source

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) | Source

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 | Source

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 | Source

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 () | Source

Add a finalizer to an `MVar` (GHC only). See Foreign.ForeignPtr and System.Mem.Weak for more about finalizers.
TVars

data STM a | Source

A monad supporting atomic memory transactions.
Instances

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

atomically :: STM a -> IO a | Source

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 | Source

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 | Source

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 | Source

Exception handling within STM actions.

alwaysSucceeds :: STM a -> STM () | Source

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.

always :: STM Bool -> STM () | Source

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.

data TVar a | Source

Shared memory locations that support atomic memory transactions.
Instances

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

newTVar :: a -> STM (TVar a) | Source

Create a new TVar holding a value supplied

newTVarIO :: a -> IO (TVar a) | Source

`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 | Source

Return the current value stored in a TVar

writeTVar :: TVar a -> a -> STM () | Source

Write the supplied value into a TVar

unsafeIOToSTM :: IO a -> STM a | Source

Unsafely performs IO in the STM monad.
Miscellaneous

signalHandlerLock :: MVar () | Source

ensureIOManagerIsRunning :: IO () | Source

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