GHC.Conc

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GHC.Conc |
Portability | non-portable (GHC extensions)
Stability | internal
Maintainer | cvs-ghc@haskell.org
Description
Basic concurrency stuff.
Synopsis
data ThreadId = ThreadId ThreadId#
myThreadId :: IO ThreadId
killThread :: ThreadId -> IO ()
throwTo :: ThreadId -> Exception -> IO ()
par :: a -> b -> b
pseq :: a -> b -> b
yield :: IO ()
labelThread :: ThreadId -> String -> IO ()
forkProcessPrim :: IO Int
forkProcess :: IO (Maybe Int)
threadDelay :: Int -> IO ()
threadWaitRead :: Int -> IO ()
threadWaitWrite :: Int -> 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 ()
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.
Constructors
ThreadId ThreadId# |
Instances
Eq ThreadId
Ord ThreadId
Show ThreadId
myThreadId :: IO ThreadId
Returns the `ThreadId` of the calling thread.
killThread :: ThreadId -> IO ()
`killThread` terminates the given thread (Note: `killThread` is not implemented in Hugs). 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 may be defined in terms of `throwTo`:
       killThread tid = throwTo tid (AsyncException ThreadKilled)
throwTo :: ThreadId -> Exception -> IO ()
`throwTo` raises an arbitrary exception in the target thread. `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.
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.
forkProcessPrim :: IO Int
This function is a replacement for Posix.forkProcess: This implementation will stop all other Concurrent Haskell threads in the (heavyweight) forked copy. `forkProcessPrim` returns the pid of the child process to the parent, 0 to the child, and a value less than 0 in case of errors. See also: `forkProcess`. Without this function, you need excessive and often impractical explicit synchronization using the regular Concurrent Haskell constructs to assure that only the desired thread is running after the fork(). The stopped threads are not garbage collected! This behaviour may change in future releases.
forkProcess :: IO (Maybe Int)
`forkProcess` is a wrapper around `forkProcessPrim` similar to the one found in Posix.forkProcess which returns a Maybe-type. The child receives `Nothing`, the parent `Just (pid::Int)`. In case of an error, an exception is thrown.
threadDelay :: Int -> IO ()
The `threadDelay` operation will cause the current thread to suspend for a given number of microseconds. Note that the resolution used by the Haskell runtime system's internal timer together with the fact that the thread may take some time to be rescheduled after the time has expired, means that the accuracy is more like 1/50 second.
threadWaitRead :: Int -> IO ()
Block the current thread until data is available to read on the given file descriptor.
threadWaitWrite :: Int -> IO ()
Block the current thread until data can be written to the given file descriptor.
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
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. If several threads are competing to take the same `MVar`, one is chosen to continue at random when the `MVar` becomes full.
putMVar :: MVar a -> a -> IO ()
Put a value into an `MVar`. If the `MVar` is currently full, `putMVar` will wait until it becomes empty. If several threads are competing to fill the same `MVar`, one is chosen to continue at random with the `MVar` becomes empty.
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`. See Foreign.ForeignPtr and System.Mem.Weak for more about finalizers.
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