Access.Control.Concurrent

Documentation

module Control.Concurrent

module Access.Control.Concurrent.MVar

module Access.Control.Concurrent.Chan

module Access.Control.Concurrent.QSem

module Access.Control.Concurrent.QSemN

class ExceptionAccess io => ThreadAccess io where

Inherits from ExceptionAccess, and gives access to Thread related functions

Minimal complete definition

myThreadId', forkIO', forkFinally', forkIOWithUnmask', killThread', forkOn', forkOnWithUnmask', getNumCapabilities', setNumCapabilities', threadCapability', yield', threadDelay', threadWaitRead', threadWaitWrite', threadWaitReadSTM', threadWaitWriteSTM'

Methods

myThreadId' :: io ThreadId

Wraps myThreadId

Returns the ThreadId of the calling thread (GHC only)

forkIO' :: io () -> io ThreadId

Wraps forkIO

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 masked state of the parent (see 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.

forkFinally' :: io a -> (Either SomeException a -> io ()) -> io ThreadId

Wraps forkFinally

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.

forkIOWithUnmask' :: ((forall a. io a -> io a) -> io ()) -> io ThreadId

Wraps forkIOWithUnmask

Like forkIO', but the child thread is passed a function that can be used to unmask asynchronous exceptions. This function is typically used in the following way

 ... mask_ $ forkIOWithUnmask' $ \unmask ->
                catch' (unmask ...) handler

so that the exception handler in the child thread is established with asynchronous exceptions masked, meanwhile the main body of the child thread is executed in the unmasked state.

Note that the unmask function passed to the child thread should only be used in that thread; the behaviour is undefined if it is invoked in a different thread.

killThread' :: ThreadId -> io ()

Wraps killThread

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

killThread' tid = throwTo' tid ThreadKilled

throwTo' :: Exception e => ThreadId -> e -> io ()

Wraps throwTo

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. However, in GHC a foreign call can be annotated as interruptible, in which case a throwTo' will cause the RTS to attempt to cause the call to return; see the GHC documentation for more details.

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'.

If the target of throwTo' is the calling thread, then the behaviour is the same as throwIO', except that the exception is thrown as an asynchronous exception. This means that if there is an enclosing pure computation, which would be the case if the current IO operation is inside unsafePerformIO or unsafeInterleaveIO, that computation is not permanently replaced by the exception, but is suspended as if it had received an asynchronous exception.

Note that if throwTo' is called with the current thread as the target, the exception will be thrown even if the thread is currently inside mask' or uninterruptibleMask'.

forkOn' :: Int -> io () -> io ThreadId

Wraps forkOn

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

The Int argument specifies a capability number (see getNumCapabilities'). Typically capabilities correspond to physical processors, but the exact behaviour is implementation-dependent. The value passed to forkOn' is interpreted modulo the total number of capabilities as returned by getNumCapabilities'.

GHC note: the number of capabilities is specified by the +RTS -N option when the program is started. Capabilities can be fixed to actual processor cores with +RTS -qa if the underlying operating system supports that, although in practice this is usually unnecessary (and may actually degrade perforamnce in some cases - experimentation is recommended).

forkOnWithUnmask' :: Int -> ((forall a. io a -> io a) -> io ()) -> io ThreadId

Wraps forkOnWithUnmask

Like forkIOWithUnmask', but the child thread is pinned to the given CPU, as with forkOn'.

getNumCapabilities' :: io Int

Wraps getNumCapabilities

Returns the number of Haskell threads that can run truly simultaneously (on separate physical processors) at any given time. To change this value, use setNumCapabilities'.

setNumCapabilities' :: Int -> io ()

Wraps setNumCapabilities

Set the number of Haskell threads that can run truly simultaneously (on separate physical processors) at any given time. The number passed to forkOn' is interpreted modulo this value. The initial value is given by the +RTS -N runtime flag.

This is also the number of threads that will participate in parallel garbage collection. It is strongly recommended that the number of capabilities is not set larger than the number of physical processor cores, and it may often be beneficial to leave one or more cores free to avoid contention with other processes in the machine.

threadCapability' :: ThreadId -> io (Int, Bool)

Wraps threadCapability

Returns the number of the capability on which the thread is currently running, and a boolean indicating whether the thread is locked to that capability or not. A thread is locked to a capability if it was created with forkOn'.

yield' :: io ()

Wraps yield

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.

threadDelay' :: Int -> io ()

Wraps threadDelay

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.

threadWaitRead' :: Fd -> io ()

Wraps threadWaitRead

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 closeFdWith.

threadWaitWrite' :: Fd -> io ()

Wraps threadWaitWrite

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 closeFdWith.

threadWaitReadSTM' :: Fd -> io (STM (), io ())

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

threadWaitWriteSTM' :: Fd -> io (STM (), io ())

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

Instances

ThreadAccess IO

class ThreadAccess io => BoundThreadAccess io where

Provides access to bounded thread functions

Methods

forkOS' :: io () -> io ThreadId

Wraps forkOS

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).

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.

isCurrentThreadBound' :: io Bool

Wraps isCurrentThreadBound

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.

runInBoundThread' :: io a -> io a

Wraps runInBoundThread

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.

runInUnboundThread' :: io a -> io a

Wraps runInUnboundThread

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 it's main thread to be bound and makes heavy use of concurrency (e.g. a web server), might want to wrap it's 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.

Instances

BoundThreadAccess IO

class WeakMemAccess io => WeakThreadAccess io where

Provides access to the mkWeakThreadId function, inherits from WeakMemAccess

Methods

mkWeakThreadId' :: ThreadId -> io (Weak ThreadId)

Wraps mkWeakThreadId

Make a weak pointer to a ThreadId. It can be important to do this if you want to hold a reference to a ThreadId while still allowing the thread to receive the BlockedIndefinitely family of exceptions (e.g. BlockedIndefinitelyOnMVar). Holding a normal ThreadId reference will prevent the delivery of BlockedIndefinitely exceptions because the reference could be used as the target of throwTo' at any time, which would unblock the thread.

Holding a Weak ThreadId, on the other hand, will not prevent the thread from receiving BlockedIndefinitely exceptions. It is still possible to throw an exception to a Weak ThreadId, but the caller must use deRefWeak' first to determine whether the thread still exists.

Instances

WeakThreadAccess IO