GHC.Conc

base-4.8.2.0: Basic libraries

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)
Safe Haskell | Unsafe
Language | Haskell2010

GHC.Conc

Contents

- Forking and suchlike
- Waiting
- Allocation counter and limit
- TVars
- Miscellaneous

Description

Basic concurrency stuff.

Synopsis

- data ThreadId = ThreadId ThreadId#
- forkIO :: IO () -> IO ThreadId
- forkIOWithUnmask :: ((forall a. IO a -> IO a) -> IO ()) -> IO ThreadId
- forkOn :: Int -> IO () -> IO ThreadId
- forkOnWithUnmask :: Int -> ((forall a. IO a -> IO a) -> IO ()) -> IO ThreadId
- numCapabilities :: Int
- getNumCapabilities :: IO Int
- setNumCapabilities :: Int -> IO ()
- getNumProcessors :: IO Int
- numSparks :: IO Int
- childHandler :: SomeException -> IO ()
- myThreadId :: IO ThreadId
- killThread :: ThreadId -> IO ()
- throwTo :: Exception e => ThreadId -> e -> IO ()
- par :: a -> b -> b
- pseq :: a -> b -> b
- runSparks :: IO ()
- yield :: IO ()
- labelThread :: ThreadId -> String -> IO ()
- mkWeakThreadId :: ThreadId -> IO (Weak ThreadId)
- data ThreadStatus
  - = ThreadRunning
  - | ThreadFinished
  - | ThreadBlocked BlockReason
  - | ThreadDied
- data BlockReason
  - = BlockedOnMVar
  - | BlockedOnBlackHole
  - | BlockedOnException
  - | BlockedOnSTM
  - | BlockedOnForeignCall
  - | BlockedOnOther
- threadStatus :: ThreadId -> IO ThreadStatus
- threadCapability :: ThreadId -> IO (Int, Bool)
- threadDelay :: Int -> IO ()
- registerDelay :: Int -> IO (TVar Bool)
- threadWaitRead :: Fd -> IO ()
- threadWaitWrite :: Fd -> IO ()
- threadWaitReadSTM :: Fd -> IO (STM (), IO ())
- threadWaitWriteSTM :: Fd -> IO (STM (), IO ())
- closeFdWith :: (Fd -> IO ()) -> Fd -> IO ()
- setAllocationCounter :: Int64 -> IO ()
- getAllocationCounter :: IO Int64
- enableAllocationLimit :: IO ()
- disableAllocationLimit :: IO ()
- newtype STM a = STM (State# RealWorld -> (#State# RealWorld, a#))
- 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 ()
- data TVar a = TVar (TVar# RealWorld a)
- newTVar :: a -> STM (TVar a)
- newTVarIO :: a -> IO (TVar a)
- readTVar :: TVar a -> STM a
- readTVarIO :: TVar a -> IO a
- writeTVar :: TVar a -> a -> STM ()
- unsafeIOToSTM :: IO a -> STM a
- withMVar :: MVar a -> (a -> IO b) -> IO b
- type Signal = CInt
- type HandlerFun = ForeignPtr Word8 -> IO ()
- setHandler :: Signal -> Maybe (HandlerFun, Dynamic) -> IO (Maybe (HandlerFun, Dynamic))
- runHandlers :: ForeignPtr Word8 -> Signal -> IO ()
- ensureIOManagerIsRunning :: IO ()
- ioManagerCapabilitiesChanged :: IO ()
- setUncaughtExceptionHandler :: (SomeException -> IO ()) -> IO ()
- getUncaughtExceptionHandler :: IO (SomeException -> IO ())
- reportError :: SomeException -> IO ()
- reportStackOverflow :: 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 |

# Forking and suchlike

forkIO :: IO () -> IO ThreadId

Creates 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, unbound thread. Foreign calls made by this thread are not guaranteed to be made by any particular OS thread; if you need foreign calls to be made by a particular OS thread, then use `forkOS` instead.

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.

forkIOWithUnmask :: ((forall a. IO a -> IO a) -> IO ()) -> IO ThreadId

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.

Since: 4.4.0.0

forkOn :: Int -> IO () -> IO ThreadId

Like `forkIO`, but lets you specify on which capability the thread should run. Unlike a `forkIO` thread, a thread created by `forkOn` will stay on the same capability for its entire lifetime (`forkIO` threads can migrate between capabilities 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 performance in some cases - experimentation is recommended).

Since: 4.4.0.0

forkOnWithUnmask :: Int -> ((forall a. IO a -> IO a) -> IO ()) -> IO ThreadId

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

Since: 4.4.0.0

numCapabilities :: Int

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 processor cores on the machine.

Strictly speaking it is better to use `getNumCapabilities`, because the number of capabilities might vary at runtime.

getNumCapabilities :: IO Int

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`.

Since: 4.4.0.0

setNumCapabilities :: Int -> IO ()

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.

Since: 4.5.0.0

getNumProcessors :: IO Int

Returns the number of CPUs that the machine has

Since: 4.5.0.0

numSparks :: IO Int

Returns the number of sparks currently in the local spark pool

childHandler :: SomeException -> IO ()

myThreadId :: IO ThreadId

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

killThread :: ThreadId -> IO ()

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

    killThread tid = throwTo tid ThreadKilled

throwTo :: Exception e => ThreadId -> e -> IO ()

`throwTo` raises an arbitrary exception in the target thread (GHC only).

Exception delivery synchronizes between the source and 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. Exception delivery is also atomic with respect to other exceptions. Atomicity is a useful property to have when dealing with race conditions: e.g. 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`.

par :: a -> b -> b infixr 0

pseq :: a -> b -> b infixr 0

runSparks :: IO ()

Internal function used by the RTS to run sparks.

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.

mkWeakThreadId :: ThreadId -> IO (Weak ThreadId)

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.

Since: 4.6.0.0

data ThreadStatus

The current status of a thread

Constructors

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

Instances

Eq ThreadStatus |
Ord ThreadStatus |
Show ThreadStatus |

data BlockReason

Constructors

BlockedOnMVar | blocked 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`.

Instances

Eq BlockReason |
Ord BlockReason |
Show BlockReason |

threadStatus :: ThreadId -> IO ThreadStatus

threadCapability :: ThreadId -> IO (Int, Bool)

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`.

Since: 4.4.0.0

# Waiting

threadDelay :: Int -> IO ()

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)

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

threadWaitRead :: Fd -> IO ()

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 ()

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.

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.

closeFdWith

Arguments

:: (Fd -> IO ()) | Low-level action that performs the real close.
-> Fd | File descriptor to close.
-> IO () |

Close a file descriptor in a concurrency-safe way (GHC only). If you are using `threadWaitRead` or `threadWaitWrite` to perform blocking I/O, you must use this function to close file descriptors, or blocked threads may not be woken.

Any threads that are blocked on the file descriptor via `threadWaitRead` or `threadWaitWrite` will be unblocked by having IO exceptions thrown.

# Allocation counter and limit

setAllocationCounter :: Int64 -> IO ()

Every thread has an allocation counter that tracks how much memory has been allocated by the thread. The counter is initialized to zero, and `setAllocationCounter` sets the current value. The allocation counter counts *down*, so in the absence of a call to `setAllocationCounter` its value is the negation of the number of bytes of memory allocated by the thread.

There are two things that you can do with this counter:

- Use it as a simple profiling mechanism, with `getAllocationCounter`.
- Use it as a resource limit. See `enableAllocationLimit`.

Allocation accounting is accurate only to about 4Kbytes.

Since: 4.8.0.0

getAllocationCounter :: IO Int64

Return the current value of the allocation counter for the current thread.

Since: 4.8.0.0

enableAllocationLimit :: IO ()

Enables the allocation counter to be treated as a limit for the current thread. When the allocation limit is enabled, if the allocation counter counts down below zero, the thread will be sent the `AllocationLimitExceeded` asynchronous exception. When this happens, the counter is reinitialised (by default to 100K, but tunable with the `+RTS -xq` option) so that it can handle the exception and perform any necessary clean up. If it exhausts this additional allowance, another `AllocationLimitExceeded` exception is sent, and so forth.

Note that memory allocation is unrelated to live memory, also known as heap residency. A thread can allocate a large amount of memory and retain anything between none and all of it. It is better to think of the allocation limit as a limit on CPU time, rather than a limit on memory.

Compared to using timeouts, allocation limits don't count time spent blocked or in foreign calls.

Since: 4.8.0.0

disableAllocationLimit :: IO ()

Disable allocation limit processing for the current thread.

Since: 4.8.0.0

# TVars

newtype STM a

A monad supporting atomic memory transactions.

Constructors

STM (State# RealWorld -> (#State# RealWorld, a#)) |

Instances

Monad STM |
Functor STM |
Applicative STM |
MonadPlus STM |
Alternative STM |

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.

throwSTM :: Exception e => e -> STM a

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.

catchSTM :: Exception e => STM a -> (e -> STM a) -> STM a

Exception handling within STM actions.

alwaysSucceeds :: STM a -> STM ()

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 ()

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

Shared memory locations that support atomic memory transactions.

Constructors

TVar (TVar# RealWorld a) |

Instances

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

readTVarIO :: TVar a -> IO a

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`.

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

Write the supplied value into a TVar

unsafeIOToSTM :: IO a -> STM a

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.

# Miscellaneous

withMVar :: MVar a -> (a -> IO b) -> IO b

type Signal = CInt

type HandlerFun = ForeignPtr Word8 -> IO ()

setHandler :: Signal -> Maybe (HandlerFun, Dynamic) -> IO (Maybe (HandlerFun, Dynamic))

runHandlers :: ForeignPtr Word8 -> Signal -> IO ()

ensureIOManagerIsRunning :: IO ()

ioManagerCapabilitiesChanged :: IO ()

setUncaughtExceptionHandler :: (SomeException -> IO ()) -> IO ()

getUncaughtExceptionHandler :: IO (SomeException -> IO ())

reportError :: SomeException -> IO ()

reportStackOverflow :: IO ()