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 |
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
- data BlockReason
- threadStatus :: ThreadId -> IO ThreadStatus
- threadCapability :: ThreadId -> IO (Int, Bool)
- newStablePtrPrimMVar :: MVar () -> IO (StablePtr PrimMVar)
- data PrimMVar
- 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
- 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 ()
- reportHeapOverflow :: IO ()
Documentation
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.
Forking and suchlike
forkIO :: IO () -> IO ThreadId Source #
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 Source #
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 Source #
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 Source #
Like forkIOWithUnmask
, but the child thread is pinned to the
given CPU, as with forkOn
.
Since: 4.4.0.0
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 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 Source #
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 () Source #
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 Source #
Returns the number of CPUs that the machine has
Since: 4.5.0.0
childHandler :: SomeException -> IO () Source #
killThread :: ThreadId -> IO () Source #
killThread
raises the ThreadKilled
exception in the given
thread (GHC only).
killThread tid = throwTo tid ThreadKilled
throwTo :: Exception e => ThreadId -> e -> IO () Source #
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
.
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.
mkWeakThreadId :: ThreadId -> IO (Weak ThreadId) Source #
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 Source #
The current status of a thread
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 Source # | Since: 4.3.0.0 |
Defined in GHC.Conc.Sync (==) :: ThreadStatus -> ThreadStatus -> Bool # (/=) :: ThreadStatus -> ThreadStatus -> Bool # | |
Ord ThreadStatus Source # | Since: 4.3.0.0 |
Defined in GHC.Conc.Sync compare :: ThreadStatus -> ThreadStatus -> Ordering # (<) :: ThreadStatus -> ThreadStatus -> Bool # (<=) :: ThreadStatus -> ThreadStatus -> Bool # (>) :: ThreadStatus -> ThreadStatus -> Bool # (>=) :: ThreadStatus -> ThreadStatus -> Bool # max :: ThreadStatus -> ThreadStatus -> ThreadStatus # min :: ThreadStatus -> ThreadStatus -> ThreadStatus # | |
Show ThreadStatus Source # | Since: 4.3.0.0 |
Defined in GHC.Conc.Sync |
data BlockReason Source #
BlockedOnMVar | blocked on |
BlockedOnBlackHole | blocked on a computation in progress by another thread |
BlockedOnException | blocked in |
BlockedOnSTM | blocked in |
BlockedOnForeignCall | currently in a foreign call |
BlockedOnOther | blocked on some other resource. Without |
Instances
Eq BlockReason Source # | Since: 4.3.0.0 |
Defined in GHC.Conc.Sync (==) :: BlockReason -> BlockReason -> Bool # (/=) :: BlockReason -> BlockReason -> Bool # | |
Ord BlockReason Source # | Since: 4.3.0.0 |
Defined in GHC.Conc.Sync compare :: BlockReason -> BlockReason -> Ordering # (<) :: BlockReason -> BlockReason -> Bool # (<=) :: BlockReason -> BlockReason -> Bool # (>) :: BlockReason -> BlockReason -> Bool # (>=) :: BlockReason -> BlockReason -> Bool # max :: BlockReason -> BlockReason -> BlockReason # min :: BlockReason -> BlockReason -> BlockReason # | |
Show BlockReason Source # | Since: 4.3.0.0 |
Defined in GHC.Conc.Sync |
threadStatus :: ThreadId -> IO ThreadStatus Source #
threadCapability :: ThreadId -> IO (Int, Bool) Source #
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
newStablePtrPrimMVar :: MVar () -> IO (StablePtr PrimMVar) Source #
Make a StablePtr that can be passed to the C function
hs_try_putmvar()
. The RTS wants a StablePtr
to the underlying
MVar#
, but a StablePtr#
can only refer to lifted types, so we
have to cheat by coercing.
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 #
Switch the value of returned TVar
from initial value False
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).
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 () Source #
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 ()) Source #
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 ()) Source #
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.
:: (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 () Source #
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 Source #
Return the current value of the allocation counter for the current thread.
Since: 4.8.0.0
enableAllocationLimit :: IO () Source #
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. Like other asynchronous exceptions, the
AllocationLimitExceeded
exception is deferred while the thread is inside
mask
or an exception handler in catch
.
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 () Source #
Disable allocation limit processing for the current thread.
Since: 4.8.0.0
TVars
A monad supporting atomic memory transactions.
atomically :: STM a -> IO a Source #
Perform a series of STM actions atomically.
Using atomically
inside an unsafePerformIO
or unsafeInterleaveIO
subverts some of guarantees that STM provides. It makes it possible to
run a transaction inside of another transaction, depending on when the
thunk is evaluated. If a nested transaction is attempted, an exception
is thrown by the runtime. It is possible to safely use atomically
inside
unsafePerformIO
or unsafeInterleaveIO
, but the typechecker does not
rule out programs that may attempt nested transactions, meaning that
the programmer must take special care to prevent these.
However, there are functions for creating transactional variables that
can always be safely called in unsafePerformIO
. See: newTVarIO
,
newTChanIO
,
newBroadcastTChanIO
,
newTQueueIO
,
newTBQueueIO
, and
newTMVarIO
.
Using unsafePerformIO
inside of atomically
is also dangerous but for
different reasons. See unsafeIOToSTM
for more on this.
Retry execution of the current memory transaction because it has seen
values in TVar
s which mean that it should not continue (e.g. the TVar
s
represent a shared buffer that is now empty). The implementation may
block the thread until one of the TVar
s that it has read from has been
updated. (GHC only)
throwSTM :: Exception e => e -> STM a Source #
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. If the exception is caught via catchSTM
, only the changes
enclosed by the catch are rolled back; changes made outside of catchSTM
persist.
If the exception is not caught inside of the STM
, it is re-thrown by
atomically
, and the entire STM
is rolled back.
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.
Shared memory locations that support atomic memory transactions.
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.
readTVarIO :: TVar a -> IO a Source #
unsafeIOToSTM :: IO a -> STM a Source #
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
type HandlerFun = ForeignPtr Word8 -> IO () Source #
setHandler :: Signal -> Maybe (HandlerFun, Dynamic) -> IO (Maybe (HandlerFun, Dynamic)) Source #
runHandlers :: ForeignPtr Word8 -> Signal -> IO () Source #
ensureIOManagerIsRunning :: IO () Source #
setUncaughtExceptionHandler :: (SomeException -> IO ()) -> IO () Source #
getUncaughtExceptionHandler :: IO (SomeException -> IO ()) Source #
reportError :: SomeException -> IO () Source #
reportStackOverflow :: IO () Source #
reportHeapOverflow :: IO () Source #