{-# LANGUAGE Unsafe #-} {-# LANGUAGE NoImplicitPrelude, MagicHash, UnboxedTuples #-} {-# OPTIONS_GHC -funbox-strict-fields #-} {-# OPTIONS_HADDOCK hide #-} ----------------------------------------------------------------------------- -- | -- Module : GHC.MVar -- Copyright : (c) The University of Glasgow 2008 -- License : see libraries/base/LICENSE -- -- Maintainer : cvs-ghc@haskell.org -- Stability : internal -- Portability : non-portable (GHC Extensions) -- -- The MVar type -- ----------------------------------------------------------------------------- module GHC.MVar ( -- * MVars MVar(..) , newMVar , newEmptyMVar , takeMVar , readMVar , putMVar , tryTakeMVar , tryPutMVar , tryReadMVar , isEmptyMVar , addMVarFinalizer ) where import GHC.Base data MVar a = MVar (MVar# RealWorld 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. -} -- pull in Eq (Mvar a) too, to avoid GHC.Conc being an orphan-instance module -- | @since 4.1.0.0 instance Eq (MVar a) where (MVar mvar1#) == (MVar mvar2#) = isTrue# (sameMVar# mvar1# mvar2#) {- M-Vars are rendezvous points for concurrent threads. They begin empty, and any attempt to read an empty M-Var blocks. When an M-Var is written, a single blocked thread may be freed. Reading an M-Var toggles its state from full back to empty. Therefore, any value written to an M-Var may only be read once. Multiple reads and writes are allowed, but there must be at least one read between any two writes. -} --Defined in IOBase to avoid cycle: data MVar a = MVar (SynchVar# RealWorld a) -- |Create an 'MVar' which is initially empty. newEmptyMVar :: IO (MVar a) newEmptyMVar = IO $ \ s# -> case newMVar# s# of (# s2#, svar# #) -> (# s2#, MVar svar# #) -- |Create an 'MVar' which contains the supplied value. newMVar :: a -> IO (MVar a) newMVar value = newEmptyMVar >>= \ mvar -> putMVar mvar value >> return mvar -- |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. -- takeMVar :: MVar a -> IO a takeMVar (MVar mvar#) = IO $ \ s# -> takeMVar# mvar# s# -- |Atomically read the contents of an 'MVar'. If the 'MVar' is -- currently empty, 'readMVar' will wait until it is full. -- 'readMVar' is guaranteed to receive the next 'putMVar'. -- -- 'readMVar' is multiple-wakeup, so when multiple readers are -- blocked on an 'MVar', all of them are woken up at the same time. -- -- /Compatibility note:/ Prior to base 4.7, 'readMVar' was a combination -- of 'takeMVar' and 'putMVar'. This mean that in the presence of -- other threads attempting to 'putMVar', 'readMVar' could block. -- Furthermore, 'readMVar' would not receive the next 'putMVar' if there -- was already a pending thread blocked on 'takeMVar'. The old behavior -- can be recovered by implementing 'readMVar as follows: -- -- @ -- readMVar :: MVar a -> IO a -- readMVar m = -- mask_ $ do -- a <- takeMVar m -- putMVar m a -- return a -- @ readMVar :: MVar a -> IO a readMVar (MVar mvar#) = IO $ \ s# -> readMVar# mvar# s# -- |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. -- putMVar :: MVar a -> a -> IO () putMVar (MVar mvar#) x = IO $ \ s# -> case putMVar# mvar# x s# of s2# -> (# s2#, () #) -- |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. tryTakeMVar :: MVar a -> IO (Maybe a) tryTakeMVar (MVar m) = IO $ \ s -> case tryTakeMVar# m s of (# s', 0#, _ #) -> (# s', Nothing #) -- MVar is empty (# s', _, a #) -> (# s', Just a #) -- MVar is full -- |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. tryPutMVar :: MVar a -> a -> IO Bool tryPutMVar (MVar mvar#) x = IO $ \ s# -> case tryPutMVar# mvar# x s# of (# s, 0# #) -> (# s, False #) (# s, _ #) -> (# s, True #) -- |A non-blocking version of 'readMVar'. The 'tryReadMVar' function -- returns immediately, with 'Nothing' if the 'MVar' was empty, or -- @'Just' a@ if the 'MVar' was full with contents @a@. -- -- @since 4.7.0.0 tryReadMVar :: MVar a -> IO (Maybe a) tryReadMVar (MVar m) = IO $ \ s -> case tryReadMVar# m s of (# s', 0#, _ #) -> (# s', Nothing #) -- MVar is empty (# s', _, a #) -> (# s', Just a #) -- MVar is full -- |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. isEmptyMVar :: MVar a -> IO Bool isEmptyMVar (MVar mv#) = IO $ \ s# -> case isEmptyMVar# mv# s# of (# s2#, flg #) -> (# s2#, isTrue# (flg /=# 0#) #) -- |Add a finalizer to an 'MVar' (GHC only). See "Foreign.ForeignPtr" and -- "System.Mem.Weak" for more about finalizers. addMVarFinalizer :: MVar a -> IO () -> IO () addMVarFinalizer (MVar m) (IO finalizer) = IO $ \s -> case mkWeak# m () finalizer s of { (# s1, _ #) -> (# s1, () #) }