2 {-# OPTIONS_GHC -fno-implicit-prelude #-}
3 -----------------------------------------------------------------------------
6 -- Copyright : (c) The University of Glasgow, 1998-2002
7 -- License : see libraries/base/LICENSE
9 -- Maintainer : cvs-ghc@haskell.org
10 -- Stability : internal
11 -- Portability : non-portable (GHC Extensions)
15 -----------------------------------------------------------------------------
22 import GHC.IOBase ( IO(..), unIO )
23 import Data.Typeable ( Typeable1(..), mkTyCon, mkTyConApp )
26 A weak pointer object with a key and a value. The value has type @v@.
28 A weak pointer expresses a relationship between two objects, the
29 /key/ and the /value/: if the key is considered to be alive by the
30 garbage collector, then the value is also alive. A reference from
31 the value to the key does /not/ keep the key alive.
33 A weak pointer may also have a finalizer of type @IO ()@; if it does,
34 then the finalizer will be run at most once, at a time after the key
35 has become unreachable by the program (\"dead\"). The storage manager
36 attempts to run the finalizer(s) for an object soon after the object
37 dies, but promptness is not guaranteed.
39 It is not guaranteed that a finalizer will eventually run, and no
40 attempt is made to run outstanding finalizers when the program exits.
41 Therefore finalizers should not be relied on to clean up resources -
42 other methods (eg. exception handlers) should be employed, possibly in
43 addition to finalisers.
45 References from the finalizer to the key are treated in the same way
46 as references from the value to the key: they do not keep the key
47 alive. A finalizer may therefore ressurrect the key, perhaps by
48 storing it in the same data structure.
50 The finalizer, and the relationship between the key and the value,
51 exist regardless of whether the program keeps a reference to the
54 There may be multiple weak pointers with the same key. In this
55 case, the finalizers for each of these weak pointers will all be
56 run in some arbitrary order, or perhaps concurrently, when the key
57 dies. If the programmer specifies a finalizer that assumes it has
58 the only reference to an object (for example, a file that it wishes
59 to close), then the programmer must ensure that there is only one
62 If there are no other threads to run, the runtime system will check
63 for runnable finalizers before declaring the system to be deadlocked.
65 data Weak v = Weak (Weak# v)
68 INSTANCE_TYPEABLE1(Weak,weakTc,"Weak")
70 -- | Establishes a weak pointer to @k@, with value @v@ and a finalizer.
72 -- This is the most general interface for building a weak pointer.
76 -> Maybe (IO ()) -- ^ finalizer
77 -> IO (Weak v) -- ^ returns: a weak pointer object
79 mkWeak key val (Just finalizer) = IO $ \s ->
80 case mkWeak# key val finalizer s of { (# s1, w #) -> (# s1, Weak w #) }
81 mkWeak key val Nothing = IO $ \s ->
82 case mkWeak# key val (unsafeCoerce# 0#) s of { (# s1, w #) -> (# s1, Weak w #) }
85 Dereferences a weak pointer. If the key is still alive, then
86 @'Just' v@ is returned (where @v@ is the /value/ in the weak pointer), otherwise
87 'Nothing' is returned.
89 The return value of 'deRefWeak' depends on when the garbage collector
90 runs, hence it is in the 'IO' monad.
92 deRefWeak :: Weak v -> IO (Maybe v)
93 deRefWeak (Weak w) = IO $ \s ->
94 case deRefWeak# w s of
95 (# s1, flag, p #) -> case flag of
96 0# -> (# s1, Nothing #)
99 -- | Causes a the finalizer associated with a weak pointer to be run
101 finalize :: Weak v -> IO ()
102 finalize (Weak w) = IO $ \s ->
103 case finalizeWeak# w s of
104 (# s1, 0#, _ #) -> (# s1, () #) -- already dead, or no finaliser
105 (# s1, _, f #) -> f s1
108 Instance Eq (Weak v) where
109 (Weak w1) == (Weak w2) = w1 `sameWeak#` w2
113 -- run a batch of finalizers from the garbage collector. We're given
114 -- an array of finalizers and the length of the array, and we just
115 -- call each one in turn.
117 -- the IO primitives are inlined by hand here to get the optimal
118 -- code (sigh) --SDM.
120 runFinalizerBatch :: Int -> Array# (IO ()) -> IO ()
121 runFinalizerBatch (I# n) arr =
122 let go m = IO $ \s ->
125 _ -> let m' = m -# 1# in
126 case indexArray# arr m' of { (# io #) ->
127 case unIO io s of { (# s, _ #) ->