Added Unknowns for higher kinds
[packages/random.git] / GHC / ForeignPtr.hs
1 {-# OPTIONS_GHC -fno-implicit-prelude #-}
2 -----------------------------------------------------------------------------
3 -- |
4 -- Module : GHC.ForeignPtr
5 -- Copyright : (c) The University of Glasgow, 1992-2003
6 -- License : see libraries/base/LICENSE
7 --
8 -- Maintainer : cvs-ghc@haskell.org
9 -- Stability : internal
10 -- Portability : non-portable (GHC extensions)
11 --
12 -- GHC's implementation of the 'ForeignPtr' data type.
13 --
14 -----------------------------------------------------------------------------
15
16 -- #hide
17 module GHC.ForeignPtr
18 (
19 ForeignPtr(..),
20 FinalizerPtr,
21 newForeignPtr_,
22 mallocForeignPtr,
23 mallocPlainForeignPtr,
24 mallocForeignPtrBytes,
25 mallocPlainForeignPtrBytes,
26 addForeignPtrFinalizer,
27 touchForeignPtr,
28 unsafeForeignPtrToPtr,
29 castForeignPtr,
30 newConcForeignPtr,
31 addForeignPtrConcFinalizer,
32 finalizeForeignPtr
33 ) where
34
35 import Control.Monad ( sequence_ )
36 import Foreign.Storable
37 import Numeric ( showHex )
38
39 import GHC.Show
40 import GHC.Num
41 import GHC.List ( null, replicate, length )
42 import GHC.Base
43 import GHC.IOBase
44 import GHC.STRef ( STRef(..) )
45 import GHC.Ptr ( Ptr(..), FunPtr, castFunPtrToPtr )
46 import GHC.Err
47
48 -- |The type 'ForeignPtr' represents references to objects that are
49 -- maintained in a foreign language, i.e., that are not part of the
50 -- data structures usually managed by the Haskell storage manager.
51 -- The essential difference between 'ForeignPtr's and vanilla memory
52 -- references of type @Ptr a@ is that the former may be associated
53 -- with /finalizers/. A finalizer is a routine that is invoked when
54 -- the Haskell storage manager detects that - within the Haskell heap
55 -- and stack - there are no more references left that are pointing to
56 -- the 'ForeignPtr'. Typically, the finalizer will, then, invoke
57 -- routines in the foreign language that free the resources bound by
58 -- the foreign object.
59 --
60 -- The 'ForeignPtr' is parameterised in the same way as 'Ptr'. The
61 -- type argument of 'ForeignPtr' should normally be an instance of
62 -- class 'Storable'.
63 --
64 data ForeignPtr a = ForeignPtr Addr# ForeignPtrContents
65 -- we cache the Addr# in the ForeignPtr object, but attach
66 -- the finalizer to the IORef (or the MutableByteArray# in
67 -- the case of a MallocPtr). The aim of the representation
68 -- is to make withForeignPtr efficient; in fact, withForeignPtr
69 -- should be just as efficient as unpacking a Ptr, and multiple
70 -- withForeignPtrs can share an unpacked ForeignPtr. Note
71 -- that touchForeignPtr only has to touch the ForeignPtrContents
72 -- object, because that ensures that whatever the finalizer is
73 -- attached to is kept alive.
74
75 data ForeignPtrContents
76 = PlainForeignPtr !(IORef [IO ()])
77 | MallocPtr (MutableByteArray# RealWorld) !(IORef [IO ()])
78 | PlainPtr (MutableByteArray# RealWorld)
79
80 instance Eq (ForeignPtr a) where
81 p == q = unsafeForeignPtrToPtr p == unsafeForeignPtrToPtr q
82
83 instance Ord (ForeignPtr a) where
84 compare p q = compare (unsafeForeignPtrToPtr p) (unsafeForeignPtrToPtr q)
85
86 instance Show (ForeignPtr a) where
87 showsPrec p f = showsPrec p (unsafeForeignPtrToPtr f)
88
89
90 -- |A Finalizer is represented as a pointer to a foreign function that, at
91 -- finalisation time, gets as an argument a plain pointer variant of the
92 -- foreign pointer that the finalizer is associated with.
93 --
94 type FinalizerPtr a = FunPtr (Ptr a -> IO ())
95
96 newConcForeignPtr :: Ptr a -> IO () -> IO (ForeignPtr a)
97 --
98 -- ^Turns a plain memory reference into a foreign object by
99 -- associating a finalizer - given by the monadic operation - with the
100 -- reference. The storage manager will start the finalizer, in a
101 -- separate thread, some time after the last reference to the
102 -- @ForeignPtr@ is dropped. There is no guarantee of promptness, and
103 -- in fact there is no guarantee that the finalizer will eventually
104 -- run at all.
105 --
106 -- Note that references from a finalizer do not necessarily prevent
107 -- another object from being finalized. If A's finalizer refers to B
108 -- (perhaps using 'touchForeignPtr', then the only guarantee is that
109 -- B's finalizer will never be started before A's. If both A and B
110 -- are unreachable, then both finalizers will start together. See
111 -- 'touchForeignPtr' for more on finalizer ordering.
112 --
113 newConcForeignPtr p finalizer
114 = do fObj <- newForeignPtr_ p
115 addForeignPtrConcFinalizer fObj finalizer
116 return fObj
117
118 mallocForeignPtr :: Storable a => IO (ForeignPtr a)
119 -- ^ Allocate some memory and return a 'ForeignPtr' to it. The memory
120 -- will be released automatically when the 'ForeignPtr' is discarded.
121 --
122 -- 'mallocForeignPtr' is equivalent to
123 --
124 -- > do { p <- malloc; newForeignPtr finalizerFree p }
125 --
126 -- although it may be implemented differently internally: you may not
127 -- assume that the memory returned by 'mallocForeignPtr' has been
128 -- allocated with 'Foreign.Marshal.Alloc.malloc'.
129 --
130 -- GHC notes: 'mallocForeignPtr' has a heavily optimised
131 -- implementation in GHC. It uses pinned memory in the garbage
132 -- collected heap, so the 'ForeignPtr' does not require a finalizer to
133 -- free the memory. Use of 'mallocForeignPtr' and associated
134 -- functions is strongly recommended in preference to 'newForeignPtr'
135 -- with a finalizer.
136 --
137 mallocForeignPtr = doMalloc undefined
138 where doMalloc :: Storable b => b -> IO (ForeignPtr b)
139 doMalloc a = do
140 r <- newIORef []
141 IO $ \s ->
142 case newPinnedByteArray# size s of { (# s, mbarr# #) ->
143 (# s, ForeignPtr (byteArrayContents# (unsafeCoerce# mbarr#))
144 (MallocPtr mbarr# r) #)
145 }
146 where (I# size) = sizeOf a
147
148 -- | This function is similar to 'mallocForeignPtr', except that the
149 -- size of the memory required is given explicitly as a number of bytes.
150 mallocForeignPtrBytes :: Int -> IO (ForeignPtr a)
151 mallocForeignPtrBytes (I# size) = do
152 r <- newIORef []
153 IO $ \s ->
154 case newPinnedByteArray# size s of { (# s, mbarr# #) ->
155 (# s, ForeignPtr (byteArrayContents# (unsafeCoerce# mbarr#))
156 (MallocPtr mbarr# r) #)
157 }
158
159 -- | Allocate some memory and return a 'ForeignPtr' to it. The memory
160 -- will be released automatically when the 'ForeignPtr' is discarded.
161 --
162 -- GHC notes: 'mallocPlainForeignPtr' has a heavily optimised
163 -- implementation in GHC. It uses pinned memory in the garbage
164 -- collected heap, as for mallocForeignPtr. Unlike mallocForeignPtr, a
165 -- ForeignPtr created with mallocPlainForeignPtr carries no finalizers.
166 -- It is not possible to add a finalizer to a ForeignPtr created with
167 -- mallocPlainForeignPtr. This is useful for ForeignPtrs that will live
168 -- only inside Haskell (such as those created for packed strings).
169 -- Attempts to add a finalizer to a ForeignPtr created this way, or to
170 -- finalize such a pointer, will throw an exception.
171 --
172 mallocPlainForeignPtr :: Storable a => IO (ForeignPtr a)
173 mallocPlainForeignPtr = doMalloc undefined
174 where doMalloc :: Storable b => b -> IO (ForeignPtr b)
175 doMalloc a = IO $ \s ->
176 case newPinnedByteArray# size s of { (# s, mbarr# #) ->
177 (# s, ForeignPtr (byteArrayContents# (unsafeCoerce# mbarr#))
178 (PlainPtr mbarr#) #)
179 }
180 where (I# size) = sizeOf a
181
182 -- | This function is similar to 'mallocForeignPtrBytes', except that
183 -- the internally an optimised ForeignPtr representation with no
184 -- finalizer is used. Attempts to add a finalizer will cause an
185 -- exception to be thrown.
186 mallocPlainForeignPtrBytes :: Int -> IO (ForeignPtr a)
187 mallocPlainForeignPtrBytes (I# size) = IO $ \s ->
188 case newPinnedByteArray# size s of { (# s, mbarr# #) ->
189 (# s, ForeignPtr (byteArrayContents# (unsafeCoerce# mbarr#))
190 (PlainPtr mbarr#) #)
191 }
192
193 addForeignPtrFinalizer :: FinalizerPtr a -> ForeignPtr a -> IO ()
194 -- ^This function adds a finalizer to the given foreign object. The
195 -- finalizer will run /before/ all other finalizers for the same
196 -- object which have already been registered.
197 addForeignPtrFinalizer finalizer fptr =
198 addForeignPtrConcFinalizer fptr
199 (mkFinalizer finalizer (unsafeForeignPtrToPtr fptr))
200
201 addForeignPtrConcFinalizer :: ForeignPtr a -> IO () -> IO ()
202 -- ^This function adds a finalizer to the given @ForeignPtr@. The
203 -- finalizer will run /before/ all other finalizers for the same
204 -- object which have already been registered.
205 --
206 -- This is a variant of @addForeignPtrFinalizer@, where the finalizer
207 -- is an arbitrary @IO@ action. When it is invoked, the finalizer
208 -- will run in a new thread.
209 --
210 -- NB. Be very careful with these finalizers. One common trap is that
211 -- if a finalizer references another finalized value, it does not
212 -- prevent that value from being finalized. In particular, 'Handle's
213 -- are finalized objects, so a finalizer should not refer to a 'Handle'
214 -- (including @stdout@, @stdin@ or @stderr@).
215 --
216 addForeignPtrConcFinalizer (ForeignPtr a c) finalizer =
217 addForeignPtrConcFinalizer_ c finalizer
218
219 addForeignPtrConcFinalizer_ f@(PlainForeignPtr r) finalizer = do
220 fs <- readIORef r
221 writeIORef r (finalizer : fs)
222 if (null fs)
223 then IO $ \s ->
224 case r of { IORef (STRef r#) ->
225 case mkWeak# r# () (foreignPtrFinalizer r) s of { (# s1, w #) ->
226 (# s1, () #) }}
227 else return ()
228 addForeignPtrConcFinalizer_ f@(MallocPtr fo r) finalizer = do
229 fs <- readIORef r
230 writeIORef r (finalizer : fs)
231 if (null fs)
232 then IO $ \s ->
233 case mkWeak# fo () (do foreignPtrFinalizer r; touch f) s of
234 (# s1, w #) -> (# s1, () #)
235 else return ()
236
237 addForeignPtrConcFinalizer_ _ _ =
238 error "GHC.ForeignPtr: attempt to add a finalizer to plain pointer"
239
240 foreign import ccall "dynamic"
241 mkFinalizer :: FinalizerPtr a -> Ptr a -> IO ()
242
243 foreignPtrFinalizer :: IORef [IO ()] -> IO ()
244 foreignPtrFinalizer r = do fs <- readIORef r; sequence_ fs
245
246 newForeignPtr_ :: Ptr a -> IO (ForeignPtr a)
247 -- ^Turns a plain memory reference into a foreign pointer that may be
248 -- associated with finalizers by using 'addForeignPtrFinalizer'.
249 newForeignPtr_ (Ptr obj) = do
250 r <- newIORef []
251 return (ForeignPtr obj (PlainForeignPtr r))
252
253 touchForeignPtr :: ForeignPtr a -> IO ()
254 -- ^This function ensures that the foreign object in
255 -- question is alive at the given place in the sequence of IO
256 -- actions. In particular 'Foreign.ForeignPtr.withForeignPtr'
257 -- does a 'touchForeignPtr' after it
258 -- executes the user action.
259 --
260 -- Note that this function should not be used to express dependencies
261 -- between finalizers on 'ForeignPtr's. For example, if the finalizer
262 -- for a 'ForeignPtr' @F1@ calls 'touchForeignPtr' on a second
263 -- 'ForeignPtr' @F2@, then the only guarantee is that the finalizer
264 -- for @F2@ is never started before the finalizer for @F1@. They
265 -- might be started together if for example both @F1@ and @F2@ are
266 -- otherwise unreachable, and in that case the scheduler might end up
267 -- running the finalizer for @F2@ first.
268 --
269 -- In general, it is not recommended to use finalizers on separate
270 -- objects with ordering constraints between them. To express the
271 -- ordering robustly requires explicit synchronisation using @MVar@s
272 -- between the finalizers, but even then the runtime sometimes runs
273 -- multiple finalizers sequentially in a single thread (for
274 -- performance reasons), so synchronisation between finalizers could
275 -- result in artificial deadlock. Another alternative is to use
276 -- explicit reference counting.
277 --
278 touchForeignPtr (ForeignPtr fo r) = touch r
279
280 touch r = IO $ \s -> case touch# r s of s -> (# s, () #)
281
282 unsafeForeignPtrToPtr :: ForeignPtr a -> Ptr a
283 -- ^This function extracts the pointer component of a foreign
284 -- pointer. This is a potentially dangerous operations, as if the
285 -- argument to 'unsafeForeignPtrToPtr' is the last usage
286 -- occurrence of the given foreign pointer, then its finalizer(s) will
287 -- be run, which potentially invalidates the plain pointer just
288 -- obtained. Hence, 'touchForeignPtr' must be used
289 -- wherever it has to be guaranteed that the pointer lives on - i.e.,
290 -- has another usage occurrence.
291 --
292 -- To avoid subtle coding errors, hand written marshalling code
293 -- should preferably use 'Foreign.ForeignPtr.withForeignPtr' rather
294 -- than combinations of 'unsafeForeignPtrToPtr' and
295 -- 'touchForeignPtr'. However, the later routines
296 -- are occasionally preferred in tool generated marshalling code.
297 unsafeForeignPtrToPtr (ForeignPtr fo r) = Ptr fo
298
299 castForeignPtr :: ForeignPtr a -> ForeignPtr b
300 -- ^This function casts a 'ForeignPtr'
301 -- parameterised by one type into another type.
302 castForeignPtr f = unsafeCoerce# f
303
304 -- | Causes the finalizers associated with a foreign pointer to be run
305 -- immediately.
306 finalizeForeignPtr :: ForeignPtr a -> IO ()
307 finalizeForeignPtr (ForeignPtr _ (PlainPtr _)) = return () -- no effect
308 finalizeForeignPtr (ForeignPtr _ foreignPtr) = do
309 finalizers <- readIORef refFinalizers
310 sequence_ finalizers
311 writeIORef refFinalizers []
312 where
313 refFinalizers = case foreignPtr of
314 (PlainForeignPtr ref) -> ref
315 (MallocPtr _ ref) -> ref
316