{-# LANGUAGE Unsafe #-} {-# LANGUAGE NoImplicitPrelude , BangPatterns , MagicHash , UnboxedTuples #-} {-# OPTIONS_HADDOCK not-home #-} {-# LANGUAGE StandaloneDeriving #-} ----------------------------------------------------------------------------- -- | -- Module : GHC.ForeignPtr -- Copyright : (c) The University of Glasgow, 1992-2003 -- License : see libraries/base/LICENSE -- -- Maintainer : cvs-ghc@haskell.org -- Stability : internal -- Portability : non-portable (GHC extensions) -- -- GHC's implementation of the 'ForeignPtr' data type. -- ----------------------------------------------------------------------------- module GHC.ForeignPtr ( -- * Types ForeignPtr(..), ForeignPtrContents(..), Finalizers(..), FinalizerPtr, FinalizerEnvPtr, -- * Create newForeignPtr_, mallocForeignPtr, mallocPlainForeignPtr, mallocForeignPtrBytes, mallocPlainForeignPtrBytes, mallocForeignPtrAlignedBytes, mallocPlainForeignPtrAlignedBytes, newConcForeignPtr, -- * Add Finalizers addForeignPtrFinalizer, addForeignPtrFinalizerEnv, addForeignPtrConcFinalizer, -- * Conversion unsafeForeignPtrToPtr, castForeignPtr, plusForeignPtr, -- * Control over lifetype withForeignPtr, unsafeWithForeignPtr, touchForeignPtr, -- * Finalization finalizeForeignPtr -- * Commentary -- $commentary ) where import Foreign.Storable import Data.Foldable ( sequence_ ) import GHC.Show import GHC.Base import GHC.IORef import GHC.STRef ( STRef(..) ) import GHC.Ptr ( Ptr(..), FunPtr(..) ) import Unsafe.Coerce ( unsafeCoerce, unsafeCoerceUnlifted ) -- |The type 'ForeignPtr' represents references to objects that are -- maintained in a foreign language, i.e., that are not part of the -- data structures usually managed by the Haskell storage manager. -- The essential difference between 'ForeignPtr's and vanilla memory -- references of type @Ptr a@ is that the former may be associated -- with /finalizers/. A finalizer is a routine that is invoked when -- the Haskell storage manager detects that - within the Haskell heap -- and stack - there are no more references left that are pointing to -- the 'ForeignPtr'. Typically, the finalizer will, then, invoke -- routines in the foreign language that free the resources bound by -- the foreign object. -- -- The 'ForeignPtr' is parameterised in the same way as 'Ptr'. The -- type argument of 'ForeignPtr' should normally be an instance of -- class 'Storable'. -- data ForeignPtr a = ForeignPtr Addr# ForeignPtrContents -- The Addr# in the ForeignPtr object is intentionally stored -- separately from the finalizer. The primary aim of the -- representation is to make withForeignPtr efficient; in fact, -- withForeignPtr should be just as efficient as unpacking a -- Ptr, and multiple withForeignPtrs can share an unpacked -- ForeignPtr. As a secondary benefit, this representation -- allows pointers to subregions within the same overall block -- to share the same finalizer (see 'plusForeignPtr'). Note -- that touchForeignPtr only has to touch the ForeignPtrContents -- object, because that ensures that whatever the finalizer is -- attached to is kept alive. -- | Functions called when a 'ForeignPtr' is finalized. Note that -- C finalizers and Haskell finalizers cannot be mixed. data Finalizers = NoFinalizers -- ^ No finalizer. If there is no intent to add a finalizer at -- any point in the future, consider 'FinalPtr' or 'PlainPtr' instead -- since these perform fewer allocations. | CFinalizers (Weak# ()) -- ^ Finalizers are all C functions. | HaskellFinalizers [IO ()] -- ^ Finalizers are all Haskell functions. -- | Controls finalization of a 'ForeignPtr', that is, what should happen -- if the 'ForeignPtr' becomes unreachable. Visually, these data constructors -- are appropriate in these scenarios: -- -- > Memory backing pointer is -- > GC-Managed Unmanaged -- > Finalizer functions are: +------------+-----------------+ -- > Allowed | MallocPtr | PlainForeignPtr | -- > +------------+-----------------+ -- > Prohibited | PlainPtr | FinalPtr | -- > +------------+-----------------+ data ForeignPtrContents = PlainForeignPtr !(IORef Finalizers) -- ^ The pointer refers to unmanaged memory that was allocated by -- a foreign function (typically using @malloc@). The finalizer -- frequently calls the C function @free@ or some variant of it. | FinalPtr -- ^ The pointer refers to unmanaged memory that should not be freed when -- the 'ForeignPtr' becomes unreachable. Functions that add finalizers -- to a 'ForeignPtr' throw exceptions when the 'ForeignPtr' is backed by -- 'PlainPtr'Most commonly, this is used with @Addr#@ literals. -- See Note [Why FinalPtr]. -- -- @since 4.15 | MallocPtr (MutableByteArray# RealWorld) !(IORef Finalizers) -- ^ The pointer refers to a byte array. -- The 'MutableByteArray#' field means that the 'MutableByteArray#' is -- reachable (by GC) whenever the 'ForeignPtr' is reachable. When the -- 'ForeignPtr' becomes unreachable, the runtime\'s normal GC recovers -- the memory backing it. Here, the finalizer function intended to be used -- to @free()@ any ancilliary *unmanaged* memory pointed to by the -- 'MutableByteArray#'. See the @zlib@ library for an example of this use. -- -- 1. Invariant: The 'Addr#' in the parent 'ForeignPtr' is an interior -- pointer into this 'MutableByteArray#'. -- 2. Invariant: The 'MutableByteArray#' is pinned, so the 'Addr#' does not -- get invalidated by the GC moving the byte array. -- 3. Invariant: A 'MutableByteArray#' must not be associated with more than -- one set of finalizers. For example, this is sound: -- -- > incrGood :: ForeignPtr Word8 -> ForeignPtr Word8 -- > incrGood (ForeignPtr p (MallocPtr m f)) = ForeignPtr (plusPtr p 1) (MallocPtr m f) -- -- But this is unsound: -- -- > incrBad :: ForeignPtr Word8 -> IO (ForeignPtr Word8) -- > incrBad (ForeignPtr p (MallocPtr m _)) = do -- > f <- newIORef NoFinalizers -- > pure (ForeignPtr p (MallocPtr m f)) | PlainPtr (MutableByteArray# RealWorld) -- ^ The pointer refers to a byte array. Finalization is not -- supported. This optimizes @MallocPtr@ by avoiding the allocation -- of a @MutVar#@ when it is known that no one will add finalizers to -- the @ForeignPtr@. Functions that add finalizers to a 'ForeignPtr' -- throw exceptions when the 'ForeignPtr' is backed by 'PlainPtr'. -- The invariants that apply to 'MallocPtr' apply to 'PlainPtr' as well. -- Note [Why FinalPtr] -- -- FinalPtr exists as an optimization for foreign pointers created -- from Addr# literals. Most commonly, this happens in the bytestring -- library, where the combination of OverloadedStrings and a rewrite -- rule overloads String literals as ByteString literals. See the -- rule "ByteString packChars/packAddress" in -- bytestring:Data.ByteString.Internal. Prior to the -- introduction of FinalPtr, bytestring used PlainForeignPtr (in -- Data.ByteString.Internal.unsafePackAddress) to handle such literals. -- With O2 optimization, the resulting Core from a GHC patched with a -- known-key cstringLength# function but without FinalPtr looked like: -- -- RHS size: {terms: 1, types: 0, coercions: 0, joins: 0/0} -- stringOne1 = "hello beautiful world"# -- RHS size: {terms: 11, types: 17, coercions: 0, joins: 0/0} -- stringOne -- = case newMutVar# NoFinalizers realWorld# of -- { (# ipv_i7b6, ipv1_i7b7 #) -> -- PS stringOne1 (PlainForeignPtr ipv1_i7b7) 0# 21# -- } -- -- After the introduction of FinalPtr, the bytestring library was modified -- so that the resulting Core was instead: -- -- RHS size: {terms: 1, types: 0, coercions: 0, joins: 0/0} -- stringOne1 = "hello beautiful world"# -- RHS size: {terms: 5, types: 0, coercions: 0, joins: 0/0} -- stringOne = PS stringOne1 FinalPtr 0# 21# -- -- This improves performance in three ways: -- -- 1. More optimization opportunities. GHC is willing to inline the FinalPtr -- variant of stringOne into its use sites. This means the offset and length -- are eligible for case-of-known-literal. Previously, this never happened. -- 2. Smaller binaries. Setting up the thunk to call newMutVar# required -- machine instruction in the generated code. On x86_64, FinalPtr reduces -- the size of binaries by about 450 bytes per ByteString literal. -- 3. Smaller memory footprint. Previously, every ByteString literal resulted -- in the allocation of a MutVar# and a PlainForeignPtr data constructor. -- These both hang around until the ByteString goes out of scope. FinalPtr -- eliminates both of these sources of allocations. The MutVar# is not -- allocated because FinalPtr does not allow it, and the data constructor -- is not allocated because FinalPtr is a nullary data constructor. -- -- For more discussion of FinalPtr, see GHC MR #2165 and bytestring PR #191. -- | @since 2.01 instance Eq (ForeignPtr a) where p == q = unsafeForeignPtrToPtr p == unsafeForeignPtrToPtr q -- | @since 2.01 instance Ord (ForeignPtr a) where compare p q = compare (unsafeForeignPtrToPtr p) (unsafeForeignPtrToPtr q) -- | @since 2.01 instance Show (ForeignPtr a) where showsPrec p f = showsPrec p (unsafeForeignPtrToPtr f) -- |A finalizer is represented as a pointer to a foreign function that, at -- finalisation time, gets as an argument a plain pointer variant of the -- foreign pointer that the finalizer is associated with. -- -- Note that the foreign function /must/ use the @ccall@ calling convention. -- type FinalizerPtr a = FunPtr (Ptr a -> IO ()) type FinalizerEnvPtr env a = FunPtr (Ptr env -> Ptr a -> IO ()) newConcForeignPtr :: Ptr a -> IO () -> IO (ForeignPtr a) -- -- ^Turns a plain memory reference into a foreign object by -- associating a finalizer - given by the monadic operation - with the -- reference. The storage manager will start the finalizer, in a -- separate thread, some time after the last reference to the -- @ForeignPtr@ is dropped. There is no guarantee of promptness, and -- in fact there is no guarantee that the finalizer will eventually -- run at all. -- -- Note that references from a finalizer do not necessarily prevent -- another object from being finalized. If A's finalizer refers to B -- (perhaps using 'touchForeignPtr', then the only guarantee is that -- B's finalizer will never be started before A's. If both A and B -- are unreachable, then both finalizers will start together. See -- 'touchForeignPtr' for more on finalizer ordering. -- newConcForeignPtr p finalizer = do fObj <- newForeignPtr_ p addForeignPtrConcFinalizer fObj finalizer return fObj mallocForeignPtr :: Storable a => IO (ForeignPtr a) -- ^ Allocate some memory and return a 'ForeignPtr' to it. The memory -- will be released automatically when the 'ForeignPtr' is discarded. -- -- 'mallocForeignPtr' is equivalent to -- -- > do { p <- malloc; newForeignPtr finalizerFree p } -- -- although it may be implemented differently internally: you may not -- assume that the memory returned by 'mallocForeignPtr' has been -- allocated with 'Foreign.Marshal.Alloc.malloc'. -- -- GHC notes: 'mallocForeignPtr' has a heavily optimised -- implementation in GHC. It uses pinned memory in the garbage -- collected heap, so the 'ForeignPtr' does not require a finalizer to -- free the memory. Use of 'mallocForeignPtr' and associated -- functions is strongly recommended in preference to -- 'Foreign.ForeignPtr.newForeignPtr' with a finalizer. -- mallocForeignPtr = doMalloc undefined where doMalloc :: Storable b => b -> IO (ForeignPtr b) doMalloc a | I# size < 0 = errorWithoutStackTrace "mallocForeignPtr: size must be >= 0" | otherwise = do r <- newIORef NoFinalizers IO $ \s -> case newAlignedPinnedByteArray# size align s of { (# s', mbarr# #) -> (# s', ForeignPtr (byteArrayContents# (unsafeCoerceUnlifted mbarr#)) (MallocPtr mbarr# r) #) } where !(I# size) = sizeOf a !(I# align) = alignment a -- | This function is similar to 'mallocForeignPtr', except that the -- size of the memory required is given explicitly as a number of bytes. mallocForeignPtrBytes :: Int -> IO (ForeignPtr a) mallocForeignPtrBytes size | size < 0 = errorWithoutStackTrace "mallocForeignPtrBytes: size must be >= 0" mallocForeignPtrBytes (I# size) = do r <- newIORef NoFinalizers IO $ \s -> case newPinnedByteArray# size s of { (# s', mbarr# #) -> (# s', ForeignPtr (byteArrayContents# (unsafeCoerceUnlifted mbarr#)) (MallocPtr mbarr# r) #) } -- | This function is similar to 'mallocForeignPtrBytes', except that the -- size and alignment of the memory required is given explicitly as numbers of -- bytes. mallocForeignPtrAlignedBytes :: Int -> Int -> IO (ForeignPtr a) mallocForeignPtrAlignedBytes size _align | size < 0 = errorWithoutStackTrace "mallocForeignPtrAlignedBytes: size must be >= 0" mallocForeignPtrAlignedBytes (I# size) (I# align) = do r <- newIORef NoFinalizers IO $ \s -> case newAlignedPinnedByteArray# size align s of { (# s', mbarr# #) -> (# s', ForeignPtr (byteArrayContents# (unsafeCoerceUnlifted mbarr#)) (MallocPtr mbarr# r) #) } -- | Allocate some memory and return a 'ForeignPtr' to it. The memory -- will be released automatically when the 'ForeignPtr' is discarded. -- -- GHC notes: 'mallocPlainForeignPtr' has a heavily optimised -- implementation in GHC. It uses pinned memory in the garbage -- collected heap, as for mallocForeignPtr. Unlike mallocForeignPtr, a -- ForeignPtr created with mallocPlainForeignPtr carries no finalizers. -- It is not possible to add a finalizer to a ForeignPtr created with -- mallocPlainForeignPtr. This is useful for ForeignPtrs that will live -- only inside Haskell (such as those created for packed strings). -- Attempts to add a finalizer to a ForeignPtr created this way, or to -- finalize such a pointer, will throw an exception. -- mallocPlainForeignPtr :: Storable a => IO (ForeignPtr a) mallocPlainForeignPtr = doMalloc undefined where doMalloc :: Storable b => b -> IO (ForeignPtr b) doMalloc a | I# size < 0 = errorWithoutStackTrace "mallocForeignPtr: size must be >= 0" | otherwise = IO $ \s -> case newAlignedPinnedByteArray# size align s of { (# s', mbarr# #) -> (# s', ForeignPtr (byteArrayContents# (unsafeCoerceUnlifted mbarr#)) (PlainPtr mbarr#) #) } where !(I# size) = sizeOf a !(I# align) = alignment a -- | This function is similar to 'mallocForeignPtrBytes', except that -- the internally an optimised ForeignPtr representation with no -- finalizer is used. Attempts to add a finalizer will cause an -- exception to be thrown. mallocPlainForeignPtrBytes :: Int -> IO (ForeignPtr a) mallocPlainForeignPtrBytes size | size < 0 = errorWithoutStackTrace "mallocPlainForeignPtrBytes: size must be >= 0" mallocPlainForeignPtrBytes (I# size) = IO $ \s -> case newPinnedByteArray# size s of { (# s', mbarr# #) -> (# s', ForeignPtr (byteArrayContents# (unsafeCoerceUnlifted mbarr#)) (PlainPtr mbarr#) #) } -- | This function is similar to 'mallocForeignPtrAlignedBytes', except that -- the internally an optimised ForeignPtr representation with no -- finalizer is used. Attempts to add a finalizer will cause an -- exception to be thrown. mallocPlainForeignPtrAlignedBytes :: Int -> Int -> IO (ForeignPtr a) mallocPlainForeignPtrAlignedBytes size _align | size < 0 = errorWithoutStackTrace "mallocPlainForeignPtrAlignedBytes: size must be >= 0" mallocPlainForeignPtrAlignedBytes (I# size) (I# align) = IO $ \s -> case newAlignedPinnedByteArray# size align s of { (# s', mbarr# #) -> (# s', ForeignPtr (byteArrayContents# (unsafeCoerceUnlifted mbarr#)) (PlainPtr mbarr#) #) } addForeignPtrFinalizer :: FinalizerPtr a -> ForeignPtr a -> IO () -- ^ This function adds a finalizer to the given foreign object. The -- finalizer will run /before/ all other finalizers for the same -- object which have already been registered. addForeignPtrFinalizer (FunPtr fp) (ForeignPtr p c) = case c of PlainForeignPtr r -> insertCFinalizer r fp 0# nullAddr# p () MallocPtr _ r -> insertCFinalizer r fp 0# nullAddr# p c _ -> errorWithoutStackTrace "GHC.ForeignPtr: attempt to add a finalizer to a plain pointer or a final pointer" -- Note [MallocPtr finalizers] (#10904) -- -- When we have C finalizers for a MallocPtr, the memory is -- heap-resident and would normally be recovered by the GC before the -- finalizers run. To prevent the memory from being reused too early, -- we attach the MallocPtr constructor to the "value" field of the -- weak pointer when we call mkWeak# in ensureCFinalizerWeak below. -- The GC will keep this field alive until the finalizers have run. addForeignPtrFinalizerEnv :: FinalizerEnvPtr env a -> Ptr env -> ForeignPtr a -> IO () -- ^ Like 'addForeignPtrFinalizer' but the finalizer is passed an additional -- environment parameter. addForeignPtrFinalizerEnv (FunPtr fp) (Ptr ep) (ForeignPtr p c) = case c of PlainForeignPtr r -> insertCFinalizer r fp 1# ep p () MallocPtr _ r -> insertCFinalizer r fp 1# ep p c _ -> errorWithoutStackTrace "GHC.ForeignPtr: attempt to add a finalizer to a plain pointer or a final pointer" addForeignPtrConcFinalizer :: ForeignPtr a -> IO () -> IO () -- ^This function adds a finalizer to the given @ForeignPtr@. The -- finalizer will run /before/ all other finalizers for the same -- object which have already been registered. -- -- This is a variant of @addForeignPtrFinalizer@, where the finalizer -- is an arbitrary @IO@ action. When it is invoked, the finalizer -- will run in a new thread. -- -- NB. Be very careful with these finalizers. One common trap is that -- if a finalizer references another finalized value, it does not -- prevent that value from being finalized. In particular, 'System.IO.Handle's -- are finalized objects, so a finalizer should not refer to a -- 'System.IO.Handle' (including 'System.IO.stdout', 'System.IO.stdin', or -- 'System.IO.stderr'). -- addForeignPtrConcFinalizer (ForeignPtr _ c) finalizer = addForeignPtrConcFinalizer_ c finalizer addForeignPtrConcFinalizer_ :: ForeignPtrContents -> IO () -> IO () addForeignPtrConcFinalizer_ (PlainForeignPtr r) finalizer = do noFinalizers <- insertHaskellFinalizer r finalizer if noFinalizers then IO $ \s -> case r of { IORef (STRef r#) -> case mkWeak# r# () (unIO $ foreignPtrFinalizer r) s of { (# s1, _ #) -> (# s1, () #) }} else return () addForeignPtrConcFinalizer_ f@(MallocPtr fo r) finalizer = do noFinalizers <- insertHaskellFinalizer r finalizer if noFinalizers then IO $ \s -> case mkWeak# fo () finalizer' s of (# s1, _ #) -> (# s1, () #) else return () where finalizer' :: State# RealWorld -> (# State# RealWorld, () #) finalizer' = unIO (foreignPtrFinalizer r >> touch f) addForeignPtrConcFinalizer_ _ _ = errorWithoutStackTrace "GHC.ForeignPtr: attempt to add a finalizer to plain pointer or a final pointer" insertHaskellFinalizer :: IORef Finalizers -> IO () -> IO Bool insertHaskellFinalizer r f = do !wasEmpty <- atomicModifyIORefP r $ \finalizers -> case finalizers of NoFinalizers -> (HaskellFinalizers [f], True) HaskellFinalizers fs -> (HaskellFinalizers (f:fs), False) _ -> noMixingError return wasEmpty -- | A box around Weak#, private to this module. data MyWeak = MyWeak (Weak# ()) insertCFinalizer :: IORef Finalizers -> Addr# -> Int# -> Addr# -> Addr# -> value -> IO () insertCFinalizer r fp flag ep p val = do MyWeak w <- ensureCFinalizerWeak r val IO $ \s -> case addCFinalizerToWeak# fp p flag ep w s of (# s1, 1# #) -> (# s1, () #) -- Failed to add the finalizer because some other thread -- has finalized w by calling foreignPtrFinalizer. We retry now. -- This won't be an infinite loop because that thread must have -- replaced the content of r before calling finalizeWeak#. (# s1, _ #) -> unIO (insertCFinalizer r fp flag ep p val) s1 -- Read the weak reference from an IORef Finalizers, creating it if necessary. -- Throws an exception if HaskellFinalizers is encountered. ensureCFinalizerWeak :: IORef Finalizers -> value -> IO MyWeak ensureCFinalizerWeak ref@(IORef (STRef r#)) value = do fin <- readIORef ref case fin of CFinalizers weak -> return (MyWeak weak) HaskellFinalizers{} -> noMixingError NoFinalizers -> IO $ \s -> case mkWeakNoFinalizer# r# (unsafeCoerce value) s of { (# s1, w #) -> -- See Note [MallocPtr finalizers] (#10904) case atomicModifyMutVar2# r# (update w) s1 of { (# s2, _, (_, (weak, needKill )) #) -> if needKill then case finalizeWeak# w s2 of { (# s3, _, _ #) -> (# s3, weak #) } else (# s2, weak #) }} where update _ fin@(CFinalizers w) = (fin, (MyWeak w, True)) update w NoFinalizers = (CFinalizers w, (MyWeak w, False)) update _ _ = noMixingError noMixingError :: a noMixingError = errorWithoutStackTrace $ "GHC.ForeignPtr: attempt to mix Haskell and C finalizers " ++ "in the same ForeignPtr" -- Swap out the finalizers with NoFinalizers and then run them. foreignPtrFinalizer :: IORef Finalizers -> IO () foreignPtrFinalizer r = do fs <- atomicSwapIORef r NoFinalizers -- atomic, see #7170 case fs of NoFinalizers -> return () CFinalizers w -> IO $ \s -> case finalizeWeak# w s of (# s1, 1#, f #) -> f s1 (# s1, _, _ #) -> (# s1, () #) HaskellFinalizers actions -> sequence_ actions newForeignPtr_ :: Ptr a -> IO (ForeignPtr a) -- ^Turns a plain memory reference into a foreign pointer that may be -- associated with finalizers by using 'addForeignPtrFinalizer'. newForeignPtr_ (Ptr obj) = do r <- newIORef NoFinalizers return (ForeignPtr obj (PlainForeignPtr r)) withForeignPtr :: ForeignPtr a -> (Ptr a -> IO b) -> IO b -- ^This is a way to look at the pointer living inside a -- foreign object. This function takes a function which is -- applied to that pointer. The resulting 'IO' action is then -- executed. The foreign object is kept alive at least during -- the whole action, even if it is not used directly -- inside. Note that it is not safe to return the pointer from -- the action and use it after the action completes. All uses -- of the pointer should be inside the -- 'withForeignPtr' bracket. The reason for -- this unsafeness is the same as for -- 'unsafeForeignPtrToPtr' below: the finalizer -- may run earlier than expected, because the compiler can only -- track usage of the 'ForeignPtr' object, not -- a 'Ptr' object made from it. -- -- This function is normally used for marshalling data to -- or from the object pointed to by the -- 'ForeignPtr', using the operations from the -- 'Storable' class. withForeignPtr fo@(ForeignPtr _ r) f = IO $ \s -> case f (unsafeForeignPtrToPtr fo) of IO action# -> keepAlive# r s action# -- | This is similar to 'withForeignPtr' but comes with an important caveat: -- the user must guarantee that the continuation does not diverge (e.g. loop or -- throw an exception). In exchange for this loss of generality, this function -- offers the ability of GHC to optimise more aggressively. -- -- Specifically, applications of the form: -- @ -- unsafeWithForeignPtr fptr ('Control.Monad.forever' something) -- @ -- -- See GHC issue #17760 for more information about the unsoundness behavior -- that this function can result in. unsafeWithForeignPtr :: ForeignPtr a -> (Ptr a -> IO b) -> IO b unsafeWithForeignPtr fo f = do r <- f (unsafeForeignPtrToPtr fo) touchForeignPtr fo return r touchForeignPtr :: ForeignPtr a -> IO () -- ^This function ensures that the foreign object in -- question is alive at the given place in the sequence of IO -- actions. However, this comes with a significant caveat: the contract above -- does not hold if GHC can demonstrate that the code preceeding -- @touchForeignPtr@ diverges (e.g. by looping infinitely or throwing an -- exception). For this reason, you are strongly advised to use instead -- 'withForeignPtr' where possible. -- -- Also, note that this function should not be used to express dependencies -- between finalizers on 'ForeignPtr's. For example, if the finalizer for a -- 'ForeignPtr' @F1@ calls 'touchForeignPtr' on a second 'ForeignPtr' @F2@, -- then the only guarantee is that the finalizer for @F2@ is never started -- before the finalizer for @F1@. They might be started together if for -- example both @F1@ and @F2@ are otherwise unreachable, and in that case the -- scheduler might end up running the finalizer for @F2@ first. -- -- In general, it is not recommended to use finalizers on separate -- objects with ordering constraints between them. To express the -- ordering robustly requires explicit synchronisation using @MVar@s -- between the finalizers, but even then the runtime sometimes runs -- multiple finalizers sequentially in a single thread (for -- performance reasons), so synchronisation between finalizers could -- result in artificial deadlock. Another alternative is to use -- explicit reference counting. -- touchForeignPtr (ForeignPtr _ r) = touch r touch :: ForeignPtrContents -> IO () touch r = IO $ \s -> case touch# r s of s' -> (# s', () #) unsafeForeignPtrToPtr :: ForeignPtr a -> Ptr a -- ^This function extracts the pointer component of a foreign -- pointer. This is a potentially dangerous operations, as if the -- argument to 'unsafeForeignPtrToPtr' is the last usage -- occurrence of the given foreign pointer, then its finalizer(s) will -- be run, which potentially invalidates the plain pointer just -- obtained. Hence, 'touchForeignPtr' must be used -- wherever it has to be guaranteed that the pointer lives on - i.e., -- has another usage occurrence. -- -- To avoid subtle coding errors, hand written marshalling code -- should preferably use 'Foreign.ForeignPtr.withForeignPtr' rather -- than combinations of 'unsafeForeignPtrToPtr' and -- 'touchForeignPtr'. However, the latter routines -- are occasionally preferred in tool generated marshalling code. unsafeForeignPtrToPtr (ForeignPtr fo _) = Ptr fo castForeignPtr :: ForeignPtr a -> ForeignPtr b -- ^This function casts a 'ForeignPtr' -- parameterised by one type into another type. castForeignPtr = coerce plusForeignPtr :: ForeignPtr a -> Int -> ForeignPtr b -- ^Advances the given address by the given offset in bytes. -- -- The new 'ForeignPtr' shares the finalizer of the original, -- equivalent from a finalization standpoint to just creating another -- reference to the original. That is, the finalizer will not be -- called before the new 'ForeignPtr' is unreachable, nor will it be -- called an additional time due to this call, and the finalizer will -- be called with the same address that it would have had this call -- not happened, *not* the new address. -- -- @since 4.10.0.0 plusForeignPtr (ForeignPtr addr c) (I# d) = ForeignPtr (plusAddr# addr d) c -- | Causes the finalizers associated with a foreign pointer to be run -- immediately. The foreign pointer must not be used again after this -- function is called. If the foreign pointer does not support finalizers, -- this is a no-op. finalizeForeignPtr :: ForeignPtr a -> IO () finalizeForeignPtr (ForeignPtr _ c) = case c of PlainForeignPtr ref -> foreignPtrFinalizer ref MallocPtr _ ref -> foreignPtrFinalizer ref PlainPtr{} -> return () FinalPtr{} -> return () {- $commentary This is a high-level overview of how 'ForeignPtr' works. The implementation of 'ForeignPtr' must accomplish several goals: 1. Invoke a finalizer once a foreign pointer becomes unreachable. 2. Support augmentation of finalizers, i.e. 'addForeignPtrFinalizer'. As a motivating example, suppose that the payload of a foreign pointer is C struct @bar@ that has an optionally NULL pointer field @foo@ to an unmanaged heap object. Initially, @foo@ is NULL, and later the program uses @malloc@, initializes the object, and assigns @foo@ the address returned by @malloc@. When the foreign pointer becomes unreachable, it is now necessary to first @free@ the object pointed to by @foo@ and then invoke whatever finalizer was associated with @bar@. That is, finalizers must be invoked in the opposite order they are added. 3. Allow users to invoke a finalizer promptly if they know that the foreign pointer is unreachable, i.e. 'finalizeForeignPtr'. How can these goals be accomplished? Goal 1 suggests that weak references and finalizers (via 'Weak#' and 'mkWeak#') are necessary. But how should they be used and what should their key be? Certainly not 'ForeignPtr' or 'ForeignPtrContents'. See the warning in "GHC.Weak" about weak pointers with lifted (non-primitive) keys. The two finalizer-supporting data constructors of 'ForeignPtr' have an @'IORef' 'Finalizers'@ (backed by 'MutVar#') field. This gets used in two different ways depending on the kind of finalizer: * 'HaskellFinalizers': The first @addForeignPtrConcFinalizer_@ call uses 'mkWeak#' to attach the finalizer @foreignPtrFinalizer@ to the 'MutVar#'. The resulting 'Weak#' is discarded (see @addForeignPtrConcFinalizer_@). Subsequent calls to @addForeignPtrConcFinalizer_@ (goal 2) just add finalizers onto the list in the 'HaskellFinalizers' data constructor. * 'CFinalizers': The first 'addForeignPtrFinalizer' call uses 'mkWeakNoFinalizer#' to create a 'Weak#'. The 'Weak#' is preserved in the 'CFinalizers' data constructor. Both the first call and subsequent calls (goal 2) use 'addCFinalizerToWeak#' to attach finalizers to the 'Weak#' itself. Also, see Note [MallocPtr finalizers] for discussion of the key and value of this 'Weak#'. In either case, the runtime invokes the appropriate finalizers when the 'ForeignPtr' becomes unreachable. -}