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{-# OPTIONS_GHC -fno-implicit-prelude #-}
-----------------------------------------------------------------------------
-- |
-- 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.
--
-----------------------------------------------------------------------------
-- #hide
module GHC.ForeignPtr
(
ForeignPtr(..),
FinalizerPtr,
newForeignPtr_,
mallocForeignPtr,
mallocForeignPtrBytes,
addForeignPtrFinalizer,
touchForeignPtr,
unsafeForeignPtrToPtr,
castForeignPtr,
newConcForeignPtr,
addForeignPtrConcFinalizer,
finalizeForeignPtr
) where
import Control.Monad ( sequence_ )
import Foreign.Ptr
import Foreign.Storable
import GHC.List ( null )
import GHC.Base
import GHC.IOBase
import GHC.Ptr ( Ptr(..) )
import GHC.Err
import GHC.Show
-- |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 ForeignObj# !(IORef [IO ()])
| MallocPtr (MutableByteArray# RealWorld) !(IORef [IO ()])
instance Eq (ForeignPtr a) where
p == q = unsafeForeignPtrToPtr p == unsafeForeignPtrToPtr q
instance Ord (ForeignPtr a) where
compare p q = compare (unsafeForeignPtrToPtr p) (unsafeForeignPtrToPtr q)
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.
--
type FinalizerPtr a = FunPtr (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.
--
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'.
mallocForeignPtr = doMalloc undefined
where doMalloc :: Storable b => b -> IO (ForeignPtr b)
doMalloc a = do
r <- newIORef []
IO $ \s ->
case newPinnedByteArray# size s of { (# s, mbarr# #) ->
(# s, MallocPtr mbarr# r #)
}
where (I# size) = sizeOf 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 (I# size) = do
r <- newIORef []
IO $ \s ->
case newPinnedByteArray# size s of { (# s, mbarr# #) ->
(# s, MallocPtr mbarr# r #)
}
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 finalizer fptr =
addForeignPtrConcFinalizer fptr
(mkFinalizer finalizer (unsafeForeignPtrToPtr fptr))
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, 'Handle's
-- are finalized objects, so a finalizer should not refer to a 'Handle'
-- (including @stdout@, @stdin@ or @stderr@).
--
addForeignPtrConcFinalizer f@(ForeignPtr fo r) finalizer = do
fs <- readIORef r
writeIORef r (finalizer : fs)
if (null fs)
then IO $ \s ->
let p = unsafeForeignPtrToPtr f in
case mkWeak# fo () (foreignPtrFinalizer r p) s of
(# s1, w #) -> (# s1, () #)
else return ()
addForeignPtrConcFinalizer f@(MallocPtr fo r) finalizer = do
fs <- readIORef r
writeIORef r (finalizer : fs)
if (null fs)
then IO $ \s ->
let p = unsafeForeignPtrToPtr f in
case mkWeak# fo () (do foreignPtrFinalizer r p
touchPinnedByteArray# fo) s of
(# s1, w #) -> (# s1, () #)
else return ()
foreign import ccall "dynamic"
mkFinalizer :: FinalizerPtr a -> Ptr a -> IO ()
foreignPtrFinalizer :: IORef [IO ()] -> Ptr a -> IO ()
foreignPtrFinalizer r p = do
fs <- readIORef r
sequence_ fs
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 []
IO $ \ s# ->
case mkForeignObj# obj s# of
(# s1#, fo# #) -> (# s1#, ForeignPtr fo# r #)
touchPinnedByteArray# :: MutableByteArray# RealWorld -> IO ()
touchPinnedByteArray# ba# = IO $ \s -> case touch# ba# s of s -> (# s, () #)
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. In particular 'Foreign.ForeignPtr.withForeignPtr'
-- does a 'touchForeignPtr' after it
-- executes the user action.
--
-- Note that this function should not be used to express liveness
-- dependencies between '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.
--
touchForeignPtr (ForeignPtr fo r)
= IO $ \s -> case touch# fo s of s -> (# s, () #)
touchForeignPtr (MallocPtr fo r)
= touchPinnedByteArray# fo
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 later routines
-- are occasionally preferred in tool generated marshalling code.
unsafeForeignPtrToPtr (ForeignPtr fo r) = Ptr (foreignObjToAddr# fo)
unsafeForeignPtrToPtr (MallocPtr fo r) = Ptr (byteArrayContents# (unsafeCoerce# fo))
castForeignPtr :: ForeignPtr a -> ForeignPtr b
-- ^This function casts a 'ForeignPtr'
-- parameterised by one type into another type.
castForeignPtr f = unsafeCoerce# f
-- | Causes a the finalizers associated with a foreign pointer to be run
-- immediately.
finalizeForeignPtr :: ForeignPtr a -> IO ()
finalizeForeignPtr foreignPtr = do
finalizers <- readIORef refFinalizers
sequence_ finalizers
writeIORef refFinalizers []
where
refFinalizers = case foreignPtr of
(ForeignPtr _ ref) -> ref
(MallocPtr _ ref) -> ref
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