    {-# 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,
            mallocPlainForeignPtr,
            mallocForeignPtrBytes,
            mallocPlainForeignPtrBytes,
            addForeignPtrFinalizer,
            touchForeignPtr,
            unsafeForeignPtrToPtr,
            castForeignPtr,
            newConcForeignPtr,
            addForeignPtrConcFinalizer,
            finalizeForeignPtr
      ) where

    import Control.Monad    ( sequence_ )
    import Foreign.Storable

    import GHC.Show
    import GHC.List         ( null )
    import GHC.Base
    import GHC.IOBase
    import GHC.STRef        ( STRef(..) )
    import GHC.Ptr          ( Ptr(..), FunPtr )
    import GHC.Err

    -- |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
            -- we cache the Addr# in the ForeignPtr object, but attach
            -- the finalizer to the IORef (or the MutableByteArray# in
            -- the case of a MallocPtr).  The 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.  Note
            -- that touchForeignPtr only has to touch the ForeignPtrContents
            -- object, because that ensures that whatever the finalizer is
            -- attached to is kept alive.

    data ForeignPtrContents
      = PlainForeignPtr !(IORef [IO ()])
      | MallocPtr      (MutableByteArray# RealWorld) !(IORef [IO ()])
      | PlainPtr       (MutableByteArray# RealWorld)

    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.
    --
    -- 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 'newForeignPtr'
    -- with a finalizer.
    --
    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, ForeignPtr (byteArrayContents# (unsafeCoerce# mbarr#))
                                  (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, ForeignPtr (byteArrayContents# (unsafeCoerce# 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 = IO $ \s ->
                case newPinnedByteArray# size s of { (# s, mbarr# #) ->
                 (# s, ForeignPtr (byteArrayContents# (unsafeCoerce# mbarr#))
                                  (PlainPtr mbarr#) #)
                }
                where (I# size) = sizeOf 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 (I# size) = IO $ \s ->
        case newPinnedByteArray# size s      of { (# s, mbarr# #) ->
           (# s, ForeignPtr (byteArrayContents# (unsafeCoerce# 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 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 (ForeignPtr a c) finalizer =
      addForeignPtrConcFinalizer_ c finalizer

    addForeignPtrConcFinalizer_ f@(PlainForeignPtr r) finalizer = do
      fs <- readIORef r
      writeIORef r (finalizer : fs)
      if (null fs)
         then IO $ \s ->
                  case r of { IORef (STRef r#) ->
                  case mkWeak# r# () (foreignPtrFinalizer r) 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 ->
                   case mkWeak# fo () (do foreignPtrFinalizer r; touch f) s of
                      (# s1, w #) -> (# s1, () #)
         else return ()

    addForeignPtrConcFinalizer_ _ _ =
      error "GHC.ForeignPtr: attempt to add a finalizer to plain pointer"

    foreign import ccall "dynamic"
      mkFinalizer :: FinalizerPtr a -> Ptr a -> IO ()

    foreignPtrFinalizer :: IORef [IO ()] -> IO ()
    foreignPtrFinalizer r = 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 []
      return (ForeignPtr obj (PlainForeignPtr 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. In particular 'Foreign.ForeignPtr.withForeignPtr'
    -- does a 'touchForeignPtr' after it
    -- executes the user action.
    --
    -- 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 fo r) = touch r

    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 later routines
    -- are occasionally preferred in tool generated marshalling code.
    unsafeForeignPtrToPtr (ForeignPtr fo r) = Ptr fo

    castForeignPtr :: ForeignPtr a -> ForeignPtr b
    -- ^This function casts a 'ForeignPtr'
    -- parameterised by one type into another type.
    castForeignPtr f = unsafeCoerce# f

    -- | Causes the finalizers associated with a foreign pointer to be run
    -- immediately.
    finalizeForeignPtr :: ForeignPtr a -> IO ()
    finalizeForeignPtr (ForeignPtr _ (PlainPtr _)) = return () -- no effect
    finalizeForeignPtr (ForeignPtr _ foreignPtr) = do
            finalizers <- readIORef refFinalizers
            sequence_ finalizers
            writeIORef refFinalizers []
            where
                    refFinalizers = case foreignPtr of
                            (PlainForeignPtr ref) -> ref
                            (MallocPtr     _ ref) -> ref