GHC/ForeignPtr.hs

    {-# 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
      (
            ForeignPtr(..),
            ForeignPtrContents(..),
            FinalizerPtr,
            FinalizerEnvPtr,
            newForeignPtr_,
            mallocForeignPtr,
            mallocPlainForeignPtr,
            mallocForeignPtrBytes,
            mallocPlainForeignPtrBytes,
            mallocForeignPtrAlignedBytes,
            mallocPlainForeignPtrAlignedBytes,
            addForeignPtrFinalizer,
            addForeignPtrFinalizerEnv,
            touchForeignPtr,
            unsafeForeignPtrToPtr,
            castForeignPtr,
            plusForeignPtr,
            newConcForeignPtr,
            addForeignPtrConcFinalizer,
            finalizeForeignPtr
      ) 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(..) )

    -- |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.

    data Finalizers
      = NoFinalizers
      | CFinalizers (Weak# ())
      | HaskellFinalizers [IO ()]

    data ForeignPtrContents
      = PlainForeignPtr !(IORef Finalizers)
      | MallocPtr      (MutableByteArray# RealWorld) !(IORef Finalizers)
      | PlainPtr       (MutableByteArray# RealWorld)

    -- | @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# (unsafeCoerce# 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# (unsafeCoerce# 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# (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
              | I# size < 0 = errorWithoutStackTrace "mallocForeignPtr: size must be >= 0"
              | otherwise = IO $ \s ->
                case newAlignedPinnedByteArray# size align s of { (# s', mbarr# #) ->
                 (# s', ForeignPtr (byteArrayContents# (unsafeCoerce# 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# (unsafeCoerce# 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# (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 (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"

    -- 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 'addForeignPtrFinalizerEnv' but allows the finalizer to be
    -- passed an additional environment parameter to be passed to the
    -- finalizer.  The environment passed to the finalizer is fixed by the
    -- second argument to 'addForeignPtrFinalizerEnv'
    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"

    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"

    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

    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"

    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))

    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 _ 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.
    finalizeForeignPtr :: ForeignPtr a -> IO ()
    finalizeForeignPtr (ForeignPtr _ (PlainPtr _)) = return () -- no effect
    finalizeForeignPtr (ForeignPtr _ foreignPtr) = foreignPtrFinalizer refFinalizers
            where
                    refFinalizers = case foreignPtr of
                            (PlainForeignPtr ref) -> ref
                            (MallocPtr     _ ref) -> ref
                            PlainPtr _            ->
                                errorWithoutStackTrace "finalizeForeignPtr PlainPtr"