-{-# OPTIONS -fno-implicit-prelude #-}
+{-# OPTIONS_GHC -fno-implicit-prelude #-}
-----------------------------------------------------------------------------
-- |
-- Module : GHC.ForeignPtr
--
-----------------------------------------------------------------------------
+-- #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.Ptr
import Foreign.Storable
-import Data.Dynamic
+import GHC.Show
import GHC.List ( null )
import GHC.Base
import GHC.IOBase
-import GHC.Ptr ( Ptr(..) )
+import GHC.STRef ( STRef(..) )
+import GHC.Ptr ( Ptr(..), FunPtr )
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 /finalisers/. A finaliser is a routine that is invoked when
+-- 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 finaliser will, then, invoke
+-- the 'ForeignPtr'. Typically, the finalizer will, then, invoke
-- routines in the foreign language that free the resources bound by
-- the foreign object.
--
-- type argument of 'ForeignPtr' should normally be an instance of
-- class 'Storable'.
--
-data ForeignPtr a
- = ForeignPtr ForeignObj# !(IORef [IO ()])
- | MallocPtr (MutableByteArray# RealWorld) !(IORef [IO ()])
+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 Show (ForeignPtr a) where
showsPrec p f = showsPrec p (unsafeForeignPtrToPtr f)
-#include "Dynamic.h"
-INSTANCE_TYPEABLE1(ForeignPtr,foreignPtrTc,"ForeignPtr")
--- |A Finaliser is represented as a pointer to a foreign function that, at
+-- |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 finaliser - given by the monadic operation
--- - with the reference. The finaliser will be executed after
--- the last reference to the foreign object is dropped. Note
--- that there is no guarantee on how soon the finaliser is
--- executed after the last reference was dropped; this depends
--- on the details of the Haskell storage manager. The only
--- guarantee is that the finaliser runs before the program
--- terminates.
---
--- The finalizer, when invoked, will run in a separate thread.
+--
+-- ^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
return fObj
mallocForeignPtr :: Storable a => IO (ForeignPtr a)
--- ^ allocates some memory and returns a ForeignPtr to it. The memory
--- will be released automatically when the ForeignPtr is discarded.
+-- ^ Allocate some memory and return a 'ForeignPtr' to it. The memory
+-- will be released automatically when the 'ForeignPtr' is discarded.
--
--- @mallocForeignPtr@ is equivalent to
+-- 'mallocForeignPtr' is equivalent to
--
--- > do { p <- malloc; newForeignPtr p free }
+-- > do { p <- malloc; newForeignPtr finalizerFree p }
--
--- although it may be implemented differently internally. You may not
+-- although it may be implemented differently internally: you may not
-- assume that the memory returned by 'mallocForeignPtr' has been
--- allocated with C's @malloc()@.
+-- 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 a => a -> IO (ForeignPtr a)
+ 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 #)
+ (# s, ForeignPtr (byteArrayContents# (unsafeCoerce# mbarr#))
+ (MallocPtr mbarr# r) #)
}
where (I# size) = sizeOf a
--- | similar to 'mallocForeignPtr', except that the size of the memory required
--- is given explicitly as a number of bytes.
+-- | 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 #)
+ (# 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 :: ForeignPtr a -> FinalizerPtr a -> IO ()
--- ^This function adds a finaliser to the given foreign object. The
+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 fptr finalizer =
+addForeignPtrFinalizer finalizer fptr =
addForeignPtrConcFinalizer fptr
(mkFinalizer finalizer (unsafeForeignPtrToPtr fptr))
addForeignPtrConcFinalizer :: ForeignPtr a -> IO () -> IO ()
--- ^This function adds a finaliser to the given @ForeignPtr@. The
+-- ^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.
--
-- is an arbitrary @IO@ action. When it is invoked, the finalizer
-- will run in a new thread.
--
-addForeignPtrConcFinalizer f@(ForeignPtr fo r) finalizer = do
+-- 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 ->
- let p = unsafeForeignPtrToPtr f in
- case mkWeak# fo () (foreignPtrFinalizer r p) s of
- (# s1, w #) -> (# s1, () #)
+ 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
+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 () (foreignPtrFinalizer r p) s of
+ 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 ()] -> Ptr a -> IO ()
-foreignPtrFinalizer r p = do
- fs <- readIORef r
- sequence_ fs
+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 []
- IO $ \ s# ->
- case mkForeignObj# obj s# of
- (# s1#, fo# #) -> (# s1#, ForeignPtr fo# r #)
+ 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 'withForeignPtr'
+-- actions. In particular 'Foreign.ForeignPtr.withForeignPtr'
-- does a 'touchForeignPtr' after it
-- executes the user action.
--
--- This function can be used to express liveness
--- dependencies between 'ForeignPtr's: for
--- example, if the finalizer for one
--- 'ForeignPtr' touches a second
--- 'ForeignPtr', then it is ensured that the
--- second 'ForeignPtr' will stay alive at
--- least as long as the first. This can be useful when you
--- want to manipulate /interior pointers/ to
--- a foreign structure: you can use
--- 'touchForeignObj' to express the
--- requirement that the exterior pointer must not be finalized
--- until the interior pointer is no longer referenced.
-touchForeignPtr (ForeignPtr fo r)
- = IO $ \s -> case touch# fo s of s -> (# s, () #)
-touchForeignPtr (MallocPtr fo r)
- = IO $ \s -> case touch# fo s of s -> (# s, () #)
+-- 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
--- occurence of the given foreign pointer, then its finaliser(s) will
+-- 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 'withForeignPtr' rather
+-- 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))
+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
+
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