Foreign.ForeignPtr.Safe.Compat

Finalised data pointers

data ForeignPtr a

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 ForeignPtrs 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.

Instances

Show (ForeignPtr a)

Since: base-2.1

Defined in GHC.ForeignPtr

Methods

showsPrec :: Int -> ForeignPtr a -> ShowS

show :: ForeignPtr a -> String

showList :: [ForeignPtr a] -> ShowS

Eq (ForeignPtr a)

Since: base-2.1

Defined in GHC.ForeignPtr

Methods

(==) :: ForeignPtr a -> ForeignPtr a -> Bool

(/=) :: ForeignPtr a -> ForeignPtr a -> Bool

Ord (ForeignPtr a)

Since: base-2.1

Defined in GHC.ForeignPtr

Methods

compare :: ForeignPtr a -> ForeignPtr a -> Ordering

(<) :: ForeignPtr a -> ForeignPtr a -> Bool

(<=) :: ForeignPtr a -> ForeignPtr a -> Bool

(>) :: ForeignPtr a -> ForeignPtr a -> Bool

(>=) :: ForeignPtr a -> ForeignPtr a -> Bool

max :: ForeignPtr a -> ForeignPtr a -> ForeignPtr a

min :: ForeignPtr a -> ForeignPtr a -> ForeignPtr a

type FinalizerPtr a = FunPtr (Ptr a -> IO ())

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 FinalizerEnvPtr env a = FunPtr (Ptr env -> Ptr a -> IO ())

Basic operations

newForeignPtr :: FinalizerPtr a -> Ptr a -> IO (ForeignPtr a)

Turns a plain memory reference into a foreign pointer, and associates a finalizer with the reference. The finalizer will be executed after the last reference to the foreign object is dropped. There is no guarantee of promptness, however the finalizer will be executed before the program exits.

newForeignPtr_ :: Ptr a -> IO (ForeignPtr a)

Turns a plain memory reference into a foreign pointer that may be associated with finalizers by using addForeignPtrFinalizer.

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.

newForeignPtrEnv :: FinalizerEnvPtr env a -> Ptr env -> Ptr a -> IO (ForeignPtr a)

This variant of newForeignPtr adds a finalizer that expects an environment in addition to the finalized pointer. The environment that will be passed to the finalizer is fixed by the second argument to newForeignPtrEnv.

addForeignPtrFinalizerEnv :: FinalizerEnvPtr env a -> Ptr env -> ForeignPtr a -> IO ()

Like addForeignPtrFinalizer but the finalizer is passed an additional environment parameter.

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.

finalizeForeignPtr :: ForeignPtr a -> IO ()

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.

Low-level operations

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 preceding 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 ForeignPtrs. 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 MVars 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.

castForeignPtr :: ForeignPtr a -> ForeignPtr b

This function casts a ForeignPtr parameterised by one type into another type.

Allocating managed memory

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 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.

mallocForeignPtrBytes :: Int -> IO (ForeignPtr a)

This function is similar to mallocForeignPtr, except that the size of the memory required is given explicitly as a number of bytes.

mallocForeignPtrArray :: Storable a => Int -> IO (ForeignPtr a)

This function is similar to mallocArray, but yields a memory area that has a finalizer attached that releases the memory area. As with mallocForeignPtr, it is not guaranteed that the block of memory was allocated by malloc.

mallocForeignPtrArray0 :: Storable a => Int -> IO (ForeignPtr a)

This function is similar to mallocArray0, but yields a memory area that has a finalizer attached that releases the memory area. As with mallocForeignPtr, it is not guaranteed that the block of memory was allocated by malloc.