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Foreign.ForeignPtr

Portability | portable
Stability | provisional
Maintainer | ffi@haskell.org

Contents

Finalised data pointers
  Basic operations
  Low-level operations
  Allocating managed memory

Description
The `ForeignPtr` type and operations. This module is part of the Foreign Function Interface (FFI) and will usually be imported via the Foreign module.
Synopsis

data ForeignPtr a
type FinalizerPtr a = FunPtr (Ptr a -> IO ())
type FinalizerEnvPtr env a = FunPtr (Ptr env -> Ptr a -> IO ())
newForeignPtr :: FinalizerPtr a -> Ptr a -> IO (ForeignPtr a)
newForeignPtr_ :: Ptr a -> IO (ForeignPtr a)
addForeignPtrFinalizer :: FinalizerPtr a -> ForeignPtr a -> IO ()
newForeignPtrEnv :: FinalizerEnvPtr env a -> Ptr env -> Ptr a -> IO (ForeignPtr a)
addForeignPtrFinalizerEnv :: FinalizerEnvPtr env a -> Ptr env -> ForeignPtr a -> IO ()
withForeignPtr :: ForeignPtr a -> (Ptr a -> IO b) -> IO b
finalizeForeignPtr :: ForeignPtr a -> IO ()
unsafeForeignPtrToPtr :: ForeignPtr a -> Ptr a
touchForeignPtr :: ForeignPtr a -> IO ()
castForeignPtr :: ForeignPtr a -> ForeignPtr b
mallocForeignPtr :: Storable a => IO (ForeignPtr a)
mallocForeignPtrBytes :: Int -> IO (ForeignPtr a)
mallocForeignPtrArray :: Storable a => Int -> IO (ForeignPtr a)
mallocForeignPtrArray0 :: Storable a => Int -> IO (ForeignPtr a)

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 `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`.

Instances

Typeable1 ForeignPtr
Typeable a => Data (ForeignPtr a)
Eq (ForeignPtr a)
Ord (ForeignPtr a)
Show (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.
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 finaliser 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.

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 `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`
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.
Low-level operations
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 `withForeignPtr` rather than combinations of `unsafeForeignPtrToPtr` and `touchForeignPtr`. However, the later routines are occasionally preferred in tool generated marshalling code.

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` 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.

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`.
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