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Foreign.ForeignPtr

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

Contents

Finalised data pointers
  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 ())
newForeignPtr :: FinalizerPtr a -> Ptr a -> IO (ForeignPtr a)
newForeignPtr_ :: Ptr a -> IO (ForeignPtr a)
addForeignPtrFinalizer :: FinalizerPtr a -> ForeignPtr a -> IO ()
withForeignPtr :: ForeignPtr a -> (Ptr a -> IO b) -> IO b
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 finalisers. A finaliser 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 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

Eq (ForeignPtr a)
Ord (ForeignPtr a)
Show (ForeignPtr a)
Typeable a => Typeable (ForeignPtr a)

type FinalizerPtr a = FunPtr (Ptr a -> IO ())
A Finaliser 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.
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 finaliser to the given foreign object. The finalizer will run before all other finalizers for the same object which have already been registered.
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 unsafety 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.

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 occurence of the given foreign pointer, then its finaliser(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.

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.

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`.

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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