4.15. ForeignPtr

This module is part of the Foreign Function Interface (FFI) and will usually be imported via the module Foreign (see Section 4.13). 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 (Section 4.29) 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 (see Section 4.35).

4.15.1. The Haskell-side Interface

data ForeignPtr a	-- abstract handle to foreign object
instance Eq (ForeignPtr a)

newForeignPtr          :: Ptr a        -> IO () -> IO (ForeignPtr a)
addForeignPtrFinalizer :: ForeignPtr a -> IO () -> IO () 
withForeignPtr         :: ForeignPtr a -> (Ptr a -> IO b) -> IO b
touchForeignPtr        :: ForeignPtr a -> IO ()
foreignPtrToPtr	       :: ForeignPtr a -> Ptr a
castForeignPtr	       :: ForeignPtr a -> ForeignPtr b

The behaviour of the functions is as follows:

newForeignPtr :: 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.

addForeignPtrFinalizer :: ForeignPtr a -> IO () -> IO ()

This function adds another finaliser to the given foreign object. No guarantees are made on the order in which multiple finalisers for a single object are run.

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.

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.

foreignPtrToPtr :: 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 foreignPtrToPtr 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 foreignPtrToPtr 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 ForeignPtrs: 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.

4.15.2. The C-side Interface

The following definition is available to C programs inter-operating with Haskell code when including the header HsFFI.h.

typedef void* HsForeignPtr;  /* C representation of a ForeignPtr */