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Stable references to Haskell values
The C-side interface
This module is part of the Foreign Function Interface (FFI) and will usually be imported via the module Foreign.
data StablePtr a
newStablePtr :: a -> IO (StablePtr a)
deRefStablePtr :: StablePtr a -> IO a
freeStablePtr :: StablePtr a -> IO ()
castStablePtrToPtr :: StablePtr a -> Ptr ()
castPtrToStablePtr :: Ptr () -> StablePtr a
Stable references to Haskell values
data StablePtr a Source

A stable pointer is a reference to a Haskell expression that is guaranteed not to be affected by garbage collection, i.e., it will neither be deallocated nor will the value of the stable pointer itself change during garbage collection (ordinary references may be relocated during garbage collection). Consequently, stable pointers can be passed to foreign code, which can treat it as an opaque reference to a Haskell value.

A value of type StablePtr a is a stable pointer to a Haskell expression of type a.

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newStablePtr :: a -> IO (StablePtr a)Source
Create a stable pointer referring to the given Haskell value.
deRefStablePtr :: StablePtr a -> IO aSource
Obtain the Haskell value referenced by a stable pointer, i.e., the same value that was passed to the corresponding call to makeStablePtr. If the argument to deRefStablePtr has already been freed using freeStablePtr, the behaviour of deRefStablePtr is undefined.
freeStablePtr :: StablePtr a -> IO ()Source
Dissolve the association between the stable pointer and the Haskell value. Afterwards, if the stable pointer is passed to deRefStablePtr or freeStablePtr, the behaviour is undefined. However, the stable pointer may still be passed to castStablePtrToPtr, but the Foreign.Ptr.Ptr () value returned by castStablePtrToPtr, in this case, is undefined (in particular, it may be Foreign.Ptr.nullPtr). Nevertheless, the call to castStablePtrToPtr is guaranteed not to diverge.
castStablePtrToPtr :: StablePtr a -> Ptr ()Source
Coerce a stable pointer to an address. No guarantees are made about the resulting value, except that the original stable pointer can be recovered by castPtrToStablePtr. In particular, the address may not refer to an accessible memory location and any attempt to pass it to the member functions of the class Foreign.Storable.Storable leads to undefined behaviour.
castPtrToStablePtr :: Ptr () -> StablePtr aSource

The inverse of castStablePtrToPtr, i.e., we have the identity

 sp == castPtrToStablePtr (castStablePtrToPtr sp)

for any stable pointer sp on which freeStablePtr has not been executed yet. Moreover, castPtrToStablePtr may only be applied to pointers that have been produced by castStablePtrToPtr.

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 *HsStablePtr;  

Note that no assumptions may be made about the values representing stable pointers. In fact, they need not even be valid memory addresses. The only guarantee provided is that if they are passed back to Haskell land, the function deRefStablePtr will be able to reconstruct the Haskell value referred to by the stable pointer.

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