Foreign.StablePtr (original) (raw)
Contents
Description
This module is part of the Foreign Function Interface (FFI) and will usually be imported via the module Foreign.
Synopsis
- 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.
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](Foreign-StablePtr.html#v: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 [Storable](Foreign-Storable.html#t:Storable) leads to undefined behaviour.
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](Foreign-StablePtr.html#v:deRefStablePtr) will be able to reconstruct the Haskell value referred to by the stable pointer.