{-# LANGUAGE Trustworthy #-}
{-# LANGUAGE NoImplicitPrelude #-}

-----------------------------------------------------------------------------
-- |
-- Module      :  Foreign.Concurrent
-- Copyright   :  (c) The University of Glasgow 2003
-- License     :  BSD-style (see the file libraries/base/LICENSE)
-- 
-- Maintainer  :  ffi@haskell.org
-- Stability   :  provisional
-- Portability :  non-portable (requires concurrency)
--
-- FFI datatypes and operations that use or require concurrency (GHC only).
--
-----------------------------------------------------------------------------

module Foreign.Concurrent
  (
        -- * Concurrency-based 'ForeignPtr' operations

        -- | These functions generalize their namesakes in the portable
        -- "Foreign.ForeignPtr" module by allowing arbitrary 'IO' actions
        -- as finalizers.  These finalizers necessarily run in a separate
        -- thread, cf. /Destructors, Finalizers and Synchronization/,
        -- by Hans Boehm, /POPL/, 2003.

        newForeignPtr,
        addForeignPtrFinalizer,
  ) where

import GHC.IO         ( IO )
import GHC.Ptr        ( Ptr )
import GHC.ForeignPtr ( ForeignPtr )
import qualified GHC.ForeignPtr

newForeignPtr :: Ptr a -> IO () -> IO (ForeignPtr a)
--
-- ^Turns a plain memory reference into a foreign object by
-- associating a finalizer - given by the monadic operation - with the
-- reference.  The storage manager will start the finalizer, in a
-- separate thread, some time after the last reference to the
-- @ForeignPtr@ is dropped.  There is no guarantee of promptness, and
-- in fact there is no guarantee that the finalizer will eventually
-- run at all.
--
-- Note that references from a finalizer do not necessarily prevent
-- another object from being finalized.  If A's finalizer refers to B
-- (perhaps using 'touchForeignPtr', then the only guarantee is that
-- B's finalizer will never be started before A's.  If both A and B
-- are unreachable, then both finalizers will start together.  See
-- 'touchForeignPtr' for more on finalizer ordering.
--
newForeignPtr = GHC.ForeignPtr.newConcForeignPtr

addForeignPtrFinalizer :: ForeignPtr a -> IO () -> IO ()
-- ^This function adds a finalizer to the given @ForeignPtr@.  The
-- finalizer will run /before/ all other finalizers for the same
-- object which have already been registered.
--
-- This is a variant of @Foreign.ForeignPtr.addForeignPtrFinalizer@,
-- where the finalizer is an arbitrary @IO@ action.  When it is
-- invoked, the finalizer will run in a new thread.
--
-- NB. Be very careful with these finalizers.  One common trap is that
-- if a finalizer references another finalized value, it does not
-- prevent that value from being finalized.  In particular, 'Handle's
-- are finalized objects, so a finalizer should not refer to a 'Handle'
-- (including @stdout@, @stdin@ or @stderr@).
--
addForeignPtrFinalizer = GHC.ForeignPtr.addForeignPtrConcFinalizer

