Foreign/Concurrent.hs

    {-# 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 'Foreign.ForeignPtr.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
    -- 'Foreign.ForeignPtr.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, 'System.IO.Handle's
    -- are finalized objects, so a finalizer should not refer to a
    -- 'System.IO.Handle' (including 'System.IO.stdout', 'System.IO.stdin', or
    -- 'System.IO.stderr').
    --
    addForeignPtrFinalizer = GHC.ForeignPtr.addForeignPtrConcFinalizer