Hi all,
GHC defines ForeignPtr as:
data ForeignPtr a = ForeignPtr Addr# ForeignPtrContents
-- we cache the Addr# in the ForeignPtr object, but attach
-- the finalizer to the IORef (or the MutableByteArray# in
-- the case of a MallocPtr). The aim of the representation
-- is to make withForeignPtr efficient; in fact, withForeignPtr
-- should be just as efficient as unpacking a Ptr, and multiple
-- withForeignPtrs can share an unpacked ForeignPtr. Note
-- that touchForeignPtr only has to touch the ForeignPtrContents
-- object, because that ensures that whatever the finalizer is
-- attached to is kept alive.
Is it ok to modify the Addr# field? Or do the libraries assume that it always
points to the start of the block described by ForeignPtrContents? Changing the
Addr# field would be quite useful for pointing to the middle of a memory block.
For instance, Storable vectors are currently defined as:
data Vector a = Vector {-# UNPACK #-} !(Ptr a)
{-# UNPACK #-} !Int
{-# UNPACK #-} !(ForeignPtr a)
If I could use the Addr# field of the ForeignPtr, I could get rid of the Ptr.
Alternatively, I could define my own version of ForeignPtr if only
ForeignPtrContents was exported (abstractedly would be enough).
Roman
_______________________________________________
Glasgow-haskell-users mailing list
Glasgow-haskell-users@haskell.org
http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
