On 01/06/2011 08:29, Roman Leshchinskiy wrote:
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:
It should be fine to use whatever Addr# you like. The only requirement
is that calling touch# on the ForeignPtrContents is enough to keep the
memory alive.
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.
Good idea!
Cheers,
Simon
_______________________________________________
Glasgow-haskell-users mailing list
Glasgow-haskell-users@haskell.org
http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
