Bug in unsafeToForeignPtr/unsafeFromForeignPtr

[patperry@…]

"unsafeToForeignPtr" and "unsafeFromForeignPtr" disagree on the definition of the offset. This leads to bugs when these functions are used in combination with "slice".

Using vector-0.6.0.2 in ghci 6.12.3 on Mac OS 10.6:

GHCi, version 6.12.3: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
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Prelude> :m + Data.Vector.Storable
Prelude Data.Vector.Storable> let x = fromList [ 0..3 ]
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Prelude Data.Vector.Storable> let y = slice 1 3 x
Prelude Data.Vector.Storable> let (f,o,n) = unsafeToForeignPtr y
Prelude Data.Vector.Storable> let y' = unsafeFromForeignPtr f o n
Prelude Data.Vector.Storable> y
fromList [1.0,2.0,3.0] :: Data.Vector.Storable.Vector
Prelude Data.Vector.Storable> y'
fromList [1.6984055785e-313,0.0,1.0] :: Data.Vector.Storable.Vector
Prelude Data.Vector.Storable> unsafeToForeignPtr y
(0x03188580,-1,3)
Prelude Data.Vector.Storable> unsafeToForeignPtr y'
(0x03188580,1,3)

The *real* source of the bug is that offsetToPtr and ptrToOffset disagree. The fix is easy: change the definition of ptrToOffset in Data.Vector.Storable.Internal from

ptrToOffset :: Storable a => ForeignPtr a -> Ptr a -> Int
{-# INLINE ptrToOffset #-}
ptrToOffset fp q = unsafeInlineIO
                 $ withForeignPtr fp $ \p -> return (distance p q)

to

ptrToOffset :: Storable a => ForeignPtr a -> Ptr a -> Int
{-# INLINE ptrToOffset #-}
ptrToOffset fp q = unsafeInlineIO
                 $ withForeignPtr fp $ \p -> return (distance q p)

[rl]

Urgh, thanks for the report. Fixed by:

Sat Aug 21 15:00:43 EST 2010 Roman Leshchinskiy <rl@…>

• Fix bad bug in ptrToOffset