On Sat, Nov 08, 2003 at 06:32:35PM -0800, Ben Escoto wrote:
Hi, I'm trying to learn about Haskell's FFI (running 6.0.1 on linux) and see the following weird behavior with ghci: Prelude> :module Data.Word Foreign.Storable Foreign.Ptr Prelude Foreign.Ptr Foreign.Storable Data.Word> sizeOf nullPtr 4 Prelude Foreign.Ptr Foreign.Storable Data.Word> sizeOf (nullPtr :: Ptr Word64) 4 Shouldn't "sizeOf nullPtr" return an error? And sizeOf a Ptr Word64 should be 8 I think. Also this program prints "4", when it seems it should print "8":
module Main where import Data.Word import Foreign.Storable import Foreign.Ptr import Foreign.StablePtr
x :: Word64 x = 5
main = do x_sptr <- newStablePtr 5 putStrLn $ show $ sizeOf x_sptr
These are artificial examples, but I originally noticed this when trying to decode a C structure. Is this a bug in ghc or am I misunderstanding what sizeOf is supposed to do?
sizeof returns the size of its argument, since the argument you passed is a pointer, so it properly returns the size of a pointer, 32 bits or 4 bytes on your architecture. to find the size of the actual datatype do something like sizeOf (undefined :: Word16) => 2 sizeOf (undefined :: Word64) => 8 sizeOf never evaluates its arguments so it is okay to pass anything to it of the type you are interested in. John -- --------------------------------------------------------------------------- John Meacham - California Institute of Technology, Alum. - john@foo.net ---------------------------------------------------------------------------