    \begin{code}
    {-# OPTIONS_GHC -fno-implicit-prelude #-}
    -----------------------------------------------------------------------------
    -- |
    -- Module      :  GHC.Ptr
    -- Copyright   :  (c) The FFI Task Force, 2000-2002
    -- License     :  see libraries/base/LICENSE
    --
    -- Maintainer  :  ffi@haskell.org
    -- Stability   :  internal
    -- Portability :  non-portable (GHC Extensions)
    --
    -- The 'Ptr' and 'FunPtr' types and operations.
    --
    -----------------------------------------------------------------------------

    -- #hide
    module GHC.Ptr where

    import GHC.Base
    import GHC.Show
    import GHC.Num
    import GHC.List ( length, replicate )
    import Numeric		( showHex )

    #include "MachDeps.h"

    ------------------------------------------------------------------------
    -- Data pointers.

    data Ptr a = Ptr Addr# deriving (Eq, Ord)
    -- ^ A value of type @'Ptr' a@ represents a pointer to an object, or an
    -- array of objects, which may be marshalled to or from Haskell values
    -- of type @a@.
    --
    -- The type @a@ will often be an instance of class
    -- 'Foreign.Storable.Storable' which provides the marshalling operations.
    -- However this is not essential, and you can provide your own operations
    -- to access the pointer.  For example you might write small foreign
    -- functions to get or set the fields of a C @struct@.

    -- |The constant 'nullPtr' contains a distinguished value of 'Ptr'
    -- that is not associated with a valid memory location.
    nullPtr :: Ptr a
    nullPtr = Ptr nullAddr#

    -- |The 'castPtr' function casts a pointer from one type to another.
    castPtr :: Ptr a -> Ptr b
    castPtr (Ptr addr) = Ptr addr

    -- |Advances the given address by the given offset in bytes.
    plusPtr :: Ptr a -> Int -> Ptr b
    plusPtr (Ptr addr) (I# d) = Ptr (plusAddr# addr d)

    -- |Given an arbitrary address and an alignment constraint,
    -- 'alignPtr' yields the next higher address that fulfills the
    -- alignment constraint.  An alignment constraint @x@ is fulfilled by
    -- any address divisible by @x@.  This operation is idempotent.
    alignPtr :: Ptr a -> Int -> Ptr a
    alignPtr addr@(Ptr a) (I# i)
      = case remAddr# a i of {
          0# -> addr;
          n -> Ptr (plusAddr# a (i -# n)) }

    -- |Computes the offset required to get from the first to the second
    -- argument.  We have
    --
    -- > p2 == p1 `plusPtr` (p2 `minusPtr` p1)
    minusPtr :: Ptr a -> Ptr b -> Int
    minusPtr (Ptr a1) (Ptr a2) = I# (minusAddr# a1 a2)

    ------------------------------------------------------------------------
    -- Function pointers for the default calling convention.

    data FunPtr a = FunPtr Addr# deriving (Eq, Ord)
    -- ^ A value of type @'FunPtr' a@ is a pointer to a function callable
    -- from foreign code.  The type @a@ will normally be a /foreign type/,
    -- a function type with zero or more arguments where
    --
    -- * the argument types are /marshallable foreign types/,
    --   i.e. 'Char', 'Int', 'Prelude.Double', 'Prelude.Float',
    --   'Bool', 'Data.Int.Int8', 'Data.Int.Int16', 'Data.Int.Int32',
    --   'Data.Int.Int64', 'Data.Word.Word8', 'Data.Word.Word16',
    --   'Data.Word.Word32', 'Data.Word.Word64', @'Ptr' a@, @'FunPtr' a@,
    --   @'Foreign.StablePtr.StablePtr' a@ or a renaming of any of these
    --   using @newtype@.
    --
    -- * the return type is either a marshallable foreign type or has the form
    --   @'Prelude.IO' t@ where @t@ is a marshallable foreign type or @()@.
    --
    -- A value of type @'FunPtr' a@ may be a pointer to a foreign function,
    -- either returned by another foreign function or imported with a
    -- a static address import like
    --
    -- > foreign import ccall "stdlib.h &free"
    -- >   p_free :: FunPtr (Ptr a -> IO ())
    --
    -- or a pointer to a Haskell function created using a /wrapper/ stub
    -- declared to produce a 'FunPtr' of the correct type.  For example:
    --
    -- > type Compare = Int -> Int -> Bool
    -- > foreign import ccall "wrapper"
    -- >   mkCompare :: Compare -> IO (FunPtr Compare)
    --
    -- Calls to wrapper stubs like @mkCompare@ allocate storage, which
    -- should be released with 'Foreign.Ptr.freeHaskellFunPtr' when no
    -- longer required.
    --
    -- To convert 'FunPtr' values to corresponding Haskell functions, one
    -- can define a /dynamic/ stub for the specific foreign type, e.g.
    --
    -- > type IntFunction = CInt -> IO ()
    -- > foreign import ccall "dynamic"
    -- >   mkFun :: FunPtr IntFunction -> IntFunction

    -- |The constant 'nullFunPtr' contains a
    -- distinguished value of 'FunPtr' that is not
    -- associated with a valid memory location.
    nullFunPtr :: FunPtr a
    nullFunPtr = FunPtr nullAddr#

    -- |Casts a 'FunPtr' to a 'FunPtr' of a different type.
    castFunPtr :: FunPtr a -> FunPtr b
    castFunPtr (FunPtr addr) = FunPtr addr

    -- |Casts a 'FunPtr' to a 'Ptr'.
    --
    -- /Note:/ this is valid only on architectures where data and function
    -- pointers range over the same set of addresses, and should only be used
    -- for bindings to external libraries whose interface already relies on
    -- this assumption.
    castFunPtrToPtr :: FunPtr a -> Ptr b
    castFunPtrToPtr (FunPtr addr) = Ptr addr

    -- |Casts a 'Ptr' to a 'FunPtr'.
    --
    -- /Note:/ this is valid only on architectures where data and function
    -- pointers range over the same set of addresses, and should only be used
    -- for bindings to external libraries whose interface already relies on
    -- this assumption.
    castPtrToFunPtr :: Ptr a -> FunPtr b
    castPtrToFunPtr (Ptr addr) = FunPtr addr

    ------------------------------------------------------------------------
    -- Show instances for Ptr and FunPtr
    -- I have absolutely no idea why the WORD_SIZE_IN_BITS stuff is here

    #if (WORD_SIZE_IN_BITS == 32 || WORD_SIZE_IN_BITS == 64)
    instance Show (Ptr a) where
       showsPrec p (Ptr a) rs = pad_out (showHex (word2Integer(int2Word#(addr2Int# a))) "") rs
         where
            -- want 0s prefixed to pad it out to a fixed length.
           pad_out ls rs =
    	  '0':'x':(replicate (2*SIZEOF_HSPTR - length ls) '0') ++ ls ++ rs
           -- word2Integer :: Word# -> Integer (stolen from Word.lhs)
           word2Integer w = case word2Integer# w of
    			(# s, d #) -> J# s d

    instance Show (FunPtr a) where
       showsPrec p = showsPrec p . castFunPtrToPtr
    #endif
    \end{code}