Chapter 37

module Foreign.Storable (  
  ) where

class Storable a where
The member functions of this class facilitate writing values of primitive types to raw memory (which may have been allocated with the above mentioned routines) and reading values from blocks of raw memory. The class, furthermore, includes support for computing the storage requirements and alignment restrictions of storable types.

Memory addresses are represented as values of type Ptr a, for some a which is an instance of class Storable. The type argument to Ptr helps provide some valuable type safety in FFI code (you can’t mix pointers of different types without an explicit cast), while helping the Haskell type system figure out which marshalling method is needed for a given pointer.

All marshalling between Haskell and a foreign language ultimately boils down to translating Haskell data structures into the binary representation of a corresponding data structure of the foreign language and vice versa. To code this marshalling in Haskell, it is necessary to manipulate primitive data types stored in unstructured memory blocks. The class Storable facilitates this manipulation on all types for which it is instantiated, which are the standard basic types of Haskell, the fixed size Int types (Int8, Int16, Int32, Int64), the fixed size Word types (Word8, Word16, Word32, Word64), StablePtr, all types from Foreign.C.Types, as well as Ptr.

Minimal complete definition: sizeOf, alignment, one of peek, peekElemOff and peekByteOff, and one of poke, pokeElemOff and pokeByteOff.


sizeOf :: a -> Int
Computes the storage requirements (in bytes) of the argument. The value of the argument is not used.
alignment :: a -> Int
Computes the alignment constraint of the argument. An alignment constraint x is fulfilled by any address divisible by x. The value of the argument is not used.
peekElemOff :: Ptr a -> Int -> IO a
Read a value from a memory area regarded as an array of values of the same kind. The first argument specifies the start address of the array and the second the index into the array (the first element of the array has index 0). The following equality holds,
 peekElemOff addr idx = IOExts.fixIO $ \result ->  
   peek (addr ‘plusPtr‘ (idx ⋆ sizeOf result))

Note that this is only a specification, not necessarily the concrete implementation of the function.

pokeElemOff :: Ptr a -> Int -> a -> IO ()
Write a value to a memory area regarded as an array of values of the same kind. The following equality holds:
 pokeElemOff addr idx x =  
   poke (addr ‘plusPtr‘ (idx ⋆ sizeOf x)) x

peekByteOff :: Ptr b -> Int -> IO a
Read a value from a memory location given by a base address and offset. The following equality holds:
 peekByteOff addr off = peek (addr ‘plusPtr‘ off)

pokeByteOff :: Ptr b -> Int -> a -> IO ()
Write a value to a memory location given by a base address and offset. The following equality holds:
 pokeByteOff addr off x = poke (addr ‘plusPtr‘ off) x

peek :: Ptr a -> IO a
Read a value from the given memory location.

Note that the peek and poke functions might require properly aligned addresses to function correctly. This is architecture dependent; thus, portable code should ensure that when peeking or poking values of some type a, the alignment constraint for a, as given by the function alignment is fulfilled.

poke :: Ptr a -> a -> IO ()
Write the given value to the given memory location. Alignment restrictions might apply; see peek.
instance Storable Bool
instance Storable Char
instance Storable Double
instance Storable Float
instance Storable Int
instance Storable Int8
instance Storable Int16
instance Storable Int32
instance Storable Int64
instance Storable Word
instance Storable Word8
instance Storable Word16
instance Storable Word32
instance Storable Word64
instance Storable WordPtr
instance Storable IntPtr
instance Storable CChar
instance Storable CSChar
instance Storable CUChar
instance Storable CShort
instance Storable CUShort
instance Storable CInt
instance Storable CUInt
instance Storable CLong
instance Storable CULong
instance Storable CLLong
instance Storable CULLong
instance Storable CFloat
instance Storable CDouble
instance Storable CPtrdiff
instance Storable CSize
instance Storable CWchar
instance Storable CSigAtomic
instance Storable CClock
instance Storable CTime
instance Storable CIntPtr
instance Storable CUIntPtr
instance Storable CIntMax
instance Storable CUIntMax
instance Storable (StablePtr a)
instance Storable (Ptr a)
instance Storable (FunPtr a)