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Foreign.Storable

Portability | portable
Stability | provisional
Maintainer | ffi@haskell.org

Description
The module Foreign.Storable provides most elementary support for marshalling and is part of the language-independent portion of the Foreign Function Interface (FFI), and will normally be imported via the Foreign module.
Synopsis

class Storable a where

sizeOf :: a -> Int
alignment :: a -> Int
peekElemOff :: Ptr a -> Int -> IO a
pokeElemOff :: Ptr a -> Int -> a -> IO ()
peekByteOff :: Ptr b -> Int -> IO a
pokeByteOff :: Ptr b -> Int -> a -> IO ()
peek :: Ptr a -> IO a
poke :: Ptr a -> a -> IO ()

Documentation
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 primtive 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`.

Methods

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`.

Instances

Storable CChar
Storable CSChar
Storable CUChar
Storable CShort
Storable CUShort
Storable CInt
Storable CUInt
Storable CLong
Storable CULong
Storable CLLong
Storable CULLong
Storable CFloat
Storable CDouble
Storable CLDouble
Storable CPtrdiff
Storable CSize
Storable CWchar
Storable CSigAtomic
Storable CClock
Storable CTime
Storable Bool
Storable Char
Storable Int
Storable Word
Storable (Ptr a)
Storable (FunPtr a)
Storable (StablePtr a)
Storable Float
Storable Double
Storable Word8
Storable Word16
Storable Word32
Storable Word64
Storable Int8
Storable Int16
Storable Int32
Storable Int64
Storable CDev
Storable CIno
Storable CMode
Storable COff
Storable CPid
Storable CSsize
Storable CGid
Storable CNlink
Storable CUid
Storable CCc
Storable CSpeed
Storable CTcflag
Storable Fd

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