34 Foreign.Marshal.Utils
Date: 2010-07-20 13:11:00

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## Chapter 34
Foreign.Marshal.Utils

module Foreign.Marshal.Utils (
with, new, fromBool, toBool, maybeNew, maybeWith, maybePeek,
withMany, copyBytes, moveBytes
) where

### 34.1 General marshalling utilities

#### 34.1.1 Combined allocation and marshalling

with :: Storable a => a -> (Ptr a -> IO b) -> IO b

  with val f executes the computation f, passing as argument a pointer to a temporarily allocated block of memory into which val has been marshalled (the combination of alloca and poke).

  The memory is freed when f terminates (either normally or via an exception), so the pointer passed to f must not be used after this.

new :: Storable a => a -> IO (Ptr a)

  Allocate a block of memory and marshal a value into it (the combination of malloc and poke). The size of the area allocated is determined by the Foreign.Storable.sizeOf method from the instance of Storable for the appropriate type.

  The memory may be deallocated using Foreign.Marshal.Alloc.free or Foreign.Marshal.Alloc.finalizerFree when no longer required.

#### 34.1.2 Marshalling of Boolean values (non-zero corresponds to True)

fromBool :: Num a => Bool -> a

  Convert a Haskell Bool to its numeric representation

toBool :: Num a => a -> Bool

  Convert a Boolean in numeric representation to a Haskell value

#### 34.1.3 Marshalling of Maybe values

maybeNew :: (a -> IO (Ptr a)) -> Maybe a -> IO (Ptr a)

  Allocate storage and marshal a storable value wrapped into a Maybe
  - the nullPtr is used to represent Nothing

maybeWith :: (a -> (Ptr b -> IO c) -> IO c)
-> Maybe a -> (Ptr b -> IO c) -> IO c

  Converts a withXXX combinator into one marshalling a value wrapped into a Maybe, using nullPtr to represent Nothing.

maybePeek :: (Ptr a -> IO b) -> Ptr a -> IO (Maybe b)

  Convert a peek combinator into a one returning Nothing if applied to a nullPtr

#### 34.1.4 Marshalling lists of storable objects

withMany :: (a -> (b -> res) -> res) -> [a] -> ([b] -> res) -> res

  Replicates a withXXX combinator over a list of objects, yielding a list of marshalled objects

#### 34.1.5 Haskellish interface to memcpy and memmove

(argument order: destination, source)

copyBytes :: Ptr a -> Ptr a -> Int -> IO ()

  Copies the given number of bytes from the second area (source) into the first (destination); the copied areas may not overlap

moveBytes :: Ptr a -> Ptr a -> Int -> IO ()

  Copies the given number of bytes from the second area (source) into the first (destination); the copied areas may overlap

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