base-4.4.0.0: Basic libraries

Portabilityportable
Stabilityprovisional
Maintainer[email protected]

Foreign.Ptr

Contents

Description

This module provides typed pointers to foreign data. It is part of the Foreign Function Interface (FFI) and will normally be imported via the Foreign module.

Synopsis

Data pointers

data Ptr a Source

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.

Instances

Typeable1 Ptr 
Eq (Ptr a) 
Typeable a => Data (Ptr a) 
Ord (Ptr a) 
Show (Ptr a) 
Storable (Ptr a) 

nullPtr :: Ptr aSource

The constant nullPtr contains a distinguished value of Ptr that is not associated with a valid memory location.

castPtr :: Ptr a -> Ptr bSource

The castPtr function casts a pointer from one type to another.

plusPtr :: Ptr a -> Int -> Ptr bSource

Advances the given address by the given offset in bytes.

alignPtr :: Ptr a -> Int -> Ptr aSource

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.

minusPtr :: Ptr a -> Ptr b -> IntSource

Computes the offset required to get from the second to the first argument. We have

 p2 == p1 `plusPtr` (p2 `minusPtr` p1)

Function pointers

data FunPtr a Source

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, Double, 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 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

Instances

nullFunPtr :: FunPtr aSource

The constant nullFunPtr contains a distinguished value of FunPtr that is not associated with a valid memory location.

castFunPtr :: FunPtr a -> FunPtr bSource

Casts a FunPtr to a FunPtr of a different type.

castFunPtrToPtr :: FunPtr a -> Ptr bSource

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.

castPtrToFunPtr :: Ptr a -> FunPtr bSource

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.

freeHaskellFunPtr :: FunPtr a -> IO ()Source

Release the storage associated with the given FunPtr, which must have been obtained from a wrapper stub. This should be called whenever the return value from a foreign import wrapper function is no longer required; otherwise, the storage it uses will leak.

Integral types with lossless conversion to and from pointers

data IntPtr Source

A signed integral type that can be losslessly converted to and from Ptr. This type is also compatible with the C99 type intptr_t, and can be marshalled to and from that type safely.

ptrToIntPtr :: Ptr a -> IntPtrSource

casts a Ptr to an IntPtr

intPtrToPtr :: IntPtr -> Ptr aSource

casts an IntPtr to a Ptr

data WordPtr Source

An unsigned integral type that can be losslessly converted to and from Ptr. This type is also compatible with the C99 type uintptr_t, and can be marshalled to and from that type safely.

ptrToWordPtr :: Ptr a -> WordPtrSource

casts a Ptr to a WordPtr

wordPtrToPtr :: WordPtr -> Ptr aSource

casts a WordPtr to a Ptr