[Haskell-cafe] Re: FFI"pointer" data types question

Ben Franksen ben.franksen at online.de
Fri Dec 28 10:57:35 EST 2007

Galchin Vasili wrote:
>      If I am calling a ANSI function that requires a pointer to a C
>      struct,
> which FFI pointer type should use?

A commonly used technique is to declare an empty data type

  data MyStruct

and then use

  Ptr MyStruct

in the FFI call. This is useful if the C struct is opaque, or at least meant
to be opaque (i.e. you are not allowed to access its elements directly).

For non-opaque structs, use a Haskell record and make it an instance of 
class Storable . An example:

struct timeval
    __time_t tv_sec;            /* Seconds.  */
    __suseconds_t tv_usec;      /* Microseconds.  */


  data Timeval = Timeval {
      tv_sec :: TimeT,
      tv_usec :: USecondsT

(with suitable definitions of TimeT and USecondsT)

  instance Storable Timeval where

and your FFI calls will get a  Ptr Timeval  as argument.


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