[Haskell-cafe] FFI Newbie: c functions that want pointers to objects

[Creighton Hogg]

On Thu, Oct 16, 2008 at 7:28 AM, Mauricio <briqueabraque at yahoo.com> wrote:
> Hi,
>
> Some functions in C changes data using pointers,
> like this example:
>
> void change_int (int *n)
> {
>  *n ++;
> }
>
> What is the proper way to handle that? I
> guess I should wrap it like this:
>
> foreign ccall "change_int" change_int
>  :: Ptr CInt -> IO ()
>
> Can I use that on a CInt in Haskell code?
> Like this obviously wrong code, but that
> probably shows what I want:
>
> do
>  let myN = 5
>  change_int myN
>
> Thanks,
> Maurício
>
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(Sending to the list as well as just Mauricio this time...oops)
You are wrapping the function correctly, but you need to actually
create the Ptr CInt using the function malloc from
Foreign.Marshal.Alloc, or more conveniently, the function new from
Foreign.Marshal.Utils.

Your code snippet would become
do
  myNPtr <- new 5
  change_int myNPtr