On Thu, Oct 16, 2008 at 7:28 AM, Mauricio 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