[Haskell-cafe] question concerning ANSI "void *"

[Galchin Vasili]

basically I am trying to implement ioctl for the Posix library .. so a
possible signtaure would be:

fdIoctl :: Fd -> Int -> Ptr Word 8 -> IO( Ptr Word8) ????


Vasili

On 2/8/08, Galchin Vasili <vigalchin at gmail.com> wrote:
>
> a couple of concrete examples:
>
> typedef struct {char a; int b; char str[8]} type1;
>
> typedef struct {long c; char d} type2;
>
> So to pthread_create (just an example function) we could be passing a
> struct of type1 or a struct of type2 .. i.e. arbitrary length and content
> ... I am trying to better understand this. I see some of the poke functions
> mentioned in the FFI. Which one are you alluding to?
>
> Regards, Vasili
>
>
>  On 2/8/08, Adam Langley <agl at imperialviolet.org> wrote:
> >
> > On Feb 8, 2008 9:13 AM, Galchin Vasili <vigalchin at gmail.com> wrote:
> > > Let's take a concrete but "made up" case .. suppose we want to call
> > through
> > > to pthread_create and pass the (void *) argument to pthread_create
> > which in
> > > turn gets interpreted by the pthread that is launched. How would one
> > > populate the C struct that is passed to the launched pthread keeping
> > in mind
> > > that this C struct is variable in length? From the FFI how would one
> > model
> > > this C struct?
> >
> > It tough to be helpful with such a generic request. Here are some
> > options that you can consider:
> >
> > 1) Write a wrapper function in C which has a nicer FFI interface to
> > call from Haskell. Using cabal this is pretty painless and, if the
> > interface suits it, it probably the easiest solution.
> > 2) Call pthread_create directly with the FFI. You can give the FFI
> > function a Haskell type with 'Ptr ()' or 'Ptr X', it doesn't really
> > matter. However the type system serves you best, do it that way. This
> > means that you need to populate the struct yourself in Haskell. The
> > issue with this is that the local system defines lots of things like
> > padding and alignment which mean that the layout of the structure in
> > memory is complex and platform specific. Tools like hsc2hs[1] or c2hs
> > will be very helpful here. Dealing with the variable length probably
> > isn't an issue. Usually variable length structures have a fixed header
> > and a variable tail, where the tail is an array of primitives. You can
> > malloc the correct sized region, use one of the previous tools to fill
> > in the fixed header and then use poke to complete the tail.
> >
> > I might be able to be more helpful if you give the actual struct and
> > function prototype that you're trying to wrap.
> >
> > Cheers
> >
> >
> >
> > [1] http://therning.org/magnus/archives/tag/hsc2hs
> > [2] http://www.cse.unsw.edu.au/~chak/haskell/c2hs/
> >
> > --
> > Adam Langley                                      agl at imperialviolet.org
> > http://www.imperialviolet.org                       650-283-9641
> >
>
>
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