[Haskell-cafe] FFI C Structs

Aleš Bizjak ales.bizjak0 at gmail.com
Fri Jul 18 14:29:22 EDT 2008


Suppose I have a C struct like
typedef {
     int a;
     int *b;
} Foo;

typedef Foo *fooptr;

and all worker functions which are all basically int foofun (fooptr ,  
fooptr, fooptr)
where the first one is a out argument and the other two are in arguments  
and the return value
is (almost) irrelevant.

Now before I can pass arguments I have to initialize them and this is done  
with void init(fooptr ) function and
all values have to be cleared which is done with void clear(fooptr ).

I tried to import this to haskell like this.

data Foo = Foo
type FFptr = ForeignPtr Foo

and because I would like to have a nice interface without manually  
worrying about allocation and freeing up I
tried something like this:

foreign import ccall "&clear" clear :: FunPtr (Ptr Foo -> IO ())

foreign import ccall "init" init :: Ptr Foo -> IO ()

foreign import ccall "foofun" foofun_ :: Ptr Foo -> Ptr Foo -> Ptr Foo ->  
IO Int

foofun :: FFptr -> FFptr -> FFptr
foofun f1 f2 = unsafePerformIO f
                where f = withForeignPtr f1 (p1 ->
                          withForeignPtr f2 (p2 ->
                          do p3 <- malloc
                             init p3
                             r <- foofun_ p3 p1 p2
                             newForeignPtr clear p3

Now should that work or do I have to do some more reading on ffi and if  
so, would anyone recommend some material
because this is all I could come up with.

PS: An example of a haskell binding like this would be even more useful,  
if anyone knows of such.

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