Foreign C with pointers

[Lemmih]

On Dec 19, 2007 1:06 AM, John Vogel <jpvogel1 at gmail.com> wrote:
>
> You do realize that the example you gave is just as general as all the
> tutorials.
>
> Here is an example I was working, but it gives a segmentation fault for some
> reason:
>
> example.h
>
>  typedef struct
> {
>  unsigned char a;
>  unsigned char b;
>  unsigned char c;
>  unsigned char d;
> } IP;
>
> IP* shiftIP(IP* addr);
>
>
> example.c
>
> #include "example.h"
>
> IP* shiftIP(IP* addr){
>  unsigned char t;
>  t = addr->a;
>  addr->a = addr->b;
>  addr->b = addr->c;
>  addr->c = addr->d;
>  addr->d = t;
> }

return addr; ?
or rather, void shiftIP.

> Example.hsc
>
> {-# OPTIONS -ffi -fglasgow-exts #-}
> module Example where
>
> import Foreign
> import Foreign.C.Types
> import Control.Monad
>
> #include "buzz.h"
>
> data MyIP = MyIP
>  { a :: CUChar
>  , b :: CUChar
>  , c :: CUChar
>  , d :: CUChar
>  } deriving (Show)
>
> instance Storable MyIP where
>  sizeOf _ = #{size IP} -- 4
>  alignment _ = alignment (undefined :: CUChar) -- 1
>  peek p = return MyIP
>    `ap` (#{peek IP, a} p)
>    `ap` (#{peek IP, b} p)
>    `ap` (#{peek IP, c} p)
>    `ap` (#{peek IP, d} p)
>  poke p ip = do
>    #{poke IP, a} p $ a ip
>    #{poke IP, b} p $ b ip
>    #{poke IP, c} p $ c ip
>    #{poke IP, d} p $ d ip
>
> foreign import ccall safe "static buzzlib.h shiftIP"
>  shiftIP :: Ptr MyIP -> Ptr MyIP

shiftIP isn't a pure function.

shiftIP :: Ptr MyIP -> IO (Ptr MyIP) or
shiftIP :: Ptr MyIP -> IO ()

> shiftMyIP :: MyIP -> MyIP
> shiftMyIP ip = unsafePerformIO . alloca $ \ptr -> poke ptr ip >> peek
> (shiftIP ptr)

shiftMyIP ip = unsafePerformIO . alloca $ \ptr -> poke ptr ip >> peek
=<< (shiftIP ptr)  or
shiftMyIP ip = unsafePerformIO . alloca $ \ptr -> poke ptr ip >>
shiftIP ptr >> peek ptr

-- 
Cheers,
  Lemmih