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Luis Cabellos | 18 Dec 19:52

Foreign C with pointers

Hi,

I am creating a binding to an existing library (lib*.a) from Haskell.

How can i bind a function that get a pointer?

e.g: I have in the c library.
int GetData( Data * d );

The steps that i need are:
1 - create the Data in Haskell
2 - create the foreign import sentence in a lib*.hs
3 - use the Data after the call

Where can i get a good tutorial about FFI? I try http://www.haskell.org/hdirect/ffi.html but it's too general (need i pair of examples, i think)

--
Thanks a lot, Luis Cabellos

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Don Stewart | 18 Dec 19:58
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Re: Foreign C with pointers

zhen.sydow:
>    Hi,
> 
>    I am creating a binding to an existing library (lib*.a) from Haskell.
> 
>    How can i bind a function that get a pointer?
> 
>    e.g: I have in the c library.
>    int GetData( Data * d );
> 
>    The steps that i need are:
>    1 - create the Data in Haskell
>    2 - create the foreign import sentence in a lib*.hs
>    3 - use the Data after the call
> 
>    Where can i get a good tutorial about FFI? I try
>    [1]http://www.haskell.org/hdirect/ffi.html but it's too general (need i
>    pair of examples, i think)

Say we have:

     #include <string.h>

     void *
     memmove(void *dst, const void *src, size_t len);

We can bind to that as:

    foreign import unsafe ccall "string.h memmove"
        c_memmove :: Ptr Word8 -> Ptr Word8 -> CSize -> IO (Ptr Word8)

You can use Foreign.Ptr.* and Foreign.ForeignPtr (or even, say,
Data.ByteString) to get at Ptr on the Haskell side.
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John Vogel | 19 Dec 01:06

Re: Foreign C with pointers

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;
}

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

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


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Lemmih | 19 Dec 02:01
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Re: Foreign C with pointers

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
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John Vogel | 19 Dec 03:05

Re: Foreign C with pointers

Lol, I am surprised that the library even compiled without the "return addr;".

But, this definition is correct:

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

The other 2 definitions don't even compile.  And since it works I'll try attaching the files.

You can also get the file here:  http://vragon2.googlepages.com/example.zip

Just run the make file to build.
Attachment (example2.zip): application/zip, 13 KiB
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Luis Cabellos | 19 Dec 09:31

Re: Foreign C with pointers

Thanks to all, that is the example that i need.
I'll test the example.zip right now.

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Luis Cabellos

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Lemmih | 19 Dec 13:54
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Re: Foreign C with pointers

On Dec 19, 2007 3:05 AM, John Vogel <jpvogel1 <at> gmail.com> wrote:
>
> >
> Lol, I am surprised that the library even compiled without the "return
> addr;".
>
> But, this definition is correct:

Well, yeah, for some value of correct. It works in this case but it
will most likely bite you if you use it in any other way.

> The other 2 definitions don't even compile.

I posted two new type-signatures and two new definitions. I'm quite
sure the right combination would work.

--

-- 
Cheers,
  Lemmih
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