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
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
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)
On Dec 19, 2007 1:06 AM, John Vogel
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
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.
Thanks to all, that is the example that i need. I'll test the example.zip right now. -- Luis Cabellos
On Dec 19, 2007 3:05 AM, John Vogel
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
participants (4)
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Don Stewart -
John Vogel -
Lemmih -
Luis Cabellos