[Haskell-cafe] FFI: Creating a Storable for a C-struct composed of char arrays

9 May 2008

      Hi all,

I'm trying to make RFC calls to SAP using the nwsaprfc library. Some structs
defined in the library contains arrays (byte or word arrays). For example:

typedef struct _RFC_ATTRIBUTES
{
    SAP_UC dest[64+1];              /* RFC destination              */
    SAP_UC host[100+1];             /* Own host name                */
    SAP_UC partnerHost[100+1];     /* Partner host name            */
    SAP_UC sysNumber[2+1];             /* R/3 system number            */
    SAP_UC sysId[8+1];              /* R/3 system name              */
    SAP_UC client[3+1];             /* Client                       */
    SAP_UC user[12+1];              /* User                         */
    SAP_UC language[2+1];           /* Language                     */
   ... continued
}}RFC_ATTRIBUTES, *P_RFC_ATTRIBUTES;

I would like to create a Haskell Storable counterpart of this structure.
Using http://therning.org/magnus/archives/tag/hsc2hs as an starting point, I
could create Storable for structures containing fields with basic types.

Before working on the real data structures I wrote a more simple example to
play with:

-- File: ArrayStruct.h --
typedef struct _ArrayStruct
{
    char a[10+1];
    char b[20+1];
    char c[30+1];
} ArrayStruct, *P_ArrayStruct;

-- File: ArrayStruct.c --
#include "ArrayStruct.h"

void
print_array_struct(ArrayStruct *f)
{
    printf("%s\n", __FUNCTION__);
    printf("f->a: %s\n", f->a);
    printf("f->b: %s\n", f->b);
    printf("f->c: %s\n", f->c);
}

-- File: HArrayStruct.hsc
{-# OPTIONS -ffi #-}
module Main
  where

import Foreign
import Foreign.C.Types

#include "ArrayStruct.h"

data HArrayStruct = HArrayStruct { a :: String, b :: String, c :: String }
type HarrayStructPtr = Ptr HArrayStruct

foreign import ccall "static ArrayStruct.h print_array_struct"
    f_print_array_struct :: ArrayStructPtr -> IO ()

instance Storable HArrayStruct where
  sizeOf _ = (#size ArrayStruct)
  alignment _ = alignment (undefined :: CInt)
  peek _ = error "peek is not implemented"
  poke ptr (HArrayStruct a' b' c') = do
    (#poke ArrayStruct, a) ptr a'
    (#poke ArrayStruct, b) ptr b'
    (#poke ArrayStruct, c) ptr c'

printArrayStruct as = with as f_print_array_struct

main = printArrayStruct $ HArrayStruct { a="some", b="test", c="data" }

-- End of files

Of course it won't work as HArrayStruct in file HArrayStruct.hs uses Strings
and String is not an instance of Storable.

Ideally I would need some sort of Storable array of char. Is
Data.Storable.Array the type I'm looking for? Could someone point me to some
code using the same kind of structures?

Thanks,

Olivier.

[Haskell-cafe] FFI: Creating a Storable for a C-struct composed of char arrays

Olivier Boudry