[Haskell-cafe] exporting Data.ByteString functions via FFI

[Jochem Berndsen]

kenny lu wrote:
> Hi,
> 
> I was trying to write a FFI wrapper for my Haskell program which manipulates
> 
> ByteString. But I am unable to compile/link it.
> 
> 
> Here is the toy program.
> 
> {-# LANGUAGE ForeignFunctionInterface #-}
> 
> module B where
> 
> import Foreign.C.Types
> import Foreign.C.String
> import qualified Data.ByteString as BS
> 
> rev :: BS.ByteString -> BS.ByteString
> rev bstr = BS.reverse bstr
> 
> rev_hs :: CString -> IO CString
> rev_hs cstr =
>     do { bstr <- BS.packCString cstr
>        ; let bstr' = rev bstr
>        ; cstr' <- newCString (show bstr')
>        ; return cstr'
>        }
> 
> foreign export ccall rev_hs :: CString -> IO CString
> 
> 
> And here is the C counter-part.
> 
> #include "B_stub.h"
> #include <stdio.h>
> 
> int main(int argc, char *argv[]) {
>   char *str;
>   hs_init(&argc, &argv);
> 
>   str = rev_hs("it works.");
>   printf("Rev: %s\n", str);
> 
>   hs_exit();
>   return 0;
> }
> 
> Compiling B.hs alone seems fine, but errors popped up when I was trying to
> compile/link it with C.
> 
> $ ghc -c -O B.hs
> 
> $ ghc -optc-O test_b.c B.o B_stub.o -o test_b
> Undefined symbols:
>   "___stginit_bytestringzm0zi9zi1zi4_DataziByteString_", referenced from:
>       ___stginit_Lib_ in B.o
>   "_bytestringzm0zi9zi1zi4_DataziByteString_zdwreverse_info", referenced
> from:
>       _s19w_info in B.o
>   "_bytestringzm0zi9zi1zi4_DataziByteStringziInternal_zdwshowsPrec_info",
> referenced from:
>       _s19v_info in B.o
>   "_bytestringzm0zi9zi1zi4_DataziByteStringziInternal_zdwshowsPrec_closure",
> referenced from:
>       _Lib_zdwa_srt in B.o
>   "_bytestringzm0zi9zi1zi4_DataziByteString_zdwa4_info", referenced from:
>       _Lib_zdwa_info in B.o
>   "_bytestringzm0zi9zi1zi4_DataziByteString_reverse_info", referenced from:
>       _Lib_rev_info in B.o
> ld: symbol(s) not found
> collect2: ld returned 1 exit status
> 
> 
> If I replace ByteString with the ordinary String, the above programs can be
> compiled and linked.
> 
> Can someone tell me what I did wrong here?

Add -package bytestring to the ghc command line options. I believe that
adding --make also may work.

Regards,
-- 
Jochem Berndsen | jochem at functor.nl
GPG: 0xE6FABFAB