[Haskell-cafe] Unable to call function from DLL using FFI

[rajendra prasad]

Hi,

I am new to Haskell and I need to call a c function by loading dynamic link
library in Haskell. I started with very simple code to create a dll in
visual studio 2008 and then trying to load it in haskell. I am listing here
the steps I follwed to achieve this:

*Step 1: *
My c++ code(HelloWorld.cpp) looks like this:

__declspec(dllexport) int doubleMe(int a)
{
    return 2*a;
}

I created the dll using following command:
cl /D_USRDLL /D_WINDLL HelloWorld.cpp  /link  /dll /out:LoadDLL.dll

*Step 2:*
My Haskell code(Main.hs) looks like this:

{-# LANGUAGE ForeignFunctionInterface #-}
module Main where
import Foreign
import Foreign.C

foreign import ccall "LoadDLL doubleMe" twice :: CInt -> IO (CInt)

main = do
  let x = 4
  y <- twice x
  print y

I am compling haskell code using following command:
ghci Main.hs -L. -lLoadDLL

I am finally getting the following error:

During interactive linking, GHCi couldn't find the following symbol:
  doubleMe
This may be due to you not asking GHCi to load extra object files,
archives or DLLs needed by your current session.  Restart GHCi, specifying
the missing library using the -L/path/to/object/dir and -lmissinglibname
flags, or simply by naming the relevant files on the GHCi command line.
Alternatively, this link failure might indicate a bug in GHCi.
If you suspect the latter, please send a bug report to:
  glasgow-haskell-bugs at haskell.org


Please help me in identifying the mistake I am doing. If possible please
let me know the exact steps to be followed to solve this issue.

Thank you so much in advance,
Regards,
Rajendra
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