Haskell DLL's on Windows

The current section of the GHC manual on creating DLL's on Windows is fairly confusing to read, and has some bugs (i.e. 3605). Since I got tripped up by the current documentation, I offered to rewrite sections 11.6.2 and 11.6.3 (merging them in the process). Creating Windows DLL's with GHC is surprisingly easy, and my revised manual section includes an example which can be called from both Microsoft Word (using VBA) and C++. I've pasted the revised manual section as the rest of this blog post. I'll shortly be submitting it to the GHC team, so any feedback is welcome.

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11.6.2. Making DLLs to be called from other languages

This section describes how to create DLLs to be called from other languages, such as Visual Basic or C++. This is a special case of Section 8.2.1.2, "Making a Haskell library that can be called from foreign code"; we'll deal with the DLL-specific issues that arise below. Here's an example:

Use foreign export declarations to export the Haskell functions you want to call from the outside. For example:

-- Adder.hs
{-# LANGUAGE ForeignFunctionInterface #-}
module Adder where

adder :: Int -> Int -> IO Int  -- gratuitous use of IO
adder x y = return (x+y)

foreign export stdcall adder :: Int -> Int -> IO Int

Add some helper code that starts up and shuts down the Haskell RTS:

// StartEnd.c
#include <Rts.h>

extern void __stginit_Adder(void);

void HsStart()
{
   int argc = 1;
   char* argv[] = {"ghcDll", NULL}; // argv must end with NULL

   // Initialize Haskell runtime
   char** args = argv;
   hs_init(&argc, &args);

   // Tell Haskell about all root modules
   hs_add_root(__stginit_Adder);
}

void HsEnd()
{
   hs_exit();
}

Here, Adder is the name of the root module in the module tree (as mentioned above, there must be a single root module, and hence a single module tree in the DLL). Compile everything up:

$ ghc -c Adder.hs
$ ghc -c StartEnd.c
$ ghc -shared -o Adder.dll Adder.o Adder_stub.o StartEnd.o

Now the file Adder.dll can be used from other programming languages. Before calling any functions in Adder it is necessary to call HsStart, and at the very end call HsEnd.

NOTE: It may appear tempting to use DllMain to call hs_init/hs_exit, but this won’t work (particularly if you compile with -threaded).

11.6.2.1. Using from VBA

An example of using Adder.dll from VBA is:

Private Declare Function Adder Lib "Adder.dll" Alias "adder@8" _
      (ByVal x As Long, ByVal y As Long) As Long

Private Declare Sub HsStart Lib "Adder.dll" ()
Private Declare Sub HsEnd Lib "Adder.dll" ()

Private Sub Document_Close()
HsEnd
End Sub

Private Sub Document_Open()
HsStart
End Sub

Public Sub Test()
MsgBox "12 + 5 = " & Adder(12, 5)
End Sub

This example uses the Document_Open/Close functions of Microsoft Word, but provided HsStart is called before the first function, and HsEnd after the last, then it will work fine.

11.6.2.2. Using from C++

An example of using Adder.dll from C++ is:

// Tester.cpp
#include "HsFFI.h"
#include "Adder_stub.h"
#include <stdio.h>

extern "C" {
    void HsStart();
    void HsEnd();
}

int main()
{
    HsStart();
    // can now safely call functions from the DLL
    printf("12 + 5 = %i\n", adder(12,5))    ;
    HsEnd();
    return 0;
}

This can be compiled and run with:

$ ghc -o tester Tester.cpp Adder.dll.a
$ tester
12 + 5 = 17

Please give feedback in the comments.