12.6. Building and using Win32 DLLs

On Win32 platforms, the compiler is capable of both producing and using dynamic link libraries (DLLs) containing ghc-compiled code. This section shows you how to make use of this facility.

There are two distinct ways in which DLLs can be used:

12.6.1. Creating a DLL

Sealing up your Haskell library inside a DLL is straightforward; compile up the object files that make up the library, and then build the DLL by issuing a command of the form:

ghc -shared -o foo.dll bar.o baz.o wibble.a -lfooble

By feeding the ghc compiler driver the option -shared, it will build a DLL rather than produce an executable. The DLL will consist of all the object files and archives given on the command line.

A couple of things to notice:

  • By default, the entry points of all the object files will be exported from the DLL when using -shared. Should you want to constrain this, you can specify the module definition file to use on the command line as follows:

    ghc -shared -o .... MyDef.def

    See Microsoft documentation for details, but a module definition file simply lists what entry points you want to export. Here's one that's suitable when building a Haskell COM server DLL:

     DllCanUnloadNow     = DllCanUnloadNow@0
     DllGetClassObject   = DllGetClassObject@12
     DllRegisterServer   = DllRegisterServer@0
     DllUnregisterServer = DllUnregisterServer@0

  • In addition to creating a DLL, the -shared option also creates an import library. The import library name is derived from the name of the DLL, as follows:

    DLL: HScool.dll  ==> import lib: libHScool.dll.a

    The naming scheme may look a bit weird, but it has the purpose of allowing the co-existence of import libraries with ordinary static libraries (e.g., libHSfoo.a and libHSfoo.dll.a. Additionally, when the compiler driver is linking in non-static mode, it will rewrite occurrence of -lHSfoo on the command line to -lHSfoo.dll. By doing this for you, switching from non-static to static linking is simply a question of adding -static to your command line.

12.6.2. 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, “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>

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

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

void HsEnd()

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.

Warning: It may appear tempting to use DllMain to call hs_init/hs_exit, but this won't work (particularly if you compile with -threaded). There are severe restrictions on which actions can be performed during DllMain, and hs_init violates these restrictions, which can lead to your dll freezing during startup (see bug #3605). 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()
End Sub

Private Sub Document_Open()
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. 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()
    // can now safely call functions from the DLL
    printf("12 + 5 = %i\n", adder(12,5))    ;
    return 0;

This can be compiled and run with:

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