11.6. Building and using Win32 DLLs
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## 11.6. Building and using Win32 DLLs

Making Haskell libraries into DLLs doesn't work on Windows at the moment; however, all the machinery is still there. If you're interested, contact the GHC team. Note that building an entire Haskell application as a single DLL is still supported: it's just multi-DLL Haskell programs that don't work. The Windows distribution of GHC contains static libraries only.

### 11.6.1. 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 ––mk-dll -o foo.dll bar.o baz.o wibble.a -lfooble

By feeding the ghc compiler driver the option `––mk-dll`, 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 `––mk-dll`. Should you want to constrain this, you can specify the module definition file to use on the command line as follows:

      ghc ––mk-dll -o .... -optdll––def -optdllMyDef.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:

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

- In addition to creating a DLL, the `––mk-dll` 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_imp.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_imp.a`. Additionally, when the compiler driver is linking in non-static mode, it will rewrite occurrence of `-lHSfoo` on the command line to `-lHSfoo_imp`. By doing this for you, switching from non-static to static linking is simply a question of adding `-static` to your command line.

### 11.6.2. Making DLLs to be called from other languages

If you want to package up Haskell code to be called from other languages, such as Visual Basic or C++, there are some extra things it is useful to know. The dirty details are in the Foreign Function Interface definition, but it can be tricky to work out how to combine this with DLL building, so here's an example:

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

      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

- Compile it up:

      ghc -c adder.hs -fglasgow-exts

  This will produce two files, adder.o and adder_stub.o

- compile up a `DllMain()` that starts up the Haskell RTS-––a possible implementation is:

      #include <windows.h>
      #include <Rts.h>

      extern void__stginit_Adder(void);

      static char* args[] = { "ghcDll", NULL };
                             /* N.B. argv arrays must end with NULL */
      BOOL
      STDCALL
      DllMain
         ( HANDLE hModule
         , DWORD reason
         , void* reserved
         )
      {
        if (reason == DLL_PROCESS_ATTACH) {
            /* By now, the RTS DLL should have been hoisted in, but we need to start it up. */
            startupHaskell(1, args, __stginit_Adder);
            return TRUE;
        }
        return TRUE;
      }

  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 this up:

      ghc -c dllMain.c

- Construct the DLL:

      ghc ––mk-dll -o adder.dll adder.o adder_stub.o dllMain.o

- Start using `adder` from VBA-––here's how I would `Declare` it:

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

  Since this Haskell DLL depends on a couple of the DLLs that come with GHC, make sure that they are in scope/visible.

  Building statically linked DLLs is the same as in the previous section: it suffices to add `-static` to the commands used to compile up the Haskell source and build the DLL.

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