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

- You can turn each Haskell package into a DLL, so that multiple Haskell executables using the same packages can share the DLL files. (As opposed to linking the libraries statically, which in effect creates a new copy of the RTS and all libraries for each executable produced.)

  That is the same as the dynamic linking on other platforms, and it is described in Section 4.12, “Using shared libraries”.

- You can package up a complete Haskell program as a DLL, to be called by some external (usually non-Haskell) program. This is usually used to implement plugins and the like, and is described below.

### 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 –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:

      EXPORTS
       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.

### 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`.

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).

#### 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

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