11.5. 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.

Until recently, strip didn't work reliably on DLLs, so you should test your version with care, or make sure you have the latest binutils. Unfortunately, we don't know exactly which version of binutils cured the problem (it was supposedly fixed some years ago).

11.5.1. Linking with DLLs

The default on Win32 platforms is to link applications in such a way that the executables will use the Prelude and system libraries DLLs, rather than contain (large chunks of) them. This is transparent at the command-line, so

sh$ cat main.hs
module Main where
main = putStrLn "hello, world!"
sh$ ghc -o main main.hs
ghc: module version changed to 1; reason: no old .hi file
sh$ strip main.exe
sh$ ls -l main.exe
-rwxr-xr-x   1 544      everyone     4608 May  3 17:11 main.exe*
sh$ ./main
hello, world!

will give you a binary as before, but the main.exe generated will use the Prelude and RTS DLLs instead of linking them in statically.

4K for a "hello, world" application—not bad, huh? :-)

11.5.2. Not linking with DLLs

If you want to build an executable that doesn't depend on any ghc-compiled DLLs, use the -static option to link in the code statically.

Notice that you cannot mix code that has been compiled with -static and not, so you have to use the -static option on all the Haskell modules that make up your application.

11.5.3. Creating a DLL

Making libraries into DLLs doesn't work on Windows at the moment (and is no longer supported); however, all the machinery is still there. If you're interested, contact the GHC team. Note that building an entire Haskell application as a DLL is still supported (it's just inter-DLL Haskell calls that don't work). 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.

To create a `static' DLL, i.e. one that does not depend on the GHC DLLs, use the -static when compiling up your Haskell code and building the DLL.

A couple of things to notice:

  • Since DLLs correspond to packages (see Section 4.8, “ Packages ”) you need to use -package-name dll-name when compiling modules that belong to a DLL if you're going to call them from Haskell. Otherwise, Haskell code that calls entry points in that DLL will do so incorrectly, and crash. For similar reasons, you can only compile a single module tree into a DLL, as startupHaskell needs to be able to call its initialisation function, and only takes one such argument (see Section 11.5.4, “Making DLLs to be called from other languages”). Hence the modules you compile into a DLL must have a common root.

  • 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:

     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.

11.5.4. 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>
    static char* args[] = { "ghcDll", NULL };
                           /* N.B. argv arrays must end with NULL */
       ( 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.