<Chapter id="win32-dlls">
<Title>Building and using Win32 DLLs
</Title>

<Para>
<IndexTerm><Primary>Dynamic link libraries, Win32</Primary></IndexTerm>
<IndexTerm><Primary>DLLs, Win32</Primary></IndexTerm>
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.
</Para>

<Para>
Until recently, <Command>strip</Command> 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).
</Para>


<Sect1 id="win32-dlls-link">
<Title>Linking with DLLs
</Title>

<Para>
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 
</Para>

<Para>
<Screen>
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!
sh$ 
</Screen>
</Para>

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

<Para>
4K for a <Literal>"hello, world"</Literal> application&mdash;not bad, huh? :-)
</Para>

</Sect1>

<Sect1 id="win32-dlls-linking-static">
<Title>Not linking with DLLs
<IndexTerm><Primary>-static option (Win32)</Primary></IndexTerm></Title>

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

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

</Sect1>

<Sect1 id="win32-dlls-create">
<Title>Creating a DLL
</Title>

<Para>
<emphasis>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).</emphasis>
<IndexTerm><Primary>Creating a Win32 DLL</Primary></IndexTerm>
<IndexTerm><Primary>&ndash;&ndash;mk-dll</Primary></IndexTerm>
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:
</Para>

<Para>
<Screen>
ghc &ndash;&ndash;mk-dll -o foo.dll bar.o baz.o wibble.a -lfooble
</Screen>
</Para>

<Para>
By feeding the ghc compiler driver the option <Option>&ndash;&ndash;mk-dll</Option>, 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.
</Para>

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

<Para>
A couple of things to notice:
</Para>

<Para>

<ItemizedList>
<ListItem>
<Para>
Since DLLs correspond to packages (see <XRef LinkEnd="packages">) you need
to use <Option>-package-name dll-name</Option> 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 <Function>startupHaskell</Function> needs to be able to call its
initialisation function, and only takes one such argument (see <XRef
LinkEnd="win32-dlls-foreign">). Hence the modules
you compile into a DLL must have a common root.
</Para>
</ListItem>

<ListItem>
<Para>
By default, the entry points of all the object files will be exported from
the DLL when using <Option>&ndash;&ndash;mk-dll</Option>. Should you want to constrain
this, you can specify the <Emphasis>module definition file</Emphasis> to use
on the command line as follows:

<Screen>
ghc &ndash;&ndash;mk-dll -o .... -optdll--def -optdllMyDef.def
</Screen>

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:

<ProgramListing>
EXPORTS
 DllCanUnloadNow     = DllCanUnloadNow@0
 DllGetClassObject   = DllGetClassObject@12
 DllRegisterServer   = DllRegisterServer@0
 DllUnregisterServer = DllUnregisterServer@0
</ProgramListing>
</Para>
</ListItem>

<ListItem>
<Para>
In addition to creating a DLL, the <Option>&ndash;&ndash;mk-dll</Option> option also
creates an import library. The import library name is derived from the
name of the DLL, as follows:

<ProgramListing>
DLL: HScool.dll  ==&#62; import lib: libHScool_imp.a
</ProgramListing>

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.,
<Filename>libHSfoo.a</Filename> and
<Filename>libHSfoo&lowbar;imp.a</Filename>.

Additionally, when the compiler driver is linking in non-static mode, it
will rewrite occurrence of <Option>-lHSfoo</Option> on the command line to
<Option>-lHSfoo&lowbar;imp</Option>. By doing this for you, switching from
non-static to static linking is simply a question of adding
<Option>-static</Option> to your command line.

</Para>
</ListItem>
</ItemizedList>
</Para>

</Sect1>


<Sect1 id="win32-dlls-foreign">
<Title>Making DLLs to be called from other languages</Title>

<Para>

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 <Emphasis>Foreign Function
Interface</Emphasis> definition, but it can be tricky to work out how to
combine this with DLL building, so here's an example:

</Para>

<ItemizedList>

<ListItem>
<Para>
Use <Literal>foreign export</Literal> declarations to export the Haskell
functions you want to call from the outside. For example,

<ProgramListing>
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
</ProgramListing>
</Para>
</ListItem>

<ListItem>
<Para>
Compile it up:

<Screen>
ghc -c adder.hs -fglasgow-exts
</Screen>
  
This will produce two files, adder.o and adder_stub.o
</Para>
</ListItem>

<ListItem>
<Para>
compile up a <Function>DllMain()</Function> that starts up the Haskell
RTS-&ndash;&ndash;a possible implementation is:

<ProgramListing>
#include &lt;windows.h&gt;
#include &lt;Rts.h&gt;

EXTFUN(__stginit_Adder);

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;
}
</ProgramListing>

Here, <Literal>Adder</Literal> 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:

<Screen>
ghc -c dllMain.c
</Screen>
</Para>
</ListItem>

<ListItem>
<Para>
Construct the DLL:

<Screen>
ghc &ndash;&ndash;mk-dll -o adder.dll adder.o adder_stub.o dllMain.o
</Screen>

</Para>
</ListItem>

<ListItem>
<Para>
Start using <Function>adder</Function> from VBA-&ndash;&ndash;here's how I would
<Constant>Declare</Constant> it:

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

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

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

</ListItem>

</ItemizedList>

</Sect1>

</Chapter>

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