%************************************************************************
%*                                                                      *
<sect>Building and using Win32 DLLs
<label id="win32-dlls">
<p>
<nidx>Dynamic link libraries, Win32</nidx>
<nidx>DLLs, Win32</nidx>
%*                                                                      *
%************************************************************************

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.

%************************************************************************
%*                                                                      *
<sect1>Linking with DLLs
<label id="win32-dlls:link">
<p>
%*                                                                      *
%************************************************************************

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 

<tscreen><verb>
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     6144 May  3 17:11 main.exe*
sh$ ./main
hello, world!
sh$ 
</verb></tscreen>

will give you a binary as before, but the <tt>main.exe</tt> generated
will use the Prelude and RTS DLLs instead.

6K for a <tt>"hello, world"</tt> application - not bad, huh? :-)

%************************************************************************
%*                                                                      *
<sect1>Not linking with DLLs
<label id="win32-dlls:linking-static">
<nidx>-static option (Win32)</nidx>
<p>
%*                                                                      *
%************************************************************************

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

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

%************************************************************************
%*                                                                      *
<sect1>Creating a DLL
<label id="win32-dlls:create">
<p>
<nidx>Creating a Win32 DLL</nidx>
<nidx>--mk-dll</nidx>
%*                                                                      *
%************************************************************************

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

<tscreen><verb>
sh$ ghc --mk-dll -o HSsuper.dll A.o Super.o B.o libmine.a -lgdi32
</verb></tscreen>

By feeding the ghc compiler driver the option <tt>--mk-dll</tt>, 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:

<itemize>
<item>
When compiling the module <tt>A</tt>, the code emitted by the compiler
differs depending on whether or not the functions and data it is
importing from other Haskell modules correspond to symbols that are
packaged up in a ghc-compiled DLL. To resolve whether such imports are
'DLL imports' or not, the following rules are used:

<itemize>
<item>
If the compiler imports from a module that's in the same directory as
the one being compiled, it is assumed to not belong to a different DLL
(or executable) than the module being processed, so none of the
same-directory imports are considered 'DLL imports'.

<item>
If a directory contains the (probably empty) file
<tt>dLL_ifs.hi</tt>, the code corresponding to the interface
files found in that directory are assumed to live in a DLL
separate from the one being compiled. 

Notice that the first rule takes precedence over this one, so if
you're compiling a module that imports from a Haskell module whose
interface file live in the same directory, <em>and</em> that directory
also contains the file <tt>dLL_ifs.hi</tt>, the import is still not
being considered to be a 'DLL import'.

<item>
If compiling with the option <tt>-static</tt>, the previous rule
is disabled.
</itemize>

So, in short, after having built your Haskell DLL, make sure you
create the file <tt>dLL_ifs.hi</tt> in the directory that contains
its interface files. If you don't, Haskell code that calls upon entry
points in that DLL, will do so incorrectly, and a crash will result.
(it is unfortunate that this isn't currently caught at compile-time).

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

<tscreen><verb>
sh$ ghc --mk-dll -o .... -optdll--def -optdllMyDef.def
</verb></tscreen>

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:

<tscreen><verb>
EXPORTS
 DllCanUnloadNow     = DllCanUnloadNow@0
 DllGetClassObject   = DllGetClassObject@12
 DllRegisterServer   = DllRegisterServer@0
 DllUnregisterServer = DllUnregisterServer@0

</verb></tscreen>

<item>
In addition to creating a DLL, the <tt>--mk-dll</tt> option will also
create an import library. The import library name is derived from the
name of the DLL, as follows:

<tscreen><verb>
  DLL: HScool.dll  ==> import lib: libHScool_imp.a
</verb></tscreen>

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., <tt>libHSfoo.a</tt> and <tt>libHSfoo_imp.a</tt>.

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

</itemize>