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:

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: