9. Building and using Win32 DLLs

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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.

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9.1 Linking with DLLs

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

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

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

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9.2 Not linking with DLLs

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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.

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9.3 Creating a DLL

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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:

sh$ ghc --mk-dll -o HSsuper.dll A.o Super.o B.o libmine.a -lgdi32

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.

A couple of things to notice:

• When compiling the module A, 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:
  • 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'.
  • If a directory contains the (probably empty) file dLL_ifs.hi, 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, and that directory also contains the file dLL_ifs.hi, the import is still not being considered to be a 'DLL import'.
  • If compiling with the option -static, the previous rule is disabled.
  So, in short, after having built your Haskell DLL, make sure you create the file dLL_ifs.hi 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).
• 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:

  sh$ 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:

  EXPORTS
   DllCanUnloadNow     = DllCanUnloadNow@0
   DllGetClassObject   = DllGetClassObject@12
   DllRegisterServer   = DllRegisterServer@0
   DllUnregisterServer = DllUnregisterServer@0
  

• In addition to creating a DLL, the --mk-dll option will also create 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.