# Running GHC on Win32 systems

## Starting GHC on Windows platforms

The installer that installs GHC on Win32 also sets up the file-suffix associations for ".hs" and ".lhs" files so that double-clicking them starts `ghci`.

Be aware of that `ghc` and `ghci` do require filenames containing spaces to be escaped using quotes:

      c:\ghc\bin\ghci "c:\\Program Files\\Haskell\\Project.hs"

If the quotes are left off in the above command, `ghci` will interpret the filename as two, "c:\\Program" and "Files\\Haskell\\Project.hs".

## Running GHCi on Windows

We recommend running GHCi in a standard Windows console: select the `GHCi` option from the start menu item added by the GHC installer, or use `Start->Run->cmd` to get a Windows console and invoke `ghci` from there (as long as it's in your `PATH`).

If you run GHCi in a Cygwin or MSYS shell, then the Control-C behaviour is adversely affected. In one of these environments you should use the `ghcii.sh` script to start GHCi, otherwise when you hit Control-C you'll be returned to the shell prompt but the GHCi process will still be running. However, even using the `ghcii.sh` script, if you hit Control-C then the GHCi process will be killed immediately, rather than letting you interrupt a running program inside GHCi as it should. This problem is caused by the fact that the Cygwin and MSYS shell environments don't pass Control-C events to non-Cygwin child processes, because in order to do that there needs to be a Windows console.

There's an exception: you can use a Cygwin shell if the `CYGWIN` environment variable does not contain `tty`. In this mode, the Cygwin shell behaves like a Windows console shell and console events are propagated to child processes. Note that the `CYGWIN` environment variable must be set before starting the Cygwin shell; changing it afterwards has no effect on the shell.

This problem doesn't just affect GHCi, it affects any GHC-compiled program that wants to catch console events. See the GHC.ConsoleHandler module.

## Interacting with the terminal

By default GHC builds applications that open a console window when they start. If you want to build a GUI-only application, with no console window, use the flag `-optl-mwindows` in the link step.

Warning: Windows GUI-only programs have no stdin, stdout or stderr so using the ordinary Haskell input/output functions will cause your program to fail with an IO exception, such as:

          Fail: <stdout>: hPutChar: failed (Bad file descriptor)

However using Debug.Trace.trace is alright because it uses Windows debugging output support rather than stderr.

For some reason, Mingw ships with the `readline` library, but not with the `readline` headers. As a result, GHC (like Hugs) does not use `readline` for interactive input on Windows. You can get a close simulation by using an emacs shell buffer!

## Differences in library behaviour

Some of the standard Haskell libraries behave slightly differently on Windows.

- On Windows, the '`^Z`' character is interpreted as an end-of-file character, so if you read a file containing this character the file will appear to end just before it. To avoid this, use `IOExts.openFileEx` to open a file in binary (untranslated) mode or change an already opened file handle into binary mode using `IOExts.hSetBinaryMode`. The `IOExts` module is part of the `lang` package.

## Using GHC (and other GHC-compiled executables) with cygwin

### Background

The cygwin tools aim to provide a unix-style API on top of the windows libraries, to facilitate ports of unix software to windows. To this end, they introduce a unix-style directory hierarchy under some root directory (typically `/` is `C:\cygwin\`). Moreover, everything built against the cygwin API (including the cygwin tools and programs compiled with cygwin's ghc) will see / as the root of their file system, happily pretending to work in a typical unix environment, and finding things like `/bin` and `/usr/include` without ever explicitly bothering with their actual location on the windows system (probably `C:\cygwin\bin` and `C:\cygwin\usr\include`).

### The problem

GHC, by default, no longer depends on cygwin, but is a native windows program. It is built using mingw, and it uses mingw's ghc while compiling your Haskell sources (even if you call it from cygwin's bash), but what matters here is that - just like any other normal windows program - neither GHC nor the executables it produces are aware of cygwin's pretended unix hierarchy. GHC will happily accept either '/' or '\' as path separators, but it won't know where to find `/home/joe/Main.hs` or `/bin/bash` or the like. This causes all kinds of fun when GHC is used from within cygwin's bash, or in make-sessions running under cygwin.

### Things to do

- Don't use absolute paths in make, configure & co if there is any chance that those might be passed to GHC (or to GHC-compiled programs). Relative paths are fine because cygwin tools are happy with them and GHC accepts '/' as path-separator. And relative paths don't depend on where cygwin's root directory is located, or on which partition or network drive your source tree happens to reside, as long as you 'cd' there first.

- If you have to use absolute paths (beware of the innocent-looking `ROOT=`pwd`` in makefile hierarchies or configure scripts), cygwin provides a tool called `cygpath` that can convert cygwin's unix-style paths to their actual windows-style counterparts. Many cygwin tools actually accept absolute windows-style paths (remember, though, that you either need to escape '\' or convert '\' to '/'), so you should be fine just using those everywhere. If you need to use tools that do some kind of path-mangling that depends on unix-style paths (one fun example is trying to interpret ':' as a separator in path lists..), you can still try to convert paths using `cygpath` just before they are passed to GHC and friends.

- If you don't have `cygpath`, you probably don't have cygwin and hence no problems with it... unless you want to write one build process for several platforms. Again, relative paths are your friend, but if you have to use absolute paths, and don't want to use different tools on different platforms, you can simply write a short Haskell program to print the current directory (thanks to George Russell for this idea): compiled with GHC, this will give you the view of the file system that GHC depends on (which will differ depending on whether GHC is compiled with cygwin's gcc or mingw's gcc or on a real unix system..) - that little program can also deal with escaping '\' in paths. Apart from the banner and the startup time, something like this would also do:

        $ echo "Directory.getCurrentDirectory >>= putStrLn . init . tail . show " | ghci

## Building and using Win32 DLLs

Making Haskell libraries into DLLs doesn't work on Windows at the moment; however, all the machinery is still there. If you're interested, contact the GHC team. Note that building an entire Haskell application as a single DLL is still supported: it's just multi-DLL Haskell programs that don't work. The Windows distribution of GHC contains static libraries only.

### Creating a DLL

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.

A couple of things to notice:

- 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:

      EXPORTS
       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&lowbar;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&lowbar;imp`. By doing this for you, switching from non-static to static linking is simply a question of adding `-static` to your command line.

### 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>

      extern void__stginit_Adder(void);

      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;
      }

  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.