13. Running GHC on Win32 systems

13.1. 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.

13.2. 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.

13.3. 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.


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!

13.4. 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.

13.5. File paths under Windows

Windows paths are not all the same. The different kinds of paths each have different meanings. The MAX_PATH limitation is not a limitation of the operating system nor the file system. It is a limitation of the default namespace enforced by the Win32 API for backwards compatibility.

The NT kernel however allows you ways to opt out of this path preprocessing by the Win32 APIs. This is done by explicitly using the desired namespace in the path.

The namespaces are:

  • file namespace: \\?\
  • device namespace: \\.\
  • NT namespace: \

Each of these turn off path processing completely by the Win32 API and the paths are passed untouched to the filesystem.

Paths with a drive letter are legacy paths. The drive letters are actually meaningless to the kernel. Just like Unix operating systems, drive letters are just a mount point. You can view your mount points by using the mountvol command.

Since GHC 8.6.1, the Haskell I/O manager automatically promotes paths in the legacy format to Win32 file namespace. By default the I/O manager will do two things to your paths:

  • replace \ with \\
  • expand relative paths to absolute paths

If you want to opt out of all preprocessing just explicitly use namespaces in your paths. Due to this change, if you need to open raw devices (e.g. COM ports) you need to use the device namespace explicitly. (e.g. \\.\COM1). GHC and Haskell programs in general no longer support opening devices in the legacy format.

See the Windows documentation for more details.

13.6. Using GHC (and other GHC-compiled executables) with Cygwin

13.6.1. 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).

13.6.2. 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.

13.6.3. 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

13.7. Building and using Win32 DLLs

Dynamic link libraries, Win32 DLLs, Win32 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.

There are two distinct ways in which DLLs can be used:

  • You can turn each Haskell package into a DLL, so that multiple Haskell executables using the same packages can share the DLL files. (As opposed to linking the libraries statically, which in effect creates a new copy of the RTS and all libraries for each executable produced.)

    That is the same as the dynamic linking on other platforms, and it is described in Using shared libraries.

  • You can package up a complete Haskell program as a DLL, to be called by some external (usually non-Haskell) program. This is usually used to implement plugins and the like, and is described below.

13.7.1. Creating a DLL

Creating a Win32 DLL -shared 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 -shared -o foo.dll bar.o baz.o wibble.a -lfooble

By feeding the ghc compiler driver the option -shared, 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 -shared. Should you want to constrain this, you can specify the module definition file to use on the command line as follows:

    ghc -shared -o .... MyDef.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:

     DllCanUnloadNow     = DllCanUnloadNow@0
     DllGetClassObject   = DllGetClassObject@12
     DllRegisterServer   = DllRegisterServer@0
     DllUnregisterServer = DllUnregisterServer@0
  • In addition to creating a DLL, the -shared 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.dll.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.dll.a. Additionally, when the compiler driver is linking in non-static mode, it will rewrite occurrence of -lHSfoo on the command line to -lHSfoo.dll. By doing this for you, switching from non-static to static linking is simply a question of adding -static to your command line.

13.7.2. Making DLLs to be called from other languages

This section describes how to create DLLs to be called from other languages, such as Visual Basic or C++. This is a special case of Making a Haskell library that can be called from foreign code; we’ll deal with the DLL-specific issues that arise below. Here’s an example:

Use foreign export declarations to export the Haskell functions you want to call from the outside. For example:

-- Adder.hs
{-# LANGUAGE ForeignFunctionInterface #-}
module Adder where

adder :: Int -> Int -> IO Int  -- gratuitous use of IO
adder x y = return (x+y)

foreign export ccall adder :: Int -> Int -> IO Int

Add some helper code that starts up and shuts down the Haskell RTS:

// StartEnd.c
#include <Rts.h>

void HsStart()
   int argc = 1;
   char* argv[] = {"ghcDll", NULL}; // argv must end with NULL

   // Initialize Haskell runtime
   char** args = argv;
   hs_init(&argc, &args);

void HsEnd()

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 everything up:

ghc -c Adder.hs
ghc -c StartEnd.c
ghc -shared -o Adder.dll Adder.o Adder_stub.o StartEnd.o

Now the file Adder.dll can be used from other programming languages. Before calling any functions in Adder it is necessary to call HsStart, and at the very end call HsEnd.


It may appear tempting to use DllMain to call hs_init/hs_exit, but this won’t work (particularly if you compile with -threaded). There are severe restrictions on which actions can be performed during DllMain, and hs_init violates these restrictions, which can lead to your DLL freezing during startup (see #3605). Using from VBA

An example of using Adder.dll from VBA is:

Private Declare Function Adder Lib "Adder.dll" Alias "adder@8" _
      (ByVal x As Long, ByVal y As Long) As Long

Private Declare Sub HsStart Lib "Adder.dll" ()
Private Declare Sub HsEnd Lib "Adder.dll" ()

Private Sub Document_Close()
End Sub

Private Sub Document_Open()
End Sub

Public Sub Test()
MsgBox "12 + 5 = " & Adder(12, 5)
End Sub

This example uses the Document_Open/Close functions of Microsoft Word, but provided HsStart is called before the first function, and HsEnd after the last, then it will work fine. Using from C++

An example of using Adder.dll from C++ is:

// Tester.cpp
#include "HsFFI.h"
#include "Adder_stub.h"
#include <stdio.h>

extern "C" {
    void HsStart();
    void HsEnd();

int main()
    // can now safely call functions from the DLL
    printf("12 + 5 = %i\n", adder(12,5))    ;
    return 0;

This can be compiled and run with:

$ ghc -o tester Tester.cpp Adder.dll.a
$ tester
12 + 5 = 17