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 Section 4.12, “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.
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:
EXPORTS 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.
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 Section 8.2.1.2, “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 stdcall 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()
{
hs_exit();
}
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.
Warning: 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
bug
#3605).
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()
HsEnd
End Sub
Private Sub Document_Open()
HsStart
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.
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()
{
HsStart();
// can now safely call functions from the DLL
printf("12 + 5 = %i\n", adder(12,5)) ;
HsEnd();
return 0;
}
This can be compiled and run with:
$ ghc -o tester Tester.cpp Adder.dll.a $ tester 12 + 5 = 17