Commit 5cf5eded authored by Neil Mitchell 's avatar Neil Mitchell
Browse files

Update the documentation on using DLL's from Windows, fixing several errors,...

Update the documentation on using DLL's from Windows, fixing several errors, in particular those relating to bug 3605
parent f8b7b3c6
......@@ -403,227 +403,150 @@ non-static to static linking is simply a question of adding
<title>Making DLLs to be called from other languages</title>
<para>
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. This is a special case of <xref linkend="ffi-library" />; we'll deal with
the DLL-specific issues that arise below. Here's an example:
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
<xref linkend="ffi-library" />; we'll deal with the DLL-specific issues that
arise below. Here's an example:
</para>
<itemizedlist>
<listitem>
<para>
Use <literal>foreign export</literal> declarations to export the Haskell
functions you want to call from the outside. For example,
Use foreign export declarations to export the Haskell functions you want to
call from the outside. For example:
</para>
<programlisting>
-- Adder.hs
{-# LANGUAGE ForeignFunctionInterface #-}
module Adder where
adder :: Int -> Int -> IO Int &ndash;&ndash; gratuitous use of IO
adder :: Int -> Int -> IO Int -- gratuitous use of IO
adder x y = return (x+y)
foreign export stdcall adder :: Int -> Int -> IO Int
</programlisting>
</para>
</listitem>
<listitem>
<para>
Compile it up:
<screen>
ghc -c adder.hs -fglasgow-exts
</screen>
This will produce two files, adder.o and adder_stub.o
Add some helper code that starts up and shuts down the Haskell RTS:
</para>
</listitem>
<listitem>
<para>
compile up a <function>DllMain()</function> that starts up the Haskell
RTS-&ndash;&ndash;a possible implementation is:
<programlisting>
#include &lt;windows.h&gt;
#include &lt;Rts.h&gt;
// StartEnd.c
#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
)
void HsStart()
{
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;
}
</programlisting>
int argc = 1;
char* argv[] = {"ghcDll", NULL}; // argv must end with NULL
Here, <literal>Adder</literal> 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:
// Initialize Haskell runtime
char** args = argv;
hs_init(&argc, &args);
<screen>
ghc -c dllMain.c
</screen>
</para>
</listitem>
// Tell Haskell about all root modules
hs_add_root(__stginit_Adder);
}
<listitem>
void HsEnd()
{
hs_exit();
}
</programlisting>
<para>
Construct the DLL:
Here, <literal>Adder</literal> 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:
</para>
<screen>
ghc &ndash;shared -o adder.dll adder.o adder_stub.o dllMain.o
ghc -c Adder.hs
ghc -c StartEnd.c
ghc -shared -o Adder.dll Adder.o Adder_stub.o StartEnd.o
</screen>
</para>
</listitem>
<listitem>
<para>
Start using <function>adder</function> from VBA-&ndash;&ndash;here's how I would
<constant>Declare</constant> it:
<programlisting>
Private Declare Function adder Lib "adder.dll" Alias "adder@8"
(ByVal x As Long, ByVal y As Long) As Long
</programlisting>
Since this Haskell DLL depends on a couple of the DLLs that come with GHC,
make sure that they are in scope/visible.
Now the file <filename>Adder.dll</filename> can be used from other
programming languages. Before calling any functions in Adder it is necessary
to call <literal>HsStart</literal>, and at the very end call
<literal>HsEnd</literal>.
</para>
<para>
Building statically linked DLLs is the same as in the previous section: it
suffices to add <option>-static</option> to the commands used to compile up
the Haskell source and build the DLL.
<emphasis>Warning:</emphasis> It may appear tempting to use
<literal>DllMain</literal> to call
<literal>hs_init</literal>/<literal>hs_exit</literal>, but this won't work
(particularly if you compile with <literal>-threaded</literal>). There are
severe restrictions on which actions can be performed during
<literal>DllMain</literal>, and <literal>hs_init</literal> violates these
restrictions, which can lead to your dll freezing during startup (see
<ulink url="http://hackage.haskell.org/trac/ghc/ticket/3605">bug
#3605</ulink>).
</para>
</listitem>
</itemizedlist>
</sect2>
<sect2>
<title>Beware of DllMain()!</title>
<para>The body of a <literal>DllMain()</literal> function is an
extremely dangerous place! This is because the order in which DLLs are
unloaded when a process is terminating is unspecified. This means that
the <literal>DllMain()</literal> for your DLL may be called when other DLLs containing
functions that you call when de-initializing your DLL have already
been unloaded. In other words, you can't put shutdown code inside
<literal>DllMain()</literal>, unless your shutdown code only requires use of certain
functions which are guaranteed to be available (see the Platform SDK
docs for more info).</para>
<para>In particular, if you are writing a DLL that's statically
linked with Haskell, it is not safe to call
<literal>hs_exit()</literal> from <literal>DllMain()</literal>, since
<literal>hs_exit()</literal> may make use of other DLLs (see also <xref
linkend="hs-exit" />). What's more, if you
wait until program shutdown to execute your deinitialisation code, Windows will have
terminated all the threads in your program except the one calling
<literal>DllMain()</literal>, which can cause even more
problems.</para>
<para>A solution is to always export <literal>Begin()</literal> and <literal>End()</literal> functions from your
DLL, and call these from the application that uses the DLL, so that
you can be sure that all DLLs needed by any shutdown code in your
End() function are available when it is called.</para>
<para>The following example is untested but illustrates the idea (please let us
know if you find problems with this example or have a better one). Suppose we have a DLL called Lewis which makes use of 2
Haskell modules <literal>Bar</literal> and <literal>Zap</literal>,
where <literal>Bar</literal> imports <literal>Zap</literal> and is
therefore the root module in the sense of <xref
linkend="using-own-main" />. Then the main C++ unit for the DLL would
look something like:</para>
<sect3 id="win32-dlls-vba">
<title>Using from VBA</title>
<para>
An example of using <filename>Adder.dll</filename> from VBA is:
</para>
<programlisting>
// Lewis.cpp -- compiled using GCC
#include &lt;Windows.h&gt;
#include "HsFFI.h"
#define __LEWIS_DLL_EXPORT
#include "Lewis.h"
#include "Bar_stub.h" // generated by GHC
#include "Zap_stub.h"
BOOL APIENTRY DllMain( HANDLE hModule,
DWORD ul_reason_for_call,
LPVOID lpReserved
){
return TRUE;
}
extern "C"{
LEWIS_API HsBool lewis_Begin(){
int argc = ...
char *argv[] = ...
// Initialize Haskell runtime
hs_init(&amp;argc, &amp;argv);
// Tell Haskell about all root modules
hs_add_root(__stginit_Bar);
// do any other initialization here and
// return false if there was a problem
return HS_BOOL_TRUE;
}
Private Declare Function Adder Lib "Adder.dll" Alias "adder@8" _
(ByVal x As Long, ByVal y As Long) As Long
LEWIS_API void lewis_End(){
hs_exit();
}
Private Declare Sub HsStart Lib "Adder.dll" ()
Private Declare Sub HsEnd Lib "Adder.dll" ()
LEWIS_API HsInt lewis_Test(HsInt x){
// use Haskell functions exported by
// modules Bar and/or Zap
Private Sub Document_Close()
HsEnd
End Sub
return ...
}
Private Sub Document_Open()
HsStart
End Sub
} // extern "C"
Public Sub Test()
MsgBox "12 + 5 = " & Adder(12, 5)
End Sub
</programlisting>
<para>
This example uses the
<literal>Document_Open</literal>/<literal>Close</literal> functions of
Microsoft Word, but provided <literal>HsStart</literal> is called before the
first function, and <literal>HsEnd</literal> after the last, then it will
work fine.
</para>
</sect3>
and some application which used the functions in the DLL would have a main() function like:
<sect3 id="win32-dlls-c++">
<title>Using from C++</title>
// MyApp.cpp
#include "stdafx.h"
#include "Lewis.h"
<para>
An example of using <filename>Adder.dll</filename> from C++ is:
</para>
int main(int argc, char *argv[]){
if (lewis_Begin()){
// can now safely call other functions
// exported by Lewis DLL
<programlisting>
// Tester.cpp
#include "HsFFI.h"
#include "Adder_stub.h"
#include <stdio.h>
extern "C" {
void HsStart();
void HsEnd();
}
}
lewis_End();
return 0;
}
int main()
{
HsStart();
// can now safely call functions from the DLL
printf("12 + 5 = %i\n", adder(12,5)) ;
HsEnd();
return 0;
}
</programlisting>
<para><literal>Lewis.h</literal> would have to have some appropriate <literal>#ifndef</literal> to ensure that the
Haskell FFI types were defined for external users of the DLL (who
wouldn't necessarily have GHC installed and therefore wouldn't have
the include files like <literal>HsFFI.h</literal> etc).
<para>
This can be compiled and run with:
</para>
<screen>
$ ghc -o tester Tester.cpp Adder.dll.a
$ tester
12 + 5 = 17
</screen>
</sect2>
</sect1>
......
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