Build and override a simple winelib DLL

[Maxime Sednaoui]

Hi,

First, I already asked for the same thing at the forum where  I've been
told to post here for my question
Link here : http://forum.winehq.org/viewtopic.php?f=8&t=18675

I have a Windows executable that load a really simple DLL (it is a test)
and I want to create a Winelib DLL that will override the Windows DLL.
Basically, I created mylib_main.c and mylib.spec to build the Winelib DLL
with the command:
winegcc -mwindows -o mylib.dll.so mylib_main.c -I${WINE_ROOT}/include
-shared mylib.spec
winebuild -w --def -o libmylib.def --export mylib.spec

Now I have mylib.dll.so and I want to override mylib.dll
What should I do ? I removed the Windows library but then I got a Page
Fault when the function is called. I also tried to configure the override
with winecfg or set environment variables like WINEDLLPATH or add a DllMain
in my library. I don't understand how to proceed.

Is this because of the way I build my library ? Or maybe I forget a step to
link it properly ?

In attachment, a tar with the sources and binaries
myapp.exe                       :    the Windows executable
mylib.dll                           :    the Windows library
mylib_main.c , mylib.spec :    the sources to build the Winelib library

[Matijn Woudt]

On Fri, Apr 5, 2013 at 6:34 PM, Maxime Sednaoui wrote:
> Hi,
>
> First, I already asked for the same thing at the forum where  I've been
> told to post here for my question
> Link here : http://forum.winehq.org/viewtopic.php?f=8&t=18675
>
> I have a Windows executable that load a really simple DLL (it is a test)
> and I want to create a Winelib DLL that will override the Windows DLL.
> Basically, I created mylib_main.c and mylib.spec to build the Winelib DLL
> with the command:
> winegcc -mwindows -o mylib.dll.so mylib_main.c -I${WINE_ROOT}/include
> -shared mylib.spec
> winebuild -w --def -o libmylib.def --export mylib.spec
>
> Now I have mylib.dll.so and I want to override mylib.dll
> What should I do ? I removed the Windows library but then I got a Page
> Fault when the function is called. I also tried to configure the override
> with winecfg or set environment variables like WINEDLLPATH or add a DllMain
> in my library. I don't understand how to proceed.
>
> Is this because of the way I build my library ? Or maybe I forget a step
> to link it properly ?

Hi,

I'm not an expert on this, but anyway: the docs say:
"If you have problems then set the WINEDEBUG=+module environment variable
before running wine to see what is actually happening."

So I would suggest to try that first.
Second, it might be that your dll is loaded, but that it crashes inside?
Have you got a trace of the crash? Or have you ran winedbg/wine with gdb to
analyze the crash?

Regards,

Matijn

[Charles Davis]

On Apr 5, 2013, at 10:34 AM, Maxime Sednaoui wrote:
> Hi,
>
> First, I already asked for the same thing at the forum where  I've been told to post here for my question
> Link here : http://forum.winehq.org/viewtopic.php?f=8&t=18675
>
> I have a Windows executable that load a really simple DLL (it is a test) and I want to create a Winelib DLL that will override the Windows DLL. Basically, I created mylib_main.c and mylib.spec to build the Winelib DLL with the command:
> winegcc -mwindows -o mylib.dll.so mylib_main.c -I${WINE_ROOT}/include -shared mylib.spec
> winebuild -w --def -o libmylib.def --export mylib.spec
>
> Now I have mylib.dll.so and I want to override mylib.dll
> What should I do ? I removed the Windows library but then I got a Page Fault when the function is called. I also tried to configure the override with winecfg or set environment variables like WINEDLLPATH or add a DllMain in my library. I don't understand how to proceed.
>
> Is this because of the way I build my library ? Or maybe I forget a step to link it properly ?

Actually, it's much simpler than that.

Your Winelib DLL doesn't export a function called myfunc(), but the native DLL does. (Try running:

$ winedump -jexport mylib.dll

to see what I mean.)

Your test app LoadLibrary()'s mylib.dll, then GetProcAddress()'s myfunc() from it. (I know this because it doesn't directly call myfunc(). Run:

$ winedump -jimport myapp.exe

to verify this.) When it does this with your Winelib DLL, GetProcAddress() returns NULL and your program crashes--probably because it doesn't check for NULL before calling the function. (Of course, the whole point of GetProcAddress() is to be able to check if a function exists in a DLL before calling it. If you don't check for NULL, then why bother with it in the first place? But I digress.)

Chip

[Maxime Sednaoui]

Hi,

It works, thank you ! Sorry, it was a silly mistake... I added
"__declspec(dllexport)" in mylib_main.c and it was done !
I really appreciate your help, I spent an awful amount of time on it.

Best,

On Sat, Apr 6, 2013 at 5:42 AM, Charles Davis  wrote:
> On Apr 5, 2013, at 10:34 AM, Maxime Sednaoui wrote:
> > Hi,
> >
> > First, I already asked for the same thing at the forum where  I've been
>
> told to post here for my question
> > Link here : http://forum.winehq.org/viewtopic.php?f=8&t=18675
> >
> > I have a Windows executable that load a really simple DLL (it is a test)
>
> and I want to create a Winelib DLL that will override the Windows DLL.
> Basically, I created mylib_main.c and mylib.spec to build the Winelib DLL
> with the command:
> > winegcc -mwindows -o mylib.dll.so mylib_main.c -I${WINE_ROOT}/include
>
> -shared mylib.spec
> > winebuild -w --def -o libmylib.def --export mylib.spec
> >
> > Now I have mylib.dll.so and I want to override mylib.dll
> > What should I do ? I removed the Windows library but then I got a Page
>
> Fault when the function is called. I also tried to configure the override
> with winecfg or set environment variables like WINEDLLPATH or add a DllMain
> in my library. I don't understand how to proceed.
> > Is this because of the way I build my library ? Or maybe I forget a step
>
> to link it properly ?
> Actually, it's much simpler than that.
>
> Your Winelib DLL doesn't export a function called myfunc(), but the native
> DLL does. (Try running:
>
> $ winedump -jexport mylib.dll
>
> to see what I mean.)
>
> Your test app LoadLibrary()'s mylib.dll, then GetProcAddress()'s myfunc()
> from it. (I know this because it doesn't directly call myfunc(). Run:
>
> $ winedump -jimport myapp.exe
>
> to verify this.) When it does this with your Winelib DLL, GetProcAddress()
> returns NULL and your program crashes--probably because it doesn't check
> for NULL before calling the function. (Of course, the whole point of
> GetProcAddress() is to be able to check if a function exists in a DLL
> before calling it. If you don't check for NULL, then why bother with it in
> the first place? But I digress.)
>
> Chip