Build and override a simple winelib DLL

[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