I have a program to which I’m adding fullscreen post-processing effects. I do not have the source for the program (it’s proprietary, although a developer did send me a copy of the debug symbols, .map format). I have the code for the effects written and working, no problems.
My issue now is linking the two.
I’ve tried two methods so far:
Use Detours to modify the original program’s import table. This works great and is guaranteed to be stable, but the user’s I’ve talked to aren’t comfortable with it, it requires installation (beyond extracting an archive), and there’s some question if patching the program with Detours is valid under the terms of the EULA. So, that option is out.
The other option is the traditional DLL-replacement. I’ve wrapped OpenGL (opengl32.dll), and I need the program to load my DLL instead of the system copy (just drop it in the program folder with the right name, that’s easy).
I then need my DLL to load the Cg framework and runtime (which relies on OpenGL) and a few other things. When Cg loads, it calls some of my functions, which call Cg functions, and I tend to get stack overflows and infinite loops. I need to be able to either include the Cg DLLs in a subdirectory and still use their functions (not sure if it’s possible to have my DLLs import table point to a DLL in a subdirectory) or I need to dynamically link them (which I’d rather not do, just to simplify the build process), something to force them to refer to the system’s file (not my custom replacement).
The entire chain is: Program loads DLL A (named opengl32.dll). DLL A loads Cg.dll and dynamically links (GetProcAddress) to sysdir/opengl32.dll. I now need Cg.dll to also refer to sysdir/opengl32.dll, not DLL A.
How would this be done?
Edit: How would this be done easily without using GetProcAddress? If nothing else works, I’m willing to fall back to that, but I’d rather not if at all possible.
Edit2: I just stumbled across the function SetDllDirectory in the MSDN docs (on a totally unrelated search). At first glance, that looks like what I need. Is that right, or am I misjudging? (off to test it now)
Edit3: I’ve solved this problem by doing thing a bit differently. Instead of dropping an OpenGL32.dll, I’ve renamed my DLL to DInput.dll. Not only does it have the advantage of having to export one function instead of well over 120 (for the program, Cg, and GLEW), I don’t have to worry about functions running back in (I can link to OpenGL as usual). To get into the calls I need to intercept, I’m using Detours. All in all, it works much better. This question, though, is still an interesting problem (and hopefully will be useful for anyone else trying to do crazy things in the future). Both the answers are good, so I’m not sure yet which to pick…
SetDllDirectory probably won’t work. Cg.dll likely just links to OpenGL.dll. When the OS loads Cg.dll, it sees that there’s already a module loaded with that name (yours), so it links Cg with that instead of going off to find some other copy. That is, the search order that SetDllDirectory modifies never even comes into play because the OS doesn’t do any searching.
I suspect your best bet will indeed be to detect re-entrant calls to your library. When you detect one, instead of doing your custom handling, forward the call directly to the real OpenGL library, which you have a reference to due to calling LoadLibrary and then GetProcAddress for each of the library’s functions.