Blizzhackers

How do people write these, what API is used in them, where can I find information on them and possible some source code?
I'd like to add a crosshair onto FPS game that I play.

Most of these are implemented using either Direct3D (if the program is rendering using D3D) or OpenGL (if the program is rendering using OpenGL) hooks. The hook function would be called by the program, then it would use the same Direct3D or OpenGL rendering context to draw the crosshair (or in the case of game overlays, usually some text on the screen), and then it'd call its original unhooked function.

The way PIX and FRAPS run their overlays in D3D mode is by implementing a v-table hook on the IDirect3DDevice9::Present() function (or D3D10 or D3D11, depends on which version you're running). This style of hooking D3D must be done immediately when the program starts (otherwise patching the v-table becomes much trickier and you might as well just use the other method).

If the overlay is not available to be installed in the program immediately when it starts (like how you can attach and detach FRAPS from a program while it's running) then you'd probably have to do a function hook on the Present() member function itself. IMO, the MS-Detours API is probably the most pain-free thing to use in this situation.

I am not entirely sure how I'd implement an OpenGL hook (I've never had to do one before), but you may be able to get away with using a proxy DLL (easiest method), or you might have to use DLL-injection + function hooking (suckier method).

I'm not great with dll injeciton, I've never beable to eject the DLL after loading it.
Specifically, it's a game that uses OpenGL. It's there any examples available? I've searched google but I only receive PHP and jQuery stuff.

I've never used hooks before, where can I learn about them?

Well this site has an explanation of how to do OpenGL function hooking in assembly. I would personally just use MS-Detours though.

What MS Api are you talking about?

This one:
http://research.microsoft.com/en-us/projects/detours/

How the hell do you compile this?!
Do I need to use Visual Studio or something? (I am using Dev-C++)

You will probably need Visual Studio to compile it without having to perform major headaches in Dev-C++

I've managed to only create a static library of it. My DLL fails to compile.



I don't even know...

Did you add createwth.cpp to the project? You are not supposed to.

I'm getting undeclared identifier errors like crazy with and without createwth.cpp. Ugh, I've never used Visual Studio and never really got into C++. dfhjsdgf

Oh I forgot that Detours is a project that is supposed to be built from the Makefile rather than from Visual Studio itself. My mistake. Just open up the Visual Studio Command Prompt from the start menu. Navigate to where you installed Detours. Go into the src folder, and do nmake Makefile

Oddly enough, I've done this too and I still receive some errors.

Which ones?

I finally got it. It was Visual C++ Express that was the problem.