Zach Burlingame

Programming, Computers, and Other Notes on Technology

Getting the Load Count for a DLL

Recently I was trying to unload a DLL from a running process so that I could delete it from the disk but it just wouldn’t delete. Looking at the Modules pane in Visual Studio, I could see that the DLL was still loaded. I doubled and tripled check all of my calls to LoadLibrary for a corresponding call to FreeLibrary, and everything checked out. I needed to figure out what was loading it and where. One of the things that I wanted to know was, “What is the current load count for the DLL?”

Windows maintains a load count for each module on a per-process basis. When the load count reaches zero, the module will be unloaded. The problem is that this load count is not accessible through documented API calls. To get it, you need to use some undocumented structures and API calls from ntdll.dll. Fortunately, like so many other issues you run into, someone else has already run into it and Google knows where they are at. In this case there is a great article here (unfortunately I couldn’t figure out who specifically was the contributing author for that article so that I could give them due props).

The securityxploded article did such a great job of explaining it that I won’t bother to duplicate it here. I based my implementation on theirs however I updated it to support both MBCS and Unicode via the TCHAR macros. It’s available as part of my ModuleUtilities library.

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