Run multiple instance of any windows application

Question

Suppose some author has enforced his Windows application to be single instance. Is there any way to make multiple instance of this application (aside from running inside a virtual machine or requesting the author to rewrite the app)?

If there is a readymade tool, I would like to know it. (I have tried sandboxie and Altiris SVS without luck).

If there is nothing out there, I want to program a tool/hack that will let me do this. I am looking for pointers where to start - where to start, what will be involved, what skills would be needed. I have moderate programming skills in C and Java.

If this cannot be done, please explain why.

EDIT: I know its a bad idea but I still need to do it (for various reasons). I want a generic way that works for any application and does not introduce errors.

Answer 1

Many applications check the global list of processes (with EnumProcesses, OpenProcess, GetModuleBaseName, and similar functions) or list of windows (with EnumWindows, EnumChildWindows).

You may try to set hook (see samples SetWindowsHookEx, CallNextHookEx, etc) to hook those specific API function calls from that application and replace requested data in response with yours to fool the application.

Answer 2

There is no generic way because different applications use different methods. A small number of apps do this simply for "usability", but for many others, there may be a good reason to enforce singletons. Bypassing that could lead to data corruption or simply failing to run.

Answer 3

Lock files, named pipes and synchronisation events are some of the common ways applications use to check they're the only running instance. To work around lock files you'd either have to virtualise the file system, hook into it or do a carefully timed delete (and that may not work if it locks the file open too). Named pipes and synchronisation events will be much more difficult because you don't have the same control from outside the application that you do with files.