Starcraft II restricts me from running multiple instances, but if I run the second instance sandboxed in Sandboxie, it works. What might account for this?
How might I replicate this behavior for my own games?
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Aside from Starcraft 2, the general answer to this is: Acquire (and lock, if the acquisition itself isn't already equivalent to locking) a specific resource from the operating system. Exit the game if the acquisition/locking fails. Example of such resources are:
- Specific TCP or UDP ports.
- Mutually exclusive locks ("mutexes") or semaphores (those two are often related on OS level)
- Files (log files are often the most obvious choice)
On the top-end, most pain-in-the-ass for the customer way, specialised hardware which (sometimes necessarily, like for some robotic control components) can only be used exclusively by a single thread can be used this way, too.
Martin Sojka
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5Also it may be useful to keep in mind possible security complications: "A single-instance program is it's own denial of service". – EnoughTea Aug 09 '11 at 13:53
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Since it works in sandboxie, it's probably not the first. 2 or 3 are good possibilities, taking a look at what files are open to their processes will probably pinpoint it. – Doug-W Aug 09 '11 at 15:42
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3@Doug-W I don't see how it working in sandboxie rules out #1. – AttackingHobo Aug 09 '11 at 17:33
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@AttackingHobo unless I miss understood the complexity of sandboxie, wouldn't the fact that a port is bound outside sandboxie preclude it from binding within? Or does sandboxie provide a virtual interface for each program running inside of it? – Doug-W Aug 10 '11 at 16:29
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@Vigil As far as I'm concerned, you may quote Raymond Chen all day, but please don't add grammatical errors to his quotes. :P (Just copy/paste them instead.) – hheimbuerger Aug 16 '11 at 09:35
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I recommend the mutex approach, but:
Another method that is often used is simply checking if another process with the same name is running.
The advantage is that it's ridiculously easy - you don't have to worry about file permissions or know what a mutex is. The downside is, you'll get a false-positive if a different program called starcraft2.exe is running on the machine.
BlueRaja - Danny Pflughoeft
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@Jonathan: Then don't hardcode the executable name... – BlueRaja - Danny Pflughoeft Aug 13 '11 at 22:28
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1I meant more like, what if the user copies the program and renames it & runs it twice? – Brandon Aug 14 '11 at 04:10
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If you are using C++ and the Windows API, one way you can do is using FindWindow, where you can pass parameters like the class name and/or the window name. Then if you find a match, just exit your program before you even load or create a window.
SourceTurtle
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