Question

Many Windows apps (like Skype or MSN for instance) don’t let you start multiple instances, rather trying to run it a 2nd time just leaves the existing version running.

Is this typically done in some simple way – the start-menu shortcut is a ‘wrapper’ app around the main app – or is there some registry magic you can do to delegate the problem to Windows itself?

Specifically dealing with Win32 here (unmanaged C++) but happy to hear more general solutions as long as they are workable on Windows XP or later.

EDIT: this seems the best duplicate.

Answer 1

1. Named Mutex or similar OS-specfic named object. If it exists – app is running.
2. Lock file somewhere (in temporary directory, etc – create it on program start, remove on program end). Linux software frequently operates this way (some programs store PID in lockfile), but it isn’t safe – if you suddenly lose power (electricity cut off), it is possible that lock file won’t be deleted.
3. And you can always enum all running processes and try to find yourself.

There could be more ways to do it, but those are the first ones I could think of.