Re: How to allow just one instance from c++ application exe?

Maxim Yegorushkin <>
Fri, 12 Jun 2009 09:51:59 +0100
James Kanze wrote:

On Jun 10, 11:29 am, Maxim Yegorushkin <>

James Kanze wrote:

On Jun 9, 9:11 pm, Magdy <> wrote:

How to allow just one instance to be running from c++
application exe?!

More details:
Platform: windows, but I prefer a cross-platform solution...

If the user ran the exe and there is another instance
already running, the new exe sends message (ex. string) to
the running one and terminate.

Any ideas and/or thoughts will be much appreciated. :-)

There's no standard solution; the usual solution under Unix
(which should also work under Windows) is to atomically
create a file with the process id of the program in a fixed
location. The "atomically" means that you'll have to use
some platform specific requests, not simple ofstream. Under
Unix, this is done by using the modes O_EXCL | O_CREAT in
the open request, under Windows, there is a flag CREATE_NEW
for CreateFile which sounds like it would do the same thing.
If the open succeeds, you're fine. If it fails, and the
error is EEXIST (Unix) or ERROR_FILE_EXISTS (Windows), then
either another instance of the executable is running, or a
previous instance didn't terminate cleanly, and failed to
delete the file. (To solve the problem concerning failing
to delete the file: use a small batch script to start your
program, which deletes the file when your program has
finished, regardless of why it finished, and add an
automatic action on booting to delete the file.) Any other
error, of course, is a serious problem, and probably means
that your program wasn't installed correctly.

As you noted, this usual solution does not work well if the
program does not delete the file for some reason.

A robust solution is to create the file with no O_EXCL flag.
And then try locking it. If locking fails, then another
instance of the application is holding it. If the application
that holds the lock terminates for any reason, the operating
system releases the lock anyway.

Yes, that sounds like it would be a better solution. I probably
should have qualified my suggestings as the "historically usual
solution"---it was certainly the usual solution before Unix
supported file locking, and probably for a long time afterwards,
but I don't know too much about today.

Too bad that today Firefox and eMule for Linux still use the old method.
Occasionally, these applications won't start because their stale pid
file have not been removed, so I have to remove these files manually. If
these applications used file locking, I would not have to know about the
existence of those files.


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