Limiting the number of instances of one application

[Chicken Blood Machine]

I know I can use process ID under Unix platform but what about others, especially Windows? Actually, even if it's not the most straightforward I think the sockect stuff is the best... I'll try to understand designerassistant sources and, well, we'll see...

http://msdn.microsoft.com/library/de...tprocessid.asp

[jpn]

Someone recently came up with this kind of solution on X11.

[wysota]

Of course the drawback is that once the application holding the semaphore crashes, you have no way of starting it again but reboot

[fullmetalcoder]

http://msdn.microsoft.com/library/de...tprocessid.asp

It's good to obtain a process ID (even though I'm pretty sure it can be done in plain Qt4) but what is important is to check that a proccess with this ID is running, as suggested...

Someone recently came up with this kind of solution on X11.

It uses something called "semaphore". Honestly I have no idea of what it is but I know that ther is a class called QSemaphore in Qt4, maybe it would fit...

[wysota]

It uses something called "semaphore". Honestly I have no idea of what it is

http://en.wikipedia.org/wiki/Semaphore_(programming)

but I know that ther is a class called QSemaphore in Qt4, maybe it would fit...

It wouldn't. QSemaphore works in scope of the application, whereas the above mentioned semaphores are system-wide and are therefore suited for IPC (Inter Process Communication) which is the basic idea of using them in this situation.

Trust me, you won't manage to do it in a system independent way - under the hood there is always platform specific code - even in Qt itself.

[fullmetalcoder]

Trust me, you won't manage to do it in a system independent way - under the hood there is always platform specific code - even in Qt itself.

Actually QTcpSocket works quite well. I've tried a few things and right now by mixing sockets, kinda lock file and a QSemaphore I'm able to prevent the creation of more than a given number of application instance. The only problem left is that communication with the working instances is not really straightforward and, ATM, buggy.

[Mike]

Did you also look at the QUdpSocket? (Qt Broadcast example)
Maybe it would also work this way:
1) Application is started.
2) Application opens a Udp port that is known to all applications.
3) The new app sends out a datagramm
4) Now the app does listen for all responses from other running applications
5) The new app does verify the number of responses and determines the total number of running instances
6.1) If there are too many applications running, the new app terminates itself.
6.2) If it is possible to start another instance, then the new app will close the Udp server socket and open a Udp client socket, listening for broadcast messages itself.

This should really work on any platform.

[wysota]

It may work but is still an ugly solution. If you only want to make sure you have a single instance of the application, you can avoid sockets. If you want your applications on a local machine to communicate, you could come up with a better solution too. Depending on high-numbered sockets is a risk, because there is no way telling if no other app (for example some ftp client or server) doesn't use the same socket.

Why do you need a QSemaphore here? Is your app accessing the socket from multiple threads?

[fullmetalcoder]

It may work but is still an ugly solution. If you only want to make sure you have a single instance of the application, you can avoid sockets. If you want your applications on a local machine to communicate, you could come up with a better solution too. Depending on high-numbered sockets is a risk, because there is no way telling if no other app (for example some ftp client or server) doesn't use the same socket.

Maybe ugly but better than nothing. Besides socket corruption is highly improbable since localhost is used and the port is dynamically assigned by Qt through the server classes (QTcpServer or QUdpServer)

Why do you need a QSemaphore here? Is your app accessing the socket from multiple threads?

The semaphore was just used as a convinient counter providing possibilities of extension to multithreaded approach...

[TheRonin]

IMHO... When I've had to limit the number of application instances in the past, I've used the port-method. With Qt it's simple to do as well. Granted, there are some problems like, what happens if another app is already using the requested port, etc. But i believe that handling those issues are simpler than creating platform specific code for locking processes or handling lock files (which in the worst case can require one or more monitoring processes to clean up after crashes etc).

Those are just my two cents, for whatever they're worth