Cocoa - Prevent System from shutting down

Hey Guys, I've another question I couldn't find an answer on the internet. I hope you can help me.

I'm writing an Cocoa Application which runs in background and does several tasks for me. If I quit the application, I show a NSAlert. That all works great. But here comes my problem. This Alert is presented from inside the applicationShouldTerminate: Method. So if I Logout, this method is triggered and the alert is shown. But OSX quits the application and logs out without waiting for a response even if I return NSTerminateCancel.

How can I force the OS to stop logging out/shutting down from within my application, so that the user (myself :-D) can decide what to do. (It just a dialog which asks if the done work shall be sa开发者_StackOverflow社区ved or not.)

I hope you can help me...

Sandro

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Mac OS X is based on UNIX. When a system shuts down, the SIGKILL signal is send that terminates all the processes. But before that, the SIGTERM signal is sent that informs the processes that a shutdown is going to take place.

I think the solution lies in handling of the signals, i.e. detecting and blocking them. Regarding this, you may find guidance in GNU C Library: Signal Handling: Blocking Signals.

The examples there maybe about BSD, but you may know that the OS X core is derived from BSD and therefore OS X supports it. Moreover, C is supported in Objective-C.