Swift on macOS: Running code on termination of background app

Question

I am writing a Mac app that runs as a daemon in the background. How can I detect when the app is killed and run some cleanup code? I have tried catching signals such as SIGTERM , SIGINT , and SIGKILL , but my app structure seems to have some problems with this.

This is the basic flow of my application start from the main thread:

func signalHandler(signal: Int32) { ... }

let signals = [SIGINT, SIGTERM, SIGKILL]
for sig in signals {
    signal(sig, signalHandler)
}

// code to kick off background execution that will occasionally need to run some stuff on the main thread

RunLoop.current.run() // SIGTERM shows up in Xcode on this line, signalHandler not called

If I stop the app using Xcode's stop button or force quit from the Activity Monitor, the app just exits with exit code 9 ( SIGKILL ). If I choose "Quit" from the activity monitor, the app receives a SIGTERM but the handler doesn't get called... Xcode just highlights the RunLoop line and says Thread 1: signal SIGTERM .

Without the RunLoop line my application will stop almost immediately but I need to keep it alive. At the same time, I am thinking perhaps that line is blocking the main thread and preventing the signal handler from being run. Are there any ways around this so I can handle my app termination gracefully?

Answer 1

A couple of notes:

• SIGKILL cannot be caught .
• if the debugger is attached, it will intercept the signal handlers instead of letting the flowing through the program; a workaround to this can be found here

Try printing the signal and launching the program from the command line

func signalHandler(signal: Int32) {
    print("Caught signal \(signal)")
}

Ctrl + C prints Caught signal 2

Terminating the process from Activity Monitor via Quit (not Force Quit ) prints Caught signal 15