One thing to add to @kshegunov, terminating these applications ungracefully may have side effects on the stability of your system.

@vlada said:

I tried to catch all possible signals but I didn't get any of them. I think it is because of the way Qt applications are run on Android. Because on Android is run a Java application which runs my app as a library.

Assuming this is true, then indeed, you'll need to write a bit of java and only if the Java runtime allows interception of the signals. I don't know if it's worth it, but it shouldn't be too hard.

@SGaist
Ok my dear friend.
Thank you again for your advise.