how to detect Linux shutdown/reboot

Question

I\'m trying to detect shut down or reboot from Linux in my c program. I found that program can use signal(SIGTERM, handler) (SIGKILL, handler). But these two will trigger if

Answer 1

Catch SIGTERM. In the handler, read the /var/run/utmp file to get the runlevel. See the source code of the runlevel(8) command for reference.

Answer 2

Your assumptions are mistaken. There is no way to catch SIGKILL; it terminates the process with no chance to respond to it.

With that said, I may have a solution: you can catch SIGTERM, and if you install the signal handler with sigaction and the SA_SIGINFO flag, you should be able to determine which process sent you the SIGTERM signal. If it came from init (pid 1), it's a shutdown. This is mildly hackish but might work for your purposes.