How systemd stop command actually works

Question

I am using a systemd service which calls a process when it\'s been \"started\" (e.g.$systemctl start test.service). As per the design, the process stays for ever in a loop,

Answer 1

By default, a SIGTERM is sent, followed by 90 seconds of waiting followed by a SIGKILL.

Killing processes with systemd is very customizable and well-documented.

I recommend reading all of man systemd.kill as well as reading about ExecStop= in man systemd.service.

To respond to those signals, refer to the signal handling documentation for the language you are using.

Answer 2

Does a systemctl stop test.service command send SIGKILL or SIGTERM signal to kill the process? How can i detect a systemctl stop operation from within a process?

Systemd sends SIGTERM signal to process. In process you have to register signals, which are "catched".

In process, eg. SIGTERM signal can be registered like this:

void signal_callback()
{
    printf("Process is going down\n");
}
signal(SIGTERM, signal_callback)

When SIGTERM is sent to process signal_callback() function is executed.