Signal

About system calls, SIGINT, SIGTERM, SIGKILL

Definition:

• System calls: communication chanel between user space program and kernel

• Signals: a different channel, used for inter-process communication

• Signals don't carry any agrgument, they are self explanatory by their name

• Some signals identified by a number, ie SIGKILL (9)

• That's why we use kill -9 <PID> to kill a process, because the kill command will send a defined signal to a process with a given identity <PID>

• when we run kill -9 <PID> command, that process is not terminate itself, instead we're telling that OS to stop running the program, no matter what the program is doing

Some useful signals

• SIGINT: is the program interrupt signal. When user presses CTRL+C, the terminal emulator sends this signal to the foreground process, it will terminate process, but it can be caught or ignored, it a graceful shutdown

• SIGTERM: is the termination signal. It used to cause process termination, this signal can be blocked, handled and ignored. It is the normal way to ask a process to terminate. The kill command generates SIGTERM by default

• SIGKILL: is an immediate termination signal. It cannot be caught or ignored by the process, because when we send a SIGKILL to a proces, we remove any chance for that process to do a tidy cleanup and shutdown.

For example, when a process does not die by using Ctrl+C (SIGINT), we should use the command kill -9 on that process PID

• SIGSTOP: is a process suspend signal, which tells the OS to stop/suspend a process. This signal cannot be caught or ignored. To resume the process, use SIGCONT signal to continue

• Ctrl+C: The interrupt signal, sends SIGINT to the job running in the foreground.

When a process is in a limbo state it is reasonable to send the process the SIGKILL signal, which can be invoked by running the kill command with the -9 flag. Unlike SIGTERM the SIGKILL signal cannot be captured by the process and thus it cannot be ignored. The SIGKILL signal is handled outside of the process completely, and is used to stop the process immediately. The problem with using SIGKILL is that it does not allow an application to close its open files or database connections cleanly and over time could cause other issues; therefor it is generally better to reserve the SIGKILL signal as a last resort.

Reference & Read more

1. http://bencane.com/2014/04/01/understanding-the-kill-command-and-how-to-terminate-processes-in-linux/
2. https://en.wikipedia.org/wiki/Unix_signal
3. http://unix.stackexchange.com/questions/149741/why-is-sigint-not-propagated-to-child-process-when-sent-to-its-parent-process
4. http://askubuntu.com/questions/890591/why-doesnt-ctrl-c-kill-the-terminal-itself/890597
5. https://www.shellscript.sh/
6. https://www.win.tue.nl/~aeb/linux/lk/lk-5.html
7. https://lasr.cs.ucla.edu/vahab/resources/signals.html
8. http://www.linuxprogrammingblog.com/all-about-linux-signals?page=show
9. ftp://ftp.gnu.org/old-gnu/Manuals/glibc-2.2.3/html_chapter/libc_24.html

10. http://askubuntu.com/questions/184071/what-is-the-purpose-of-the-9-option-in-the-kill-command
11. https://www.digitalocean.com/community/tutorials/how-to-use-ps-kill-and-nice-to-manage-processes-in-linux
12. https://s905060.gitbooks.io/site-reliability-engineer-handbook/content/signals.html
13. http://bencane.com/2014/04/01/understanding-the-kill-command-and-how-to-terminate-processes-in-linux/
14. https://major.io/2010/03/18/sigterm-vs-sigkill/