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If you’ve ever used kill you’ve used signals
person, angrily: DIE!!!
process, sad: okay
the Linux kernel sends processes signals in lots of situations
- your child terminated
- the timer you set expired
- that pipe is closed
- illegal instruction
- segmentation fault
you can send signals yourself with the kill system call or command
SIGINT Ctrl-C
SIGTERM kill
SIGKILL kill -9
SIGHUP kill -HUP
(various levels of “die”)
SIGHUP is often interpreted as “reload config”, e.g. by nginx.
Every signal has a default action, which is one of:
- ignore
- kill process
- kill process AND make core dump file
- stop process
- resume process
Your program can set Custom handlers for almost any signal
person: SIGTERM (terminate)
process: okay! I’ll (clean up and then exit!
exceptions:
SIGSTOP & SIGKILL can’t be ignored
dead program: got SIGKILLed
Signals can be hard to handle correctly since they can happen at ANY time
process: handling a signal
person: SURPRISE! another signal!