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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

SIGKILL kill -9

(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!

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