Beej’s Guide to Interprocess Communication

My notes on Beej’s Guide to Interprocess Communication by Beej

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Notes

• Repo to follow along with code examples

Chapter 2. A fork() Primer

• A parent process fork()‘s a child process (the new one) and the child process gets a copy of the parents’ data.
• When a process dies it doesn’t really go away, but instead a small remnant is waiting around for the parent process to pick up. The remnant contains the return value from the child process. So after a parent process fork()s a child process, it must wait() or waitpid() for that child process to exit. This is what allows all remnants of a child to vanish.
• The parent can ignore the SIGCHLD signal and then it won’t have to wait()
  
• If a chile process dies and has not been wait()ed on, it will show up in ps listing as <defunct>
• Oh interesting, so if a the parent dies before it wait()s for the child, the child is reparented to the init process (PID 1).
• Always ensure your parent process either ignores SIGCHLD or wait()s for all child it fork()s.
• wait() waits for whatever child process exists first, use waitpid() if you want to wait for a specific child

Chapter 3. Signals

• A signal is just a way to interrupt another process.
• SIGSTOP is one signal you can send to stop that process and you can continue with SIGCONT.
• Many signals are predefined and the process has a default signal handler to deal with it.
• SIGINT is the interrupt signal that a process receives when the user hits CTRL-C. The default handler causes the process to exit.
• SIGKILL is a signal you can’t add your own handler for. Same with the SIGTSTOP.
• SIGTERM is the default signal that gets sent from the kill command
• SIGUSR1 and SIGUSR2 are not reserved and are free to use for whatever.
• kill() is also the name of the sys call used to sends signals
• raise() is another way to raise a signal within the same process
• sigaction() is used to catch signals
• write() is async signal safe while printf() is not.