Intro to Signals

Objectives

1. Learn the basics of signals.

Notes

• This is chapter 20.
  • I know Michael wanted chapter 24, but it relies on chapters 20,21,22
  • And it is also needed for the Daemon chapter.
  • And this is good stuff.
• A signal is a notification to a process that an event has occurred
  • Think of it as a software exception.
  • They are delivered by the OS.
• kill is the way a user can ask for a signal to be sent to a process
  • But in general you can only send signals to your process
  • Unless you are root, then you can send a signal to any process.
  • kill -l
    • Signal numbers are not uniform across all UNIX implementations.
    • Signal numbers are not ranked
      • Signal 2 is not stronger than signal 1.
• Signals occur when the problem occurs
  • They can occur at any time.
  • And this leads to all sorts of problems.
  • But more of that later.
• When a signal is sent, it is described as pending.
• Some signals can be blocked, or not received until later.
• When a signal is received it can
  • Ignored: IE thrown away
  • Cause the process to terminate
  • Cause the process to save a core dump file
  • Cause the process to stop
  • Cause the process to resume
• Some signals (like kill) can not be ignored or blocked.
• Section 20.2 provides a list of signals and their cause.
  • 1 hup is a terminal disconnect.
  • 9 kill is a termination signal
  • 18 SIGCONT and 19-20 are for process control (stop, continue)
• This is forced, but here are some programs.
• int kill(int pid, int sig)
  • Delivers signal sig to process pid
  • If you are allow to send that signal to that process.
  • If pid = -1, it sends the signal to every process to which the program can send a signal.
  • If sig = 0, no signal is sent, but permission is checked
    • This is one way to check for process existence.
  • See killer.cpp.
• Declaring a signal handler
  • static void
  • parameters depend on how it is called.
  • just a int signal.
    • int sig - the signal that caused the handler to be called.
    • (optional) siginfo_t * si - a structure holding information about the signal
    • (optional) void * ucontext - information about the context of the signal.
• Installing a signal handler
• int sigaction(int signum, const struct sigaction *_Nullable restrict act, struct sigaction *_Nullable restrict oldact);
  • signum : the signal nummber
  • act - a structure that defines the action to take place
    • restrict says that fields in this can not overlap
    • sa_flags modify the behavior of the signal
      • SA_SIGINFO says give me the three parameter version of the signal handler.
      • SA_NOCLDWAIT and SA_NOCLDSTOP control the behavior of SIGCHILD
        • SA_NOCLDWAIT - don't make my children zombies
        • SA_NOCLDSTOP - don't tell me about the child change of state
      • Many others.
  • oldact is the old action that was registered.
• By default most signals lead to termination when not handled.