4061 Session 14 (3/1)

1. 4061 Session 14 (3/1)

2. Today • Signals • Process Groups and Sessions

3. Today’s Objectives • Define a signal and explain its purpose • Explain what happens in the operating system and in a user process when a signal is generated • Write and read code to raise, catch, block, and ignore signals in C • Understand process groups and sessions • Explain how a shell assigns process groups to a pipeline of processes, and why

4. Admin • Quiz 2: Answers online • Quiz 3: Next Thursday

5. Killing Processes • kill [ -s signal ] pid …

6. What’s Going on Here? • Kill is sending a “signal” to the process • A signal is the operating system telling a process to stop whatever it is doing and deal with a message • Thus, signals are asynchronous with the process • By default, the process terminates • But we can override the default behavior

7. What are Signals Used For? • Used by the OS to notify processes that an event has occurred • This is nice because the process does not have to poll for events

8. Signal Types • There are many signal types • man 7 signal • Signals can be generated interactively with special chars in the terminal • ctrl-c: SIGINT (terminate) • ctrl-z: SIGTSTP (suspend) • Others defined: stty -a • How do these keyboard-generated signals (asynchonously) find their way to the program immediately?

9. Signal Vocabulary • A signal is generated when the event causing the signal occurs • A signal is delivered when the process actually takes action • Note: there can be real time between generation and delivery. (This gap is known as the signal’s lifetime) • A process can catch a signal (by defining a signal handler), block a signal (queue the signal for later delivery), or ignore a signal (ignore the signal permanently).

10. Defaults • If we don’t catch a signal, a default action takes place • abort • abort w/ core dump • stop • ignore • Override the defaults by catching, blocking, or ignoring particular signals • Except SIGKILL and SIGSTOP: defaults cannot be overridden

11. Sending Signals • We’ve seen how to do it from the command line: kill • Exclude the “SIG” prefix for the signal name • E.g. kill -KILL 7236 • In C: • int kill(pid_t pid, int signal) • pid > 0: specified process id • pid == 0: current process group • pid == -1: all procs except init • pid < -1: process group -pid

12. Raising Signals • Sending a signal to the current process is called “raising a signal” int raise(int signal); • Just like: kill(getpid(), signal); • In a single-threaded program, raising a signal is synchronous • If it’s not blocked or ignored, delivered before next instruction

13. Blocking Signals • Blocked signals remain pending until unblocked • Note: we can only remember last signal • A pending signal is immediately delivered when unblocked • int sigprocmask(int how, sigset_t *set, sigset_t *oldset) • How is: • SIG_BLOCK • SIG_UNBLOCK • SIG_SETMASK

14. Signal Sets • Manipulating signal sets • sigaddset • sigdelset • sigemptyset • sigfillset • sigismember

15. Handling Signals • The easy way to do it is with “signal”sighandler_t signal(int signum, sighandler_t handler); • But this way has some serious problems • So, before we get to the more complex (and better) way, some more structure

16. sigaction int sigaction(int signum, const struct sigaction *act, struct sigaction *oldact); struct sigaction { void (*sa_handler)(int);void (*sa_sigaction)(int, siginfo_t *, void *);sigset_t sa_mask; int sa_flags; void (*sa_restorer)(void); }

17. Waiting for a Signal • pause, sigsuspend, and sigwait

18. System Shutdown • First send a TERM signal to all processes, wait for a grace period, then send a KILL signal. • KILL is a signal that cannot be handled

19. Signals • More information: http://users.actcom.co.il/~choo/lupg/tutorials/signals/signals-programming.html#sending

20. Process Groups • Used for the distribution of signals • A process is always a member of a single process group • A process group ID is the same as the PID of the group’s initial member • Thus, they share a “name space” with PIDs • When created, each process is placed into the process group of its parent

21. Sessions • A set of one or more process groups associated with a terminal • Uses: • Group user terminal sessions together • Isolate daemons • setsid • A process can only join process groups in its own session

22. Shells and Process Groups • Shells, though, create new process groups, placing related processes (e.g. those connected with a pipe) into the same group • Steps (for first process in pipeline): • Fork (initially has same PG ID as the shell) • Child issues setpgid() to set its PG ID = PID, creating a new process group. Child is now a “process leader” • Redirect I/O to pipe • Exec • Subsequent processes in pipeline: • Same, but children set their PG ID to that of the first child

23. Some Shell Tricks • What if the parent creates the second child in the pipeline before the first child becomes the session leader? • What if the first child exits before the second child issues setpgid?

24. Process Groups • Read Robbins 11.5 • Additional Reading • Process Groups in BSD: http://www.informit.com/articles/article.asp?p=366888&seqNum=8&rl=1