Introduction to Signals

• A signal is used to notify a process that an event has occurred.
• Sometimes called software interrupts
• Processes can, if given permission, send signals to other processes.
  • This is used by the shell to kill background jobs.
  • This can be a primitive form of inter process communications.
• It is normal, however for signals to be generated by the kernel.
  • A hardware exception occurred.
    • An illegal instruction
    • A floating point exception
  • The user typed one of the special characters that generates a signal
    • ctrl-c: interrupt
    • ctrl-z: suspend
  • A software event occurred
    • SIGCHILD, timer, IO change, ...
• Signals are a small, unique integer
  • man 7 signal
  • They are not UNIVERSAL across unix/linux implementations.
  • Some are everywhere (1-9 or so) others are not.
• In general
  • An event causes a signal to be generated
  • It is then delivered to a process
  • A signal is pending between the time it is generated and the time it is delivered.
  • Usually pending signals are acted upon immediately when the process runs next.
  • The process may block some signals.
  • Or the process may handle the signal.
  • Some signals my not be blocked or handled
• By default signals , depending on the signal
  • Cause the program to terminate or exit
  • Cause the program to dump core and exit.
    • on most linux systems core dumps are set to 0 by default.
    • ulimit -c size
    • ulimit -c unlimited
    • ulimit -c 0
  • Cause the program to suspend
  • Cause a suspended program to resume
  • Are ignored
• Default actions can be overridden by a signal handler
  • Signal handlers are installed
  • when a signal handler is called the signal is caught
• Some signals can't be caught, blocked, or ignored
  • SIGSTOP - suspend the process
  • SIGKILL - cause the process to exit
• There is a full list of signals
  • In the book, pages 390 - 395
  • man 7 signal
• Some important signals are
  • SIGHUP (1) - hangup
  • SIGINT - interrupt from keyboard
  • SIGKILL (9) - kills processes dead
  • SIGCHLD - child termination
  • SIGUSR1, SIGUSR2 - reserved for user signals.
• The numbers differ on different architectures.
• The numbers are just numbers, there is no priority.
• Sending a signal to a process (command line)
  • /bin/kill and the bash builtin kill
  • kill signal pid
  • kill -9 12345
  • kill -KILL 12345
  • Best to use -1 first then -9
    • Many processes catch -1 (HUP) and clean up
    • Editors will save the current version in a backup file
    • Others might close log files.
  • In bash %n corresponds to the number assigned to background processes.
• Play with usr.C