Recipe 16.14 Sending a Signal

16.14.1 Problem

You want to send a signal to a process. This could be sent to your own process or to another on the same system. For instance, you caught SIGINT and want to pass it on to your children.

16.14.2 Solution

Use kill to send a signal by name or number to the process IDs listed in the remaining arguments:

kill  9     => $pid;                    # send $pid a signal 9
kill -1     => $pgrp;                   # send whole job a signal 1
kill  USR1  => $$;                      # send myself a SIGUSR1
kill  HUP   => @pids;                   # send a SIGHUP to processes in @pids

16.14.3 Discussion

Perl's kill function is an interface to the syscall of the same name. The first argument is the signal to send, identified by number or by name; subsequent arguments are process IDs to send the signal to. It returns the count of processes successfully signaled. You can only send signals to processes running under the same real or saved UID as your real or effective UID—unless you're the superuser.

If the signal number is negative, Perl interprets remaining arguments as process group IDs and sends that signal to all those groups' processes using the killpg(2) syscall.

A process group is essentially a job. It's how the operating system ties related processes together. For example, when you use your shell to pipe one command into another, you've started two processes, but only one job. When you use Ctrl-C to interrupt the current job or Ctrl-Z to suspend it, this sends the appropriate signals to the entire job, which may be more than one process.

kill can also check whether a process is alive. Sending the special pseudo-signal number 0 checks whether it's legal for you to send a signal to the process—without actually sending one. If it returns true, the process is still alive. If it returns false, the process either has changed its effective UID (in which case $! will be set to EPERM) or no longer exists (and $! is ESRCH). Zombie processes (as described in Recipe 16.19) also report back as ESRCH.

use POSIX qw(:errno_h);

if (kill 0 => $minion) {
    print "$minion is alive!\n";
} elsif ($! =  = EPERM) {             # changed uid
    print "$minion has escaped my control!\n";
} elsif ($! =  = ESRCH) {
    print "$minion is deceased.\n";  # or zombied
} else {
    warn "Odd; I couldn't check on the status of $minion: $!\n";
}

16.14.4 See Also

The "Signals" sections in Chapter 16 of Programming Perl and in perlipc(1); your system's sigaction(2), signal(3), and kill(2) manpages (if you have them); the kill function in Chapter 29 of Programming Perl and perlfunc(1)