Event-driven child termination

So far we've had to lock up our parent script while we wait for our children to finish executing, but we can combine the new-found knowledge of forking with the signals we saw earlier in this chapter to allow our parent process to execute freely until it gets notified that a child by the OS that a child finished.

To accomplish this feat we need to catch a new signal, SIGCHLD, which is sent when one or more child process has terminated. I say "one or more" on purpose: you may well get only one SIGCHLD call for several processes terminating, so you need to use the same pcntl_waitpid() while loop to ensure you catch them all in the same SIGCHLD handler.

So, what we're going to do is install a signal handler for SIGCHLD, use it to call pcntl_waitpid() to print out return information from the child scripts, then let the parent execute freely. Make sense? Good - here's how it looks in PHP:

<?php
    declare(ticks = 1);

    pcntl_signal(SIGCHLD, "signal_handler");

    function signal_handler($signal) {
        switch($signal) {
            case SIGCHLD:
                while (pcntl_waitpid(0, $status) != -1) {
                    $status = pcntl_wexitstatus($status);
                    echo "Child $status completed\n";
                }

                exit;
        }
    }

    for ($i = 1; $i <= 5; ++$i) {
        $pid = pcntl_fork();

        if (!$pid) {
            sleep(1);
            print "In child $i\n";
            exit($i);
        }
    }

    while(1) {
        // parent does processing here...
    }
?>

The nice thing about that script is that it's simply a combination of our signal-handling script and our forking script: the only addition is the new SIGCHLD action. The advantages to forking this way should be clear: the parent process isn't left sitting around while the children are off doing cool stuff; instead, it can carry on processing a work load independently of its children, and gracefully handle child termination when ready. This actually works out as a smart way to write interactive applications: rather than have your users wait for an action to complete, why not fork it and let them continue using the parent, then notify them when the child has finished?

Copyright ©2015 Paul Hudson. Follow me: @twostraws.