pcntl_fork() results in defunct parent process - php

Question

So, I have this PHP daemon worker that listens to IPC messages.
Weird thing is that the parent process (result from pcntl_fork) leaves a [php] < defunct> process untill the child process is done but ONLY when the script is started form a cronjob, not directly from command line.
I know < defunct> processes aren't evil, but I can't figure out why it's happening only when running from a cronjob.
Command
/path/to/php/binary/php /path/to/php/file/IpcServer.php
Forking code:
$iParent = posix_getpid();
$iChild = pcntl_fork();
if ($iChild == -1)
throw new Exception("Unable to fork into child process.");
elseif ($iChild)
{
echo "Forking into background [{$iChild}].\n";
Log::d('Killing parent process (' . $iParent. ').');
exit;
}
Output
Forking into background [20835].
Killing parent process (20834).
ps aux | grep php
root 20834 0.0 0.0 0 0 ? Zs 14:28 0:00 [php] <defunct>
root 20835 0.0 0.2 275620 8064 ? Ss 15:35 0:00 /path/to/php/binary/php /path/to/php/file/IpcServer.php
I've found out that it's a known apache bug, but then why do I get this bug when running from cronjob?

Answer 1

In PHP the child will become a zombie process unless the parent waits for it to return with a pcntl_wait() or pcntl_waitpid(). The zombies will be destroyed once all processes have ended or are handled. It looks like the parent will become a zombie too if children aren't handled and a child runs longer than the parent.
Example from pcntl_fork page:
$pid = pcntl_fork();
if ($pid == -1) {
die('could not fork');
} else if ($pid) {
// we are the parent
pcntl_wait($status); //Protect against Zombie children
} else {
// we are the child
}
Or use signal handling like so to prevent waiting on the main thread:
$processes = array(); // List of running processes
$signal_queue = array(); // List of signals for main thread read
// register signal handler
pcntl_signal(SIGCHLD, 'childSignalHandler');
// fork. Can loop around this for lots of children too.
switch ($pid = pcntl_fork()) {
case -1: // FAILED
break;
case 0: // CHILD
break;
default: // PARENT
// ID the process. Auto Increment or whatever unique thing you want
$processes[$pid] = $someID;
if(isset($signal_queue[$pid])){
childSignalHandler(SIGCHLD, $pid, $signal_queue[$pid]);
unset($signal_queue[$pid]);
}
break;
}
function childSignalHandler($signo, $pid=null, $status=null){
global $processes, $signal_queue;
// If no pid is provided, Let's wait to figure out which child process ended
if(!$pid){
$pid = pcntl_waitpid(-1, $status, WNOHANG);
}
// Get all exited children
while($pid > 0){
if($pid && isset($processes[$pid])){
// I don't care about exit status right now.
// $exitCode = pcntl_wexitstatus($status);
// if($exitCode != 0){
// echo "$pid exited with status ".$exitCode."\n";
// }
// Process is finished, so remove it from the list.
unset($processes[$pid]);
}
else if($pid){
// Job finished before the parent process could record it as launched.
// Store it to handle when the parent process is ready
$signal_queue[$pid] = $status;
}
$pid = pcntl_waitpid(-1, $status, WNOHANG);
}
return true;
}