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POPHP forking and multiple child signals
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copied!<p>I'm trying to write a script which creates a number of forked child processes using the <a href="http://www.php.net/manual/en/ref.pcntl.php" rel="noreferrer"><code>pcntl_*</code> functions</a>.</p> <p>Basically, there is a single script which runs in a loop for about a minute, periodically polling a database to see if there is a task to be run. If there is one, it should fork and run the task in a separate process so that the parent isn't held up by a long-running task.</p> <p>Since there possibly could be a large number of tasks ready to be run, I want to limit the number of child processes that are created. Therefore, I am keeping track of the number of processes by incrementing a variable each time one is created (and then pausing if there's too many), and then decrementing it in a signal handler. Kind of like this:</p> <pre><code>define(ticks = 1); $openProcesses = 0; // how many we have open $max = 3; // the most we want open at a time pcntl_signal(SIGCHLD, "childFinished"); while (!time_is_up()) { if (there_is_something_to_do()) { $pid = pcntl_fork(); if (!$pid) { // I am the child foo(); // run the long-running task exit(0); // and exit } else { // I am the parent ++$openProcesses; if ($openProcesses >= $max) { pcntl_wait($status); // wait for any child to exit } // before continuing } } else { sleep(3); } } function childFinished($signo) { global $openProcesses; --$openProcesses; } </code></pre> <p>This works pretty much ok most of the time, except for when two or more processes finish simultaneously - the signal handler function is only called once, which throws out my counter. The reason for this is explained by "Anonymous" in the <a href="http://www.php.net/manual/en/function.pcntl-signal.php" rel="noreferrer">notes of the PHP manual</a>:</p> <blockquote> <p>Multiple children return less than the number of children exiting at a given moment SIGCHLD signals is normal behavior for Unix (POSIX) systems. SIGCHLD might be read as "one or more children changed status -- go examine your children and harvest their status values". </p> </blockquote> <p>My question is this: <strong>How do I examine the children and harvest their status?</strong> Is there any reliable way to check how many child processes are open at any given time?</p> <p><sub><em>Using PHP 5.2.9</em></sub></p>

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