Calling rand / mt_rand on forked children gives the same results
I am writing a script that needs to do parallel tasks in PHP.
I ran a little test and came across a strange result. I am using pcntl_fork to generate a child. The parent process does nothing but wait for the child to finish.
I am generating 5 children, each of which performs a function that generates a random number (in seconds) and sleeps for a long time. For some reason, all children generate the same number.
Here's some sample code:
private $_child_count = 0;
private function _fork_and_exec($func)
{
$cid = ++$this->_child_count;
$pid = pcntl_fork();
if ($pid){ // parent
return $pid;
} else { // child
$func($cid);
//pcntl_waitpid(-1, $status);
exit;
}
}
public function parallel_test()
{
$func = function($id){
echo 'child ' . $id . ' starts'."\n";
$wait_time = mt_rand(1,4);
echo 'sleeping for '.$wait_time."\n";
sleep($wait_time);
echo 'child ' . $id . ' ends'."\n";
};
$children = [];
for ($i=0; $i<5; $i++){
$children[] = $this->_fork_and_exec($func) ."\n";
}
pcntl_wait($status);
echo 'done' ."\n";
exit;
}
Output example:
child 1 starts
sleeping for 1
child 2 starts
sleeping for 1
child 3 starts
sleeping for 1
child 4 starts
sleeping for 1
child 5 starts
sleeping for 1
child 1 ends
child 2 ends
child 3 ends
child 4 ends
child 5 ends
done
early
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This is because all children start from the same state (fork () duplicates code and data segments). And since rand and mt_rand are pseudo-random generators, they will all generate the same sequence.
You will need to reinitialize the random generator, for example with the process / thread ID, or read a few bytes from / dev / urandom.
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I really think you should take a look pthreads
which provides PHP compatible multithreading based on Posix streams.
Just how
class AsyncOperation extends Thread {
public function __construct($arg) {
$this->arg = $arg;
}
public function run() {
if ($this->arg) {
echo 'child ' . $this->arg . ' starts' . "\n";
$wait_time = mt_rand(1, 4);
echo 'sleeping for ' . $wait_time . "\n";
sleep($wait_time);
echo 'child ' . $this->arg . ' ends' . "\n";
}
}
}
$t = microtime(true);
$g = array();
foreach(range("A","D") as $i) {
$g[] = new AsyncOperation($i);
}
foreach ( $g as $t ) {
$t->start();
}
Output
child B starts
sleeping for 3
child B ends
child C starts
sleeping for 3
child C ends
child A starts
sleeping for 4
child A ends
child D starts
sleeping for 4
child D ends
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