对分叉的孩子调用rand/mt_rand会产生相同的结果 [英] Calling rand/mt_rand on forked children yields identical results

问题描述

我正在编写一个脚本，该脚本需要在PHP中执行并发任务.

I'm writing a script that needs to execute concurrent tasks in PHP.

我进行了一些测试，并遇到了奇怪的结果.我正在使用pcntl_fork生成一个孩子.父进程什么也没做，只是等待子进程完成.

I ran a little test and ran into strange result. I'm using pcntl_fork to generate a child. The parent process does nothing but wait for the child to complete.

我要生5个孩子，每个孩子都运行一个函数，该函数生成一个随机数(秒)，并睡眠那么长时间.由于某些原因-所有孩子都会产生相同的数字.

I'm generating 5 children, each child runs a function that generates a random number (of seconds) and sleeps for that long. For some reason - all children generate the same number.

这是一个代码示例:

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;
}

示例输出:

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

预先感谢

推荐答案

这是因为所有子级都以相同的状态开始(fork()复制了代码段和数据段).而且由于rand和mt_rand是伪随机生成器，因此它们都将生成相同的序列.

That is because all children start with the same state (fork() duplicates the code and data segments). And since rand and mt_rand are pseudorandom generators, they will all generate the same sequence.

您将不得不重新初始化随机生成器，例如，使用进程/线程ID或从/dev/urandom中读取一些字节.

You will have to re-initialize the random generator, for example with the process/thread ID or read a few bytes from /dev/urandom.

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