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是否存在没有父进程的进程？

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是否存在没有父进程的进程？

在排名最高的问题答案中：

如果计算机从 0 开始计数，为什么 init 进程的 pid 为 1？

据说，每个进程都有一个 PPID（父进程）。

然而我读到（稍后将提供链接），有很多进程没有父进程。

有人能将这些相互矛盾的陈述放在合理的背景下吗？

答案1

进程总是有一个父进程。然而，当现有进程死亡时，成为新父进程的进程不一定是 PID 1。Linux 是这样做的：

/*
 * When we die, we re-parent all our children, and try to:
 * 1. give them to another thread in our thread group, if such a member exists
 * 2. give it to the first ancestor process which prctl'd itself as a
 *    child_subreaper for its children (like a service manager)
 * 3. give it to the init process (PID 1) in our pid namespace
 */
static struct task_struct *find_new_reaper(struct task_struct *father,
                       struct task_struct *child_reaper)
{
    struct task_struct *thread, *reaper;

    thread = find_alive_thread(father);
    if (thread)
        return thread;

    if (father->signal->has_child_subreaper) {
        unsigned int ns_level = task_pid(father)->level;
        /*
         * Find the first ->is_child_subreaper ancestor in our pid_ns.
         * We can't check reaper != child_reaper to ensure we do not
         * cross the namespaces, the exiting parent could be injected
         * by setns() + fork().
         * We check pid->level, this is slightly more efficient than
         * task_active_pid_ns(reaper) != task_active_pid_ns(father).
         */
        for (reaper = father->real_parent;
             task_pid(reaper)->level == ns_level;
             reaper = reaper->real_parent) {
            if (reaper == &init_task)
                break;
            if (!reaper->signal->is_child_subreaper)
                continue;
            thread = find_alive_thread(reaper);
            if (thread)
                return thread;
        }
    }

    return child_reaper;
}

答案2

当一个进程的Parent死亡时，该进程可以说是“没有父进程”。当这种情况发生时，PPIDPID 被设置为 1 init。

每个进程$STATUS 在退出时都会返回一个值。父进程可以用这个值做一些事情，但它必须存储存储的free内存$STATUS，并释放内核中的进程数据。

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