fork（）：子进程检查父进程是否已死的方法？

Question

Background

I am writing a shared library in C, dynamically linked with LD_PRELOAD , that is meant to intercept and override network calls from the application preloading it, such as socket() , connect() , recv() , send() , etc.

In the init of the library I launch a thread with pthread_create() . This thread polls some kernel memory, which is mapped to user space.

The library is meant to be generic enough to deal with - but not limited to - networking benchmark applications such as netperf , iperf , sockperf .

My issue

Everything works fine, life is sweet in most of the cases except one. Deamonized applications. For instance, if I launch netserver (the server side of the netperf benchmarking application) as a deamon, ie without the -D parameter, one of the first things the application does is call fork() . On fork, the parent is closed using exit(EXIT_SUCCESS) and the child listens for connections. The fact that the parent exits kills my polling thread.

So what I'd like to achieve is have the child spawn a new polling thread if the parent has gone. I can intercept and override the fork() call but fundamentally, how can I have the child know whether the parent has gone or not? I can't take any assumption because the library has to be generic.

Thanks.

Answer 1

You can poll periodically getppid() function. As soon as it starts to return '1' (the id of init process) - your parent is dead.

Update

excerpt from 'man pthread_create':

The new thread terminates in one of the following ways: ...

• Any of the threads in the process calls exit(3), or the main thread performs a return from main(). This causes the termination of all threads in the process.

So, if your thread is created by the netserver process that calls exit - yes this thread will be terminated

Answer 2

你可以使用ptread_atfork(3)在孩子中启动你的线程。

Answer 3

About a year later, I have the answer to my own question, which was "how does a child process know that its parent is dead?"

Well, the child can ask what the PID of its parent is, using getppid() . If the parent PID is 1, then it means the child process was reparented to the init process. Therefore, the parent has gone.

For instance:

void child_fork(void)
{
    int parent_pid;
    usleep(1);
    parent_pid = (int)getppid();

    if (parent_pid == 1)
        run_my_polling_thread();
}

Note: when a process forks the parent is put to sleep. The child process runs first, then the parent is woken up (for Copy-on-Write optimization, I won't go into details here). The usleep(1) call is here to put the child immediately to sleep so that the parent has time to wake up and terminate. Before the child wakes up it will be reparented to init .

Now, at lib init time I just need to call:

__register_atfork(NULL, NULL, child_fork, NULL)

However, this solution does not cover all cases. For instance, what if the parent has multiple children? Do I really want them all to launch a polling thread?

Right now it's an acceptable solution for me, although it's not ideal.