Is the parent process init not a zombie process?

Is the parent process init not a zombie process?

Is the parent process init not a zombie process?
-- Lvyilong316

There was a "Z" (zombie) process found when I was operating the online environment for several times at work. The first thought at that time was to kill the parent process of the process. However, in ps, the parent process of the process is init. This situation is confusing to me, because in general, once the init process takes over the Z state process, it will call wait to recycle it. This is an important way to avoid zombie processes. How can this happen?

Later, I thought about the process of terminating the process and found out the problem. We know that the kernel function do_exit will be called when any process ends. This function has two important tasks to complete:

(1) (as a parent process) find a new parent process for its child process (if any;

(2) notify the parent process (as a sub-process) to "collect dead ";

The first thing is relatively simple. If you want to exit a multi-threaded process, you can entrust the sub-process to your brother thread. If there is no such thread, it is entrusted to the init process;

The second thing is relatively simple for single-threaded processes, but multi-threaded processes will be more complex. Only the main thread in the thread group is eligible to notify the parent process. When other threads in the thread group terminate, the parent process is not notified, and there is no need to reserve resources to enter the botnet state, call the release_task function to release all resources.

Because the parent process only recognizes the main thread of the child process, if the main thread is terminated in the thread group but there are other threads in the thread group, the parent process will not be notified to collect the dead, until the last thread in the thread group exits. In other words: at the user level, you can call pthread_exit to make the main thread "dead" first, but in the kernel state, the task_struct of the main thread must be kept, even if it becomes a zombie.

So when the main thread exits but other threads are still running, the main thread will change to Z state, even if init takes over the main thread.

L test code

 
 

  
  
# Include  
 
  
  
# Include  
 
  
  
# Include  
 
  
  
# Include  
 
  
  
Void thread_fun (void)

  
  
{

  
  
Printf ("begin thread... \ n ");

  
  
Sleep (100 );

  
  
Printf ("end thread... \ n ");

  
  
}

  
  
Int main (int argc, char * argv [])

  
  
{

  
  
Pthread_t tid;

  
  
Int ret;

  
  
Daemon (0, 0); // call deamon to let init take over the main thread

  
  
Printf ("process begin... \ n ");

  
  
Ret = pthread_create (& tid, NULL, (void *) thread_fun, NULL );

  
  
If (ret! = 0)

  
  
{

  
  
Printf ("pthread error \ n ");

  
  
Exit (1 );

  
  
}

  
  
Printf ("main thread is end... \ n ");

  
  
Pthread_exit (0 );

  
  
Printf ("never exce \ n ");

  
  
}
 
 

We create a thread to execute sleep, and then the main thread exits. When we use ps to view the status of the main thread, the results are as follows:

18099 1 Zsl a. out 0.0 0.0

As you can see, although init takes over the main thread, the main thread is still in Z state. There are two ways to make the Z process disappear: one is to kill the entire process, that is, kill 18099, and the other is ps-eLf to find other threads and kill them.