Zombie process even though threads are still running

Question

Why does Linux consider a process whose main thread has terminated to be a zombie process, and is there any way to avoid this?

In the code below I:

1. Create a process with one main thread
2. Create a new detached thread
3. pthread_exit the main thread
4. pthread_exit the detached thread

Before #3, ps(1) shows my process as a normal process. After #3, however, ps(1) shows my process as a zombie (e.g., 2491 pts/0 00:00:00 thread-app <defunct>) even though it still has running threads.

Is it possible to quit the main thread but avoid going into a zombie state?

Code:

#include <stdio.h>
#include <pthread.h>
#include <unistd.h>
#include <stdlib.h>

void *thread_function(void *args)
{
printf("The is new thread! Sleep 20 seconds...\n");
sleep(20);
printf("Exit from thread\n");
pthread_exit(0);
}

int main(int argc, char **argv)
{
pthread_t thrd;
pthread_attr_t attr;
int res = 0;
res = pthread_attr_init(&attr);
res = pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED);
res = pthread_create(&thrd, &attr, thread_function, NULL);
res = pthread_attr_destroy(&attr);
printf("Main thread. Sleep 5 seconds\n");
sleep(5);
printf("Exit from main process\n");
pthread_exit(0);
}

# ./thread-app

Answer 1

This is a known issue. A fix proposed a while ago by Kaz wasn't accepted by Ulrich Drepper. His comment on this was:

I haven't looked at the patch nor tried it.

If the patch changes the behavior that the main thread, after calling
sys_exit, still react to signals sent to this thread or to the process
as a whole, then the patch is wrong. The userlevel context of the
thread is not usable anymore. It will have run all kinds of
destructors. The current behavior is AFAIK that the main thread won't
react to any signal anymore. That is absolutely required.

Read the mail chain for more discussion on this here:

http://lkml.iu.edu/hypermail/linux/kernel/0902.0/00153.html

Answer 2

The operating-system thinks your process is a zombie because the main thread, which was started by the operating-system, returned (i.e., exited). If you don't want this behavior, then don't have the main thread exit.