How to join a thread which have fork in it?

Question

Following is the case.

{
    ...
    pthread_create(thread_id, NULL, thread_fun, NULL);
    pthread_join(thread_id, NULL);
    ...
}
void * thread_fun(void * arg)
{
    if(fork())
    {
        printf("In Parent\n");
        pthread_exit(NULL);
    }
    else
    {
         printf("In Child\n");
         pthread_exit(NULL);
    }
}

How to use pthread_join() in such a way that I can wait for both child and parent process in thread?

Answer 1

Put all threads into endless loops and then inspect the whole program state with ps, top, htop or similar tools. You will find that the child process only has a single thread, while the parent process has (at least) two.

Now, concerning your question how to use pthread_join() to wait for the child process, you simply can't, because you can only use it to wait for threads in the same process. You could wait for the child process to terminate (waitpid()).

If the above doesn't answer your question (it's a bit unclear because you want to "wait for both child and parent process", which is where I wonder from which process' context) take a step back instead and describe on a higher level what you're trying to achieve instead. In other words, this could be a so-called "XY Problem". Do a bit of research on that term, it's a good thing to learn and understand in any case.