Sunday, December 13, 2009

double fork to avoid zombie process

It is a common mistake to fork a child process without calling waitpid() to wait for the termination of the child process. Without a wait() call, the child process will become a zombie process after its termination because its parent process does not cleanup its process information in the system. A zombie process occupies a pid in the system, decrease the available pids in the system. Zombies are mark as "defunct" if you check the process by the "ps" command.

However, sometimes we do not want the parent process to wait for its child process for a long time. There is a way to achieve both "not create zombie process" and "not wait for the child process to its termination", and the way is to do a double fork.

The idea is simple, when a parent process (say A) want to fork a child process to do "something". Process A does not fork a process to do "something" directly. Process A first forks a child process (say B), and process then forks its child process (say C) to do "something" and process B terminates as soon as process C is created. In this way, process A only has to wait for process B for a short time. In the same time, since it has no parent process (process B is dead), the system will "rechild" process C to the init process. The init process calls wait() for its child process, solving the zombie process problem.

The program looks like


void func()
{
pit_t pid1;
pit_t pid2;
int status;

if (pid1 = fork()) {
/* parent process A */
waitpid(pid1, &status, NULL);
} else if (!pit1) {
/* child process B */
if (pid2 = fork()) {
exit(0);
} else if (!pid2) {
/* child process C */
execvp("something");
} else {
/* error */
}
} else {
/* error */
}
}

16 comments:

  1. it's the best explanation I've ever read. i knew i had to use double fork, but i didn't get why. thanx

    ReplyDelete
  2. Thank you for your explanation. I've used this solution in an project and it worked well (also if it has an expensive overhead). If you don't want manage signals, this is the best way i think..

    ReplyDelete
  3. This comment has been removed by the author.

    ReplyDelete
  4. In my case, I catch signal SIGCHLD and then perform wait(). Give you as reference.

    ReplyDelete
  5. good references when I visited to get good information

    Mengobati Asam Urat

    ReplyDelete
  6. Thanks for the explanation. I'll use this solution in my project.

    ReplyDelete
  7. Thank you it works perfectly!

    ReplyDelete
  8. Nice tutorial, but there are some bugs in your code.

    Firstly it should be pid_t.

    But more importantly the if,else if,else statement will never reach else.

    When fork fails you get -1, -1 is true in C. So your error handling does not work.

    Thanks for the article though

    ReplyDelete
  9. Thank you, just what I needed. I added a `signal ( SIGHUP, SIG_IGN ) ;` in the grandchild for good measure.

    ReplyDelete
  10. MH Chen
    Thank you for this explanation!

    ReplyDelete
  11. Best explanation I have found on Internet for this. Thank You.

    ReplyDelete
  12. Why not A just died let the B being adopted by system?

    ReplyDelete