Zombie Processes

• If a child process terminates while its parent is calling a wait function, the child process vanishes and its termination status is passed to its parent via the wait call.
• What happens when a child process terminates and the parent is not calling wait?
• The information about its termination, such as whether it exited normally and, if so, what its exit status is would be lost.
• A zombie process is a process that has terminated but has not been cleaned up yet. It is the responsibility of the parent process to clean up its zombie children.The wait functions do this.
• Suppose that a program forks a child process, performs some other computations, and then calls wait.
  • If the child process has not terminated at that point, the parent process will block in the wait call until the child process finishes.
  • If the child process finishes before the parent process calls wait, the child process becomes a zombie.
• When the parent process calls wait, the zombie child's termination status is extracted, the child process is deleted, and the wait call returns immediately.
• The following program forks a child process, which terminates immediately and then goes to sleep for a minute, without ever cleaning up the child process.

  #include <stdlib.h>
  #include <sys/types.h>
  #include <unistd.h>
  
  int main ()
  {
    pid_t child_pid;
  
    /* Create a child process.  */
    child_pid = fork ();
    if (child_pid > 0) {
      /* This is the parent process.  Sleep for a minute.  */
      sleep (60);
    }
    else {
      /* This is the child process.  Exit immediately.  */
      exit (0);
    }
    return 0;
  }
  

• Run it, and while it's still running, list the processes on the system by invoking the following command in another window:

  $ ps -e -o pid,ppid,stat,cmd
  

• What happens when the main make-zombie program ends when the parent process exits, without ever calling wait? Does the zombie process stay around? The init process automatically cleans up any zombie child processes that it inherits.