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clinuxoperating-systemforkzombie-process

Create zombie process


I am interested in creating a zombie process. To my understanding, zombie process happens when the parent process exits before the children process. However, I tried to recreate the zombie process using the following code:

#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>

int main ()
{
  pid_t child_pid;

  child_pid = fork ();
  if (child_pid > 0) {
    exit(0);
  }
  else {
    sleep(100);
    exit (0);
  }
  return 0;
}

However, this code exits right after execute which is expected. However, as I do

ps aux | grep a.out

I found a.out is just running as a normal process, rather than a zombie process as I expected.

The OS I am using is ubuntu 14.04 64 bit


Solution

  • Quoting:

    To my understanding, zombie process happens when the parent process exits before the children process.

    This is wrong. According to man 2 wait (see NOTES) :

    A child that terminates, but has not been waited for becomes a "zombie".

    So, if you want to create a zombie process, after the fork(2), the child-process should exit(), and the parent-process should sleep() before exiting, giving you time to observe the output of ps(1).

    For instance, you can use the code below instead of yours, and use ps(1) while sleep()ing:

    #include <stdio.h>
    #include <stdlib.h>
    #include <unistd.h>
    #include <sys/types.h>
    #include <sys/wait.h>
    
    int main(void)
    {
        pid_t pid;
        int status;
    
        if ((pid = fork()) < 0) {
            perror("fork");
            exit(1);
        }
    
        /* Child */
        if (pid == 0)
            exit(0);
    
        /* Parent
         * Gives you time to observe the zombie using ps(1) ... */
        sleep(100);
    
        /* ... and after that, parent wait(2)s its child's
         * exit status, and prints a relevant message. */
        pid = wait(&status);
        if (WIFEXITED(status))
            fprintf(stderr, "\n\t[%d]\tProcess %d exited with status %d.\n",
                    (int) getpid(), pid, WEXITSTATUS(status));
    
        return 0;
    }