Double forking to prevent Zombie process

We have discussed Three methods of Zombie Prevention. This article is about one more method of zombie prevention.

Zombie Process: A process which has finished the execution but still has entry in the process table to report to its parent process is known as a zombie process. A child process always first becomes a zombie before being removed from the process table.

How Creating a Grandchild / Double Forking helps?

  1. The parent calls wait and creates a child. The child creates a grandchild and exits.
  2. The grandchild executes its instruction(task) and eventually it terminates. As the child has already exited, the grandchild will be taken care by init process.
  3. Init collect the exit status of grandchild. Hence the grandchild is not a zombie.

Note: Child is not a zombie as the parent called wait. Also in this case, the parent cannot verify the exit status of grandchild.

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// C program of zombie prevention by
// creating grandchild or double forking
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/wait.h>
  
int main()
{
    pid_t pid;
  
    // fork first time
    pid = fork();
  
    if (pid == 0)
    {
        // double fork
        pid = fork();
        if (pid == 0)
            printf("Grandchild pid : %d\n Child"
               " pid : %d\n", getpid(), getppid());
    }
  
    else
    {
        wait(NULL);
        sleep(10);
    }
}

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