Zombie Process code in Linux operating system

#include <unistd.h>
#include <stdio.h>

int main() {
  int ret;
  printf("welcome..pid=%d\n", getpid());
  ret = fork();
  if (ret < 0) {
    perror("fork");
    exit(1);
  }
  if (ret == 0) {
    printf("child--welcome,pid=%d,ppid=%d\n", getpid(), getppid());
    exit(0);
  } else // ret>0
  {
    printf("parent--hello,pid=%d,ppid=%d\n", getpid(), getppid());
    printf("enter any char to terminate\n");
    getchar(); // sleep(5);
    // no waitpid, parent is not cleaning up child's PCB
  }
  return 0;
}

 
Notes

• A process got terminated, but not cleaned up by parent (no waitpid call) is known as zombie process (in zombie state)
• Typically exit will release all resources, except PCB/PD and entry in process table
• waitpid by parent is expected to clear these
• Too many zombie process causes process table to be full and failure of future fork request
• It’s adviced that parent should use waitpid, even blocking & exit status collection not required

 
Observations & Understanding

• Observing child in zombie state , ps command output (Symbol Z, defunct)
• Two phase termination of any process
  • Running to Zombie, release all resources by exit system calls, except PCB and entry in table
  • Zombie to Terminated, clean up PCB & process table entry by parent using waitpid
• What’ll happen to Zombie processes when parent terminates
• Problems with too many Zombie processes when parent doesn’t terminate for sometime.
• How to avoid zombies — use waitpid by parent even if not intrested in exit status of child