zombie process can't be killed

Question

Is there a way to kill a zombie process? I've tried calling exit to kill the process and even sending SIGINT signal to the process, but it seems that nothing can kill it. I'm programming for Linux.

Sethrida

Oh, I had forgotten to make the obvious joke: the way to kill a zombie is a shoot in the head. – Pachton

When will kill PID fail? – Shoshana

See also What does reaping children imply? – Personate

Answer 1

Zombie processes are already dead, so they cannot be killed, they can only be reaped, which has to be done by their parent process via wait*(). This is usually called the child reaper idiom, in the signal handler for SIGCHLD:

while (wait*(... WNOHANG ...)) {
    ...
}

Pachton

I got to use zombie, dead, killed and child reaper in this answer :) – Pachton

This code is incorrect. If you put that in your SIGCHLD handler, you risk blocking and never returning. – Morose

@R..: you're supposed to use one of the wait() variants that support WNOHANG – Pachton

You can also have the parent process ignore the child status via signal(SIGCHLD, SIG_IGN). The child process goes away (leaves no zombie) as soon as it exits because the parent has registered a lack of interest in the well being of its children. – Phony

Answer 2

Here is a script I created to kill ALL zombie processes. It uses the GDB debugger to attach to the parent process and send a waitpid to kill the zombie process. This will leave the parent live and only slay the zombie.

GDB debugger will need to be installed and you will need to be logged in with permissions to attach to a process. This has been tested on Centos 6.3.

#!/bin/bash
##################################################################
# Script: Zombie Slayer
# Author: Mitch Milner
# Date:   03/13/2013 ---> A good day to slay zombies
#
# Requirements: yum install gdb
#               permissions to attach to the parent process
#
# This script works by using a debugger to
# attach to the parent process and then issuing
# a waitpid to the dead zombie. This will not kill
# the living parent process.
##################################################################

clear
# Wait for user input to proceed, give user a chance to cancel script
echo "***********************************************************"
echo -e "This script will terminate all zombie process."
echo -e "Press [ENTER] to continue or [CTRL] + C to cancel:"
echo "***********************************************************"
read cmd_string
echo -e "\n"

# initialize variables
intcount=0
lastparentid=0

# remove old gdb command file
rm -f /tmp/zombie_slayer.txt

# create the gdb command file
echo "***********************************************************"
echo "Creating command file..."
echo "***********************************************************"
ps -e -o ppid,pid,stat,command | grep Z | sort | while read LINE; do
  intcount=$((intcount+1))
  parentid=`echo $LINE | awk '{print $1}'`
  zombieid=`echo $LINE | awk '{print $2}'`
  verifyzombie=`echo $LINE | awk '{print $3}'`

  # make sure this is a zombie file and we are not getting a Z from
  # the command field of the ps -e -o ppid,pid,stat,command
  if [ "$verifyzombie" == "Z" ]
  then
    if [ "$parentid" != "$lastparentid" ]
    then
      if [ "$lastparentid" != "0" ]
      then
        echo "detach" >> /tmp/zombie_slayer.txt
      fi
    echo "attach $parentid" >> /tmp/zombie_slayer.txt
    fi
    echo "call waitpid ($zombieid,0,0)" >> /tmp/zombie_slayer.txt
    echo "Logging: Parent: $parentid  Zombie: $zombieid"
    lastparentid=$parentid
  fi
done
if [ "$lastparentid" != "0" ]
then
  echo "detach" >> /tmp/zombie_slayer.txt
fi

# Slay the zombies with gdb and the created command file
echo -e "\n\n"
echo "***********************************************************"
echo "Slaying zombie processes..."
echo "***********************************************************"
gdb -batch -x /tmp/zombie_slayer.txt
echo -e "\n\n"
echo "***********************************************************"
echo "Script complete."
echo "***********************************************************"

Enjoy.

Haugen

Didn't run on Ubuntu 14.04: warning: /tmp/zombie_slayer.txt: No such file or directory. – Byproduct

Answer 3

A zombie process is a process id (and associated termination status and resource usage information) that has not yet been waited for by its parent process. The only ways to eliminate it are to get its parent to wait for it (sometimes this can be achieved by sending SIGCHLD to the parent manually if the parent was just buggy and had a race condition where it missed the chance to wait) but usually you're out of luck unless you forcibly terminate the parent.

Edit: Another way, if you're desperate and don't want to kill the parent, is to attach to the parent with gdb and forcibly call waitpid on the zombie child.

Morose

Answer 4

kill -17 ZOMBIE_PID

OR

kill -SIGCHLD ZOMBIE_PID

would possibly work, bu tlike everyone else said, it is waiting for the parent to call wait() so unless the parent dies without reaping, and it got stuck there for some reason you might not want to kill it.

Slackjawed

but i think this is wrong, you might have to send that to the parent, idk – Slackjawed

Answer 5

if I recall correctly, killing the parent of a zombie process will allow the zombie process to die.

use ps faux to get a nice hierarchical tree of your running processes showing parent/child relationships.

Jessiejessika

It actually allows it to be reaped by init. – Pachton

Answer 6

See unix-faqs "How do I get rid of zombie processes that persevere?"

You cannot kill zombies, as they are already dead. But if you have too many zombies then kill parent process or restart service.

You can try to kill zombie process using its pid

kill -9 pid

Please note that kill -9 does not guarantee to kill a zombie process

Attack

Not only does it not guarantee to kill a zombie process. kill -9 can never kill a zombie process. – Morose