38.16 Why You Can't Kill a Zombie

[Processes in your ps output that are in the <exiting> or Z status are called zombies . -JP]

You cannot kill zombies; they are already dead.

"What is a zombie?" I hear you ask. "Why should a dead process stay around?"

Dead processes stick around for two principal reasons. The lesser of these is that they provide a sort of "context" for closing open file descriptors (38.3 ) , and shutting down other resources (memory, swap space, and so forth). This generally happens immediately, and the process remains only for its major purpose: to hold on to its name and exit status (44.7 ) .

A process is named by its process ID or PID. Each process also has associated with it a Parent Process ID . The parent PID is the PID of the process that created it via fork (38.2 ) , or, if that particular process has since vanished, 1 (the PID of init (38.2 ) ). While the original parent is around, it can remember the PIDs of its children. These PIDs cannot be re-used until the parent knows the children are done. The parent can also get a single byte of status (44.7 ) from each child. The wait system call looks for a zombie child, then "collects" it, making its PID available and returning that status. The init (8) program is always waiting, so that once a parent exits, init will collect all its children as they exit, and promptly ignore each status.

So, to get rid of a zombie, you must wait for it. If you have already done so or if the process' PPID is 1, the process is almost certainly stuck in a device driver (42.1 ) close routine, and if it remains that way forever, the driver has a bug.