Zombie process
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On Unix operating systems, a zombie process or defunct process is a process that has completed execution but still has an entry in the process table, allowing the process that started it to read its exit status. In the term's colorful metaphor, the child process has died but has not yet been reaped. When a process ends, all of the memory and resources associated with it are deallocated so they can be used by other processes. However, the process's entry in the process table remains. The parent is sent a SIGCHLD signal indicating that a child has died; the handler for this signal will typically execute the wait system call, which reads the exit status and removes the zombie. The zombie's process ID and entry in the process table can then be reused. However, if a parent ignores the SIGCHLD, the zombie will be left in the process table. In some situations this may be desirable, for example if the parent creates another child process it ensures that it will not be allocated the same process ID. A zombie process is not the same as an orphan process. Orphan processes don't become zombie processes; instead, they are adopted by init (process ID 1), which waits on its children. The term zombie process derives from the common definition of zombie—an undead person. Zombies can be identified in the output from the Unix ps command by the presence of a "Z" in the STAT column. Zombies that exist for more than a short period of time typically indicate a bug in the parent program. As with other leaks, the presence of a few zombies isn't worrisome in itself, but may indicate a problem that would grow serious under heavier loads. To remove zombies from a system, the SIGCHLD signal can be sent to the parent manually, using the kill command. If the parent process still refuses to reap the zombie, the next step would be to remove the parent process. When a process loses its parent, init becomes its new parent. Init periodically executes the wait system call to reap any zombies with init as parent. Darryl Gregorash wrote:
On 20/10/06 14:47, Mike Noble wrote:
Anders Johansson wrote:
On Fri, 2006-10-20 at 14:45 -0500, Gilberto I. Monroy Lopez wrote:
Hi
Does anybody knows how to kill a zombie process? I know that the only way to kill the zombie process is when you reboot the computer, but if I couldnt do that? what other options does I have?, somebody told me that there is another way and it is sending a signal to the process zombie because it is in a state of wait4(), but I dont know it is true but if it were, how could i do that? because the parent is the init process pid 1..
A zombie is already dead, so it can't be killed, and it is in no state
The parent process is grimly responsible for reaping its dead children (aren't UNIX metaphors fun? :) and if it doesn't, you're stuck with them, but it's nothing really to worry about unless there are so many of them that they fill up the process table (highly unlikely)
The init process will sooner or later get rid of them if it is the parent
This is not true, a zombie process will stay around for ever. A zombie is caused by the parent process dying and leaving the child process still running.
The OP has already said that init is the parent of his zombie, so your statement here is already disproved.
Under Unix (Solaris) and some other Linux versions I have worked with offer the -f or -F flag to kill which would remove zombie processes. OK it worked better in Solaris, Linux was always kinda a crap shoot, which is probably why they removed the flag (if Suse ever had it).
Mike