SIGKILL

POSIX signals

• Zombie processes cannot be killed since they are already dead and waiting for their parent processes to reap them.
• Processes that are in the blocked state will not die until they wake up again.
• The init process is special: It does not get signals that it does not want to handle, and thus it can ignore SIGKILL. An exception from this rule is while init is ptraced on Linux.
• An uninterruptibly sleeping process may not terminate (and free its resources) even when sent SIGKILL. This is one of the few cases in which a UNIX system may have to be rebooted to solve a temporary software problem.