Why doesn't kill -9 work?

From http://pcunix.com/SCOFAQ/scotec6.html#cantkill :


One of the early things people learn about Unix is that a "kill -9" is invincible- that a process must die if you send it a KILL (-9). However, that's not entirely true:

A process can be sleeping in kernel code. Usually that's because of faulty hardware or a badly written driver- or maybe a little of both. A device that isn't set to the interrupt the driver thinks it is can cause this, for example- the driver is waiting for something its never going to get. The process doesn't ignore your signal- it just never gets it.

A zombie process doesn't react to signals because it's not really a process at all- it's just what's left over after it died.

What's supposed to happen is that its parent process was
to issue a "wait()" to collect the information about its exit. If the parent doesn't (programming error or just bad programming), you get a zombie. The zombie will go away if its parent dies- it will be "adopted" by init which will do the wait()- so if you see one hanging about, check its parent; if it is init, it will be gone soon, if not the only recourse is to kill the parent..which you may or may not want to do.

Finally, a process that is being traced (by a debugger, for example) won't react to the KILL either.

See SCO TA 104438 for more details.