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(libc.info)Orphaned Process Groups

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Orphaned Process Groups
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   When a controlling process terminates, its terminal becomes free and
a new session can be established on it.  (In fact, another user could
log in on the terminal.)  This could cause a problem if any processes
from the old session are still trying to use that terminal.

   To prevent problems, process groups that continue running even after
the session leader has terminated are marked as "orphaned process
groups".

   When a process group becomes an orphan, its processes are sent a
`SIGHUP' signal.  Ordinarily, this causes the processes to terminate.
However, if a program ignores this signal or establishes a handler for
it (Note: Signal Handling), it can continue running as in the orphan
process group even after its controlling process terminates; but it
still cannot access the terminal any more.