Re: busyloop in sigchld_handler

[David Kastrup <dak@gnu.org>]

Andreas Schwab <schwab@suse.de> writes:

> David Kastrup <dak@gnu.org> writes:
>
>> Andreas Schwab <schwab@suse.de> writes:
>>
>>> If you don't use WNOHANG you open up a race where several processes may
>>> have their status changed, but only one signal is sent (non-realtime
>>> signals are not queued).
>>
>> How does WNOHANG protect against that?
>
> It makes it possible to loop around without blocking.

Too bad you snipped the details of my question.  It does not make it
possible to loop around without preemption.  In fact, it _forces_
preemption at some point of time (this is the only way to exit the
infinite loop on a single-processor system), and when preemption
happens, of course several signals may be delivered together: after
all, a preempted process is not so likely to get scheduled right
again.

> On system without WNOHANG there needs to be other mechanisms to
> garantee one signal per child (they probably redeliver the signal as
> long as such children exist).

Again: I don't see that this guaranteed one signal per child, and you
did not explain how it could.

-- 
David Kastrup