Re: Signals on Win32 (yet again)

[Andrew Dunstan <andrew(at)dunslane(dot)net>]

Merlin Moncure wrote:

> Magnus Hagander wrote:
>  
>
> > Correct, SleepEx(0) with nothing to do will only incur the cost of a
> > switch to kernel mode and back. If other threads are runnable at the
> > same priority level, they will get scheduled, but no actual sleeping
> >    
> is
>  
>
> > done.
> >    
>
> Is that 100% correct?  The way I read MSDN docs for SleepEx() is that if
> there is at least one other thread running at any priority in the same
> process, the thread that calls SleepEx() gives up the remainder of its
> timeslice to that thread.  This could have performance implications if
> other threads have to be created for some reason.
>  

If we catch the events in the main thread then there is nothing else to 
yield to. If we use a separate thread to catch them then it should call 
WaitForSingleObject instead of SleepEx, ISTM. In any case, this couldn't 
be handled by appropriate setting of thread priorities?

> I think WaitForSingleObject(INFINITE) (in the backend thread) is more
> appropriate.  This will return immediately if the object is signaled but
> unlike SleepEx() does not give up the remainder of its timeslice.  We
> can turn on the event when the backend starts up and leave it on until a
> signal thread turns it off (meaning, the backend must stop and take
> another action).  Also, the signal processing will occur in the main
> thread, not the worker thread.  As soon as the backend enters the signal
> handling routine, it resets the event status to 'on' meaning it can
> accept more signal interrupts.  In the rare case that signals stack up,
> a FIFO stack could hold them (is this allowed in unix? I honestly don't
> know).
>
>  

On Unix IIRC you should not assume that you will get a signal more than 
once. If 2 signals are generated before the receiving process catches 
one, it might only see the signal once. That's why it is common to see a 
loop in a SIGCHLD handler, to catch all the children that might have died.

cheers

andrew