Re: Events

[Neil Brown]

On  June 27, [e-mail] wrote:
> Alan Cox <[e-mail]> writes:
> 
> > > For example we have a thread, this thread handle the file
> > > IO. When I'm wating for some data I can use the select
> > > function to check it out if there is anything for me. If I
> > > want to stop the thread during the select, I have no other
> > > way except for the signal handlers functions.
> > > It is not really handy, because the example above is too
> > > simple. In our case is much more difficulter.
> > 
> > Signal handling should do the job fine.
> 
> No, that's not right (and would be _really_ happy to stand
> corrected). select and signals for interruption are not
> compatible. You always have a race condition between unblocking the
> signal and actually getting into select. AFAIK POSIX defines the
> pselect call for this problem which also introduces a signal mask
> argument. Without this, signals are useless to interrupt select.
> 
> Greetings
>           Christoph
> 

I hit just such a race in xdm with xdmcp enabled.  Because of the
particular way I was using it (forcibly logging students in labs off by
signalling xdm) I was caught by this race moderately often.

What I did was make the "struct timeval" which was passed to select a
global, and had the signal handler set it to 0,0.  If the signal came
in before the select, the select would exit immediately due to a
timeout.

Works well.

NeilBrown