Julio Merino wrote:

> > The only problem I can see is that you're reading the dynamic state
> > from the kernel.  Once you return to userland, there is no guarantee
> > that the returned value is correct any more.  You can tell if an
> > event occurred, but not infer any precise kernel state.
> 
> Well, in fact this does not matter from the point of view of the mouse
> daemon. All I need to know is when events happen...
>
> Though I think that event's data is quite trustable. Can you explain
> why it may be wrong?

Your example code said this:

                if (poll(pfd, 1, INFTIM) <= 0)
                        warn("failed");
                read(fd, &evt, sizeof(evt));
                /* XXX */
                if (evt.type == WSCONS_EVENT_DISPLAY_SWITCH)
                        printf("New display: %d\n", evt.value);

Which isn't necessarily true.  If the display is changed at /* XXX */
then evt will hold stale information and the printf() statement is
incorrect.

	-- Gregory McGarry <g.mcgarry@ieee.org>