When I ported this dockapp 20 years ago, it was not uncommon to
peek inside ``struct vmmeter'' and other kernel structures with
kvm_read(3) to obtain various data (e.g. page counters).
These interfaces had always been fragile and potentially unsafe
(due to sgid-to-kmem requirement), so let's switch to sysctl(3)
instead as it provides the same data via documented, stable API.
Address another problem while here: the dockapp tried to update
the window from the signal handler, and no Xlib function can be
safely called within a signal handler*. Do as advised and only
raise a "need update" flag upon receiving a signal, then gather
new data and redraw the window in the main loop. This approach,
however, cannot work without modifying process_events() because
of the blocking nature of XNextEvent(3) -- put another select(2)
call in front so it can relinquish control to the outer loop.
[*] http://www-h.eng.cam.ac.uk/help/tpl/graphics/X/signals.html
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