AFAIK, there's also a signal that gets sent in event of a power fluctuation. From there, you can use "ispoweron()" to tell the difference between a momentary power blip, and a real outage condition.
You'll only bother to poll when you've got a signal that said there was a potential outage issue, and then only to check to see if the situation has resolved itself (power back on or not). Depending on that knowledge, you can implement a variety of recovery policies (shutdown immediately; wait 10 minutes, then shutdown if the power is still out; etc).
From what I remember, it's not a useless function; albeit rarely very useful, either. AFAIK, it's been implemented on one of the BSD varients (BSDI Unix, perhaps?)
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