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Comment by eminence32 | Hacker News Reader

Hmm, it seems like you need to make sure the computer is on before calling this function, or else you run the risk of getting back undefined data. Seems like we also need a function that can be used to tell if the computer is on...

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There's a syscall to check whether the computer is on so just call that before this and you'll be fine

That introduces a time-of-check to time-of-use bug (https://en.wikipedia.org/wiki/Time-of-check_to_time-of-use).

Ah, good ole TOCTOU

Aha. If only the API was "Execute a user-provided function in a context where the computer is guaranteed to be on".

Nah. Just bump that responsibility to the caller, and power supply manufacturer. Have the caller call power_on_computer() first always. This will ensure you always get defined state!

...What do you mean not every power supply manufacturer implements the power_on_computer() function as a NOOP if the computer is already on?!

Some implement it that way, some it returns and error message or throws an NMI; then ya got the cheap clones that use it to frob a toggle without regard to the state it was in (which there's no way to read)... Then ya got the weird ones (HP) where it only works right if you call it twice with a 40ms delay between while streaming a sample from "Swan Lake" over their NDA'd 1wire thingy.

I know that at least Solaris implemented SIGPWR signal to inform programs of a power failure: https://community.bmc.com/s/article/Inquira-KA334556

TIL: linux also implements SIGPWR. Using "kill -PWR `pidof dd`" killed it with "power failure".

You need to halt this thinking at once

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