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Comment by h2odragon | Hacker News Reader

> If the computer isn't on, the value returned by this function is undefined.

Shouldn't it just block until the computer turns back on? Probably interferes with pthreads.

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...

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).

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

Ah, good ole TOCTOU

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

Seems like a reasonable compiler would inline the most likely implementation as 1. So, while technically undefined, it's safe to assume that it will 'return' 1 regardless of the actual on/off state of the computer.

I would be unreasonable. The power state appears to me as highly asynchronous to program execution. I see no explicit guarantees on how the state is retrieved or synchronized, without synchronization the state wont be properly updated. We know that the computer was of at some point, so it starts of as undefined. Given undefined as starting state and the lack of update guarantee we can assume that it will always return an undefined value. This opens up a lot more optimization possibilities than assuming a 1.

I agree on the implementation, but I'm pretty sure if the computer is off you aren't getting a 1.

{return 1;} is clearly a faulty implementation, it's entirely possible that the computer is already turned off by the time the function is executed using power stored in the decoupling capacitors of the CPU.

Since the behavior is undefined when the computer is off this implementation seems correct in your hypothetical as well.

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