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Subject: Re: [std-proposals] Remove infinite loop UB
From: Thiago Macieira (thiago_at_[hidden])
Date: 2020-05-08 21:45:41


On Friday, 8 May 2020 18:02:59 PDT connor horman via Std-Proposals wrote:
> Currently in the C++ language, it is undefined behaviour to have an
> infinite loop without observable side effects. While this is nice on paper,
> it has a few issues. Any time you need to loop forever, you basically need
> to insert a (potentially costly) side effect.

Any time? How often do you need to do that? The only case I can think of is of
an OS that has finished running and doesn't know how to turn the device off.

Can you give an example different from the above?

> A further case would be an init process on linux, which cannot terminate
> (doing so results in a kernel panic), nor can just call hlt in a loop, as
> that's one way to get a #GP which probably translates to SIGILL.

It should call reboot(2). This is not a valid example.

> Once an
> init process is done setting up, it could want to just spin forever, and do
> so using as little system resources as possible. The logical idea would be
> just for(;;) std::this_thread::yield();, but that would be UB as yield() is
> not observable (correct me if I'm wrong).

It can use less resources by calling a function that will sleep forever, like
poll() with no timeout and no file descriptors. In any case, I don't know for
sure, but I think yield() counts as observable side-effect.

> Having the ability to spin forever, without wasting time actually doing
> stuff, seems like a reasonable thing to have in low-level code

Yes, in low-level code. Assembly.

-- 
Thiago Macieira - thiago (AT) macieira.info - thiago (AT) kde.org
   Software Architect - Intel System Software Products

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