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Subject: Re: [std-proposals] Remove infinite loop UB
From: David Brown (david_at_[hidden])
Date: 2020-05-11 02:39:58


On 09/05/2020 15:18, Tony V E via Std-Proposals wrote:
> The basic idea is a program that looks like:
>
> int main()
> {
>    setupSignalHandlers();
>    for(;;)
>        ;
> }
>
> The real work of the program happens in the signal handlers.
> Or interrupts. Etc.
>
> Nothing happens on the main thread. But it needs to be running for the
> signal handlers to have somewhere to run. 
>
> We could have a special function for this:
>
>    for (;;)
>       std::magic_pretend_im_doing_something();
>
> Not sure how we specify it though.
>

In gcc and clang, you can use:

        asm volatile ("" ::: );

It is "volatile", but does absolutely nothing.

It is standard, of course. But I can't see it being difficult to specify.

However, what you /really/ want in loops like this is something like:

        while (true) sleep();

where "sleep" is highly cpu specific. Some cpu's have a "sleep"
instruction, others have a "wait for events", "halt", "HCF", etc.

This should only be used at the lowest level of the system - in the main
loop of a bare-metal embedded system, or in the idle task of an OS. Any
other task in a system with an OS should block waiting on some event, or
at the very least "yield" its time-slice.

And these sorts of things are all observable events.

I think the only use I have had for a genuinely empty infinite loop is
in handling restarts in an embedded system, where the processor is
twiddling its thumbs waiting for the watchdog to cause a restart.


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