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Re: Remove infinite loop UB

From: David Brown <david_at_[hidden]>
Date: Mon, 11 May 2020 09:39:58 +0200
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.

Received on 2020-05-11 02:43:09