Re: [Xenomai-help] some questions about alarms

[Philippe Gerum <rpm@xenomai.org>]

Tobias Marschall wrote:
> Hello,
> 
> my application needs (besides doing some other stuff) to set some output 
> signals on an interface card at specific points in time. I think such a 
> simple thing doesn't deserve its own thread and am considering to use alarms 
> for this.
> 
> Is it correct that the handler of an alarm is executed with the same priority 
> as the thread that created it? Meaning that the alarm execution will be 
> delayed if a higher-priority (primary mode) thread is running at the time the 
> alarm expires?
>

Alarms used in kernel context are run on behalf of the Xenomai timer ISR 
context. In user-space, real-time alarms are currently handled by server 
threads you create, which call the rt_alarm_wait() service to wait for 
the next alarm shot, then process the timeout from their own context. As 
a side-effect, this service boosts the calling thread priority above all 
regular Xenomai threads, i.e. to some kind of pseudo-interrupt level.

> In my case it would be most convenient to be able to program an alarm timer 
> using an absolute time (date). I didn't find such a function in the API 
> documentation (native skin). Is the absense of such a function intended or 
> did simply nobody implement it? Is it possible to implement such a function 
> by just modifying the native skin?
> 

Yes, but I don't think that alarms are the most appropriate feature to 
use in your case. Alarms are basically provided for setting watchdogs 
that would handle timeout conditions.

> But perhaps there is a better way to solve my issue. Is there any "best 
> practice" for my problem? By now I see to possibilities: 1) Use a 
> rt_timer_read in connection with alarms. 2) Spawn a whole thread and use 
> rt_task_sleep_until.
>

Spawning a thread and use rt_task_set_periodic()+rt_task_wait_period() 
would be better here. You would be able to specify an absolute starting 
date, and a fixed period.

> Any Ideas?
> 
> Best regards and thanks in advance,
> Tobias
> 
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-- 

Philippe.