[Xenomai-help] rt_timer_read() value

[Antoine Nourry <nourry@domain.hid>]

>
> rt_timer_read value should never be decreasing, except maybe from time
> to time when an overflow takes place. A decreasing clock would really be
> something unatural to work with. Overflow may take place more often if you
> cast the return value of rt_timer_read to 32 bits: it is a 64 bits
> value, and I would not expect it to overflow before years of uptime.
> So, there is probably something wrong with your program. Have you
> included native/timer.h ? Do you store the result of rt_timer_read in a
> 64 bits variable ?
yes and no, the worst is that i was focused on the trivial periodic 
example where time is casted to long type. I think i had tried with long 
double, which type do you typically use for this value ?

> The only function which you can call without breaking real-time are
> those documented as such in Xenomai native skin (or posix skin, for that
> matter) documentation. Everything else needs careful inspection.
> Everything that uses Linux system calls, such as for instance, functions
> accessing non real-time drivers such as a keyboard driver, will break
> real-time. The only functions that have a chance not to break real-time
> are those which do not use system calls (and do not call functions which
> use system calls), the function "sin" comes to my mind as an example of
> such a function.
Another thing, as timer is running in parallel to every tasks, would it 
be possible to measure time elapsed from the moment picture appears
to the keypress event ? If i can't be sure to benefit from very low 
latency during all this period, can i rely on this ns-scale timer to 
inform me on how much time elapsed in each functions, in order to deduct 
it from my raw reaction time ? I have another time reaction program but 
its time pace is too high for high accuracy...
How would you resolve this problem and is real-time really the answer ?

Thanks for your answer Sir Gilles,
Antoine