[Xenomai] Possible sync problem with timerfd and application

Gilles Chanteperdrix gilles.chanteperdrix at xenomai.org
Sun Dec 20 16:03:33 CET 2015

On Thu, Dec 17, 2015 at 02:07:20PM +0000, Umair Ali wrote:
> Hello there,
> I am using the timerfd to produce the required sleep function for
> sending the raw ethernet packets using rtnet driver periodically.
> I read the file using the mmap() function and
> mlockall(MCL_CURRENT|MCL_FUTURE). My code is attached with this
> email. Gilles have already explained me with the jitter problem
> with timerfd but it is acceptable so far with me. Now the problem
> is that the code runs ok for more or less 30 secs. For this period
> of running of code the jitter is acceptable. But after some time
> the jitter increases very much and causes problem. I thought that
> this may be because the timerfd has lost the synchronization with
> application.

The "read" call on a timerfd reads the number of timer ticks since
the last read, so you can know whether an overrun occurred by
checking the variable named "exp" in your program.

> Or timerfd is not working fine. I tried to use the
> poll() and epoll() also but the problem persist. Can you guide me

poll() is not a wrapped call, so causes a switch to secondary mode.
Did not you see that with slackspot? Xenomai only wraps the "select"

Also note that by using the AF_INET family, you are no longer
sending raw packets but UDP packets.

And finally I do not know what your ugly memcpy with hardcoded
lengths do exactly, but the check (j < size) looks insufficient, 
as the memcpys are accessing data beyond j. If you receive a signal doing
this, this would certainly cause a switch to secondary mode.


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