Re: [Xenomai-help] Shared memory between a Xenomai user task and a standard Linux process?

[Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>]

Bob Feretich wrote:
>   Thank you for the solution. Using Linux shm shared memory should have 
> been obvious. :-[
> 
> I was still thinking of Xenomai user tasks as running in a walled of 
> environment. I've got to remember that when a rt user task is in 
> secondary mode it has full access to Linux facilities.
> 
> When my rt user task starts and ends it is in secondary mode. It can 
> easily shmat() and shmdt() during these start and end periods.

Yes, and a process running Xenomai threads may also run non-Xenomai
threads, they will share the same memory, without needing to resort to
shared memories.

As for the shared memory, you may also use the POSIX interface:
shm_open/ftruncate/mmap/shm_close/shm_unlink

You may find it more convenient.

> 
> As Philippe states, I just need to be careful about the synchronization 
> method I use. The Linux process must never be able to block the rt task. 
> (If the rt task wraps the buffer, we'll just throw away the history data 
> until space becomes available and report a "data overrun" on the next 
> block written.) As I said before, the buffer should be large enough so 
> that never happens anyway.

You can use a FIFO, with a periodic non-rt thread polling one of the
FIFO's pointers. This would look a lot like what rt_printf does (file
src/rtdk/rt_print.c in Xenomai sources).


-- 
					    Gilles.