Re: [Xenomai-help] [RT_PIPE] - User space read() not waking up

[Philippe Gerum <rpm@xenomai.org>]

On Sat, 2007-03-24 at 13:07 +0100, Viktor STARK wrote:
> >   
> That looks like a pretty good solution. I'll be able to test this out on 
> Monday. I'll keep the list posted about my results.

Yeah, well, except that my last post was utter crap actually. I should
really consider having sleep because I don't even remember how my code
works, it seems. Ok, here is a fixed version: to sum up, you need to use
rt_queues to do what you want to do, i.e. exchanging data in primary
mode between real-time tasks. Performance-wise, there is no downside in
using queues instead of pipes anyway, except when rt_pipe_stream() is
actually appropriate for the sending side (i.e. when data is produced
incrementally by small chunks, and the reader has lower priority than
the sender).

What I sketched would not work because rt_pipe_stream/read/write used
from user-space would still feed the real-time endpoint of a pipe, which
could only be read from a non real-time endpoint. IOW, there is no way
to use pipes for RT to RT communication, because data sent using
rt_pipe_write() is only readable by read(), and data wrote by write() is
only readable from rt_pipe_read(). The rt_pipe API is available from
user-space in order to feed regular Linux tasks with real-time data
produced from a user-space context, that's all.

Sorry for the noise. Yes Dmitry, I'm going to take a nap right now, in
order to fix this annoying -ENOBRAIN error that surges from time to
time. Gasp...

-- 
Philippe.