Re: [rtl] long user writes to rtl fifos

[Trevor Woolven <trevw@zentropix.com>]

Please note that POSIX Message Queues have been available in RTAI for
the last three months.
Could someone please point me towards the specification from which the
API call 'mq_recv' was taken so that I can update my understanding of
these beasts? 'mq_recv' does not feature in any POSIX spec. I've seen so
far, I use 'mq_receive'. Should it be 'mq_recv_np'? :-)

Best regards

Trevor Woolven.

yodaiken@fsmlabs.com wrote:
> 
> On Mon, Mar 13, 2000 at 10:28:30AM -0500, Paul Koning wrote:
> > I was looking at the rtl_fifo.c code and something struck me...  If
> > you write to /dev/rtf<n>, you can specify any length you like,
> > including one longer than the fifo buffer size.  Fine, the user
> > process stalls in mid-write until the real time thread makes more
> > room, then continues, repeating until the write count has been
> > satisfied.
> >
> > The question: it seems likely that a real time thread will be using
> > the rtf handler mechanism to trigger reading from the fifo.  Normally
> > that's not a problem, but in the case I mentioned the handler isn't
> > invoked until the write is done.  That's consistent... but it means
> > that you may have a deadlock.
> >
> > Even though it's slightly inconsistent, it seems appropriate for this
> > situation to call the handler when the user process is about to be
> > stalled due to lack of room.
> 
> Seems reasonable. Since user fifo writes are streams of bytes ...
> 
> I'd like to fold fifos into the POSIX mq. Fifos would still be files
> on the user side, but would be message queues on the RT side.
> Then if there is a structure, mq_recv on RT side will simply fail
> if a partial message structure is there.
> My idea is to extend so that mq_recv(md,&x,n*msize) can be used to read
> "n" messages. For standard fifos, message size is 1, and things will
> still work the same.
> 
> >
> >       paul
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-- 
Trevor Woolven - Director of Customer Applications Engineering
Zentropix Inc - a Lineo company

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