Re: [rtl] RT FIFOs and select system call.

[Michael Barabanov <baraban@fsmlabs.com>]

Hi Stuart,

There is no code in RTLinux's rtl_fifo.c to support FIONREAD
(can be easily added though). So I don't see how it can work at all.

Michael.

Stuart Warren (SWarren@btr-auto.aust.com) wrote:
> Hi Laurent,
> 
> I noticed the other day that if I did a (rtl 3.0 pre6d)
> 
> 	ioctl(fifo_fd, FIONREAD, unread);
> 
> it would work okay until I emptied the fifo but after that (and after
> some more data accumulated in the fifo), it would continue to return 0.
> 
> I haven't looked through the fifo code but it sounds similar to what you
> are experiencing with RAT.
> 
> cheers,
> 
> Stuart Warren
> 
> Laurent.Kersten@alcatel.be wrote:
> > 
> > Hello,
> > 
> > I've found a "undocumented feature" in the RTAI fifos  (new fifos (both version 0.9a and 1.5)). We are unable to perform a select system call from a user-space program (actually this is a TCL script that use the filevent TCL feature, but if you look
> > through strace you can actually see that this is a call to select with a filedescriptor corresponding to the FIFO). Although the real-time process perform a write on the FIFO, the user-space process doesn't wake-up. We observed that it sleeps on the select
> > system call.
> > 
> > The only work-around we had, is to have a separate user-space process that does a blocking read on the RT-FIFO and then we something is written, reads the RT-FIFO and rewrite the information on named pipe on which the script is now sleeping. This works
> > but, it's not very elegant and it wastes CPU power.
> > 
> > Thanks for your help.
> > 
> > Laurent Kersten
> >