Look up all IDDP/XDDP labels in the registry from userspace

Sam Daniel daniel.jacob.samuel at gmail.com
Tue Jan 26 20:44:37 CET 2021

On Tue, Jan 26, 2021 at 10:07 AM Jan Kiszka <jan.kiszka at siemens.com> wrote:

> On 25.01.21 22:07, Sam Daniel via Xenomai wrote:
> > On Mon, Jan 25, 2021 at 12:20 PM Sam Daniel <
> daniel.jacob.samuel at gmail.com>
> > wrote:
> >
> >> Is it possible to look up from userspace code all of the IDDP/XDDP
> labels
> >> that exist in the registry without leaving the primary domain?
> >>
> >> I have tried a few different ways of reading the contents of
> >> /proc/xenomai/registry/rtipc/iddp. The inotify API causes mode switches
> (it
> >> relies on select or poll), not to mention that inotify does not work
> well
> >> with virtual filesystems. And using the C++ filesystem library to read
> the
> >> contents of the directory also causes mode switches (filesystem
> >> interactions).
> >>
> >> Is there a real-time API function that I can use to query these labels?
> >>
> >
> > C++ filesystem library is causing mode switches because of an mmap deep
> in
> > its call stack, not because of "filesystem interactions" like I initially
> > thought.
> >
> What is the use case you need label listing from RT context for?
> Jan
> --
> Siemens AG, T RDA IOT
> Corporate Competence Center Embedded Linux

My use case is an IDDP-based pub/sub messaging system for real-time threads.

Threads publish and subscribe to a channel ("xyz"). Any number of threads
can publish to any channel. And any number of threads can subscribe to any
channel (and each one should receive every message published to that

Subscribers bind labeled IDDP sockets for each channel using labels like
so: <channel name>-<thread name>. If three threads subscribe to channel
"xyz" then I would expect /proc/xenomai/registry/rtipc/iddp to show:
xyz-threadA -> 0
xyz-threadB -> 1
xyz-threadC -> 2

For the pub/sub implementation to work, when thread X calls publish("xyz",
&message) the publish method would need to multicast the message to all
three of the IDDP sockets above.

I would like to look up all labels in the registry from within the publish
method to check if any subscribers have gone away or if any new subscribers
have appeared and then adjust the multicast of the message accordingly.

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