[zeromq-dev] What are the “serious caveats” with ZMQ_FD?

Kalle Huttunen khuttun at gmail.com
Mon May 2 10:24:45 CEST 2016


So basically I will just write (I use the C++ bindings):

mySocket.send(myMsg);
mySocket.getsockopt(ZMQ_EVENTS); // Update the file descriptor state

?

-- 
Kalle Huttunen


ma 2. toukokuuta 2016 klo 11.15 Justin Karneges <justin at affinix.com>
kirjoitti:

> "update the state" is strange wording but it means you need to query
> ZMQ_EVENTS.
>
> And yes, you should assume that even a call to zmq_recv() might yield a
> situation where there is more to read, but the fd doesn't become readable.
> Basically, check ZMQ_EVENTS after every call to zmq_send or zmq_recv, and
> whenever the fd becomes readable, regardless of which events you are
> interested in.
>
> On Mon, May 2, 2016, at 01:07 AM, Kalle Huttunen wrote:
>
> Okay, this one I missed because I was looking at an old version of the man
> page (http://api.zeromq.org/master:zmq-getsockopt takes you to 2.2.0 man
> page).
>
> Full quote from the 4.1.5 man page (
> http://api.zeromq.org/4-1:zmq-getsockopt):
>
> "The returned file descriptor is also used internally by the zmq_send and
> zmq_recv functions. As the descriptor is edge triggered, applications must
> update the state of ZMQ_EVENTS after each invocation of zmq_send or
> zmq_recv.To be more explicit: after calling zmq_send the socket may become
> readable (and vice versa) without triggering a read event on the file
> descriptor."
>
> This definitely sounds like a caveat I should handle. The quote raises
> some questions, though:
>
> - What exactly does it mean to "update the state of ZMQ_EVENTS"?
>
> - Do both zmq_send() and zmq_recv() reset all events in the FD? So if I'm
> only interested on events about incoming messages (I check only ZMQ_POLLIN
> from ZMQ_EVENTS), do I need to "update the state of ZMQ_EVENTS" after
> both zmq_send() and zmq_recv(), or is it enough to do it only after
> zmq_send()?
>
> --
> Kalle Huttunen
>
> ma 2. toukokuuta 2016 klo 4.16 KIU Shueng Chuan <nixchuan at gmail.com>
> kirjoitti:
>
> From the man page for zmq_getsockopt:
>
> "after calling zmq_send the socket may become readable (and vice versa)
> without triggering a read event on the file descriptor."
>
> On 1 May 2016 3:33 am, "Kalle Huttunen" <khuttun at gmail.com> wrote:
>
> The ZeroMQ FAQ (http://zeromq.org/area:faq#toc5) states in the "Why can't
> I use standard I/O multiplexing functions such as select() or poll() on
> ZeroMQ sockets?" question:
>
> > Note that there's a way to retrieve a file descriptor from ZeroMQ socket
> (ZMQ_FD socket option) that you can poll on from version 2.1 onwards,
> however, there are some serious caveats when using it. Check the
> documentation carefully before using this feature.
>
> I've prototyped integrating ZeroMQ socket receiving to Qt's and custom
> select() based event loops, and on the first glance everything seems to
> work.
>
> From the documentation I have identified two "caveats" that I handle in my
> code:
>
> 1. The ability to read from the returned file descriptor does not
> necessarily indicate that messages are available to be read from the socket
>
> This I have solved by checking ZMQ_EVENTS before reading from the socket.
>
> 2. Events are signaled in edge-triggered fashion
>
> This one I have solved by always receiving all the messages from the
> socket when the file descriptor signals.
>
> Are there some caveats that I'm missing?
>
> --
> Kalle Huttunen
>
>
> _______________________________________________
> zeromq-dev mailing list
> zeromq-dev at lists.zeromq.org
> http://lists.zeromq.org/mailman/listinfo/zeromq-dev
>
> _______________________________________________
> zeromq-dev mailing list
> zeromq-dev at lists.zeromq.org
> http://lists.zeromq.org/mailman/listinfo/zeromq-dev
>
> *_______________________________________________*
> zeromq-dev mailing list
> zeromq-dev at lists.zeromq.org
> http://lists.zeromq.org/mailman/listinfo/zeromq-dev
>
>
> _______________________________________________
> zeromq-dev mailing list
> zeromq-dev at lists.zeromq.org
> http://lists.zeromq.org/mailman/listinfo/zeromq-dev
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.zeromq.org/pipermail/zeromq-dev/attachments/20160502/ced9eeb3/attachment.htm>


More information about the zeromq-dev mailing list