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

KIU Shueng Chuan nixchuan at gmail.com
Mon May 2 03:16:31 CEST 2016


>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
>
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> zeromq-dev at lists.zeromq.org
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>
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