[zeromq-dev] missing events ZMQ_FD / ZMQ_EVENTS

Paul Colomiets paul at colomiets.name
Wed Jun 27 22:19:19 CEST 2012


On Wed, Jun 27, 2012 at 10:57 PM, Justin Karneges <justin at affinix.com> wrote:
> On Wednesday, June 27, 2012 12:44:45 PM Paul Colomiets wrote:
>> On Tue, Jun 26, 2012 at 2:16 AM, Justin Karneges <justin at affinix.com> wrote:
>> > Does this mean that maybe I need to check ZMQ_EVENTS not only after read
>> > indications on the fd, but also after anytime I call zmq_recv() ?
>> I've not tried REP sockets with asynchronous event loop (XREP usually
>> needed). But I'm pretty sure, you're right. You need to recheck
>> ZMQ_EVENTS after doing zmq_recv(), as the state of the socket changes
>> at that time (it's not writable before not because of network issues
>> but because of state machine).
> Yeah I understand the ability to write is part of the state change that occurs
> by reading. I just wonder why the ZMQ_FD isn't triggered internally by
> zmq_recv(). That would have been more intuitive I think.

For performance reasons: its cheaper to call zmq_getsockopt, than to
write to, poll and read from fd.

>> However, checking ZMQ_EVENTS after each zmq_recv and zmq_send is
>> needed anyway, as described in current documentation and in this ML
>> thread.
> In which document is this described? I do not see this in the ZMQ_EVENTS
> section of the zmq_getsockopt man page in 2.2.0.

It's seems it was late for 2.2, but it is in current master both for
2.x and 3.x series.

> In any case, thanks for clarifying. I'd actually gone ahead and changed my
> code to check ZMQ_EVENTS after all three scenarios (post zmq_recv, post
> zmq_send, and upon read indication of the ZMQ_FD), and that managed to get
> things to work properly.



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