[zeromq-dev] ZMQ_FD with request sockets

[Yi Ding]

On 12/8/2011 6:22 PM, Chuck Remes wrote:
> On Dec 8, 2011, at 6:14 PM, Francois-Xavier Bourlet wrote:
>
>> You don't need to call recv right after any send, but when you want to
>> receive something, you need to call recv before trying to wait for any
>> events. In the case recv failed with a tryagain error, you can then
>> wait for an event.
> This isn't correct. You should be able to call zmq_poll() on the socket and detect any read/write events.
>
> ZMQ_FD is a bit tricky. Even if it returns that there was an event, you *must* check the socket with ZMQ_EVENTS to determine if it was a read or write event. Sometimes it is neither because the library uses that FD to signal some internal things too. That's why it is important to check the event with ZMQ_EVENTS.
>
> Also, as Francois correctly stated earlier in this thread, zmq_poll() is edge triggered. What this means in practice is that once you receive an event on a socket, you *must* process all *messages* on that socket before zmq_poll() will trigger it again.
>
> cr
I guess there is a difference in behavior to how ZMQ_FD works and
zmq_poll.  When using ZMQ_FD I don't even get the signal if I don't call
recv first (the timing doesn't matter, I can send out a reply 10 seconds
later and I still won't get a notification on the ZMQ_FD unless I've
called recv before.

Another thing is that calling recv on a ZMQ_REQ socket that I've already
recv'd from, even if NOBLOCK is enabled, will return EFSM.