[zeromq-dev] about using ZMQ_FD in an external select loop

[Justin Karneges]

The ZMQ_FD is a special fd you use to get notified of events on a zmq socket. 
It is not the actual underlying network socket that receives messages. 
Remember a zmq socket is "virtual", and under the hood it may be managing many 
network sockets (or none at all, if you consider the ipc type).

I don't believe ZMQ_FD will indicate readability if a partial message is 
received over the network. At least I don't see a reason why zmq would want to 
bother the app about this. That said, even if ZMQ_FD gets indicated as 
readable more often than necessary, you should simply get the resulting 
ZMQ_EVENTS and act on them.

On Thursday, September 06, 2012 04:24:43 PM diffuser78 wrote:
> I did not frame my question correctly and subject was misleading too. Let
> me put that correctly.
> 
> I am using a ZMQ FD in an external select() loop, i,.e, a ZMQ FD is added
> to the set of fds (regular unix fds) in an external select() loop.
> 
> Consider a situation, for a given message (in a router dealer pattern),
> only a few bytes are received at the receiver end and not the complete
> message, so the external select() loop would wake up the ZMQ FD since it
> can read atleast few bytes, but when we call zmq_getsockopt() API with an
> option of ZMQ_EVENTS to get the list of events on the socket, ZMQ doesn't
> return any FD since no complete message is recvd (only a few bytes have
> been recvd). My question is:
> 
> During the next select() iteration, would external select() wake up ZMQ FD
> again because we did not read the few bytes that were ready to be read on
> that fd OR will it NOT wake up ZMQ FD until new bytes are available for
> reading on this fd? What is ZMQ behavior in this case.
> 
> Any help is greatly appreciated.
> 
> Thanks.
> 
> On Wed, Sep 5, 2012 at 5:41 PM, diffuser78 <diffuser78 at gmail.com> wrote:
> > Hi,
> > 
> > I am using a ZMQ FD in an external select() loop, that is, a ZMQ FD is
> > added to the set of fds (regular unix fds) in an external select loop.
> > 
> > Consider a situation, for a given message (in a router dealer pattern),
> > only a few bytes are received at the receiver end and not the complete
> > message, so the external select() loop would wake up the ZMQ FD since it
> > can read atleast few bytes, but when we do a ZMQ_POLL, ZMQ  doesn't return
> > any FD since no complete message is recvd (only a few bytes have been
> > recvd). My question is:
> > 
> > Next time, would external select() wake up ZMQ FD again because it is
> > level triggered or does ZMQ internally reads those bytes so that external
> > select loop won't wake up ZMQ FD until new set of bytes are received ?
> > 
> > Please let me know your thoughts.
> > 
> > DJ