EAGAIN should also be thrown

[rafaelroquetto]

Hello,

perhaps I am missing the rationale behind the decision to return 0 (or false) inside recv and send when an EAGAIN is spotted, instead of throwing - and I apologize in advance if this is the case.

The problem I encountered with this approach is that, when returning 0 only (instead of explicitly throwing EAGAIN like the other errors), I cannot tell, specially in the context of recv, whether I am reading a valid message with an empty frame (with size 0), or if this is an actual case of EAGAIN.

Is there a way to tell apart a genuine EAGAIN error versus reading an empty frame of a multipart message? Or should EAGAIN really be thrown?

Thanks,
Rafael

[sigiesec]

EAGAIN is intentionally not thrown, since it is a normal result rather than an error in nonblocking calls.
However, the API might not be designed properly to be (easily) usable. Unfortunately, there are no tests for this. I will try to add some, but for now I only added the basic skeleton for implementing tests with googletest. If you want to contribute some, this would be very welcome.

[dgsteffen]

Hi -- I just ran into the same problem, but the other way around. :-) I misread the documentation and was expecting a return value of -1 if something went wrong. But, I wasn't expecting exceptions to be thrown -- the text about "will throw exceptions" is elsewhere in the documentation.

I think the interface is fine -- return true if you got a message, return false if you didn't, throw if something really went wrong. However, I haven't fiddled with multipart messages, so I don't know if there's something else we need to do there.

However, I'd recommend adding some details to the documentation. For example, under "bool socket_t::recv(message_t *msg, int flags = 0)" I'd put the "maps to zmq_recv" information last, since that's the least important thing. Put "Returns true if..." part first, and add a statement "throws on error condition" there.

I'll submit a patch against the docs if you'll point me to where they are.... :-)

I want to add to this question. I was under the assumption, that recv would throw in nonblocking mode if nothing is in the queue.

try
{
    while (true)
    {

      recv = skt_recv.recv(&zmq_topic,ZMQ_NOBLOCK);
      skt_recv.getsockopt(ZMQ_RCVMORE, &more, &more_size)
      while (more)
      {
           recv3 = skt_recv.recv(&zmq_overtaking_status,ZMQ_NOBLOCK);
           skt_recv.getsockopt(ZMQ_RCVMORE, &more, &more_size);
      }
    }
}
catch(const std::exception &)
{}

I want to always empty the queue so I have the newest message, even if the sender might be faster. However without the throwing how can I decide if its just nothing published or if i emptied the whole queue?