ZMQ C++ Req to Router issues

Question

I am using ZeroMQ for my network layer and so far everything works except when it comes to ROUTER sockets. In particular I receive the expected message on the ROUTER but when I try send an answer back to my REQ socket the message is never received.

Here is a relatively simple test I wrote which tries to send a "HELLO" message to the ROUTER and expects a message to come back.

here the client code :

    try
    {
        zmq::context_t myContext;

        zmq::socket_t reqSocket(myContext, ZMQ_REQ);
        reqSocket.setsockopt(ZMQ_IDENTITY, "REQ", 3);
        reqSocket.connect(gpRouterAddress);

        //request delimiter
        zmq::message_t zmqMsgReqDelimiter(1);
        memcpy ((void *) zmqMsgReqDelimiter.data(), "\0", 1);
        reqSocket.send(zmqMsgReqDelimiter, ZMQ_SNDMORE);

        //some message
        zmq::message_t reqMsg(5);
        memcpy ((void *) reqMsg.data(), "HELLO", 5);
        reqSocket.send(reqMsg, 0);

        int rcvMore = 0;
        size_t sizeInt = sizeof(int);
        bool bRcvMore = true;
        std::vector<std::string> history;

        while(bRcvMore)
        {
            zmq::message_t zmqMsg;

            reqSocket.recv(&zmqMsg, rcvMore);
            const char* pMsgStr = static_cast<char*>(zmqMsg.data());
            history.push_back(pMsgStr);

            reqSocket.getsockopt(ZMQ_RCVMORE, &rcvMore, &sizeInt);

            bRcvMore = (rcvMore == 1);
        }
    }
    catch (zmq::error_t error)
    {
        std::string errorStr = error.what();
    }

and here is my Router code (can run in a different thread , in which case theContext would be the same as "myContext" from the code above) or a entirely different application :

    try
    {
        zmq::context_t theContext;

        zmq::socket_t router (theContext, ZMQ_ROUTER);
        int value = 1;
        router.setsockopt(ZMQ_ROUTER_MANDATORY, &value, sizeof(int));
        router.setsockopt(ZMQ_IDENTITY, "ROUT", 4);
        router.bind(gpRouterAddress);

        zmq::message_t zmqMsgInternalAddress;
        router.recv(&zmqMsgInternalAddress, 0);
        const char* pAddressStr = static_cast<char*>(zmqMsgInternalAddress.data());

        zmq::message_t zmqMsgDelimiter;
        router.recv(&zmqMsgDelimiter, ZMQ_RCVMORE);
        const char* pDelimiterStr = static_cast<char*>(zmqMsgDelimiter.data());

        int rcvMore = 0;
        size_t sizeInt = sizeof(int);
        bool bRcvMore = true;

        router.getsockopt(ZMQ_RCVMORE, &rcvMore, &sizeInt);
        bRcvMore = (rcvMore == 1);

        std::vector<std::string> history;

        while(bRcvMore)
        {
            zmq::message_t zmqMsg;

            router.recv(&zmqMsg, rcvMore);
            const char* pMsgStr = static_cast<char*>(zmqMsg.data());
            history.push_back(pMsgStr);

            router.getsockopt(ZMQ_RCVMORE, &rcvMore, &sizeInt);

            bRcvMore = (rcvMore == 1);
        }


        //reply address

        size_t len = strlen(pAddressStr) - 1; //if I don't subtract 1 char here, an exception will be raised 

        zmq::message_t replyAddress(len);
        memcpy ((void *) replyAddress.data(), pAddressStr, len);
        router.send(replyAddress, ZMQ_SNDMORE);

        //reply delimiter
        zmq::message_t zmqMsgReplyDelimiter(1);
        memcpy ((void *) zmqMsgReplyDelimiter.data(), "\0", 1);
        router.send(zmqMsgReplyDelimiter, ZMQ_SNDMORE);

        //some message
        zmq::message_t replyMsg(5);
        memcpy ((void *) replyMsg.data(), "WORLD", 5);
        router.send(replyMsg, 0);

    }
    catch (zmq::error_t error)
    {
        std::string errorStr = error.what();
    }

I receive the "HELLO" message on the router and I can step through the ROUTER send and everything seems to be sent okay (i.e. no exception is being raised) but I never receive a message on the REQ socket which will keep waiting indefinitely.

According to the ZeroMQ Guide I should expect the ROUTER to receive the following messages :

The REQ socket sends

empty
HELLO

the ROUTER receives

REQ
empty
HELLO

but I receive

REQ
some binary message
empty
HELLO

and I send

REQ
empty
WORLD

which I would expect to arrive at REQ as

empty
WORLD

If I connect to a REP socket instead (using a simple REQ-REP topology everything works fine).

Can anyone see what I am missing/doing wrong?

Answer 1

I found the issue.

Basically the error was in how I sent the delimiter

    zmq::message_t zmqMsgReplyDelimiter(1);
    memcpy ((void *) zmqMsgReplyDelimiter.data(), "\0", 1);
    router.send(zmqMsgReplyDelimiter, ZMQ_SNDMORE);

it should just be

    zmq::message_t zmqMsgReplyDelimiter(0);
    router.send(zmqMsgReplyDelimiter, ZMQ_SNDMORE);