ZMQ can communicate with non-ZMQ TCP peers using the ZMQ_STREAM socket type. This is a silly example that uses the CZMQ library to create a non-ZMTP TCP server and a non-ZMTP TCP client. It was created to investigate how one might make use of the ZMQ_STREAM socket type.

czmq-stream-client.c

//
//  A minimal TCP client using the ZMQ API. This demonstrates a
//  useful mechanism for bridging a ZMQ based system out to other
//  TCP based systems.
//

#include "czmq.h"


int main (void)
{
    zctx_t *ctx = zctx_new();
    void *sock = zsocket_new(ctx, ZMQ_STREAM);
    int rc = zsocket_connect(sock, "tcp://127.0.0.1:5555");
    if (rc != 0)
        printf ("%s", zmq_strerror(zmq_errno()));
    assert(rc == 0);

    // Retrieve the socket identity. This is important in this
    // particular scenario because when sending data using a
    // 'ZMQ_STREAM' socket the implementation uses the first
    // frame as the *identity* to route the message to. The ZMQ
    // implementation strips off this first frame before sending
    // the data to the endpoint.
    //char *identity = zsocket_identity (sock);

    // Must currently resort to the libzmq low-level lib to obtain
    // raw identity information because CZMQ is returning the binary
    // identity data as a char* and often there is 0 in the data which
    // prematurely terminates the char* data.
    uint8_t id [256];
    size_t id_size = 256;
    rc = zmq_getsockopt (sock, ZMQ_IDENTITY, id, &id_size);
    assert (rc == 0);

    zframe_t *frame;
    while (!zctx_interrupted) {

        zmsg_t *msg = zmsg_new ();
        // Supply ZMQ with the identity to route the message to.
        zmsg_addmem (msg, id, id_size);
        // add message data.
        zmsg_addstr (msg, "Hello");
        zmsg_send(&msg, sock);


        // When receiving TCP data, a 'ZMQ_STREAM' socket shall 
        // prepend a frame containing the *identity* of the 
        // originating peer to the message before passing it to 
        // the application. Messages received are fair-queued from 
        // among all connected peers.
        //
        // So in a multi-connection environment our simple
        // assumption that we will receive a reply from the
        // endpoint we just sent a message to is naive but will
        // suffice for this simple test.

        // Read off the *identity* first
        frame = zframe_recv (sock);
        if (!frame)
            break; // interrupted
        zframe_destroy (&frame);

        // Now read off the message.
        char *response = zstr_recv (sock);
        printf ("Response: %s\n", response);
        free (response);

        // crude delay between consequtive requests
        zclock_sleep (2000);
    }

    zctx_destroy (&ctx);
    return 0;
}

czmq-stream-server.c

//
//  Minimal TCP server using 0MQ
//

#include "czmq.h"


int main (void)
{
    zctx_t *ctx = zctx_new();
    void *sock = zsocket_new (ctx, ZMQ_STREAM);

    int rc = zsocket_bind (sock, "tcp://*:5555");
    assert (rc != -1);

    while (!zctx_interrupted) {

        //  Get connection client identity so we can instruct
        // router socket which client to send the response to.
        zframe_t *frame = zframe_recv(sock);
        if (!frame)
            break;       //  Ctrl-C interrupt

        // Read the message
        char *request = zstr_recv (sock);
        printf ("Received: %s\n", request);
        free (request);

        // Send response
        zframe_send (&frame, sock, ZFRAME_MORE + ZFRAME_REUSE);
        zstr_send (sock, "World!");
    }
    zctx_destroy (&ctx);
    return 0;
}

I find the wording "non-ZMTP TCP client" very misleading when you still need to send the ZMQ frame containing the identity. The server doesn't work for example with a netcat client or telnet client that simply sends the text or binary stream.