title: Zero-copy receive and message size
body: <p>Is there a way to query the size of the received message, when using zero-copy for receiving?</p>
  
  <p>With this I (try to) achieve zero-copy:</p>
  
  <pre><code>zmq_recv(sock, buf, sizeof(buf), 0);
  </code></pre>
  
  <p>I also tried:</p>
  
  <pre><code>zmq_msg_t msg;
  zmq_msg_init_data (&amp;msg, buf, sizeof(buf), nullptr, NULL);
  zmq_msg_recv(&amp;msg, sock, 0);
  size_t len = zmq_msg_size(&amp;msg);
  </code></pre>
  
  <p>This returns correct size, but does not fill <code>buf</code>. I think <code>zmq_msg_init_data</code> is not intended for use with <code>zmq_msg_recv</code> and the message rebuilds on receive.</p>