为什么zmq_stream客户端需要recv四次才能获得实际数据？

Question

端口9070是简单的http服务器（没有zmq的tcp）
我有一个问题：

1. 为什么第二次recv为0？它来自哪里？

感谢您的帮助

void test_stream_to_tcp()
{
    uint8_t id [256];
    size_t id_size = 256;
    uint8_t buffer [256];
    void *ctx = zmq_ctx_new ();
    void *client = zmq_socket (ctx, ZMQ_STREAM);
    int rc = 0;
    rc = zmq_connect (client, "tcp://localhost:9070");
    rc = zmq_getsockopt (client, ZMQ_IDENTITY, id, &id_size);
    rc = zmq_send (client, id, id_size, ZMQ_SNDMORE);
    rc = zmq_send (client, "GET /\n\n", 7, 0);
    assert (rc == 7);
    rc = zmq_recv (client, buffer, 256, 0);
    cout <<"first recv "<<  buffer << "rc = "<< rc<<endl;
    rc = zmq_recv (client, buffer, 256, 0);
    cout <<"second recv "<<  buffer<< "rc = "<< rc<<endl;
    rc = zmq_recv (client, buffer, 256, 0);
    cout <<"third recv "<<  buffer<< "rc = "<< rc<<endl;
    rc = zmq_recv (client, buffer, 256, 0);
    buffer[rc] = '\0';
    cout <<"forth  recv "<<  buffer<< "rc = "<< rc<<endl;
}

Answer 1

我在zmq :: stream_engine_t :: plug中得到了它 对于原始套接字，将初始0长度消息发送到应用程序，以便它知道对等方已连接。