Disconnecting ZeroMQ socket and ZMQ_EVENT_DISCONNECTED

Question

The ZeroMQ API documentation states that the event (source here):

ZMQ_EVENT_DISCONNECTED
The socket was disconnected unexpectedly. The event value is the FD of the underlying network socket. Warning: this socket will be closed.

How is "unexpected" defined here, and how would it be possible to trigger a "clean" disconnect on, say, a REQ-REP pair?

I've tried:

• zmq_close(client) signals ZMQ_EVENT_DISCONNECTED, although I would have expected this to be clean
• zmq_disconnect(client, addr) before the above does not change the monitored events (i.e., I still get a signal for an "unexpected disconnect" when the explicit disconnect is successful)

Is there a way to distinguish a clean socket closing from, say, a kill -9 on the client? (the latter still signals FIN, so this should theoretically be possible to detect and distinguish)

Code for illustration below (modified from here; compile and run with gcc -lzmq zmq_test.c && ./a.out)

#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <assert.h>
#include <zmq.h>

// Read one event off the monitor socket; return value and address
// by reference, if not null, and event number by value. Returns -1
// in case of error.
static int get_monitor_event (void *monitor, int *value, char **address)
{
    // First frame in message contains event number and value
    zmq_msg_t msg;
    zmq_msg_init (&msg);
    if (zmq_msg_recv (&msg, monitor, 0) == -1)
        return -1; // Interruped, presumably
    assert (zmq_msg_more (&msg));

    uint8_t *data = (uint8_t *) zmq_msg_data (&msg);
    uint16_t event = *(uint16_t *) (data);
    if (value)
        *value = *(uint32_t *) (data + 2);

    // Second frame in message contains event address
    zmq_msg_init (&msg);
    if (zmq_msg_recv (&msg, monitor, 0) == -1)
        return -1; // Interruped, presumably
    assert (!zmq_msg_more (&msg));

    if (address) {
        uint8_t *data = (uint8_t *) zmq_msg_data (&msg);
        size_t size = zmq_msg_size (&msg);
        *address = (char *) malloc (size + 1);
        memcpy (*address, data, size);
        *address [size] = 0;
    }
    printf("event: %d\n", event);
    return event;
}

static void close_zero_linger(void *socket)
{
    int linger = 0;
    zmq_setsockopt(socket, ZMQ_LINGER, &linger, sizeof(linger));
    zmq_close(socket);
}

static void bounce(void *client, void *server)
{
    char send_buf[] = "Hello";
    char recv_buf[sizeof(send_buf) / sizeof(send_buf[0])];

    zmq_send(client, send_buf, sizeof(send_buf), 0);
    zmq_recv(server, recv_buf, sizeof(recv_buf), 0);
    zmq_send(server, send_buf, sizeof(send_buf), 0);
    zmq_recv(client, recv_buf, sizeof(recv_buf), 0);
}

int main (void)
{
    void *ctx = zmq_ctx_new ();
    void *client = zmq_socket (ctx, ZMQ_REQ);
    void *server = zmq_socket (ctx, ZMQ_REP);
    int rc = zmq_socket_monitor (client, "tcp://127.0.0.1:9999", 0);

    // Monitor all events on client and server sockets
    rc = zmq_socket_monitor (server, "inproc://monitor-server", ZMQ_EVENT_ALL)
    void *server_mon = zmq_socket (ctx, ZMQ_PAIR);
    rc = zmq_connect (server_mon, "inproc://monitor-server");

    // Now do a basic ping test
    rc = zmq_bind (server, "tcp://127.0.0.1:9998");
    rc = zmq_connect (client, "tcp://127.0.0.1:9998");
    bounce (client, server);

//  Close client
    rc = zmq_disconnect(client, "tcp://127.0.0.1:9998");
    printf("disconnect: %d\n", rc);
//    close_zero_linger (client);
    rc = zmq_close(client);
    printf("close: %d\n", rc);
//   sleep(1); // no difference

    // This is the flow of server events
    event = get_monitor_event (server_mon, NULL, NULL);
    assert (event == ZMQ_EVENT_LISTENING);
    event = get_monitor_event (server_mon, NULL, NULL);
    assert (event == ZMQ_EVENT_ACCEPTED);
    event = get_monitor_event (server_mon, NULL, NULL);
    assert (event == ZMQ_EVENT_HANDSHAKE_SUCCEEDED);
//   sleep(1); // no difference
    event = get_monitor_event (server_mon, NULL, NULL);
//    assert (event == ZMQ_EVENT_CLOSED);
//    event = get_monitor_event (server_mon, NULL, NULL);
//    assert (event == ZMQ_EVENT_MONITOR_STOPPED);
    assert (event == ZMQ_EVENT_DISCONNECTED); // why??

    // Close down the sockets
    close_zero_linger (server);
    close_zero_linger (server_mon);
    zmq_ctx_term (ctx);

    return 0;
}