Monitoring events are only generated by some transports: At the moment these
are SOCKS, TCP, IPC, and TIPC. Note that it is not an error to call
'zmq_socket_monitor_versioned' on a socket that is connected or bound to some
other transport, as this may not be known at the time
'zmq_socket_monitor_versioned' is called. Also, a socket can be connected/bound
to multiple endpoints using different transports.

ZMQ_EVENT_CONNECTED
~~~~~~~~~~~~~~~~~~~
The socket has successfully connected to a remote peer. The event value
is the file descriptor (FD) of the underlying network socket. Warning:
there is no guarantee that the FD is still valid by the time your code
receives this event.
ZMQ_EVENT_CONNECT_DELAYED
~~~~~~~~~~~~~~~~~~~~~~~~~
A connect request on the socket is pending. The event value is unspecified.
ZMQ_EVENT_CONNECT_RETRIED
~~~~~~~~~~~~~~~~~~~~~~~~~
A connect request failed, and is now being retried. The event value is the
reconnect interval in milliseconds. Note that the reconnect interval is
recalculated for each retry.
ZMQ_EVENT_LISTENING
~~~~~~~~~~~~~~~~~~~
The socket was successfully bound to a network interface. The event value
is the FD of the underlying network socket. Warning: there is no guarantee
that the FD is still valid by the time your code receives this event.
ZMQ_EVENT_BIND_FAILED
~~~~~~~~~~~~~~~~~~~~~
The socket could not bind to a given interface. The event value is the
errno generated by the system bind call.
ZMQ_EVENT_ACCEPTED
~~~~~~~~~~~~~~~~~~
The socket has accepted a connection from a remote peer. The event value is
the FD of the underlying network socket. Warning: there is no guarantee that
the FD is still valid by the time your code receives this event.
ZMQ_EVENT_ACCEPT_FAILED
~~~~~~~~~~~~~~~~~~~~~~~
The socket has rejected a connection from a remote peer. The event value is
the errno generated by the accept call.
ZMQ_EVENT_CLOSED
~~~~~~~~~~~~~~~~
The socket was closed. The event value is the FD of the (now closed) network
socket.
ZMQ_EVENT_CLOSE_FAILED
~~~~~~~~~~~~~~~~~~~~~~
The socket close failed. The event value is the errno returned by the system
call. Note that this event occurs only on IPC transports.
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.
ZMQ_EVENT_MONITOR_STOPPED
~~~~~~~~~~~~~~~~~~~~~~~~~
Monitoring on this socket ended.
ZMQ_EVENT_HANDSHAKE_FAILED_NO_DETAIL
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Unspecified error during handshake.
The event value is an errno.
ZMQ_EVENT_HANDSHAKE_SUCCEEDED
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The ZMTP security mechanism handshake succeeded.
The event value is unspecified.
ZMQ_EVENT_HANDSHAKE_FAILED_PROTOCOL
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The ZMTP security mechanism handshake failed due to some mechanism protocol
error, either between the ZMTP mechanism peers, or between the mechanism
server and the ZAP handler. This indicates a configuration or implementation
error in either peer resp. the ZAP handler.
The event value is one of the ZMQ_PROTOCOL_ERROR_* values:
ZMQ_PROTOCOL_ERROR_ZMTP_UNSPECIFIED
ZMQ_PROTOCOL_ERROR_ZMTP_UNEXPECTED_COMMAND
ZMQ_PROTOCOL_ERROR_ZMTP_INVALID_SEQUENCE
ZMQ_PROTOCOL_ERROR_ZMTP_KEY_EXCHANGE
ZMQ_PROTOCOL_ERROR_ZMTP_MALFORMED_COMMAND_UNSPECIFIED
ZMQ_PROTOCOL_ERROR_ZMTP_MALFORMED_COMMAND_MESSAGE
ZMQ_PROTOCOL_ERROR_ZMTP_MALFORMED_COMMAND_HELLO
ZMQ_PROTOCOL_ERROR_ZMTP_MALFORMED_COMMAND_INITIATE
ZMQ_PROTOCOL_ERROR_ZMTP_MALFORMED_COMMAND_ERROR
ZMQ_PROTOCOL_ERROR_ZMTP_MALFORMED_COMMAND_READY
ZMQ_PROTOCOL_ERROR_ZMTP_MALFORMED_COMMAND_WELCOME
ZMQ_PROTOCOL_ERROR_ZMTP_INVALID_METADATA
ZMQ_PROTOCOL_ERROR_ZMTP_CRYPTOGRAPHIC
ZMQ_PROTOCOL_ERROR_ZMTP_MECHANISM_MISMATCH
ZMQ_PROTOCOL_ERROR_ZAP_UNSPECIFIED
ZMQ_PROTOCOL_ERROR_ZAP_MALFORMED_REPLY
ZMQ_PROTOCOL_ERROR_ZAP_BAD_REQUEST_ID
ZMQ_PROTOCOL_ERROR_ZAP_BAD_VERSION
ZMQ_PROTOCOL_ERROR_ZAP_INVALID_STATUS_CODE
ZMQ_PROTOCOL_ERROR_ZAP_INVALID_METADATA
ZMQ_EVENT_HANDSHAKE_FAILED_AUTH
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The ZMTP security mechanism handshake failed due to an authentication failure.
The event value is the status code returned by the ZAP handler (i.e. 300,
400 or 500).

ZMQ_EVENT_PIPE_STATS
~~~~~~~~~~~~~~~~~~~~
This event provides two values, the number of messages in each of the two
queues associated with the returned endpoint (respectively egress and ingress).
This event only triggers after calling the function
_zmq_socket_monitor_pipes_stats()_.
NOTE: this measurement is asynchronous, so by the time the message is received
the internal state might have already changed.
NOTE: when the monitored socket and the monitor are not used in a poll, the
event might not be delivered until an API has been called on the monitored
socket, like zmq_getsockopt for example (the option is irrelevant).
NOTE: in DRAFT state, not yet available in stable releases.
RETURN VALUE

The 0MQ context associated with the specified socket was terminated.

The transport protocol of the monitor endpoint is not supported. Monitor sockets are required to use the inproc:// transport.

The monitor endpoint supplied does not exist or the specified socket type is not supported.

The socket parameter was not a valid 0MQ socket.

The socket did not have monitoring enabled.

The monitored socket did not have any connections to monitor yet.

//  Read one event off the monitor socket; return values and addresses
//  by reference, if not null, and event number by value. Returns -1
//  in case of error.
static uint64_t
get_monitor_event (void *monitor, uint64_t *value, char **local_address, char **remote_address)
{


    //  First frame in message contains event number


    zmq_msg_t msg;


    zmq_msg_init (&msg);


    if (zmq_msg_recv (&msg, monitor, 0) == -1)


        return -1;              //  Interrupted, presumably


    assert (zmq_msg_more (&msg));


    uint64_t event;


    memcpy (&event, zmq_msg_data (&msg), sizeof (event));


    zmq_msg_close (&msg);


    //  Second frame in message contains the number of values


    zmq_msg_init (&msg);


    if (zmq_msg_recv (&msg, monitor, 0) == -1)


        return -1;              //  Interrupted, presumably


    assert (zmq_msg_more (&msg));


    uint64_t value_count;


    memcpy (&value_count, zmq_msg_data (&msg), sizeof (value_count));


    zmq_msg_close (&msg);


    for (uint64_t i = 0; i < value_count; ++i) {


        //  Subsequent frames in message contain event values


        zmq_msg_init (&msg);


        if (zmq_msg_recv (&msg, monitor, 0) == -1)


            return -1;              //  Interrupted, presumably


        assert (zmq_msg_more (&msg));


        if (value_ && value_ + i)


            memcpy (value_ + i, zmq_msg_data (&msg), sizeof (*value_));


        zmq_msg_close (&msg);


    }


    //  Second-to-last frame in message contains local address


    zmq_msg_init (&msg);


    if (zmq_msg_recv (&msg, monitor, 0) == -1)


        return -1;              //  Interrupted, presumably


    assert (zmq_msg_more (&msg));


    if (local_address_) {


        uint8_t *data = (uint8_t *) zmq_msg_data (&msg);


        size_t size = zmq_msg_size (&msg);


        *local_address_ = (char *) malloc (size + 1);


        memcpy (*local_address_, data, size);


        (*local_address_)[size] = 0;


    }


    zmq_msg_close (&msg);


    //  Last frame in message contains remote address


    zmq_msg_init (&msg);


    if (zmq_msg_recv (&msg, monitor, 0) == -1)


        return -1;              //  Interrupted, presumably


    assert (!zmq_msg_more (&msg));


    if (remote_address_) {


        uint8_t *data = (uint8_t *) zmq_msg_data (&msg);


        size_t size = zmq_msg_size (&msg);


        *remote_address_ = (char *) malloc (size + 1);


        memcpy (*remote_address_, data, size);


        (*remote_address_)[size] = 0;


    }


    zmq_msg_close (&msg);


    return event;
}
int main (void)
{


    void *ctx = zmq_ctx_new ();


    assert (ctx);


    //  We'll monitor these two sockets


    void *client = zmq_socket (ctx, ZMQ_DEALER);


    assert (client);


    void *server = zmq_socket (ctx, ZMQ_DEALER);


    assert (server);


    //  Socket monitoring only works over inproc://


    int rc = zmq_socket_monitor_versioned (client, "tcp://127.0.0.1:9999", 0, 2);


    assert (rc == -1);


    assert (zmq_errno () == EPROTONOSUPPORT);


    //  Monitor all events on client and server sockets


    rc = zmq_socket_monitor_versioned (client, "inproc://monitor-client", ZMQ_EVENT_ALL, 2);


    assert (rc == 0);


    rc = zmq_socket_monitor_versioned (server, "inproc://monitor-server", ZMQ_EVENT_ALL, 2);


    assert (rc == 0);


    //  Create two sockets for collecting monitor events


    void *client_mon = zmq_socket (ctx, ZMQ_PAIR);


    assert (client_mon);


    void *server_mon = zmq_socket (ctx, ZMQ_PAIR);


    assert (server_mon);


    //  Connect these to the inproc endpoints so they'll get events


    rc = zmq_connect (client_mon, "inproc://monitor-client");


    assert (rc == 0);


    rc = zmq_connect (server_mon, "inproc://monitor-server");


    assert (rc == 0);


    //  Now do a basic ping test


    rc = zmq_bind (server, "tcp://127.0.0.1:9998");


    assert (rc == 0);


    rc = zmq_connect (client, "tcp://127.0.0.1:9998");


    assert (rc == 0);


    bounce (client, server);


    //  Close client and server


    close_zero_linger (client);


    close_zero_linger (server);


    //  Now collect and check events from both sockets


    int event = get_monitor_event (client_mon, NULL, NULL);


    if (event == ZMQ_EVENT_CONNECT_DELAYED)


        event = get_monitor_event (client_mon, NULL, NULL);


    assert (event == ZMQ_EVENT_CONNECTED);


    event = get_monitor_event (client_mon, NULL, NULL);


    assert (event == ZMQ_EVENT_MONITOR_STOPPED);


    //  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_CLOSED);


    event = get_monitor_event (server_mon, NULL, NULL);


    assert (event == ZMQ_EVENT_MONITOR_STOPPED);


    //  Close down the sockets


    close_zero_linger (client_mon);


    close_zero_linger (server_mon);


    zmq_ctx_term (ctx);


    return 0 ;
}