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ZMQ_SOCKET_MONITOR

Section: 0MQ Manual (3)
Updated: 12/31/2016
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NAME

zmq_socket_monitor - monitor socket events

SYNOPSIS

int zmq_socket_monitor (void *socket, char *endpoint, int events);

DESCRIPTION

The zmq_socket_monitor() method lets an application thread track socket events (like connects) on a ZeroMQ socket. Each call to this method creates a ZMQ_PAIR socket and binds that to the specified inproc:// endpoint. To collect the socket events, you must create your own ZMQ_PAIR socket, and connect that to the endpoint.

The events argument is a bitmask of the socket events you wish to monitor, see Supported events below. To monitor all events, use the event value ZMQ_EVENT_ALL. NOTE: as new events are added, the catch-all value will start returning them. An application that relies on a strict and fixed sequence of events must not use ZMQ_EVENT_ALL in order to guarantee compatibility with future versions.

Each event is sent as two frames. The first frame contains an event number (16 bits), and an event value (32 bits) that provides additional data according to the event number. The second frame contains a string that specifies the affected TCP or IPC endpoint.

The _zmq_socket_monitor()_ method supports only connection-oriented
transports, that is, TCP, IPC, and TIPC.

SUPPORTED EVENTS

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 at 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

The ZMTP security mechanism handshake failed. The event value is unspecified. NOTE: in DRAFT state, not yet available in stable releases.

ZMQ_EVENT_HANDSHAKE_SUCCEED

The ZMTP security mechanism handshake succeeded. The event value is unspecified. NOTE: in DRAFT state, not yet available in stable releases.

RETURN VALUE

The zmq_socket_monitor() function returns a value of 0 or greater if successful. Otherwise it returns -1 and sets errno to one of the values defined below.

ERRORS

ETERM

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

EPROTONOSUPPORT

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

EINVAL

: The endpoint supplied is invalid.

EXAMPLE

Monitoring client and server sockets.

//  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;              //  Interrupted, 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;              //  Interrupted, 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;
    }
    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 (client, "tcp://127.0.0.1:9999", 0);
    assert (rc == -1);
    assert (zmq_errno () == EPROTONOSUPPORT);

    //  Monitor all events on client and server sockets
    rc = zmq_socket_monitor (client, "inproc://monitor-client", ZMQ_EVENT_ALL);
    assert (rc == 0);
    rc = zmq_socket_monitor (server, "inproc://monitor-server", ZMQ_EVENT_ALL);
    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 ;
}

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