# zmq_socket_monitor(3) Manual Page

## 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.

Note that there is also a DRAFT function zmq_socket_monitor_versioned(3), which allows to subscribe to events that provide more information. Calling zmq_socket_monitor is equivalent to calling zmq_socket_monitor_versioned with the event_version parameter set to 1, with the exception of error cases.

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 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_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).

## 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 ØMQ 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_HANDSHAKE_SUCCEEDED);
        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_HANDSHAKE_SUCCEEDED);
        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 ;
    }

## SEE ALSO

zmq(7)

## AUTHORS

This page was written by the ØMQ community. To make a change please read the ØMQ Contribution Policy at http://www.zeromq.org/docs:contributing.