= zmq_socket_monitor(3)


== 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 xref:zmq_socket_monitor_versioned.adoc[zmq_socket_monitor_versioned],
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 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_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
* xref:zmq.adoc[zmq]


== AUTHORS
This page was written by the 0MQ community. To make a change please
read the 0MQ Contribution Policy at <https://zeromq.org/how-to-contribute/>.
