zmq_ctx_socket_monitor(3)
=========================


NAME
----

zmq_socket_monitor - register a monitoring callback


SYNOPSIS
--------
*int zmq_socket_monitor (void '*socket', char * '*addr', int 'events');*


DESCRIPTION
-----------
The _zmq_socket_monitor()_ function shall spawn a 'PAIR' socket that publishes
socket state changes (events) over the inproc:// transport to a given endpoint.
Messages are 'zmq_event_t' structs. It's recommended to connect via a 'PAIR'
socket in another application thread and handle monitoring events there. It's
possible to also supply a bitmask ('ZMQ_EVENT_ALL' or any combination of the
'ZMQ_EVENT_*' constants) of the events you're interested in.

----
// monitoring thread
static void *req_socket_monitor (void *ctx)
{
    zmq_event_t event;
    int rc;

    void *s = zmq_socket (ctx, ZMQ_PAIR);
    assert (s);

    rc = zmq_connect (s, "inproc://monitor.req");
    assert (rc == 0);
    while (true) {
        zmq_msg_t msg;
        zmq_msg_init (&msg);
        rc = zmq_msg_recv (s, &msg, 0);
        if (rc == -1 && zmq_errno() == ETERM) break;
        assert (rc != -1);
        memcpy (&event, zmq_msg_data (&msg), sizeof (event));
        switch (event.event) {
        case ZMQ_EVENT_CONNECTED:
            // handle socket connected event
            break;
        case ZMQ_EVENT_CLOSED:
            // handle socket closed event
            break;
        }
    }
    zmq_close (s);
    return NULL;
}

// register a monitor endpoint for all socket events
rc = zmq_socket_monitor (req, "inproc://monitor.req", ZMQ_EVENT_ALL);
assert (rc == 0);

// spawn a monitoring thread
rc = pthread_create (&threads [0], NULL, req_socket_monitor, ctx);
assert (rc == 0);
----

Only connection oriented (tcp and ipc) transports are supported in this initial
implementation.

Supported events are:


ZMQ_EVENT_CONNECTED: connection established
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The 'ZMQ_EVENT_CONNECTED' event triggers when a connection has been established
to a remote peer. This can happen either synchronous or asynchronous.

.Event metadata:
----
data.connected.addr // peer address
data.connected.fd // socket descriptor
----

ZMQ_EVENT_CONNECT_DELAYED: synchronous connect failed, it's being polled
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The 'ZMQ_EVENT_CONNECT_DELAYED' event triggers when an immediate connection
attempt is delayed and it's completion's being polled for.

.Event metadata:
----
data.connect_delayed.addr // peer address
data.connect_delayed.err // errno value
----

ZMQ_EVENT_CONNECT_RETRIED: asynchronous connect / reconnection attempt
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The 'ZMQ_EVENT_CONNECT_RETRIED' event triggers when a connection attempt
is being handled by reconnect timer. The reconnect interval's recomputed
for each attempt.

.Event metadata:
----
data.connect_retried.addr // peer address
data.connect_retried.interval // computed reconnect interval
----

ZMQ_EVENT_LISTENING: socket bound to an address, ready to accept connections
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The 'ZMQ_EVENT_LISTENING' event triggers when a socket's successfully bound
to a an interface.

.Event metadata:
----
data.listening.addr //  listen address
data.listening.fd // socket descriptor
----

ZMQ_EVENT_BIND_FAILED: socket could not bind to an address
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The 'ZMQ_EVENT_BIND_FAILED' event triggers when a socket could not bind to
a given interface.

.Event metadata:
----
data.bind_failed.addr // listen address
data.bind_failed.err // errno value
----

ZMQ_EVENT_ACCEPTED: connection accepted to bound interface
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The 'ZMQ_EVENT_ACCEPTED' event triggers when a connection from a remote peer
has been established with a socket's listen address.

.Event metadata:
----
data.accepted.addr // listen address
data.accepted.fd // socket descriptor
----

ZMQ_EVENT_ACCEPT_FAILED: could not accept client connection
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The 'ZMQ_EVENT_ACCEPT_FAILED' event triggers when a connection attempt to
a socket's bound address fails.

.Event metadata:
----
data.accept_failed.addr // listen address
data.accept_failed.err // errno value
----

ZMQ_EVENT_CLOSED: connection closed
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The 'ZMQ_EVENT_CLOSED' event triggers when a connection's underlying descriptor
has been closed. 

.Event metadata:
----
data.closed.addr // address
data.closed.fd // socket descriptor
----

ZMQ_EVENT_CLOSE_FAILED: connection couldn't be closed
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The 'ZMQ_EVENT_CLOSE_FAILED' event triggers when a descriptor could not be
released back to the OS.

.Event metadata:
----
data.close_failed.addr // address
data.close_failed.err // errno value
----

ZMQ_EVENT_DISCONNECTED: broken session
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The 'ZMQ_EVENT_DISCONNECTED' event triggers when the stream engine (tcp and ipc
specific) detects a corrupted / broken session.

.Event metadata:
----
data.disconnected.addr // address
data.disconnected.fd // socket descriptor
----

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
-------
.Observing a 'REP' socket's connection state 
----
// REP socket monitor thread
static void *rep_socket_monitor (void *ctx)
{
    zmq_event_t event;
    int rc;

    void *s = zmq_socket (ctx, ZMQ_PAIR);
    assert (s);

    rc = zmq_connect (s, "inproc://monitor.rep");
    assert (rc == 0);
    while (true) {
        zmq_msg_t msg;
        zmq_msg_init (&msg);
        rc = zmq_msg_recv (s, &msg, 0);
        if (rc == -1 && zmq_errno() == ETERM) break;
        assert (rc != -1);
        memcpy (&event, zmq_msg_data (&msg), sizeof (event));
        switch (event.event) {
        case ZMQ_EVENT_LISTENING:
            printf ("listening socket descriptor %d\n", event.data.listening.fd);
            printf ("listening socket address %s\n", event.data.listening.addr);
            break;
        case ZMQ_EVENT_ACCEPTED:
            printf ("accepted socket descriptor %d\n", event.data.accepted.fd);
            printf ("accepted socket address %s\n", event.data.accepted.addr);
            break;
        case ZMQ_EVENT_CLOSE_FAILED:
            printf ("socket close failure error code %d\n", event.data.close_failed.err);
            printf ("socket address %s\n", event.data.close_failed.addr);
            break;
        case ZMQ_EVENT_CLOSED:
            printf ("closed socket descriptor %d\n", event.data.closed.fd);
            printf ("closed socket address %s\n", event.data.closed.addr);
            break;
        case ZMQ_EVENT_DISCONNECTED:
            printf ("disconnected socket descriptor %d\n", event.data.disconnected.fd);
            printf ("disconnected socket address %s\n", event.data.disconnected.addr);
            break;
        }
        zmq_msg_close (&msg);
    }
    zmq_close (s);
    return NULL;
}


//  Create the infrastructure
void *ctx = zmq_init (1);
assert (ctx);

// REP socket
rep = zmq_socket (ctx, ZMQ_REP);
assert (rep);

// REP socket monitor, all events
rc = zmq_socket_monitor (rep, "inproc://monitor.rep", ZMQ_EVENT_ALL);
assert (rc == 0);
rc = pthread_create (&threads [0], NULL, rep_socket_monitor, ctx);
assert (rc == 0);

rc = zmq_bind (rep, addr);
assert (rc == 0);

// Allow some time for event detection
zmq_sleep (1);

// Close the REP socket
rc = zmq_close (rep);
assert (rc == 0);

zmq_term (ctx);
----


SEE ALSO
--------
linkzmq:zmq[7]


AUTHORS
-------
This 0MQ manual page was written by Lourens Naudé <lourens@methodmissing.com>