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_recvmsg (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.

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_recvmsg (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);

AUTHORS

This 0MQ manual page was written by Lourens Naudé <m[blue][email protected]m[][1]>

NOTES

1.

[email protected]

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