How to monitor whether a ZeroMQ server exists?

I want to check the existence ( state ) of a server before I send a ZeroMQ request, but I have no idea how to do it.

Q : I want to check the existence ( state ) of a server before I send a ZeroMQ request

The solution is to setup and use the services of a zmq_socket_monitor()

// 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
                    )
{   
    zmq_msg_t msg;
    zmq_msg_init ( &msg );                                       // First frame in message contains event number and value
    if ( zmq_msg_recv ( &msg, monitor, 0 ) == -1 ) return -1;    // Interrupted, presumably
    assert ( zmq_msg_more ( &msg )              & "REASON: Frame #1 FAILED TO SIG 2nd, EXPECTED, FRAME TO COME" );

    uint8_t  *data  =  ( uint8_t  * ) zmq_msg_data ( &msg );
    uint16_t  event = *( uint16_t * ) ( data );

    if ( value )
        *value = *( uint32_t * ) ( data + 2 );


    zmq_msg_init ( &msg );                                      // Second frame in message contains event address
    if ( zmq_msg_recv ( &msg, monitor, 0 ) == -1 ) return -1;   // Interrupted, presumably
    assert ( !zmq_msg_more ( &msg )             & "REASON: Frame #2 FAILED TO SIG more, NOT EXPECTED, FRAMEs TO COME" );

    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                                & "REASON: Context FAILED to instantiate" );

    void    *client = zmq_socket ( ctx, ZMQ_DEALER );
    assert ( client                             & "REASON: Socket FAILED to instantiate" );

 // Socket monitoring only works over inproc://
    int      rc = zmq_socket_monitor ( client, "inproc://monitor-client-side", ZMQ_EVENT_ALL );
    assert ( rc == 0                            & "REASON: socket_monitor FAILED to instantiate over INPROC:// transport-class" );

 // Create socket for collecting monitor events
    void    *client_side_mon = zmq_socket ( ctx, ZMQ_PAIR );
    assert ( client_side_mon                    & "REASON: socket_monitor receiving Socket FAILED to instantiate " );

 // Connect these to the inproc endpoints so they'll get events
             rc = zmq_connect ( client_side_mon, "inproc://monitor-client-side" );
    assert ( rc == 0                            & "REASON: .connect()-method FAILED to get connected" );

 // Now do whatever you need
    ...

 // Close client
    close_zero_linger ( client );

 // --------------------------------------------------------------------
 // How to collect and check events from socket_monitor:
    int  event =  get_monitor_event ( client_side_mon, NULL, NULL );

    if ( event == ZMQ_EVENT_CONNECT_DELAYED )
         event =  get_monitor_event ( client_side_mon, NULL, NULL );

    assert ( event == ZMQ_EVENT_CONNECTED       & "REASON: [client]-socket still not in an expected, .connect()-ed, state" );
    ...

    ...
    event = get_monitor_event ( client_side_mon, NULL, NULL );
    assert ( event == ZMQ_EVENT_MONITOR_STOPPED & "REASON: [client]-socket not in an expected, .close()-ed, state" );

 // --------------------------------------------------------------------
 // FINALLY:
 // --------------------------------------------------------------------
 // Close down the sockets
    close_zero_linger ( client_side_mon );

    zmq_ctx_term ( ctx );

    return 0;
    }

_{( included in API since v3.2+ )}