for most transports and socket types the connection is not performed immediately but as needed by 0MQ. Thus a successful call to zmq_connect() does not mean that the connection was or could actually be established. Because of this, for most transports and socket types the order in which a server socket is bound and a client socket is connected to it does not matter. The ZMQ_PAIR and ZMQ_CHANNEL sockets are an exception, as they do not automatically reconnect to endpoints.

following a zmq_connect(), for socket types except for ZMQ_ROUTER, the socket enters its normal ready state. By contrast, following a zmq_bind() alone, the socket enters a mute state in which the socket blocks or drops messages according to the socket type, as defined in zmq_socket(3). A ZMQ_ROUTER socket enters its normal ready state for a specific peer only when handshaking is complete for that peer, which may take an arbitrary time.

for some socket types, multiple connections to the same endpoint don’t really make sense (see https://github.com/zeromq/libzmq/issues/788). For those socket types, any attempt to connect to an already connected endpoint is silently ignored (i.e., returns zero). This behavior applies to ZMQ_DEALER, ZMQ_SUB, ZMQ_PUB, and ZMQ_REQ socket types.

/* Create a ZMQ_SUB socket */
void *socket = zmq_socket (context, ZMQ_SUB);
assert (socket);
/* Connect it to an in-process transport with the address 'my_publisher' */
int rc = zmq_connect (socket, "inproc://my_publisher");
assert (rc == 0);
/* Connect it to the host server001, port 5555 using a TCP transport */
rc = zmq_connect (socket, "tcp://server001:5555");
assert (rc == 0);