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zmq_proxy - start built-in 0MQ proxy


int zmq_proxy (void *frontend, void *backend, void *capture);


The zmq_proxy() function starts the built-in 0MQ proxy in the current application thread.

The proxy connects a frontend socket to a backend socket. Conceptually, data flows from frontend to backend. Depending on the socket types, replies may flow in the opposite direction. The direction is conceptual only; the proxy is fully symmetric and there is no technical difference between frontend and backend.

Before calling zmq_proxy() you must set any socket options, and connect or bind both frontend and backend sockets. The two conventional proxy models are:

zmq_proxy() runs in the current thread and returns only if/when the current context is closed.

If the capture socket is not NULL, the proxy shall send all messages, received on both frontend and backend, to the capture socket. The capture socket should be a ZMQ_PUB, ZMQ_DEALER, ZMQ_PUSH, or ZMQ_PAIR socket.

Refer to zmq_socket(3) for a description of the available socket types.


Shared Queue

When the frontend is a ZMQ_ROUTER socket, and the backend is a ZMQ_DEALER socket, the proxy shall act as a shared queue that collects requests from a set of clients, and distributes these fairly among a set of services. Requests shall be fair-queued from frontend connections and distributed evenly across backend connections. Replies shall automatically return to the client that made the original request.


When the frontend is a ZMQ_XSUB socket, and the backend is a ZMQ_XPUB socket, the proxy shall act as a message forwarder that collects messages from a set of publishers and forwards these to a set of subscribers. This may be used to bridge networks transports, e.g. read on tcp:// and forward on pgm://.


When the frontend is a ZMQ_PULL socket, and the backend is a ZMQ_PUSH socket, the proxy shall collect tasks from a set of clients and forwards these to a set of workers using the pipeline pattern.


The zmq_proxy() function always returns -1 and errno set to ETERM or EINTR (the 0MQ context associated with either of the specified sockets was terminated) or EFAULT (the provided frontend or backend was invalid).


Creating a shared queue proxy.

//  Create frontend and backend sockets
void *frontend = zmq_socket (context, ZMQ_ROUTER);
assert (frontend);
void *backend = zmq_socket (context, ZMQ_DEALER);
assert (backend);
//  Bind both sockets to TCP ports
assert (zmq_bind (frontend, "tcp://*:5555") == 0);
assert (zmq_bind (backend, "tcp://*:5556") == 0);
//  Start the queue proxy, which runs until ETERM
zmq_proxy (frontend, backend, NULL);


zmq_bind(3) zmq_connect(3) zmq_socket(3) zmq(7)


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10/09/2023 0MQ 4.3.5