C++/Java Sockets automatic Port allocation

I've a server (Java) and a number of clients (c++), connected 开发者_运维百科by sockets. I would like to set the ports automatically. Assuming the IP is already known.

In the Java side I can make :

ServerSocket s = new ServerSocket(0);

So now I've a random free port on the server.

How can I know in the C++ side, what port is the server listening to?

I think is not possible, if you want establish a connection with a server, you must know in which port is the server listening, there are programs like nmap that shows you a list of opened ports in a server, but a server can have many opened ports at the same time and then, How do you know what is the port opened by your server? and in any case, is too slow and inefficient to call external tool, read and parse its output. For what reason do you need a random port service?

Other option can be get the opened socket in the server side calling to s.getLocalPort() and send it via UDP for any listening node in the network with broadcasting, and re-program the client side to listen in broadcast and when it receives a message, check if it is a port number and connect to the server using that port.

You can't, not reliably. In IP, a machine is identified by an address. A server (ie, a service) is identified by an address and a port. You clients need some form of "known service" that they can connect to.

If you, for whatever reason, absolutely want to have dynamic listening port, you could combine it with a "locator" service on a known port. For instance, have a web service/servlet on the standard http port (80). Your clients connect to the "locator" service (always on port 80) and asks which port your application is currently listening on. This is a not entirely uncommon pattern. RMI works is a similar way where you have a registry on a known port. Clients connect to the registry and asks for the location of RMI endpoints.