I'm writing a client-server application where client should be able to automatically detect server presence in the network so that user would not have to mess with manual configuration.
I think that server should broadcast its presence somehow. What is the best way to do it?
Server is to be run on either Windows / Linux or OS X, so the solution should be cross-platform. Client and server are written in C++.
You should look into ZeroConf as a generic way of broadcasting this information. It's also known as Bonjour (formerly Rendezvous) on Apple systems, but it's not an Apple-specific thing - plenty of other devices (both embedded and OSs) broadcast/use it.
There are plenty of libraries that can deal with ZeroConf in plenty of languages - finding one is a matter of Googling it. Bear in mind that ZeroConf itself is the advertising/discovery part of the process and identifies a machine and a socket - but you can then do whatever you want over the socket(s) that you identify.
For example, Apache mod_zeroconf allows you to broadcast Apache sites. Your solution will be dependent on what licensing and usage rights you're dependent on.
If you want to do it yourself you can use multicasting. Some (braindead) ways to do that are: 1) have the server periodically advertise it's presence to the subscribed clients, followed by unicast transmissions between the client & the server; 2) have the client signal (a number of times) it's presence to the subscribed server(s), again followed by unicast transmissions between server & client. One more thing that needs to be considered it that the multicast will be all UDP and the unicast can be either TCP or UDP.