What is the best way to implement a cross-platform, multi-threaded server in C/C++?

I've used Boost.Thread & Boost.Asio to build a multi-threaded server on Windows & Linux systems. The tutorials made it easy to get started.

The best way to write such a server is not to write one, and to rearchitect your system so it is not necessary, and/or to reuse components that already exist. Because:

Someone would connect a sensor device to their machine in one part of the building and run our server, thus sharing the device(s) with the rest of the network.

This also has the potential to share the entire machine with rest of the network, if your code has a vulnerability (which it probably will, as you're writing it in C++ from scratch and inventing a new protocol).

So, do it the other way around. Install a simple client on the machine that has the sensor hardware, then run it either all the time, or periodically, and have it push (post) results to a central server. The central server could even be a standard web server. Or it could be a database. (Notice that both of these have been written already - no need to reinvent the wheel ;-)

Your application then works the same way you have in mind now, however it collects data from the database rather than the sensors. The part running on the machine with the sensor, however, has shrunk from a multi-threaded custom server nightmare, to a nice little single threaded command line client that only makes outgoing connections, and which can be run from cron (or equivalent on windows).

Even if you need real time data collection (and from your description it sounds like you do not) it still may be better for the sensor collector be a client and not a server. Let it open a long lived connection to a central collector (or a group of them) and await instructions to provide its data.

edit: ceretullis and pukku's answers suggest a nice variation on this using multicast - see this answer and the comments

Douglas Schmidt's ACE (Adaptive Communications Environment) is a mature, highly portable open-source framework for building high-performance multithreaded servers. It's mainly aimed at telecommunication applications, but has been used for a variety of projects. It also comes with an object request broker called TAO (if you're into CORBA)

One claim to fame of the framework is that it supports many threading models (thread pool, thread per request, asynchronous + threads etc.), so you can use its thread management in a way that's optimal for your application. This is actually the most interesting feature of the system - the server framework functionality comes out of the box. Most of the other libraries I've seen suggested here would still require you to implement much of this functionality yourself.

There is quite a bit of electronic documentation and also several books written about it. It's not the most warm and fluffy of systems and is really built for speed rather than comfort - but we are using C++. You will probably find that it's much less effort to get your head around ACE than try to rebuild the functionality and debug all the synchronisation.

Oh, and by the way, it's free as in speech - and as in beer. If you want to a commercial route there is also an ecosystem of consultants that will provide support and mentoring services for it. A tutorial with some code snippets can be found here.