Cross-platform application-service. What to write on?

Question

Programming

Please tell me the easiest way to create a service application (without a GUI, except maybe a start / stop button) for as many platforms as possible with minimal costs for finishing for a specific platform?
Interested in Win, *nix, MacOS, iOS, Android.
From the functionality there will be network access (client and server), multithreading, SQLite. As an analogue - a torrent client with a web interface.

The language is preferably C++.
Maybe someone knows what kind of open source project is similar to peep how it's done.

Answer 1

I recommend Java. If you know C, then there will be no problems with Java syntax. Multithreading, SQLite, whatever.
For example, you can look at the Vuze torrent client ( www.vuze.com/ ), its sources are open - you can study as much as you like.
I myself am currently writing a cross-platform GUI application in JavaSE

Answer 2

Some incomprehensible wording of the problem.
If we talk about services (services) on Windows and daemons on Linux, then you can use either QtService or mono-service in C #. But something tells me that these solutions will not work on iOS and Android (although I can not vouch).
In general, Windows services are a rather specific thing, and if you suddenly want to somehow access the GUI through it, then nothing will come of it.
You can take some library (POCO, for example) and write a regular application on it that will run in the background.