civetweb will run cross platform as a http server.

the issue is when you need write code on each OS, e.g. a cgi for windows/macos/linux/bsd, you will have to know the native API for each OS and use them, which could be hard to maintain if cross-platform is a must.

I wish there is something lower level that is truly cross platform, e.g. a C library runs on each OS and let me do filesystem/network/etc, it does not seem exist.