Odin code:

    package main

    import "core:fmt"

    @export
    sayhi :: proc() {
      fmt.println("Hello World")
    }

Key is the "@export" statement. Then build Linux dynamic library by doing "odin build main.odin -file -build-mode:shared". -file flag is since I'm just building a single Odin file, didn't set up a project structure or anything. -build-mode:shared is pretty obvious, there's separate build mode flags for Windows or Mac platforms (called "dll" and "dynamic" I think).

Ruby code:

    require 'ffi'
    module MyLib
      extend FFI::Library
      ffi_lib "./main.so"
      attach_function :sayhi, [], :void
    end

    MyLib.sayhi

That's literally all there is to it. Odin lib is a single file. Ruby code is a single file. Don't even need a project setup since Odin can compile single files into programs or libraries and Ruby can of course link to a shared lib that's anywhere.

Thank you so much, that was way easier than what I expected, amazing!