Cargo.toml

[dependencies]
libc = "0.2"

[lib]
name = ""
crate-type = ["dylib"]

---

lib.rs

extern crate libc;
use std::ffi::{CStr, CString};

#[no_mangle]
pub extern fn lib_fn(name: *const libc::c_char) {
	let name = unsafe { CStr::from_ptr(name) };
	// implementation
}

---

test.rb

require 'ffi'

module Test
	extend FFI::Library
	ffi_lib 'path/to/lib'
	attach_function :lib_fn, [:string], :void
end

Test.lib_fn(str)