callback.rs

fn main() {
  extern "C" fn callback(msg: c_uint, user_data: c_long, p1: c_long, p2: c_long) -> c_int {
    let ptr = p1 as *const _;
    // turn information into a Rust String
    let information = str::from_utf8(unsafe { CStr::from_ptr(ptr) }.to_bytes()).unwrap();
    // get user_data (our passed String):
    let our_string = str::from_utf8(unsafe { slice::from_raw_parts(user_data as *const u8, 1024) } ).unwrap();
    // write information to our_string here...
  }
  let user_data = String::with_capacity(1024);
  native_c_function(callback, user_data.as_ptr());
  println!("{}", user_data);
}