use std::net::TcpListener;
 
fn create_app_lock(port: u16) -> TcpListener {
    match TcpListener::bind(("0.0.0.0", port)) {
        Ok(socket) => {
            socket
        },
        Err(_) => {
            panic!("Couldn't lock port {}: another instance already running?", port);
        }
    }
}
 
fn remove_app_lock(socket: TcpListener) {
    drop(socket);
}
 
fn main() {
    let lock_socket = create_app_lock(12345);
    // ...
    // your code here
    // ...
    remove_app_lock(lock_socket);
}