enable SO_PEERCRED in Rust on a UNIX domain socket

enable_so_peercred_dgram.rs

extern crate libc;
extern crate tokio_uds;

use std::os::unix::io::AsRawFd;

// this is automatically enabled on stream socket - only for
// datagram sockets do you need to manually enable it.
fn enable_so_passcred(unix_socket: &tokio_uds::UnixListener) {
    let fd = unix_socket.as_raw_fd();
    let val: libc::c_int = 1;
    let r = unsafe {
        libc::setsockopt(fd,
                        libc::SOL_SOCKET,
                        libc::SO_PASSCRED,
                        &val as *const libc::c_int as *const libc::c_void,
                        std::mem::size_of::<libc::c_int>() as libc::socklen_t)
    };
    if r != 0 {
        eprintln!("cannot set SO_PASSCRED");
        std::process::exit(1);
    }
}