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    // Copyright 2015 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    //! Networking primitives for TCP/UDP communication.

    #![stable(feature = "rust1", since = "1.0.0")]

    use fmt;
    use io::{self, Error, ErrorKind};
    use sys_common::net as net_imp;

    #[stable(feature = "rust1", since = "1.0.0")]
    pub use self::ip::{IpAddr, Ipv4Addr, Ipv6Addr, Ipv6MulticastScope};
    #[stable(feature = "rust1", since = "1.0.0")]
    pub use self::addr::{SocketAddr, SocketAddrV4, SocketAddrV6, ToSocketAddrs};
    #[stable(feature = "rust1", since = "1.0.0")]
    pub use self::tcp::{TcpStream, TcpListener, Incoming};
    #[stable(feature = "rust1", since = "1.0.0")]
    pub use self::udp::UdpSocket;
    #[stable(feature = "rust1", since = "1.0.0")]
    pub use self::parser::AddrParseError;

    mod ip;
    mod addr;
    mod tcp;
    mod udp;
    mod parser;
    #[cfg(test)]
    mod test;

    /// Possible values which can be passed to the [`shutdown`] method of
    /// [`TcpStream`].
    ///
    /// [`shutdown`]: struct.TcpStream.html#method.shutdown
    /// [`TcpStream`]: struct.TcpStream.html
    #[derive(Copy, Clone, PartialEq, Eq, Debug)]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub enum Shutdown {
        /// Indicates that the reading portion of this stream/socket should be shut
        /// down. All currently blocked and future reads will return `Ok(0)`.
        #[stable(feature = "rust1", since = "1.0.0")]
        Read,
        /// Indicates that the writing portion of this stream/socket should be shut
        /// down. All currently blocked and future writes will return an error.
        #[stable(feature = "rust1", since = "1.0.0")]
        Write,
        /// Shut down both the reading and writing portions of this stream.
        ///
        /// See `Shutdown::Read` and `Shutdown::Write` for more information.
        #[stable(feature = "rust1", since = "1.0.0")]
        Both,
    }

    #[doc(hidden)]
    trait NetInt {
        fn from_be(i: Self) -> Self;
        fn to_be(&self) -> Self;
    }
    macro_rules! doit {
        ($($t:ident)*) => ($(impl NetInt for $t {
            fn from_be(i: Self) -> Self { <$t>::from_be(i) }
            fn to_be(&self) -> Self { <$t>::to_be(*self) }
        })*)
    }
    doit! { i8 i16 i32 i64 isize u8 u16 u32 u64 usize }

    fn hton<I: NetInt>(i: I) -> I { i.to_be() }
    fn ntoh<I: NetInt>(i: I) -> I { I::from_be(i) }

    fn each_addr<A: ToSocketAddrs, F, T>(addr: A, mut f: F) -> io::Result<T>
        where F: FnMut(&SocketAddr) -> io::Result<T>
    {
        let mut last_err = None;
        for addr in addr.to_socket_addrs()? {
            match f(&addr) {
                Ok(l) => return Ok(l),
                Err(e) => last_err = Some(e),
            }
        }
        Err(last_err.unwrap_or_else(|| {
            Error::new(ErrorKind::InvalidInput,
                       "could not resolve to any addresses")
        }))
    }

    /// An iterator over `SocketAddr` values returned from a host lookup operation.
    #[unstable(feature = "lookup_host", reason = "unsure about the returned \
                                                  iterator and returning socket \
                                                  addresses",
               issue = "27705")]
    pub struct LookupHost(net_imp::LookupHost);

    #[unstable(feature = "lookup_host", reason = "unsure about the returned \
                                                  iterator and returning socket \
                                                  addresses",
               issue = "27705")]
    impl Iterator for LookupHost {
        type Item = SocketAddr;
        fn next(&mut self) -> Option<SocketAddr> { self.0.next() }
    }

    #[stable(feature = "std_debug", since = "1.15.0")]
    impl fmt::Debug for LookupHost {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.pad("LookupHost { .. }")
        }
    }

    /// Resolve the host specified by `host` as a number of `SocketAddr` instances.
    ///
    /// This method may perform a DNS query to resolve `host` and may also inspect
    /// system configuration to resolve the specified hostname.
    ///
    /// The returned iterator will skip over any unknown addresses returned by the
    /// operating system.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// #![feature(lookup_host)]
    ///
    /// use std::net;
    ///
    /// # fn foo() -> std::io::Result<()> {
    /// for host in net::lookup_host("rust-lang.org")? {
    ///     println!("found address: {}", host);
    /// }
    /// # Ok(())
    /// # }
    /// ```
    #[unstable(feature = "lookup_host", reason = "unsure about the returned \
                                                  iterator and returning socket \
                                                  addresses",
               issue = "27705")]
    pub fn lookup_host(host: &str) -> io::Result<LookupHost> {
        net_imp::lookup_host(host).map(LookupHost)
    }

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