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    use crate::ffi::CStr;
    use crate::io::{self, IoVec, IoVecMut};
    use crate::mem;
    use crate::net::{SocketAddr, Shutdown};
    use crate::str;
    use crate::sys::fd::FileDesc;
    use crate::sys_common::{AsInner, FromInner, IntoInner};
    use crate::sys_common::net::{getsockopt, setsockopt, sockaddr_to_addr};
    use crate::time::{Duration, Instant};
    use crate::cmp;

    use libc::{c_int, c_void, size_t, sockaddr, socklen_t, EAI_SYSTEM, MSG_PEEK};

    pub use crate::sys::{cvt, cvt_r};

    #[allow(unused_extern_crates)]
    pub extern crate libc as netc;

    pub type wrlen_t = size_t;

    // See below for the usage of SOCK_CLOEXEC, but this constant is only defined on
    // Linux currently (e.g., support doesn't exist on other platforms). In order to
    // get name resolution to work and things to compile we just define a dummy
    // SOCK_CLOEXEC here for other platforms. Note that the dummy constant isn't
    // actually ever used (the blocks below are wrapped in `if cfg!` as well.
    #[cfg(target_os = "linux")]
    use libc::SOCK_CLOEXEC;
    #[cfg(not(target_os = "linux"))]
    const SOCK_CLOEXEC: c_int = 0;

    // Another conditional constant for name resolution: Macos et iOS use
    // SO_NOSIGPIPE as a setsockopt flag to disable SIGPIPE emission on socket.
    // Other platforms do otherwise.
    #[cfg(target_vendor = "apple")]
    use libc::SO_NOSIGPIPE;
    #[cfg(not(target_vendor = "apple"))]
    const SO_NOSIGPIPE: c_int = 0;

    pub struct Socket(FileDesc);

    pub fn init() {}

    pub fn cvt_gai(err: c_int) -> io::Result<()> {
        if err == 0 {
            return Ok(())
        }

        // We may need to trigger a glibc workaround. See on_resolver_failure() for details.
        on_resolver_failure();

        if err == EAI_SYSTEM {
            return Err(io::Error::last_os_error())
        }

        let detail = unsafe {
            str::from_utf8(CStr::from_ptr(libc::gai_strerror(err)).to_bytes()).unwrap()
                .to_owned()
        };
        Err(io::Error::new(io::ErrorKind::Other,
                           &format!("failed to lookup address information: {}",
                                    detail)[..]))
    }

    impl Socket {
        pub fn new(addr: &SocketAddr, ty: c_int) -> io::Result<Socket> {
            let fam = match *addr {
                SocketAddr::V4(..) => libc::AF_INET,
                SocketAddr::V6(..) => libc::AF_INET6,
            };
            Socket::new_raw(fam, ty)
        }

        pub fn new_raw(fam: c_int, ty: c_int) -> io::Result<Socket> {
            unsafe {
                // On linux we first attempt to pass the SOCK_CLOEXEC flag to
                // atomically create the socket and set it as CLOEXEC. Support for
                // this option, however, was added in 2.6.27, and we still support
                // 2.6.18 as a kernel, so if the returned error is EINVAL we
                // fallthrough to the fallback.
                if cfg!(target_os = "linux") {
                    match cvt(libc::socket(fam, ty | SOCK_CLOEXEC, 0)) {
                        Ok(fd) => return Ok(Socket(FileDesc::new(fd))),
                        Err(ref e) if e.raw_os_error() == Some(libc::EINVAL) => {}
                        Err(e) => return Err(e),
                    }
                }

                let fd = cvt(libc::socket(fam, ty, 0))?;
                let fd = FileDesc::new(fd);
                fd.set_cloexec()?;
                let socket = Socket(fd);
                if cfg!(target_vendor = "apple") {
                    setsockopt(&socket, libc::SOL_SOCKET, SO_NOSIGPIPE, 1)?;
                }
                Ok(socket)
            }
        }

        pub fn new_pair(fam: c_int, ty: c_int) -> io::Result<(Socket, Socket)> {
            unsafe {
                let mut fds = [0, 0];

                // Like above, see if we can set cloexec atomically
                if cfg!(target_os = "linux") {
                    match cvt(libc::socketpair(fam, ty | SOCK_CLOEXEC, 0, fds.as_mut_ptr())) {
                        Ok(_) => {
                            return Ok((Socket(FileDesc::new(fds[0])), Socket(FileDesc::new(fds[1]))));
                        }
                        Err(ref e) if e.raw_os_error() == Some(libc::EINVAL) => {},
                        Err(e) => return Err(e),
                    }
                }

                cvt(libc::socketpair(fam, ty, 0, fds.as_mut_ptr()))?;
                let a = FileDesc::new(fds[0]);
                let b = FileDesc::new(fds[1]);
                a.set_cloexec()?;
                b.set_cloexec()?;
                Ok((Socket(a), Socket(b)))
            }
        }

        pub fn connect_timeout(&self, addr: &SocketAddr, timeout: Duration) -> io::Result<()> {
            self.set_nonblocking(true)?;
            let r = unsafe {
                let (addrp, len) = addr.into_inner();
                cvt(libc::connect(self.0.raw(), addrp, len))
            };
            self.set_nonblocking(false)?;

            match r {
                Ok(_) => return Ok(()),
                // there's no ErrorKind for EINPROGRESS :(
                Err(ref e) if e.raw_os_error() == Some(libc::EINPROGRESS) => {}
                Err(e) => return Err(e),
            }

            let mut pollfd = libc::pollfd {
                fd: self.0.raw(),
                events: libc::POLLOUT,
                revents: 0,
            };

            if timeout.as_secs() == 0 && timeout.subsec_nanos() == 0 {
                return Err(io::Error::new(io::ErrorKind::InvalidInput,
                                          "cannot set a 0 duration timeout"));
            }

            let start = Instant::now();

            loop {
                let elapsed = start.elapsed();
                if elapsed >= timeout {
                    return Err(io::Error::new(io::ErrorKind::TimedOut, "connection timed out"));
                }

                let timeout = timeout - elapsed;
                let mut timeout = timeout.as_secs()
                    .saturating_mul(1_000)
                    .saturating_add(timeout.subsec_nanos() as u64 / 1_000_000);
                if timeout == 0 {
                    timeout = 1;
                }

                let timeout = cmp::min(timeout, c_int::max_value() as u64) as c_int;

                match unsafe { libc::poll(&mut pollfd, 1, timeout) } {
                    -1 => {
                        let err = io::Error::last_os_error();
                        if err.kind() != io::ErrorKind::Interrupted {
                            return Err(err);
                        }
                    }
                    0 => {}
                    _ => {
                        // linux returns POLLOUT|POLLERR|POLLHUP for refused connections (!), so look
                        // for POLLHUP rather than read readiness
                        if pollfd.revents & libc::POLLHUP != 0 {
                            let e = self.take_error()?
                                .unwrap_or_else(|| {
                                    io::Error::new(io::ErrorKind::Other, "no error set after POLLHUP")
                                });
                            return Err(e);
                        }

                        return Ok(());
                    }
                }
            }
        }

        pub fn accept(&self, storage: *mut sockaddr, len: *mut socklen_t)
                      -> io::Result<Socket> {
            // Unfortunately the only known way right now to accept a socket and
            // atomically set the CLOEXEC flag is to use the `accept4` syscall on
            // Linux. This was added in 2.6.28, however, and because we support
            // 2.6.18 we must detect this support dynamically.
            if cfg!(target_os = "linux") {
                syscall! {
                    fn accept4(
                        fd: c_int,
                        addr: *mut sockaddr,
                        addr_len: *mut socklen_t,
                        flags: c_int
                    ) -> c_int
                }
                let res = cvt_r(|| unsafe {
                    accept4(self.0.raw(), storage, len, SOCK_CLOEXEC)
                });
                match res {
                    Ok(fd) => return Ok(Socket(FileDesc::new(fd))),
                    Err(ref e) if e.raw_os_error() == Some(libc::ENOSYS) => {}
                    Err(e) => return Err(e),
                }
            }

            let fd = cvt_r(|| unsafe {
                libc::accept(self.0.raw(), storage, len)
            })?;
            let fd = FileDesc::new(fd);
            fd.set_cloexec()?;
            Ok(Socket(fd))
        }

        pub fn duplicate(&self) -> io::Result<Socket> {
            self.0.duplicate().map(Socket)
        }

        fn recv_with_flags(&self, buf: &mut [u8], flags: c_int) -> io::Result<usize> {
            let ret = cvt(unsafe {
                libc::recv(self.0.raw(),
                           buf.as_mut_ptr() as *mut c_void,
                           buf.len(),
                           flags)
            })?;
            Ok(ret as usize)
        }

        pub fn read(&self, buf: &mut [u8]) -> io::Result<usize> {
            self.recv_with_flags(buf, 0)
        }

        pub fn peek(&self, buf: &mut [u8]) -> io::Result<usize> {
            self.recv_with_flags(buf, MSG_PEEK)
        }

        pub fn read_vectored(&self, bufs: &mut [IoVecMut<'_>]) -> io::Result<usize> {
            self.0.read_vectored(bufs)
        }

        fn recv_from_with_flags(&self, buf: &mut [u8], flags: c_int)
                                -> io::Result<(usize, SocketAddr)> {
            let mut storage: libc::sockaddr_storage = unsafe { mem::zeroed() };
            let mut addrlen = mem::size_of_val(&storage) as libc::socklen_t;

            let n = cvt(unsafe {
                libc::recvfrom(self.0.raw(),
                            buf.as_mut_ptr() as *mut c_void,
                            buf.len(),
                            flags,
                            &mut storage as *mut _ as *mut _,
                            &mut addrlen)
            })?;
            Ok((n as usize, sockaddr_to_addr(&storage, addrlen as usize)?))
        }

        pub fn recv_from(&self, buf: &mut [u8]) -> io::Result<(usize, SocketAddr)> {
            self.recv_from_with_flags(buf, 0)
        }

        pub fn peek_from(&self, buf: &mut [u8]) -> io::Result<(usize, SocketAddr)> {
            self.recv_from_with_flags(buf, MSG_PEEK)
        }

        pub fn write(&self, buf: &[u8]) -> io::Result<usize> {
            self.0.write(buf)
        }

        pub fn write_vectored(&self, bufs: &[IoVec<'_>]) -> io::Result<usize> {
            self.0.write_vectored(bufs)
        }

        pub fn set_timeout(&self, dur: Option<Duration>, kind: libc::c_int) -> io::Result<()> {
            let timeout = match dur {
                Some(dur) => {
                    if dur.as_secs() == 0 && dur.subsec_nanos() == 0 {
                        return Err(io::Error::new(io::ErrorKind::InvalidInput,
                                                  "cannot set a 0 duration timeout"));
                    }

                    let secs = if dur.as_secs() > libc::time_t::max_value() as u64 {
                        libc::time_t::max_value()
                    } else {
                        dur.as_secs() as libc::time_t
                    };
                    let mut timeout = libc::timeval {
                        tv_sec: secs,
                        tv_usec: (dur.subsec_nanos() / 1000) as libc::suseconds_t,
                    };
                    if timeout.tv_sec == 0 && timeout.tv_usec == 0 {
                        timeout.tv_usec = 1;
                    }
                    timeout
                }
                None => {
                    libc::timeval {
                        tv_sec: 0,
                        tv_usec: 0,
                    }
                }
            };
            setsockopt(self, libc::SOL_SOCKET, kind, timeout)
        }

        pub fn timeout(&self, kind: libc::c_int) -> io::Result<Option<Duration>> {
            let raw: libc::timeval = getsockopt(self, libc::SOL_SOCKET, kind)?;
            if raw.tv_sec == 0 && raw.tv_usec == 0 {
                Ok(None)
            } else {
                let sec = raw.tv_sec as u64;
                let nsec = (raw.tv_usec as u32) * 1000;
                Ok(Some(Duration::new(sec, nsec)))
            }
        }

        pub fn shutdown(&self, how: Shutdown) -> io::Result<()> {
            let how = match how {
                Shutdown::Write => libc::SHUT_WR,
                Shutdown::Read => libc::SHUT_RD,
                Shutdown::Both => libc::SHUT_RDWR,
            };
            cvt(unsafe { libc::shutdown(self.0.raw(), how) })?;
            Ok(())
        }

        pub fn set_nodelay(&self, nodelay: bool) -> io::Result<()> {
            setsockopt(self, libc::IPPROTO_TCP, libc::TCP_NODELAY, nodelay as c_int)
        }

        pub fn nodelay(&self) -> io::Result<bool> {
            let raw: c_int = getsockopt(self, libc::IPPROTO_TCP, libc::TCP_NODELAY)?;
            Ok(raw != 0)
        }

        pub fn set_nonblocking(&self, nonblocking: bool) -> io::Result<()> {
            let mut nonblocking = nonblocking as libc::c_int;
            cvt(unsafe { libc::ioctl(*self.as_inner(), libc::FIONBIO, &mut nonblocking) }).map(|_| ())
        }

        pub fn take_error(&self) -> io::Result<Option<io::Error>> {
            let raw: c_int = getsockopt(self, libc::SOL_SOCKET, libc::SO_ERROR)?;
            if raw == 0 {
                Ok(None)
            } else {
                Ok(Some(io::Error::from_raw_os_error(raw as i32)))
            }
        }
    }

    impl AsInner<c_int> for Socket {
        fn as_inner(&self) -> &c_int { self.0.as_inner() }
    }

    impl FromInner<c_int> for Socket {
        fn from_inner(fd: c_int) -> Socket { Socket(FileDesc::new(fd)) }
    }

    impl IntoInner<c_int> for Socket {
        fn into_inner(self) -> c_int { self.0.into_raw() }
    }

    // In versions of glibc prior to 2.26, there's a bug where the DNS resolver
    // will cache the contents of /etc/resolv.conf, so changes to that file on disk
    // can be ignored by a long-running program. That can break DNS lookups on e.g.
    // laptops where the network comes and goes. See
    // https://sourceware.org/bugzilla/show_bug.cgi?id=984. Note however that some
    // distros including Debian have patched glibc to fix this for a long time.
    //
    // A workaround for this bug is to call the res_init libc function, to clear
    // the cached configs. Unfortunately, while we believe glibc's implementation
    // of res_init is thread-safe, we know that other implementations are not
    // (https://github.com/rust-lang/rust/issues/43592). Code here in libstd could
    // try to synchronize its res_init calls with a Mutex, but that wouldn't
    // protect programs that call into libc in other ways. So instead of calling
    // res_init unconditionally, we call it only when we detect we're linking
    // against glibc version < 2.26. (That is, when we both know its needed and
    // believe it's thread-safe).
    #[cfg(target_env = "gnu")]
    fn on_resolver_failure() {
        use crate::sys;

        // If the version fails to parse, we treat it the same as "not glibc".
        if let Some(version) = sys::os::glibc_version() {
            if version < (2, 26) {
                unsafe { libc::res_init() };
            }
        }
    }

    #[cfg(not(target_env = "gnu"))]
    fn on_resolver_failure() {}

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