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    #[cfg(any(target_os = "linux", target_os = "android"))]
    mod eventfd {
        use crate::sys::Selector;
        use crate::{Interest, Token};

        use std::fs::File;
        use std::io::{self, Read, Write};
        use std::os::unix::io::FromRawFd;

        /// Waker backed by `eventfd`.
        ///
        /// `eventfd` is effectively an 64 bit counter. All writes must be of 8
        /// bytes (64 bits) and are converted (native endian) into an 64 bit
        /// unsigned integer and added to the count. Reads must also be 8 bytes and
        /// reset the count to 0, returning the count.
        #[derive(Debug)]
        pub struct Waker {
            fd: File,
        }

        impl Waker {
            pub fn new(selector: &Selector, token: Token) -> io::Result<Waker> {
                syscall!(eventfd(0, libc::EFD_CLOEXEC | libc::EFD_NONBLOCK)).and_then(|fd| {
                    // Turn the file descriptor into a file first so we're ensured
                    // it's closed when dropped, e.g. when register below fails.
                    let file = unsafe { File::from_raw_fd(fd) };
                    selector
                        .register(fd, token, Interest::READABLE)
                        .map(|()| Waker { fd: file })
                })
            }

            pub fn wake(&self) -> io::Result<()> {
                let buf: [u8; 8] = 1u64.to_ne_bytes();
                match (&self.fd).write(&buf) {
                    Ok(_) => Ok(()),
                    Err(ref err) if err.kind() == io::ErrorKind::WouldBlock => {
                        // Writing only blocks if the counter is going to overflow.
                        // So we'll reset the counter to 0 and wake it again.
                        self.reset()?;
                        self.wake()
                    }
                    Err(err) => Err(err),
                }
            }

            /// Reset the eventfd object, only need to call this if `wake` fails.
            fn reset(&self) -> io::Result<()> {
                let mut buf: [u8; 8] = 0u64.to_ne_bytes();
                match (&self.fd).read(&mut buf) {
                    Ok(_) => Ok(()),
                    // If the `Waker` hasn't been awoken yet this will return a
                    // `WouldBlock` error which we can safely ignore.
                    Err(ref err) if err.kind() == io::ErrorKind::WouldBlock => Ok(()),
                    Err(err) => Err(err),
                }
            }
        }
    }

    #[cfg(any(target_os = "linux", target_os = "android"))]
    pub use self::eventfd::Waker;

    #[cfg(any(target_os = "freebsd", target_os = "ios", target_os = "macos"))]
    mod kqueue {
        use crate::sys::Selector;
        use crate::Token;

        use std::io;

        /// Waker backed by kqueue user space notifications (`EVFILT_USER`).
        ///
        /// The implementation is fairly simple, first the kqueue must be setup to
        /// receive waker events this done by calling `Selector.setup_waker`. Next
        /// we need access to kqueue, thus we need to duplicate the file descriptor.
        /// Now waking is as simple as adding an event to the kqueue.
        #[derive(Debug)]
        pub struct Waker {
            selector: Selector,
            token: Token,
        }

        impl Waker {
            pub fn new(selector: &Selector, token: Token) -> io::Result<Waker> {
                selector.try_clone().and_then(|selector| {
                    selector
                        .setup_waker(token)
                        .map(|()| Waker { selector, token })
                })
            }

            pub fn wake(&self) -> io::Result<()> {
                self.selector.wake(self.token)
            }
        }
    }

    #[cfg(any(target_os = "freebsd", target_os = "ios", target_os = "macos"))]
    pub use self::kqueue::Waker;

    #[cfg(any(
        target_os = "dragonfly",
        target_os = "illumos",
        target_os = "netbsd",
        target_os = "openbsd",
        target_os = "solaris"
    ))]
    mod pipe {
        use crate::sys::unix::Selector;
        use crate::{Interest, Token};

        use std::fs::File;
        use std::io::{self, Read, Write};
        use std::os::unix::io::FromRawFd;

        /// Waker backed by a unix pipe.
        ///
        /// Waker controls both the sending and receiving ends and empties the pipe
        /// if writing to it (waking) fails.
        #[derive(Debug)]
        pub struct Waker {
            sender: File,
            receiver: File,
        }

        impl Waker {
            pub fn new(selector: &Selector, token: Token) -> io::Result<Waker> {
                let mut fds = [-1; 2];
                syscall!(pipe2(fds.as_mut_ptr(), libc::O_NONBLOCK | libc::O_CLOEXEC))?;
                // Turn the file descriptors into files first so we're ensured
                // they're closed when dropped, e.g. when register below fails.
                let sender = unsafe { File::from_raw_fd(fds[1]) };
                let receiver = unsafe { File::from_raw_fd(fds[0]) };
                selector
                    .register(fds[0], token, Interest::READABLE)
                    .map(|()| Waker { sender, receiver })
            }

            pub fn wake(&self) -> io::Result<()> {
                // The epoll emulation on some illumos systems currently requires
                // the pipe buffer to be completely empty for an edge-triggered
                // wakeup on the pipe read side.
                #[cfg(target_os = "illumos")]
                self.empty();

                match (&self.sender).write(&[1]) {
                    Ok(_) => Ok(()),
                    Err(ref err) if err.kind() == io::ErrorKind::WouldBlock => {
                        // The reading end is full so we'll empty the buffer and try
                        // again.
                        self.empty();
                        self.wake()
                    }
                    Err(ref err) if err.kind() == io::ErrorKind::Interrupted => self.wake(),
                    Err(err) => Err(err),
                }
            }

            /// Empty the pipe's buffer, only need to call this if `wake` fails.
            /// This ignores any errors.
            fn empty(&self) {
                let mut buf = [0; 4096];
                loop {
                    match (&self.receiver).read(&mut buf) {
                        Ok(n) if n > 0 => continue,
                        _ => return,
                    }
                }
            }
        }
    }

    #[cfg(any(
        target_os = "dragonfly",
        target_os = "illumos",
        target_os = "netbsd",
        target_os = "openbsd",
        target_os = "solaris"
    ))]
    pub use self::pipe::Waker;