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    #![unstable(reason = "not public", issue = "none", feature = "fd")]

    #[cfg(test)]
    mod tests;

    use crate::cmp;
    use crate::io::{self, Initializer, IoSlice, IoSliceMut, Read};
    use crate::mem;
    use crate::sys::cvt;
    use crate::sys_common::AsInner;

    use libc::{c_int, c_void};

    #[derive(Debug)]
    pub struct FileDesc {
        fd: c_int,
    }

    // The maximum read limit on most POSIX-like systems is `SSIZE_MAX`,
    // with the man page quoting that if the count of bytes to read is
    // greater than `SSIZE_MAX` the result is "unspecified".
    //
    // On macOS, however, apparently the 64-bit libc is either buggy or
    // intentionally showing odd behavior by rejecting any read with a size
    // larger than or equal to INT_MAX. To handle both of these the read
    // size is capped on both platforms.
    #[cfg(target_os = "macos")]
    const READ_LIMIT: usize = c_int::MAX as usize - 1;
    #[cfg(not(target_os = "macos"))]
    const READ_LIMIT: usize = libc::ssize_t::MAX as usize;

    #[cfg(any(
        target_os = "dragonfly",
        target_os = "freebsd",
        target_os = "ios",
        target_os = "macos",
        target_os = "netbsd",
        target_os = "openbsd",
    ))]
    const fn max_iov() -> usize {
        libc::IOV_MAX as usize
    }

    #[cfg(any(target_os = "android", target_os = "emscripten", target_os = "linux"))]
    const fn max_iov() -> usize {
        libc::UIO_MAXIOV as usize
    }

    #[cfg(not(any(
        target_os = "android",
        target_os = "dragonfly",
        target_os = "emscripten",
        target_os = "freebsd",
        target_os = "ios",
        target_os = "linux",
        target_os = "macos",
        target_os = "netbsd",
        target_os = "openbsd",
    )))]
    const fn max_iov() -> usize {
        16 // The minimum value required by POSIX.
    }

    impl FileDesc {
        pub fn new(fd: c_int) -> FileDesc {
            FileDesc { fd }
        }

        pub fn raw(&self) -> c_int {
            self.fd
        }

        /// Extracts the actual file descriptor without closing it.
        pub fn into_raw(self) -> c_int {
            let fd = self.fd;
            mem::forget(self);
            fd
        }

        pub fn read(&self, buf: &mut [u8]) -> io::Result<usize> {
            let ret = cvt(unsafe {
                libc::read(self.fd, buf.as_mut_ptr() as *mut c_void, cmp::min(buf.len(), READ_LIMIT))
            })?;
            Ok(ret as usize)
        }

        pub fn read_vectored(&self, bufs: &mut [IoSliceMut<'_>]) -> io::Result<usize> {
            let ret = cvt(unsafe {
                libc::readv(
                    self.fd,
                    bufs.as_ptr() as *const libc::iovec,
                    cmp::min(bufs.len(), max_iov()) as c_int,
                )
            })?;
            Ok(ret as usize)
        }

        #[inline]
        pub fn is_read_vectored(&self) -> bool {
            true
        }

        pub fn read_to_end(&self, buf: &mut Vec<u8>) -> io::Result<usize> {
            let mut me = self;
            (&mut me).read_to_end(buf)
        }

        pub fn read_at(&self, buf: &mut [u8], offset: u64) -> io::Result<usize> {
            #[cfg(target_os = "android")]
            use super::android::cvt_pread64;

            #[cfg(not(target_os = "android"))]
            unsafe fn cvt_pread64(
                fd: c_int,
                buf: *mut c_void,
                count: usize,
                offset: i64,
            ) -> io::Result<isize> {
                #[cfg(not(target_os = "linux"))]
                use libc::pread as pread64;
                #[cfg(target_os = "linux")]
                use libc::pread64;
                cvt(pread64(fd, buf, count, offset))
            }

            unsafe {
                cvt_pread64(
                    self.fd,
                    buf.as_mut_ptr() as *mut c_void,
                    cmp::min(buf.len(), READ_LIMIT),
                    offset as i64,
                )
                .map(|n| n as usize)
            }
        }

        pub fn write(&self, buf: &[u8]) -> io::Result<usize> {
            let ret = cvt(unsafe {
                libc::write(self.fd, buf.as_ptr() as *const c_void, cmp::min(buf.len(), READ_LIMIT))
            })?;
            Ok(ret as usize)
        }

        pub fn write_vectored(&self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
            let ret = cvt(unsafe {
                libc::writev(
                    self.fd,
                    bufs.as_ptr() as *const libc::iovec,
                    cmp::min(bufs.len(), max_iov()) as c_int,
                )
            })?;
            Ok(ret as usize)
        }

        #[inline]
        pub fn is_write_vectored(&self) -> bool {
            true
        }

        pub fn write_at(&self, buf: &[u8], offset: u64) -> io::Result<usize> {
            #[cfg(target_os = "android")]
            use super::android::cvt_pwrite64;

            #[cfg(not(target_os = "android"))]
            unsafe fn cvt_pwrite64(
                fd: c_int,
                buf: *const c_void,
                count: usize,
                offset: i64,
            ) -> io::Result<isize> {
                #[cfg(not(target_os = "linux"))]
                use libc::pwrite as pwrite64;
                #[cfg(target_os = "linux")]
                use libc::pwrite64;
                cvt(pwrite64(fd, buf, count, offset))
            }

            unsafe {
                cvt_pwrite64(
                    self.fd,
                    buf.as_ptr() as *const c_void,
                    cmp::min(buf.len(), READ_LIMIT),
                    offset as i64,
                )
                .map(|n| n as usize)
            }
        }

        #[cfg(target_os = "linux")]
        pub fn get_cloexec(&self) -> io::Result<bool> {
            unsafe { Ok((cvt(libc::fcntl(self.fd, libc::F_GETFD))? & libc::FD_CLOEXEC) != 0) }
        }

        #[cfg(not(any(
            target_env = "newlib",
            target_os = "solaris",
            target_os = "illumos",
            target_os = "emscripten",
            target_os = "fuchsia",
            target_os = "l4re",
            target_os = "linux",
            target_os = "haiku",
            target_os = "redox"
        )))]
        pub fn set_cloexec(&self) -> io::Result<()> {
            unsafe {
                cvt(libc::ioctl(self.fd, libc::FIOCLEX))?;
                Ok(())
            }
        }
        #[cfg(any(
            target_env = "newlib",
            target_os = "solaris",
            target_os = "illumos",
            target_os = "emscripten",
            target_os = "fuchsia",
            target_os = "l4re",
            target_os = "linux",
            target_os = "haiku",
            target_os = "redox"
        ))]
        pub fn set_cloexec(&self) -> io::Result<()> {
            unsafe {
                let previous = cvt(libc::fcntl(self.fd, libc::F_GETFD))?;
                let new = previous | libc::FD_CLOEXEC;
                if new != previous {
                    cvt(libc::fcntl(self.fd, libc::F_SETFD, new))?;
                }
                Ok(())
            }
        }

        #[cfg(target_os = "linux")]
        pub fn set_nonblocking(&self, nonblocking: bool) -> io::Result<()> {
            unsafe {
                let v = nonblocking as c_int;
                cvt(libc::ioctl(self.fd, libc::FIONBIO, &v))?;
                Ok(())
            }
        }

        #[cfg(not(target_os = "linux"))]
        pub fn set_nonblocking(&self, nonblocking: bool) -> io::Result<()> {
            unsafe {
                let previous = cvt(libc::fcntl(self.fd, libc::F_GETFL))?;
                let new = if nonblocking {
                    previous | libc::O_NONBLOCK
                } else {
                    previous & !libc::O_NONBLOCK
                };
                if new != previous {
                    cvt(libc::fcntl(self.fd, libc::F_SETFL, new))?;
                }
                Ok(())
            }
        }

        pub fn duplicate(&self) -> io::Result<FileDesc> {
            // We want to atomically duplicate this file descriptor and set the
            // CLOEXEC flag, and currently that's done via F_DUPFD_CLOEXEC. This
            // is a POSIX flag that was added to Linux in 2.6.24.
            let fd = cvt(unsafe { libc::fcntl(self.raw(), libc::F_DUPFD_CLOEXEC, 0) })?;
            Ok(FileDesc::new(fd))
        }
    }

    impl<'a> Read for &'a FileDesc {
        fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
            (**self).read(buf)
        }

        #[inline]
        unsafe fn initializer(&self) -> Initializer {
            Initializer::nop()
        }
    }

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

    impl Drop for FileDesc {
        fn drop(&mut self) {
            // Note that errors are ignored when closing a file descriptor. The
            // reason for this is that if an error occurs we don't actually know if
            // the file descriptor was closed or not, and if we retried (for
            // something like EINTR), we might close another valid file descriptor
            // opened after we closed ours.
            let _ = unsafe { libc::close(self.fd) };
        }
    }