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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.

    #![unstable(reason = "not public", issue = "0", feature = "fd")]

    use prelude::v1::*;

    use io::{self, Read};
    use libc::{self, c_int, size_t, c_void};
    use mem;
    use sync::atomic::{AtomicBool, Ordering};
    use sys::cvt;
    use sys_common::AsInner;
    use sys_common::io::read_to_end_uninitialized;

    pub struct FileDesc {
        fd: c_int,
    }

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

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

        /// Extracts the actual filedescriptor 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,
                           buf.len() as size_t)
            })?;
            Ok(ret as usize)
        }

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

        #[cfg(not(any(target_env = "newlib", target_os = "solaris", target_os = "emscripten")))]
        pub fn set_cloexec(&self) {
            unsafe {
                let ret = libc::ioctl(self.fd, libc::FIOCLEX);
                debug_assert_eq!(ret, 0);
            }
        }
        #[cfg(any(target_env = "newlib", target_os = "solaris", target_os = "emscripten"))]
        pub fn set_cloexec(&self) {
            unsafe {
                let previous = libc::fcntl(self.fd, libc::F_GETFD);
                let ret = libc::fcntl(self.fd, libc::F_SETFD, previous | libc::FD_CLOEXEC);
                debug_assert_eq!(ret, 0);
            }
        }

        pub fn set_nonblocking(&self, nonblocking: bool) {
            unsafe {
                let previous = libc::fcntl(self.fd, libc::F_GETFL);
                debug_assert!(previous != -1);
                let new = if nonblocking {
                    previous | libc::O_NONBLOCK
                } else {
                    previous & !libc::O_NONBLOCK
                };
                let ret = libc::fcntl(self.fd, libc::F_SETFL, new);
                debug_assert!(ret != -1);
            }
        }

        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
            // flag, however, isn't supported on older Linux kernels (earlier than
            // 2.6.24).
            //
            // To detect this and ensure that CLOEXEC is still set, we
            // follow a strategy similar to musl [1] where if passing
            // F_DUPFD_CLOEXEC causes `fcntl` to return EINVAL it means it's not
            // supported (the third parameter, 0, is always valid), so we stop
            // trying that.
            //
            // Also note that Android doesn't have F_DUPFD_CLOEXEC, but get it to
            // resolve so we at least compile this.
            //
            // [1]: http://comments.gmane.org/gmane.linux.lib.musl.general/2963
            #[cfg(target_os = "android")]
            use libc::F_DUPFD as F_DUPFD_CLOEXEC;
            #[cfg(not(target_os = "android"))]
            use libc::F_DUPFD_CLOEXEC;

            let make_filedesc = |fd| {
                let fd = FileDesc::new(fd);
                fd.set_cloexec();
                fd
            };
            static TRY_CLOEXEC: AtomicBool =
                AtomicBool::new(!cfg!(target_os = "android"));
            let fd = self.raw();
            if TRY_CLOEXEC.load(Ordering::Relaxed) {
                match cvt(unsafe { libc::fcntl(fd, F_DUPFD_CLOEXEC, 0) }) {
                    // We *still* call the `set_cloexec` method as apparently some
                    // linux kernel at some point stopped setting CLOEXEC even
                    // though it reported doing so on F_DUPFD_CLOEXEC.
                    Ok(fd) => {
                        return Ok(if cfg!(target_os = "linux") {
                            make_filedesc(fd)
                        } else {
                            FileDesc::new(fd)
                        })
                    }
                    Err(ref e) if e.raw_os_error() == Some(libc::EINVAL) => {
                        TRY_CLOEXEC.store(false, Ordering::Relaxed);
                    }
                    Err(e) => return Err(e),
                }
            }
            cvt(unsafe { libc::fcntl(fd, libc::F_DUPFD, 0) }).map(make_filedesc)
        }
    }

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

        fn read_to_end(&mut self, buf: &mut Vec<u8>) -> io::Result<usize> {
            unsafe { read_to_end_uninitialized(self, buf) }
        }
    }

    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) };
        }
    }

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