diff --git a/src/libstd/fs.rs b/src/libstd/fs.rs
index 2f2969b110db1..5bb5183fd6a93 100644
--- a/src/libstd/fs.rs
+++ b/src/libstd/fs.rs
@@ -1903,6 +1903,130 @@ mod tests {
         check!(fs::remove_file(filename));
     }
 
+    #[test]
+    fn file_test_io_eof() {
+        let tmpdir = tmpdir();
+        let filename = tmpdir.join("file_rt_io_file_test_eof.txt");
+        let mut buf = [0; 256];
+        {
+            let oo = OpenOptions::new().create_new(true).write(true).read(true).clone();
+            let mut rw = check!(oo.open(&filename));
+            assert_eq!(check!(rw.read(&mut buf)), 0);
+            assert_eq!(check!(rw.read(&mut buf)), 0);
+        }
+        check!(fs::remove_file(&filename));
+    }
+
+    #[test]
+    #[cfg(unix)]
+    fn file_test_io_read_write_at() {
+        use os::unix::fs::FileExt;
+
+        let tmpdir = tmpdir();
+        let filename = tmpdir.join("file_rt_io_file_test_read_write_at.txt");
+        let mut buf = [0; 256];
+        let write1 = "asdf";
+        let write2 = "qwer-";
+        let write3 = "-zxcv";
+        let content = "qwer-asdf-zxcv";
+        {
+            let oo = OpenOptions::new().create_new(true).write(true).read(true).clone();
+            let mut rw = check!(oo.open(&filename));
+            assert_eq!(check!(rw.write_at(write1.as_bytes(), 5)), write1.len());
+            assert_eq!(check!(rw.seek(SeekFrom::Current(0))), 0);
+            assert_eq!(check!(rw.read_at(&mut buf, 5)), write1.len());
+            assert_eq!(str::from_utf8(&buf[..write1.len()]), Ok(write1));
+            assert_eq!(check!(rw.seek(SeekFrom::Current(0))), 0);
+            assert_eq!(check!(rw.read_at(&mut buf[..write2.len()], 0)), write2.len());
+            assert_eq!(str::from_utf8(&buf[..write2.len()]), Ok("\0\0\0\0\0"));
+            assert_eq!(check!(rw.seek(SeekFrom::Current(0))), 0);
+            assert_eq!(check!(rw.write(write2.as_bytes())), write2.len());
+            assert_eq!(check!(rw.seek(SeekFrom::Current(0))), 5);
+            assert_eq!(check!(rw.read(&mut buf)), write1.len());
+            assert_eq!(str::from_utf8(&buf[..write1.len()]), Ok(write1));
+            assert_eq!(check!(rw.seek(SeekFrom::Current(0))), 9);
+            assert_eq!(check!(rw.read_at(&mut buf[..write2.len()], 0)), write2.len());
+            assert_eq!(str::from_utf8(&buf[..write2.len()]), Ok(write2));
+            assert_eq!(check!(rw.seek(SeekFrom::Current(0))), 9);
+            assert_eq!(check!(rw.write_at(write3.as_bytes(), 9)), write3.len());
+            assert_eq!(check!(rw.seek(SeekFrom::Current(0))), 9);
+        }
+        {
+            let mut read = check!(File::open(&filename));
+            assert_eq!(check!(read.read_at(&mut buf, 0)), content.len());
+            assert_eq!(str::from_utf8(&buf[..content.len()]), Ok(content));
+            assert_eq!(check!(read.seek(SeekFrom::Current(0))), 0);
+            assert_eq!(check!(read.seek(SeekFrom::End(-5))), 9);
+            assert_eq!(check!(read.read_at(&mut buf, 0)), content.len());
+            assert_eq!(str::from_utf8(&buf[..content.len()]), Ok(content));
+            assert_eq!(check!(read.seek(SeekFrom::Current(0))), 9);
+            assert_eq!(check!(read.read(&mut buf)), write3.len());
+            assert_eq!(str::from_utf8(&buf[..write3.len()]), Ok(write3));
+            assert_eq!(check!(read.seek(SeekFrom::Current(0))), 14);
+            assert_eq!(check!(read.read_at(&mut buf, 0)), content.len());
+            assert_eq!(str::from_utf8(&buf[..content.len()]), Ok(content));
+            assert_eq!(check!(read.seek(SeekFrom::Current(0))), 14);
+            assert_eq!(check!(read.read_at(&mut buf, 14)), 0);
+            assert_eq!(check!(read.read_at(&mut buf, 15)), 0);
+            assert_eq!(check!(read.seek(SeekFrom::Current(0))), 14);
+        }
+        check!(fs::remove_file(&filename));
+    }
+
+    #[test]
+    #[cfg(windows)]
+    fn file_test_io_seek_read_write() {
+        use os::windows::fs::FileExt;
+
+        let tmpdir = tmpdir();
+        let filename = tmpdir.join("file_rt_io_file_test_seek_read_write.txt");
+        let mut buf = [0; 256];
+        let write1 = "asdf";
+        let write2 = "qwer-";
+        let write3 = "-zxcv";
+        let content = "qwer-asdf-zxcv";
+        {
+            let oo = OpenOptions::new().create_new(true).write(true).read(true).clone();
+            let mut rw = check!(oo.open(&filename));
+            assert_eq!(check!(rw.seek_write(write1.as_bytes(), 5)), write1.len());
+            assert_eq!(check!(rw.seek(SeekFrom::Current(0))), 9);
+            assert_eq!(check!(rw.seek_read(&mut buf, 5)), write1.len());
+            assert_eq!(str::from_utf8(&buf[..write1.len()]), Ok(write1));
+            assert_eq!(check!(rw.seek(SeekFrom::Current(0))), 9);
+            assert_eq!(check!(rw.seek(SeekFrom::Start(0))), 0);
+            assert_eq!(check!(rw.write(write2.as_bytes())), write2.len());
+            assert_eq!(check!(rw.seek(SeekFrom::Current(0))), 5);
+            assert_eq!(check!(rw.read(&mut buf)), write1.len());
+            assert_eq!(str::from_utf8(&buf[..write1.len()]), Ok(write1));
+            assert_eq!(check!(rw.seek(SeekFrom::Current(0))), 9);
+            assert_eq!(check!(rw.seek_read(&mut buf[..write2.len()], 0)), write2.len());
+            assert_eq!(str::from_utf8(&buf[..write2.len()]), Ok(write2));
+            assert_eq!(check!(rw.seek(SeekFrom::Current(0))), 5);
+            assert_eq!(check!(rw.seek_write(write3.as_bytes(), 9)), write3.len());
+            assert_eq!(check!(rw.seek(SeekFrom::Current(0))), 14);
+        }
+        {
+            let mut read = check!(File::open(&filename));
+            assert_eq!(check!(read.seek_read(&mut buf, 0)), content.len());
+            assert_eq!(str::from_utf8(&buf[..content.len()]), Ok(content));
+            assert_eq!(check!(read.seek(SeekFrom::Current(0))), 14);
+            assert_eq!(check!(read.seek(SeekFrom::End(-5))), 9);
+            assert_eq!(check!(read.seek_read(&mut buf, 0)), content.len());
+            assert_eq!(str::from_utf8(&buf[..content.len()]), Ok(content));
+            assert_eq!(check!(read.seek(SeekFrom::Current(0))), 14);
+            assert_eq!(check!(read.seek(SeekFrom::End(-5))), 9);
+            assert_eq!(check!(read.read(&mut buf)), write3.len());
+            assert_eq!(str::from_utf8(&buf[..write3.len()]), Ok(write3));
+            assert_eq!(check!(read.seek(SeekFrom::Current(0))), 14);
+            assert_eq!(check!(read.seek_read(&mut buf, 0)), content.len());
+            assert_eq!(str::from_utf8(&buf[..content.len()]), Ok(content));
+            assert_eq!(check!(read.seek(SeekFrom::Current(0))), 14);
+            assert_eq!(check!(read.seek_read(&mut buf, 14)), 0);
+            assert_eq!(check!(read.seek_read(&mut buf, 15)), 0);
+        }
+        check!(fs::remove_file(&filename));
+    }
+
     #[test]
     fn file_test_stat_is_correct_on_is_file() {
         let tmpdir = tmpdir();
@@ -2221,8 +2345,8 @@ mod tests {
         check!(fs::set_permissions(&out, attr.permissions()));
     }
 
-    #[cfg(windows)]
     #[test]
+    #[cfg(windows)]
     fn copy_file_preserves_streams() {
         let tmp = tmpdir();
         check!(check!(File::create(tmp.join("in.txt:bunny"))).write("carrot".as_bytes()));
diff --git a/src/libstd/sys/unix/android.rs b/src/libstd/sys/unix/android.rs
index abbe3fc1846bd..10436723a81d0 100644
--- a/src/libstd/sys/unix/android.rs
+++ b/src/libstd/sys/unix/android.rs
@@ -28,10 +28,11 @@
 
 #![cfg(target_os = "android")]
 
-use libc::{c_int, sighandler_t};
+use libc::{c_int, c_void, sighandler_t, size_t, ssize_t};
+use libc::{ftruncate, pread, pwrite};
 
 use io;
-use sys::cvt_r;
+use super::{cvt, cvt_r};
 
 // The `log2` and `log2f` functions apparently appeared in android-18, or at
 // least you can see they're not present in the android-17 header [1] and they
@@ -96,13 +97,10 @@ pub unsafe fn signal(signum: c_int, handler: sighandler_t) -> sighandler_t {
 //
 // If it doesn't we just fall back to `ftruncate`, generating an error for
 // too-large values.
+#[cfg(target_pointer_width = "32")]
 pub fn ftruncate64(fd: c_int, size: u64) -> io::Result<()> {
     weak!(fn ftruncate64(c_int, i64) -> c_int);
 
-    extern {
-        fn ftruncate(fd: c_int, off: i32) -> c_int;
-    }
-
     unsafe {
         match ftruncate64.get() {
             Some(f) => cvt_r(|| f(fd, size as i64)).map(|_| ()),
@@ -117,3 +115,56 @@ pub fn ftruncate64(fd: c_int, size: u64) -> io::Result<()> {
         }
     }
 }
+
+#[cfg(target_pointer_width = "64")]
+pub fn ftruncate64(fd: c_int, size: u64) -> io::Result<()> {
+    unsafe {
+        cvt_r(|| ftruncate(fd, size as i64)).map(|_| ())
+    }
+}
+
+#[cfg(target_pointer_width = "32")]
+pub unsafe fn cvt_pread64(fd: c_int, buf: *mut c_void, count: size_t, offset: i64)
+    -> io::Result<ssize_t>
+{
+    use convert::TryInto;
+    weak!(fn pread64(c_int, *mut c_void, size_t, i64) -> ssize_t);
+    pread64.get().map(|f| cvt(f(fd, buf, count, offset))).unwrap_or_else(|| {
+        if let Ok(o) = offset.try_into() {
+            cvt(pread(fd, buf, count, o))
+        } else {
+            Err(io::Error::new(io::ErrorKind::InvalidInput,
+                               "cannot pread >2GB"))
+        }
+    })
+}
+
+#[cfg(target_pointer_width = "32")]
+pub unsafe fn cvt_pwrite64(fd: c_int, buf: *const c_void, count: size_t, offset: i64)
+    -> io::Result<ssize_t>
+{
+    use convert::TryInto;
+    weak!(fn pwrite64(c_int, *const c_void, size_t, i64) -> ssize_t);
+    pwrite64.get().map(|f| cvt(f(fd, buf, count, offset))).unwrap_or_else(|| {
+        if let Ok(o) = offset.try_into() {
+            cvt(pwrite(fd, buf, count, o))
+        } else {
+            Err(io::Error::new(io::ErrorKind::InvalidInput,
+                               "cannot pwrite >2GB"))
+        }
+    })
+}
+
+#[cfg(target_pointer_width = "64")]
+pub unsafe fn cvt_pread64(fd: c_int, buf: *mut c_void, count: size_t, offset: i64)
+    -> io::Result<ssize_t>
+{
+    cvt(pread(fd, buf, count, offset))
+}
+
+#[cfg(target_pointer_width = "64")]
+pub unsafe fn cvt_pwrite64(fd: c_int, buf: *const c_void, count: size_t, offset: i64)
+    -> io::Result<ssize_t>
+{
+    cvt(pwrite(fd, buf, count, offset))
+}
diff --git a/src/libstd/sys/unix/ext/fs.rs b/src/libstd/sys/unix/ext/fs.rs
index 77587918ac94b..fcfab05158841 100644
--- a/src/libstd/sys/unix/ext/fs.rs
+++ b/src/libstd/sys/unix/ext/fs.rs
@@ -20,6 +20,51 @@ use sys;
 use sys_common::{FromInner, AsInner, AsInnerMut};
 use sys::platform::fs::MetadataExt as UnixMetadataExt;
 
+/// Unix-specific extensions to `File`
+#[unstable(feature = "file_offset", issue = "35918")]
+pub trait FileExt {
+    /// Reads a number of bytes starting from a given offset.
+    ///
+    /// Returns the number of bytes read.
+    ///
+    /// The offset is relative to the start of the file and thus independent
+    /// from the current cursor.
+    ///
+    /// The current file cursor is not affected by this function.
+    ///
+    /// Note that similar to `File::read`, it is not an error to return with a
+    /// short read.
+    #[unstable(feature = "file_offset", issue = "35918")]
+    fn read_at(&self, buf: &mut [u8], offset: u64) -> io::Result<usize>;
+
+    /// Writes a number of bytes starting from a given offset.
+    ///
+    /// Returns the number of bytes written.
+    ///
+    /// The offset is relative to the start of the file and thus independent
+    /// from the current cursor.
+    ///
+    /// The current file cursor is not affected by this function.
+    ///
+    /// When writing beyond the end of the file, the file is appropiately
+    /// extended and the intermediate bytes are initialized with the value 0.
+    ///
+    /// Note that similar to `File::write`, it is not an error to return a
+    /// short write.
+    #[unstable(feature = "file_offset", issue = "35918")]
+    fn write_at(&self, buf: &[u8], offset: u64) -> io::Result<usize>;
+}
+
+#[unstable(feature = "file_offset", issue = "35918")]
+impl FileExt for fs::File {
+    fn read_at(&self, buf: &mut [u8], offset: u64) -> io::Result<usize> {
+        self.as_inner().read_at(buf, offset)
+    }
+    fn write_at(&self, buf: &[u8], offset: u64) -> io::Result<usize> {
+        self.as_inner().write_at(buf, offset)
+    }
+}
+
 /// Unix-specific extensions to `Permissions`
 #[stable(feature = "fs_ext", since = "1.1.0")]
 pub trait PermissionsExt {
diff --git a/src/libstd/sys/unix/ext/mod.rs b/src/libstd/sys/unix/ext/mod.rs
index 1be3d75d866dd..b2483f4e20937 100644
--- a/src/libstd/sys/unix/ext/mod.rs
+++ b/src/libstd/sys/unix/ext/mod.rs
@@ -50,6 +50,8 @@ pub mod prelude {
     pub use super::fs::{PermissionsExt, OpenOptionsExt, MetadataExt, FileTypeExt};
     #[doc(no_inline)] #[stable(feature = "rust1", since = "1.0.0")]
     pub use super::fs::DirEntryExt;
+    #[doc(no_inline)] #[unstable(feature = "file_offset", issue = "35918")]
+    pub use super::fs::FileExt;
     #[doc(no_inline)] #[stable(feature = "rust1", since = "1.0.0")]
     pub use super::thread::JoinHandleExt;
     #[doc(no_inline)] #[stable(feature = "rust1", since = "1.0.0")]
diff --git a/src/libstd/sys/unix/fd.rs b/src/libstd/sys/unix/fd.rs
index eadf98867a6b7..41bb96fed16cb 100644
--- a/src/libstd/sys/unix/fd.rs
+++ b/src/libstd/sys/unix/fd.rs
@@ -50,6 +50,30 @@ impl FileDesc {
         (&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(any(target_os = "linux", target_os = "emscripten"))]
+            use libc::pread64;
+            #[cfg(not(any(target_os = "linux", target_os = "emscripten")))]
+            use libc::pread as pread64;
+            cvt(pread64(fd, buf, count, offset))
+        }
+
+        unsafe {
+            cvt_pread64(self.fd,
+                        buf.as_mut_ptr() as *mut c_void,
+                        buf.len(),
+                        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,
@@ -59,6 +83,30 @@ impl FileDesc {
         Ok(ret as usize)
     }
 
+    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(any(target_os = "linux", target_os = "emscripten"))]
+            use libc::pwrite64;
+            #[cfg(not(any(target_os = "linux", target_os = "emscripten")))]
+            use libc::pwrite as pwrite64;
+            cvt(pwrite64(fd, buf, count, offset))
+        }
+
+        unsafe {
+            cvt_pwrite64(self.fd,
+                         buf.as_ptr() as *const c_void,
+                         buf.len(),
+                         offset as i64)
+                .map(|n| n as usize)
+        }
+    }
+
     #[cfg(not(any(target_env = "newlib",
                   target_os = "solaris",
                   target_os = "emscripten",
diff --git a/src/libstd/sys/unix/fs.rs b/src/libstd/sys/unix/fs.rs
index 606e2c2264ad0..fe8cbc8421572 100644
--- a/src/libstd/sys/unix/fs.rs
+++ b/src/libstd/sys/unix/fs.rs
@@ -483,10 +483,18 @@ impl File {
         self.0.read_to_end(buf)
     }
 
+    pub fn read_at(&self, buf: &mut [u8], offset: u64) -> io::Result<usize> {
+        self.0.read_at(buf, offset)
+    }
+
     pub fn write(&self, buf: &[u8]) -> io::Result<usize> {
         self.0.write(buf)
     }
 
+    pub fn write_at(&self, buf: &[u8], offset: u64) -> io::Result<usize> {
+        self.0.write_at(buf, offset)
+    }
+
     pub fn flush(&self) -> io::Result<()> { Ok(()) }
 
     pub fn seek(&self, pos: SeekFrom) -> io::Result<u64> {
diff --git a/src/libstd/sys/windows/ext/fs.rs b/src/libstd/sys/windows/ext/fs.rs
index 4388a0bdff2cf..1e2b8bf38fa71 100644
--- a/src/libstd/sys/windows/ext/fs.rs
+++ b/src/libstd/sys/windows/ext/fs.rs
@@ -12,12 +12,61 @@
 
 #![stable(feature = "rust1", since = "1.0.0")]
 
-use fs::{OpenOptions, Metadata};
+use fs::{self, OpenOptions, Metadata};
 use io;
 use path::Path;
 use sys;
 use sys_common::{AsInnerMut, AsInner};
 
+/// Windows-specific extensions to `File`
+#[unstable(feature = "file_offset", issue = "35918")]
+pub trait FileExt {
+    /// Seeks to a given position and reads a number of bytes.
+    ///
+    /// Returns the number of bytes read.
+    ///
+    /// The offset is relative to the start of the file and thus independent
+    /// from the current cursor. The current cursor **is** affected by this
+    /// function, it is set to the end of the read.
+    ///
+    /// Reading beyond the end of the file will always return with a length of
+    /// 0.
+    ///
+    /// Note that similar to `File::read`, it is not an error to return with a
+    /// short read. When returning from such a short read, the file pointer is
+    /// still updated.
+    #[unstable(feature = "file_offset", issue = "35918")]
+    fn seek_read(&self, buf: &mut [u8], offset: u64) -> io::Result<usize>;
+
+    /// Seeks to a given position and writes a number of bytes.
+    ///
+    /// Returns the number of bytes written.
+    ///
+    /// The offset is relative to the start of the file and thus independent
+    /// from the current cursor. The current cursor **is** affected by this
+    /// function, it is set to the end of the write.
+    ///
+    /// When writing beyond the end of the file, the file is appropiately
+    /// extended and the intermediate bytes are left uninitialized.
+    ///
+    /// Note that similar to `File::write`, it is not an error to return a
+    /// short write. When returning from such a short write, the file pointer
+    /// is still updated.
+    #[unstable(feature = "file_offset", issue = "35918")]
+    fn seek_write(&self, buf: &[u8], offset: u64) -> io::Result<usize>;
+}
+
+#[unstable(feature = "file_offset", issue = "35918")]
+impl FileExt for fs::File {
+    fn seek_read(&self, buf: &mut [u8], offset: u64) -> io::Result<usize> {
+        self.as_inner().read_at(buf, offset)
+    }
+
+    fn seek_write(&self, buf: &[u8], offset: u64) -> io::Result<usize> {
+        self.as_inner().write_at(buf, offset)
+    }
+}
+
 /// Windows-specific extensions to `OpenOptions`
 #[stable(feature = "open_options_ext", since = "1.10.0")]
 pub trait OpenOptionsExt {
diff --git a/src/libstd/sys/windows/ext/mod.rs b/src/libstd/sys/windows/ext/mod.rs
index c3578fdfdb16b..932bb5e956405 100644
--- a/src/libstd/sys/windows/ext/mod.rs
+++ b/src/libstd/sys/windows/ext/mod.rs
@@ -36,4 +36,6 @@ pub mod prelude {
     pub use super::ffi::{OsStrExt, OsStringExt};
     #[doc(no_inline)] #[stable(feature = "rust1", since = "1.0.0")]
     pub use super::fs::{OpenOptionsExt, MetadataExt};
+    #[doc(no_inline)] #[unstable(feature = "file_offset", issue = "35918")]
+    pub use super::fs::FileExt;
 }
diff --git a/src/libstd/sys/windows/fs.rs b/src/libstd/sys/windows/fs.rs
index 90a16853d56dd..a927eae020dcf 100644
--- a/src/libstd/sys/windows/fs.rs
+++ b/src/libstd/sys/windows/fs.rs
@@ -311,6 +311,10 @@ impl File {
         self.handle.read(buf)
     }
 
+    pub fn read_at(&self, buf: &mut [u8], offset: u64) -> io::Result<usize> {
+        self.handle.read_at(buf, offset)
+    }
+
     pub fn read_to_end(&self, buf: &mut Vec<u8>) -> io::Result<usize> {
         self.handle.read_to_end(buf)
     }
@@ -319,6 +323,10 @@ impl File {
         self.handle.write(buf)
     }
 
+    pub fn write_at(&self, buf: &[u8], offset: u64) -> io::Result<usize> {
+        self.handle.write_at(buf, offset)
+    }
+
     pub fn flush(&self) -> io::Result<()> { Ok(()) }
 
     pub fn seek(&self, pos: SeekFrom) -> io::Result<u64> {
diff --git a/src/libstd/sys/windows/handle.rs b/src/libstd/sys/windows/handle.rs
index 97e746ee34576..10b86ba44bc79 100644
--- a/src/libstd/sys/windows/handle.rs
+++ b/src/libstd/sys/windows/handle.rs
@@ -104,6 +104,23 @@ impl RawHandle {
         }
     }
 
+    pub fn read_at(&self, buf: &mut [u8], offset: u64) -> io::Result<usize> {
+        let mut read = 0;
+        let len = cmp::min(buf.len(), <c::DWORD>::max_value() as usize) as c::DWORD;
+        let res = unsafe {
+            let mut overlapped: c::OVERLAPPED = mem::zeroed();
+            overlapped.Offset = offset as u32;
+            overlapped.OffsetHigh = (offset >> 32) as u32;
+            cvt(c::ReadFile(self.0, buf.as_mut_ptr() as c::LPVOID,
+                            len, &mut read, &mut overlapped))
+        };
+        match res {
+            Ok(_) => Ok(read as usize),
+            Err(ref e) if e.raw_os_error() == Some(c::ERROR_HANDLE_EOF as i32) => Ok(0),
+            Err(e) => Err(e),
+        }
+    }
+
     pub unsafe fn read_overlapped(&self,
                                   buf: &mut [u8],
                                   overlapped: *mut c::OVERLAPPED)
@@ -174,6 +191,19 @@ impl RawHandle {
         Ok(amt as usize)
     }
 
+    pub fn write_at(&self, buf: &[u8], offset: u64) -> io::Result<usize> {
+        let mut written = 0;
+        let len = cmp::min(buf.len(), <c::DWORD>::max_value() as usize) as c::DWORD;
+        unsafe {
+            let mut overlapped: c::OVERLAPPED = mem::zeroed();
+            overlapped.Offset = offset as u32;
+            overlapped.OffsetHigh = (offset >> 32) as u32;
+            cvt(c::WriteFile(self.0, buf.as_ptr() as c::LPVOID,
+                             len, &mut written, &mut overlapped))?;
+        }
+        Ok(written as usize)
+    }
+
     pub fn duplicate(&self, access: c::DWORD, inherit: bool,
                      options: c::DWORD) -> io::Result<Handle> {
         let mut ret = 0 as c::HANDLE;