/// Get directory entries.
///
/// # Example
///
/// ```
/// let path = "/etc";
/// let ret = unsafe { nc::openat(nc::AT_FDCWD, path, nc::O_DIRECTORY, 0) };
/// assert!(ret.is_ok());
/// let fd = ret.unwrap();
///
/// const BUF_SIZE: usize = 4 * 1024;
/// loop {
///     // TODO(Shaohua): Only allocate one buf block.
///     let mut buf: Vec<u8> = vec![0; BUF_SIZE];
///     let ret = unsafe { nc::getdents(fd, buf.as_mut_ptr() as usize, BUF_SIZE) };
///     assert!(ret.is_ok());
///
///     let buf_box = buf.into_boxed_slice();
///     let buf_box_ptr = Box::into_raw(buf_box) as *mut u8 as usize;
///     let nread = ret.unwrap() as usize;
///     if nread == 0 {
///         break;
///     }
///
///     let mut bpos: usize = 0;
///     while bpos < nread {
///         let d = (buf_box_ptr + bpos) as *mut nc::linux_dirent_t;
///         let d_ref = unsafe { &(*d) };
///         let mut name_vec: Vec<u8> = vec![];
///         // TODO(Shaohua): Calculate string len of name.
///         for i in 0..nc::PATH_MAX {
///             let c = d_ref.d_name[i as usize];
///             if c == 0 {
///                 break;
///             }
///             name_vec.push(c);
///         }
///         let _name = String::from_utf8(name_vec).unwrap();
///
///         bpos += d_ref.d_reclen as usize;
///     }
/// }
///
/// let ret = unsafe { nc::close(fd) };
/// assert!(ret.is_ok());
/// ```
pub unsafe fn getdents(fd: i32, dirp: usize, count: size_t) -> Result<ssize_t, Errno> {
    let fd = fd as usize;
    syscall3(SYS_GETDENTS, fd, dirp, count).map(|ret| ret as ssize_t)
}
