/// Get directory entries.
///
/// # Examples
///
/// ```rust
/// const BUF_SIZE: usize = 1 * 1024;
///
/// let path = "/etc";
/// let ret = unsafe { nc::openat(nc::AT_FDCWD, path, nc::O_DIRECTORY, 0) };
/// assert!(ret.is_ok());
/// let fd = ret.unwrap();
/// let mut buf = [0; BUF_SIZE];
///
/// loop {
///     let ret = unsafe { nc::getdents64(fd, &mut buf) };
///     assert!(ret.is_ok());
///     let nread = ret.unwrap() as usize;
///     if nread == 0 {
///         break;
///     }
///
///     let buf_ptr = buf.as_ptr() as usize;
///     let mut bpos: usize = 0;
///
///     println!("--------------- nread={nread} ---------------");
///     println!("inode#    file type  d_reclen  d_off   d_name");
///     while bpos < nread {
///         let d = (buf_ptr + bpos) as *mut nc::linux_dirent64_t;
///         let d_ref: &nc::linux_dirent64_t = unsafe { &(*d) };
///         let d_type = match d_ref.d_type {
///             nc::DT_REG => "regular",
///             nc::DT_DIR => "directory",
///             nc::DT_FIFO => "FIFO",
///             nc::DT_SOCK => "socket",
///             nc::DT_LNK => "symlink",
///             nc::DT_BLK => "block-dev",
///             nc::DT_CHR => "char-dev",
///             _ => "unknown",
///         };
///
///         let name = std::str::from_utf8(d_ref.name()).unwrap();
///         println!(
///             "{: >8}  {:<10} {: >4} {: >12}  {}",
///             d_ref.d_ino, d_type, d_ref.d_reclen, d_ref.d_off as u32, name
///         );
///
///         bpos += d_ref.d_reclen as usize;
///     }
/// }
///
/// let ret = unsafe { nc::close(fd) };
/// assert!(ret.is_ok());
/// ```
pub unsafe fn getdents64(fd: i32, dir_buf: &mut [u8]) -> Result<ssize_t, Errno> {
    let fd = fd as usize;
    let count = dir_buf.len();
    let dir_buf_ptr = dir_buf.as_mut_ptr() as usize;
    unsafe { syscall3(SYS_GETDENTS64, fd, dir_buf_ptr, count).map(|ret| ret as ssize_t) }
}
