/// Read value of a symbolic link.
///
/// # Examples
///
/// ```
/// let oldname = "/etc/passwd";
/// let newname = "/tmp/nc-readlink";
/// let ret = unsafe { nc::symlinkat(oldname, nc::AT_FDCWD, newname) };
/// assert!(ret.is_ok());
/// let mut buf = [0_u8; nc::PATH_MAX as usize + 1];
/// let ret = unsafe { nc::readlink(newname, &mut buf) };
/// assert!(ret.is_ok());
/// let n_read = ret.unwrap() as usize;
/// assert_eq!(n_read, oldname.len());
/// assert_eq!(oldname.as_bytes(), &buf[0..n_read]);
/// let ret = unsafe { nc::unlinkat(nc::AT_FDCWD, newname, 0) };
/// assert!(ret.is_ok());
/// ```
pub unsafe fn readlink<P: AsRef<Path>>(filename: P, buf: &mut [u8]) -> Result<ssize_t, Errno> {
    let filename = CString::new(filename.as_ref());
    let filename_ptr = filename.as_ptr() as usize;
    let buf_ptr = buf.as_mut_ptr() as usize;
    let buf_len = buf.len();
    unsafe { syscall3(SYS_READLINK, filename_ptr, buf_ptr, buf_len).map(|ret| ret as ssize_t) }
}
