Deference question

[papand0pul0@yahoo.com (Papandopulo)]

Maybe simple question, but I am just starting with Ada.

I try to read an array from file using streams:

declare
  type X_Value is mod 2**8;
  type X_Buffer is array (Positive range <>) of X_Value;
  type X_Buffer_Access is access all X_Buffer;
  type X_Buffer_Sz is new X_Buffer (1 .. Positive(Q_Bytes));
  type X_Buffer_Sz_Access is access all X_Buffer_Sz;
  Buff    : X_Buffer_Sz_Access;
begin
  Buff := new X_Buffer_Sz;
  X_Buffer_Sz'Read (X_Stream, Buff'all);
end;

But gnat tells me:
attribute designator expected
on the dereference (Buff'all)

What's wrong ?

Thank you,
George