This is very similar to the @code{write} function; see @ref{I/O
Primitives}. Your function should transfer up to @var{size} bytes from
the buffer, and return the number of bytes written. You can return a
-value of @code{-1} to indicate an error.
+value of @code{0} to indicate an error. You must not return any
+negative value.
You should define the function to perform seek operations on the cookie
as: