function returns the final return status for the asynchronous I/O request
with control block pointed to by
This function should be called only once for any given request, after
returns something other than
If the asynchronous I/O operation has completed, this function returns
the value that would have been returned in case of a synchronous
Otherwise the return value is undefined.
On error, the error value is returned.
does not point at a control block for an asynchronous I/O request
of which the return status has not been retrieved yet.