H5A_OPEN opens an existing attribute,
attr_name, that is attached to an object specified by an object identifier,
The attribute access property list,
aapl_id, is currently unused and should be H5P_DEFAULT.
This function, H5A_OPEN_BY_IDX , or H5A_OPEN_BY_NAME must be called before an attribute can be accessed for any further purpose, including reading, writing, or any modification.
The attribute identifier returned by this function must be released with H5A_CLOSE or resource leaks will develop.