store multiple contiguous data elements in a single request. This flag may only be specified if the database was opened with MDB_DUPFIXED. The data argument must be an array of two MDB_vals. The mv_size of the first MDB_val must be the size of a single data element. The mv_data of the first MDB_val must point to the beginning of the array of contiguous data elements. The mv_size of the second MDB_val must be the count of the number of data elements to store. On return this field will be set to the count of the number of elements actually written. The mv_data of the second MDB_val is unused.