CAT subroutine interface

The NULL and EXCEPT attributes of a column (see Section ) control the way that null or locum values are handled for that column. They are specified when the column is created and are immutable thereafter. The NULL attribute defines the type of null or locum which the column supports. The possibilities are:

• standard, HDS bad value (CAT__NULLD),

• explicitly specified null value (CAT__NULLS),

• locum (CAT__LOCUM).

If the standard HDS null value is being used then it is not necessary to associate its actual value with the column; knowing that the standard value is to be used is adequate. Conversely, if either:

• the value of the null has been specified explicitly,

• a locum value has been specified,

then the required value must be associated with the column. In either case, this value is stored in the exception attribute of the column.

In the routines to GET and PUT values through the CAT subroutine interface, null values are indicated by flags which appear as separate arguments in the calling sequences for the GET and PUT subroutines. However, the appropriate Starlink bad value is also substituted for the value in the actual data variable or array. The advantages of this approach are:

• the separate flag for nulls is convenient for processing in database applications,

• the embedded values in the data variable or array mean that arrays (or variables, though variables are less likely to be important in practice) can be passed straight into existing Starlink subroutines.