User Controlled Support for Null-Capable Fields and Key Fields

When an externally described file contains null-capable fields and the ALWNULL(*USRCTL) keyword is specified on a control specification, you can do the following:

Read, write, update, and delete records with null values from externally described database files.
Retrieve and position-to records with null keys using keyed operations, by specifying an indicator in factor 2 of the KFLD associated with the field.
Determine whether a null-capable field is actually null using the %NULLIND built-in function on the right-hand-side of an expression.
Set a null-capable field to be null for output or update using the %NULLIND built-in function on the left-hand-side of an expression.

You are responsible for ensuring that fields containing null values are used correctly within the program. For example, if you use a null-capable field as factor 2 of a MOVE operation, you should first check if it is null before you do the MOVE, otherwise you may corrupt your result field value. You should also be careful when outputting a null-capable field to a file that does not have the field defined as null-capable, for example a WORKSTN or PRINTER file, or a program-described file.

Note:

The value of the null indicator for a null-capable field is only considered for these operations: input, output and file-positioning. Here are some examples of operations where the the null indicator is not taken into consideration:

DSPLY of a null-capable field shows the contents of the field even if the null indicator is on.
If you move a null-capable field to another null-capable field, and the factor 2 field has the null indicator on, the the result field will get the data from the factor 2 field. The corresponding null indicator for the result field will not be set on.
Comparison operations, including SORTA and LOOKUP, with null capable fields do not consider the null indicators.

A field is considered null-capable if it is null-capable in any externally described database record and is not defined as a constant in the program.

When a field is considered null-capable in an RPG program, a null indicator is associated with the field. Note the following:

If the field is a multiple-occurrence data structure or a table, an array of null indicators will be associated with the field. Each null indicator corresponds to an occurrence of the data structure or element of the table.
If the field is an array element, the entire array will be considered null-capable. An array of null indicators will be associated with the array, each null indicator corresponds to an array element.
If the field is an element of an array subfield of a multiple-occurrence data structure, an array of null indicators will be associated with the array for each occurrence of the data structure.

Null indicators are initialized to zeros during program initialization and thus null-capable fields do not contain null values when the program starts execution.

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