Describing Field Use for Logical Files

You can specify that fields in database files are to be input-only, both (input/output), or neither fields. Do this by specifying one of the following in position 38:

Entry

Meaning

Blank

For simple or multiple format logical files, defaults to B (both) For join logical files, defaults to I (input only)

B

Both input and output allowed; not valid for join logical files

I

Input only (read only)

N

Neither input nor output; valid only for join logical files

Note:

The usage value (in position 38) is not used on a reference function. When another file refers to a field (using a REF or REFFLD keyword) in a logical file, the usage value is not copied into that file.

Describing Field Use for Logical Files: Both

A both field can be used for both input and output operations. Your program can read data from the field and write data to the field. Both fields are not valid for join logical files, because join logical files are read-only files.

Describing Field Use for Logical Files: Input Only

An input only field can be used for read operations only. Your program can read data from the field, but cannot update the field in the file. Typical cases of input-only fields are key fields (to reduce maintenance of access paths by preventing changes to key field values), sensitive fields that a user can see but not update (for example, salary), and fields for which either the translation table (TRNTBL) keyword or the substring (SST) keyword is specified.

If your program updates a record in which you have specified input-only fields, the input-only fields are not changed in the file. If your program adds a record that has input-only fields, the input-only fields take default values (DFT keyword).

Describing Field Use for Logical Files: Neither

A neither field is used neither for input nor for output. It is valid only for join logical files. A neither field can be used as a join field in a join logical file, but your program cannot read or update a neither field.

Use neither fields when the attributes of join fields in the physical files do not match. In this case, one or both join fields must be defined again. However, you cannot include these redefined fields in the record format (the application program does not see the redefined fields.) Therefore, redefined join fields can be coded N so that they do not appear in the record format.

A field with N in position 38 does not appear in the buffer used by your program. However, the field description is displayed with the Display File Field Description (DSPFFD) command.

Neither fields cannot be used as select/omit or key fields.

For an example of a neither field, see Describing Fields That Never Appear in the Record Format (Example 5).