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Using Externally Described Disk Files
Using Externally Described Disk Files
Externally described DISK files are identified by an E in position 22 of the file
description specifications. The E indicates that the compiler is to retrieve the
external description of the file from the system when the program is compiled.
Therefore, you must create the file before the program is compiled.
The external description for a DISK file includes:
The record-format specifications that contain a description of the fields in a
record
Access path specifications that describe how the records are to be retrieved.
These specifications result from the DDS for the file and the OS/400 create file
command that is used for the file.
Record Format Specifications
The record-format specifications allow you to describe the fields in a record and the
location of the fields in a record. The fields are located in the record in the order
specified in the DDS. The field description generally includes the field name, the
field type, and the field length (including the number of decimal positions in a
numeric field). Instead of specifying the field attributes in the record format for a
physical or logical file, you can define them in a field-reference file.
In addition, the DDS keywords can be used to:
Specify that duplicate key values are not allowed for the file (UNIQUE)
Specify a text description for a record format or a field (TEXT).
For a complete list of the DDS keywords that are valid for a database file, see the
DB2 UDB for AS/400 Database Programming
.
Figure 139 on page 283 shows an example of the DDS for a database file, and
Figure 140 on page 284 for a field-reference file that defines the attributes for the
fields used in the database file. See the
DDS Reference
for more information on a
field-reference file.
Access Path
The description of an externally described file contains the access path that
describes how records are to be retrieved from the file. Records can be retrieved
based on an arrival sequence (non-keyed) access path or on a keyed-sequence
access path.
The arrival sequence access path is based on the order in which the records are
stored in the file. Records are added to the file one after another.
For the keyed-sequence access path, the sequence of records in the file is based
on the contents of the key field that is defined in the DDS for the file. For example,
in the DDS shown in Figure 139 on page 283, CUST is defined as the key field.
The keyed-sequence access path is updated whenever records are added, deleted,
or when the contents of a key field change.
For a complete description of the access paths for an externally described data-
base file, see the
DB2 UDB for AS/400 Database Programming
manual.
282 ILE RPG for AS/400 Programmer's Guide