Using Program-Described Disk Files - IBM - RPG

Program-described files, which are identified by an F in position 22 of the file description specifications, can be described as indexed files, as sequential files, or as record-address files.

Indexed File
An indexed file is a program-described DISK file whose access path is built on key values. You must create the access path for an indexed file by using data description specifications.

An indexed file is identified by an I in position 35 of the file description specifications.

The key fields identify the records in an indexed file. You specify the length of the key field in positions 29 through 33, the format of the key field in position 34, and the starting location of the key field in the KEYLOC keyword of the file description specifications.

An indexed file can be processed sequentially by key, sequentially within limits, or randomly by key.

Valid Search Arguments
For a program-described file, a search argument must be a single field. For the CHAIN and DELETE operations, the search argument must be the same length as the key field that is defined on the file description specifications for the indexed file. For the other file operations, the search argument may be a partial field.

The DDS specifies the fields to be used as a key field. The KEYLOC keyword of the file description specifications specify the starting position of the first key field. The entry in positions 29 through 33 of the file description specifications must specify the length of the key as defined in the DDS.

This show examples of how to use the DDS to describe the access path for indexed files.

DDS and corresponding File-Description Specification Detail Flow of RPG IV Exception/Error Handling

DDS and corresponding File-Description Specification Detail Flow of RPG IV Exception/Error Handling

You must use data description specifications to create the access path for a program-described indexed file.

In the DDS for the record format FORMATA for the logical file ORDDTLL, the field ORDER, which is five digits long, is defined as the key field, and is in packed format. The definition of ORDER as the key field establishes the keyed access for this file. Two other fields, FLDA and FLDB, describe the remaining positions in this record as character fields.

The program-described input file ORDDTLL is described on the file description specifications as an indexed file. Positions 29 through 33 must specify the number of positions in the record required for the key field as defined in the DDS: three positions. The KEYLOC keyword specifies position 15 as the starting position of the key field in the record. Because the file is defined as program-described by the F in position 22, the ILE RPG compiler does not retrieve the external field-level description of the file at compilation time. Therefore, you must describe the fields in the record on the input specifications.

Using Data Description Specifications to Define the Access Path (Composite Key) for an Indexed File

Using Data Description Specifications to Define the Access Path (Composite Key) for an Indexed File

In this example, the data description specifications define two key fields for the record format FORMAT in the logical file ORDDTLL. For the two fields to be used as a composite key for a program described indexed file, the key fields must be contiguous in the record.

On the file description specifications, the length of the key field is defined as 10 in positions 29 through 33 (the combined number of positions required for the ORDER and ITEM fields). The starting position of the key field is described as 15 using the keyword KEYLOC (starting in position 44). The starting position must specify the first position of the first key field.

Using Data Description Specifications to Define the Access Path (Composite Key) for an Indexed File

Using Data Description Specifications to Define the Access Path (Composite Key) for an Indexed File

When the DDS specifies a composite key, you must build a search argument in the program to CHAIN to the file. (A KLIST cannot be used for a program-described file.) One way is to create a data structure (using definition specifications) with subfields equal to the key fields defined in the DDS. Then, in the calculations, set the subfields equal to the value of the key fields, and use the data-structure name as the search argument in the CHAIN operation.

In this example, the MOVE operations set the subfields K1 and K2 equal to the value of ORDER and ITEM, respectively. The data-structure name (KEY) is then used as the search argument in the CHAIN operation.

Sequential File
Sequential files are files where the order of the records in the file is based on the order the records are placed in the file (that is, in arrival sequence). For example, the tenth record placed in the file occupies the tenth record position.

Sequential files can be processed randomly by relative record number, consecutively, or by a record-address file. You can use either the SETLL or SETGT operation code to set limits on the file.

Record Address File
You can use a record-address file to process another file. A record-address file can contain (1) limits records that are used to process a file sequentially within limits, or (2) relative record numbers that are used to process a file by relative record numbers. The record-address file itself must be processed sequentially.

A record-address file is identified by an R in position 18 of the file description specifications. If the record-address file contains relative record numbers, position 35 must contain a T. The name of the file to be processed by the record-address file must be specified on the file description specification. You identify the file using the keyword RAFDATA(file-name).

Limits Records
For sequential-within-limits processing, the record-address file contains limits records. A limits record contains the lowest record key and the highest record key of the records in the file to be read.

The format of the limits records in the record-address file is as follows:

  • The low key begins in position 1 of the record; the high key immediately follows the low key. No blanks can appear between the keys.
  • Each record in the record-address file can contain only one set of limits. The record length must be greater than or equal to twice the length of the record key.
  • The low key and the high key in the limits record must be the same length. The length of the keys must be equal to the length of the key field of the file to be processed.
  • A blank entry equal in length to the record key field causes the ILE RPG compiler to read the next record in the record-address file.

Relative Record Numbers
For relative-record-number processing, the record-address file contains relative record numbers. Each record retrieved from the file being processed is based on a relative record number in the record-address file. A record-address file containing relative record numbers cannot be used for limits processing. Each relative record number in the record-address file is a multi-byte binary field where each field contains a relative record number.

You can specify the record-address file length as 4, 3, or blank, depending on the source of the file. When using a record-address file from the iSeriesenvironment, specify the record-address file length as 4, since each field is 4 bytes in length. When using a record-address file created for the System/36 Environment™, specify the record-address file length as 3, since each field is 3 bytes in length. If you specify the record-address file length as blank, the compiler will check the primary record length at run time and determine whether to treat the record-address file as 3 byte or as 4 byte.

A minus 1 (-1 or hexadecimal FFFFFFFF) relative-record-number value stops the use of a relative-record-address file record. End of file occurs when all records from the record-address file have been processed.


All rights reserved © 2018 Wisdom IT Services India Pvt. Ltd DMCA.com Protection Status

IBM - RPG Topics