COBOL Tutorial

COBOL REDEFINES Clause - Quick Reference

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Overview

The REDEFINES clause allows multiple data layouts to share the same storage area, creating union-like structures in COBOL. This enables different interpretations of the same memory location and is commonly used for multiple record layouts and data type conversions.

Purpose and Usage

Multiple data layouts - Different structures for same storage
Union-like behavior - Share memory between different fields
Record type handling - Different record layouts in single file
Data interpretation - View same data in different formats
Memory efficiency - Reduce storage requirements

Storage Sharing Concept

Original Field: [Field1][Field2][Field3][Field4]

REDEFINES A: [Alt1][Alt2][Alt3][Alt4]

REDEFINES B: [Diff1][Diff2][Diff3][Diff4]

All share the same memory location

Multiple REDEFINES fields occupy the same storage area as the original field.

Syntax

The REDEFINES clause follows specific syntax patterns and can be used at various levels in data definitions.

Basic Syntax

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* Basic REDEFINES clause syntax
level-number data-name REDEFINES original-field-name
 
* Examples
05  ALTERNATE-LAYOUT REDEFINES ORIGINAL-FIELD.
05  NUMERIC-VALUE REDEFINES CHARACTER-FIELD.
05  DATE-FIELD REDEFINES TIMESTAMP-FIELD.
 
* Complete example
01  CUSTOMER-RECORD.
    05  RECORD-TYPE        PIC X.
    05  CUSTOMER-DATA      PIC X(99).
 
01  CUSTOMER-HEADER REDEFINES CUSTOMER-RECORD.
    05  HR-RECORD-TYPE     PIC X.
    05  HR-FILE-DATE       PIC 9(8).
    05  HR-RECORD-COUNT    PIC 9(5).
    05  HR-FILLER          PIC X(86).
 
01  CUSTOMER-DETAIL REDEFINES CUSTOMER-RECORD.
    05  DR-RECORD-TYPE     PIC X.
    05  DR-CUSTOMER-ID     PIC 9(5).
    05  DR-CUSTOMER-NAME   PIC X(30).
    05  DR-CUSTOMER-ADDR   PIC X(50).
    05  DR-STATUS          PIC X.
    05  DR-FILLER          PIC X(13).

REDEFINES can be used at any level except 01 (record level).

Level Restrictions

Level	Can Use REDEFINES	Common Usage
01	No	Record level - cannot redefine
02-49	Yes	Field and group levels
66	No	RENAMES level - different purpose
77	No	Independent item level
88	No	Condition name level

Multiple REDEFINES

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* Multiple REDEFINES of the same field
01  SHARED-STORAGE.
    05  ORIGINAL-FIELD      PIC X(10).
 
05  ALTERNATE-1 REDEFINES ORIGINAL-FIELD.
    10  ALT1-PART1         PIC X(5).
    10  ALT1-PART2         PIC X(5).
 
05  ALTERNATE-2 REDEFINES ORIGINAL-FIELD.
    10  ALT2-NUMERIC       PIC 9(10).
 
05  ALTERNATE-3 REDEFINES ORIGINAL-FIELD.
    10  ALT3-DATE          PIC 9(8).
    10  ALT3-TIME          PIC 9(2).
 
* All three REDEFINES share the same 10-byte storage area

Multiple REDEFINES can share the same original field storage.

Common Use Cases

REDEFINES is commonly used in specific scenarios where different data layouts need to share storage or where data interpretation varies.

Multiple Record Types in File

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* File with different record types
01  INVENTORY-RECORD.
    05  RECORD-TYPE        PIC X.
        88  HEADER-RECORD     VALUE 'H'.
        88  DETAIL-RECORD     VALUE 'D'.
        88  TRAILER-RECORD    VALUE 'T'.
    05  RECORD-DATA        PIC X(99).
 
01  HEADER-RECORD REDEFINES INVENTORY-RECORD.
    05  HR-TYPE            PIC X.
    05  HR-FILE-DATE       PIC 9(8).
    05  HR-COMPANY-NAME    PIC X(50).
    05  HR-VERSION         PIC 9(2).
    05  HR-FILLER          PIC X(39).
 
01  DETAIL-RECORD REDEFINES INVENTORY-RECORD.
    05  DR-TYPE            PIC X.
    05  DR-ITEM-NUMBER     PIC 9(10).
    05  DR-ITEM-NAME       PIC X(30).
    05  DR-QUANTITY        PIC 9(5).
    05  DR-UNIT-PRICE      PIC 9(5)V99.
    05  DR-LOCATION        PIC X(10).
    05  DR-STATUS          PIC X.
    05  DR-FILLER          PIC X(36).
 
01  TRAILER-RECORD REDEFINES INVENTORY-RECORD.
    05  TR-TYPE            PIC X.
    05  TR-RECORD-COUNT    PIC 9(10).
    05  TR-TOTAL-ITEMS     PIC 9(10).
    05  TR-TOTAL-VALUE     PIC 9(10)V99.
    05  TR-FILLER          PIC X(66).

Different record layouts share the same storage area based on record type.

Data Type Conversion

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* Converting between data types
01  FLEXIBLE-DATA.
    05  CHARACTER-VALUE    PIC X(8).
 
05  NUMERIC-VALUE REDEFINES CHARACTER-VALUE.
    10  NUM-PART           PIC 9(8).
 
05  DATE-VALUE REDEFINES CHARACTER-VALUE.
    10  YEAR               PIC 9(4).
    10  MONTH              PIC 9(2).
    10  DAY                PIC 9(2).
 
05  TIME-VALUE REDEFINES CHARACTER-VALUE.
    10  HOUR               PIC 9(2).
    10  MINUTE             PIC 9(2).
    10  SECOND             PIC 9(2).
    10  HUNDREDTH          PIC 9(2).
 
* Usage example
PROCEDURE DIVISION.
    * Store as character
    MOVE "20231225" TO CHARACTER-VALUE
    
    * Access as date components
    DISPLAY "Year: " YEAR
    DISPLAY "Month: " MONTH
    DISPLAY "Day: " DAY
    
    * Access as numeric
    DISPLAY "Numeric: " NUMERIC-VALUE

Same data can be interpreted as different types using REDEFINES.

Bit-Level Manipulation

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* Bit-level access using REDEFINES
01  FLAG-WORD.
    05  FLAG-BYTE         PIC X.
 
05  FLAG-BITS REDEFINES FLAG-BYTE.
    10  BIT-0             PIC 1.
    10  BIT-1             PIC 1.
    10  BIT-2             PIC 1.
    10  BIT-3             PIC 1.
    10  BIT-4             PIC 1.
    10  BIT-5             PIC 1.
    10  BIT-6             PIC 1.
    10  BIT-7             PIC 1.
 
* Usage example
PROCEDURE DIVISION.
    * Set individual bits
    MOVE 1 TO BIT-0
    MOVE 1 TO BIT-3
    MOVE 1 TO BIT-7
    
    * Check bit values
    IF BIT-0 = 1
        DISPLAY "Bit 0 is set"
    END-IF
    
    * Access as byte
    DISPLAY "Flag byte value: " FLAG-BYTE

REDEFINES enables bit-level access to byte data.

Best Practices and Tips

Following these best practices ensures effective and safe use of the REDEFINES clause in COBOL applications.

REDEFINES Design Principles

Ensure size compatibility - Total REDEFINES size should not exceed original
Use condition names - 88-level items to determine which layout to use
Document relationships - Clearly document which fields share storage
Validate data integrity - Ensure data makes sense in all layouts
Consider data alignment - Be aware of potential alignment issues
Test thoroughly - Verify all REDEFINES layouts work correctly

Common Pitfalls to Avoid

Pitfall	Problem	Solution
Size mismatch	Data corruption, program errors	Ensure total size doesn\'t exceed original
Incorrect field access	Wrong data interpretation	Use condition names to determine layout
Data corruption	Changes affect all REDEFINES fields	Be careful when modifying shared data
Complex structures	Hard to maintain and debug	Keep REDEFINES structures simple
No documentation	Confusion about data relationships	Document all REDEFINES relationships

Performance Considerations

Memory efficiency - REDEFINES reduces memory usage
No runtime overhead - REDEFINES is compile-time only
Data access patterns - Consider how data will be accessed
Alignment issues - Some platforms may have alignment requirements
Debugging complexity - REDEFINES can make debugging harder
Maintenance considerations - Changes affect multiple layouts

When to Use REDEFINES

Use Case	REDEFINES Suitability	Reasoning
Multiple record types	Excellent	Natural fit for different layouts
Data type conversion	Good	Efficient for type reinterpretation
Bit manipulation	Good	Provides bit-level access
Simple field aliases	Poor	Use RENAMES instead
Independent data	Poor	Use separate fields

REDEFINES Clause Quick Reference

Usage	Syntax	Example
Basic redefinition	level data-name REDEFINES original-field	05 ALT-LAYOUT REDEFINES ORIG-FIELD
Multiple redefinitions	Multiple REDEFINES of same field	05 ALT1 REDEFINES FIELD 05 ALT2 REDEFINES FIELD
Record layouts	01 record-name REDEFINES original-record	01 HEADER-REC REDEFINES DATA-REC
Data type conversion	05 numeric-field REDEFINES char-field	05 NUM-VAL REDEFINES CHAR-VAL
Bit manipulation	05 bit-fields REDEFINES byte-field	05 BITS REDEFINES FLAG-BYTE

Test Your Knowledge

1. What is the primary purpose of the REDEFINES clause in COBOL?

To define data types
To allow multiple data layouts to share the same storage area
To control file operations
To perform calculations

2. At what level can the REDEFINES clause be used?

Only at the 01 level
Only at the 05 level
At any level except 01
At any level including 01

3. What happens when you modify data in a REDEFINES field?

Only that field is changed
All REDEFINES fields sharing the same storage are affected
The program terminates with an error
Nothing happens

4. Which of the following is a common use case for REDEFINES?

Simple arithmetic operations
Multiple record layouts in a single file
File I/O operations
Screen display

5. What is the relationship between the original field and REDEFINES fields?

They are completely independent
They share the same storage location
They are automatically synchronized
They have different data types

COBOL Tutorial

COBOL REDEFINES Clause - Quick Reference

Overview

Purpose and Usage

Storage Sharing Concept

Syntax

Basic Syntax

Level Restrictions

Multiple REDEFINES

Common Use Cases

Multiple Record Types in File

Data Type Conversion

Bit-Level Manipulation

Best Practices and Tips

REDEFINES Design Principles

Common Pitfalls to Avoid

Performance Considerations

When to Use REDEFINES

REDEFINES Clause Quick Reference

Test Your Knowledge

Frequently Asked Questions

Related Concepts

RENAMES Clause

Data Description Entries

File Description (FD)

Condition Names

Data Types

Related Pages

COBOL Tutorial

COBOL REDEFINES Clause - Quick Reference

Overview

Purpose and Usage

Storage Sharing Concept

Syntax

Basic Syntax

Level Restrictions

Multiple REDEFINES

Common Use Cases

Multiple Record Types in File

Data Type Conversion

Bit-Level Manipulation

Best Practices and Tips

REDEFINES Design Principles

Common Pitfalls to Avoid

Performance Considerations

When to Use REDEFINES

REDEFINES Clause Quick Reference

Test Your Knowledge

Frequently Asked Questions

What is the difference between REDEFINES and RENAMES in COBOL?

Can you use REDEFINES with different data types and sizes?

How do you determine which REDEFINES field to use in your program logic?

What are the potential risks and pitfalls of using REDEFINES?

When should you use REDEFINES versus creating separate fields?

Related Concepts

RENAMES Clause

Data Description Entries

File Description (FD)

Condition Names

Data Types

Related Pages