COBOL Tutorial

COBOL DEBUG-SUB-1 Clause - Quick Reference

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Overview

The DEBUG-SUB-1 clause is used to define debugging data items that track the first subscript value being used in array or table operations during program debugging. This clause is part of COBOL's debugging facility and provides essential subscript tracking for array debugging and analysis.

Purpose and Usage

First subscript tracking - Track first subscript in array operations
Array access monitoring - Monitor array element access
Bounds checking - Identify array bounds issues
Access pattern analysis - Analyze array access patterns
Development support - Aid in array debugging

Subscript Tracking Concept

Array Access: ARRAY[1] → ARRAY[2] → ARRAY[3] → ...

DEBUG-SUB-1: [Current First Subscript Value]

Debugging Info: [Array[5]: Value = 100]

Tracks first subscript value in array operations

DEBUG-SUB-1 provides real-time tracking of the first subscript value in array operations.

Syntax

The DEBUG-SUB-1 clause follows specific syntax patterns and is typically used at the 77 level for independent debugging data items.

Basic Syntax

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* Basic DEBUG-SUB-1 clause syntax
77 debug-sub-1-item DEBUG-SUB-1.
 
* Examples
77 DEBUG-FIRST-SUB DEBUG-SUB-1.
77 DEBUG-SUB-1-VALUE DEBUG-SUB-1.
77 DEBUG-ARRAY-SUB-1 DEBUG-SUB-1.
 
* Complete example with data description
77 DEBUG-FIRST-SUB DEBUG-SUB-1 PIC 9(5).
77 DEBUG-SUB-1-VALUE DEBUG-SUB-1 PIC 9(6).
77 DEBUG-ARRAY-SUB-1 DEBUG-SUB-1 COMP.
 
* Usage in program
PROCEDURE DIVISION.
    * Display current first subscript value
    DISPLAY "First subscript: " DEBUG-FIRST-SUB
    
    * Store subscript for later use
    MOVE DEBUG-FIRST-SUB TO SAVED-SUBSCRIPT
    
    * Check if at specific subscript
    IF DEBUG-FIRST-SUB = 100
        DISPLAY "Accessing element 100"
    END-IF

DEBUG-SUB-1 is typically used at the 77 level for independent debugging items.

Data Type Considerations

Data Type	Size	Use Case
PIC 9(5)	5 digits	Arrays up to 99,999 elements
PIC 9(6)	6 digits	Large arrays up to 999,999 elements
COMP	Binary	Efficient storage and processing
COMP-3	Packed decimal	Mainframe compatibility

Debugging Mode Requirements

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* Enabling debugging mode for DEBUG-SUB-1
* Compiler option
* Example: cobc -fdebug program.cob
 
* Runtime debugging activation
* Set environment variable
* Example: COBOL_DEBUG=1 ./program
 
* Program-level debugging declaration
PROGRAM-ID. DEBUG-PROGRAM.
ENVIRONMENT DIVISION.
CONFIGURATION SECTION.
SPECIAL-NAMES.
    DEBUGGING MODE IS ON.
 
DATA DIVISION.
WORKING-STORAGE SECTION.
77 DEBUG-FIRST-SUB DEBUG-SUB-1 PIC 9(5).

DEBUG-SUB-1 requires debugging mode to be enabled for subscript tracking functionality.

Common Use Cases

DEBUG-SUB-1 is commonly used in specific debugging scenarios where first subscript tracking is needed for array analysis.

Array Access Tracking

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* Tracking array access patterns
77 DEBUG-FIRST-SUB DEBUG-SUB-1 PIC 9(5).
 
PROCEDURE DIVISION.
    DISPLAY "Starting array access at subscript: " DEBUG-FIRST-SUB
    
    * Process array elements
    PERFORM VARYING I FROM 1 BY 1 UNTIL I > 100
        * Access array element
        MOVE ARRAY(I) TO TEMP-VALUE
        DISPLAY "Accessing element at subscript: " DEBUG-FIRST-SUB
    END-PERFORM
    
    DISPLAY "Array processing completed at subscript: " DEBUG-FIRST-SUB

Track which array elements are being accessed.

Array Bounds Monitoring

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* Monitoring array bounds
77 DEBUG-FIRST-SUB DEBUG-SUB-1 PIC 9(5).
77 ARRAY-SIZE PIC 9(5) VALUE 100.
 
PROCEDURE DIVISION.
    * Check for bounds violations
    IF DEBUG-FIRST-SUB > ARRAY-SIZE
        DISPLAY "WARNING: Array bounds violation at subscript: " DEBUG-FIRST-SUB
        DISPLAY "Array size is: " ARRAY-SIZE
    END-IF
    
    * Monitor specific subscript ranges
    IF DEBUG-FIRST-SUB >= 90 AND DEBUG-FIRST-SUB <= 100
        DISPLAY "Accessing high-index elements at subscript: " DEBUG-FIRST-SUB
        * Add extra validation here
    END-IF
    
    * Track zero-based access
    IF DEBUG-FIRST-SUB = 0
        DISPLAY "WARNING: Zero-based array access detected"
    END-IF

Monitor array bounds and identify potential violations.

Conditional Array Debugging

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* Conditional debugging based on subscripts
77 DEBUG-FIRST-SUB DEBUG-SUB-1 PIC 9(5).
 
PROCEDURE DIVISION.
    * Only show debugging info for specific subscript ranges
    IF DEBUG-FIRST-SUB >= 1 AND DEBUG-FIRST-SUB <= 10
        DISPLAY "Debug info for low subscripts: " DEBUG-FIRST-SUB
        DISPLAY "Array value = " ARRAY(DEBUG-FIRST-SUB)
    END-IF
    
    * Break execution at specific subscript
    IF DEBUG-FIRST-SUB = 50
        DISPLAY "Breakpoint reached at subscript 50"
        * Add breakpoint logic here
    END-IF
    
    * Track access frequency
    IF DEBUG-FIRST-SUB = 25
        ADD 1 TO ACCESS-COUNT
        DISPLAY "Element 25 accessed " ACCESS-COUNT " times"
    END-IF

Use subscripts to implement conditional debugging logic.

Best Practices and Tips

Following these best practices ensures effective use of the DEBUG-SUB-1 clause for array debugging and subscript tracking.

DEBUG-SUB-1 Design Principles

Choose appropriate size - Select data type that accommodates your largest array
Use meaningful names - Choose descriptive names for debugging subscript items
Minimize overhead - Don\'t overuse subscript tracking to avoid performance impact
Document array sizes - Document expected array bounds for validation
Consider production impact - Remember that subscript tracking can affect performance
Test debugging functionality - Verify that subscript tracking works correctly

Common Pitfalls to Avoid

Pitfall	Problem	Solution
Insufficient field size	Subscript values overflow	Choose appropriate PIC size for largest array
Overuse in production	Performance degradation	Use conditional compilation or separate versions
Not enabling debug mode	No subscript tracking information	Ensure debugging mode is properly enabled
Poor naming conventions	Confusing debugging output	Use clear, descriptive names
Not documenting usage	Difficult to understand subscript tracking	Document all subscript tracking items and their purpose

Performance Considerations

Subscript tracking overhead - DEBUG-SUB-1 adds runtime overhead when active
Memory usage - Subscript tracking items consume additional memory
I/O impact - Subscript tracking output may affect I/O performance
Production considerations - Disable subscript tracking in production environments
Selective tracking - Use conditional logic for selective subscript tracking
Debugging levels - Consider different levels of subscript tracking detail

When to Use DEBUG-SUB-1

Use Case	DEBUG-SUB-1 Suitability	Reasoning
Array debugging	Excellent	Essential for array subscript tracking
Complex array operations	Good	Helps track complex array access patterns
Bounds checking	Good	Provides array bounds validation
Production monitoring	Poor	Performance impact outweighs benefits
Simple programs	Poor	Unnecessary overhead for simple logic

DEBUG-SUB-1 Clause Quick Reference

Usage	Syntax	Example
Basic subscript tracking	77 debug-sub-1-item DEBUG-SUB-1	77 DEBUG-SUB-1 DEBUG-SUB-1
With data description	77 debug-sub-1-item DEBUG-SUB-1 PIC...	77 DEBUG-SUB-1 DEBUG-SUB-1 PIC 9(5)
Multiple subscript trackers	Multiple 77 items with DEBUG-SUB-1	77 DEBUG1 DEBUG-SUB-1 77 DEBUG2 DEBUG-SUB-1
Numeric subscript tracking	77 debug-sub-1-item DEBUG-SUB-1 PIC 9(n)	77 DEBUG-SUB-1 DEBUG-SUB-1 PIC 9(6)
Binary subscript tracking	77 debug-sub-1-item DEBUG-SUB-1 COMP	77 DEBUG-SUB-1 DEBUG-SUB-1 COMP

Test Your Knowledge

1. What is the primary purpose of the DEBUG-SUB-1 clause in COBOL?

To define data types
To track the first subscript value during debugging
To control file operations
To perform calculations

2. At what level is the DEBUG-SUB-1 clause typically used?

01 level only
88 level only
77 level only
Any level except 01

3. What information does DEBUG-SUB-1 provide during debugging?

Only variable names
Only program flow
First subscript value in array operations
Only error messages

4. When is DEBUG-SUB-1 most useful?

During production runs
During program development and debugging with arrays
Only during compilation
Only during program termination

5. How does DEBUG-SUB-1 relate to array debugging?

They are completely independent
DEBUG-SUB-1 provides essential subscript tracking for array debugging
DEBUG-SUB-1 overrides other debugging features
They serve the same purpose

COBOL Tutorial

COBOL DEBUG-SUB-1 Clause - Quick Reference

Overview

Purpose and Usage

Subscript Tracking Concept

Syntax

Basic Syntax

Data Type Considerations

Debugging Mode Requirements

Common Use Cases

Array Access Tracking

Array Bounds Monitoring

Conditional Array Debugging

Best Practices and Tips

DEBUG-SUB-1 Design Principles

Common Pitfalls to Avoid

Performance Considerations

When to Use DEBUG-SUB-1

DEBUG-SUB-1 Clause Quick Reference

Test Your Knowledge

Frequently Asked Questions

Related Concepts

DEBUG-SUB-2 Clause

DEBUG-SUB-3 Clause

OCCURS Clause

Table Handling

Data Types

Related Pages

COBOL Tutorial

COBOL DEBUG-SUB-1 Clause - Quick Reference

Overview

Purpose and Usage

Subscript Tracking Concept

Syntax

Basic Syntax

Data Type Considerations

Debugging Mode Requirements

Common Use Cases

Array Access Tracking

Array Bounds Monitoring

Conditional Array Debugging

Best Practices and Tips

DEBUG-SUB-1 Design Principles

Common Pitfalls to Avoid

Performance Considerations

When to Use DEBUG-SUB-1

DEBUG-SUB-1 Clause Quick Reference

Test Your Knowledge

Frequently Asked Questions

What is the difference between DEBUG-SUB-1 and DEBUG-SUB-2?

Can DEBUG-SUB-1 be used in production programs?

What types of array operations can DEBUG-SUB-1 track?

What data type should be used for DEBUG-SUB-1?

Are there performance implications when using DEBUG-SUB-1?

Related Concepts

DEBUG-SUB-2 Clause

DEBUG-SUB-3 Clause

OCCURS Clause

Table Handling

Data Types

Related Pages