Question 1

How do you debug COBOL programs?

Accepted Answer

COBOL debugging involves several techniques: 1) Using DISPLAY statements to trace program execution and show variable values, 2) Using debugging tools like IBM Debug Tool, Xpediter, or mainframe debuggers, 3) Analyzing compiler listings and error messages, 4) Using file status codes and exception handling, 5) Adding conditional debug output, 6) Using dump analysis for abends, 7) Tracing program flow with PERFORM statements, 8) Validating data at key points. Effective debugging requires understanding program flow, data values, and system behavior.

Question 2

What is the best way to add debug output in COBOL?

Accepted Answer

The best way to add debug output in COBOL is to use DISPLAY statements conditionally based on a debug flag. Define a debug switch (using SPECIAL-NAMES or a working storage flag), then wrap debug DISPLAY statements in IF conditions. For example: "IF DEBUG-MODE DISPLAY 'DEBUG: Variable = ' VARIABLE-NAME END-IF". This allows you to enable/disable debug output without code changes. Use consistent prefixes like "DEBUG:" to identify debug output, and include context like procedure names and variable values.

Question 3

What are common COBOL debugging errors?

Accepted Answer

Common COBOL debugging errors include: 1) Data type mismatches (moving alphanumeric to numeric), 2) Uninitialized variables causing unpredictable behavior, 3) Off-by-one errors in loops and array access, 4) File status not checked after I/O operations, 5) Incorrect data sizes (PIC clauses), 6) Logic errors in conditional statements, 7) PERFORM loop issues (infinite loops, wrong termination), 8) String overflow in STRING/UNSTRING, 9) Division by zero, 10) Subscript out of bounds. Understanding these common issues helps focus debugging efforts.

Question 4

How do you use file status codes for debugging?

Accepted Answer

File status codes provide information about I/O operation results. After each file operation (READ, WRITE, OPEN, CLOSE), check the file status code. Status "00" means success, "10" means end of file, "23" means duplicate key, etc. Always check file status after I/O operations: "READ FILE AT END ... NOT AT END ... END-READ" and check FILE-STATUS. Log file status codes in debug output to understand I/O behavior. Invalid file status often indicates data or file definition problems.

Question 5

What debugging tools are available for COBOL?

Accepted Answer

COBOL debugging tools include: 1) IBM Debug Tool - interactive debugging with breakpoints and variable inspection, 2) Xpediter - comprehensive debugging and testing tool, 3) Compiler listings - detailed compilation information and cross-references, 4) Dump analysis tools - analyze abend dumps, 5) File comparison tools - compare expected vs actual output, 6) Performance monitors - identify bottlenecks, 7) Code coverage tools - ensure all code paths are tested, 8) Static analysis tools - find potential issues before execution. Many mainframe environments provide integrated debugging capabilities.

Question 6

How do you debug infinite loops in COBOL?

Accepted Answer

To debug infinite loops: 1) Add a loop counter that increments each iteration, 2) Display the counter and loop variables in debug output, 3) Check the loop termination condition - ensure it can actually be met, 4) Verify loop control variables are being modified correctly, 5) Check for off-by-one errors in loop bounds, 6) Ensure PERFORM UNTIL conditions will eventually become true, 7) Add a maximum iteration limit as a safety check, 8) Use debugging tools to set breakpoints in the loop. Infinite loops often result from conditions that never become true or variables that aren't updated.

Question 7

What is a COBOL abend and how do you debug it?

Accepted Answer

An abend (abnormal end) is an unexpected program termination. To debug abends: 1) Review the abend code and message (like S0C7 for data exception), 2) Analyze the dump - it shows the program state at failure, 3) Check the statement causing the abend (usually shown in dump), 4) Verify data values - abends often result from invalid data, 5) Check for uninitialized variables, 6) Verify array bounds and subscripts, 7) Check for division by zero, 8) Review file operations and status codes. Common abend codes include S0C7 (data exception), S0C4 (protection exception), and S0C1 (operation exception).

Question 8

How do you trace program execution flow in COBOL?

Accepted Answer

To trace program execution: 1) Add DISPLAY statements at the start of each major procedure, 2) Display entry and exit points of PERFORM statements, 3) Show key decision points and which branch was taken, 4) Display loop iterations and control variables, 5) Use a trace level or debug flag to control verbosity, 6) Log procedure names with timestamps if needed, 7) Show data values at key transformation points, 8) Use debugging tools that provide execution traces. Consistent tracing helps understand program flow and identify where logic diverges from expectations.

Status Code	Meaning	Typical Action
00	Successful operation	Continue processing
10	End of file reached	Normal termination condition
23	Duplicate key (for indexed files)	Handle duplicate key error
30	Permanent error	Check file definition and data
92	Logic error	Review program logic

Abend Code	Meaning	Common Cause
S0C7	Data exception	Invalid data in numeric operation
S0C4	Protection exception	Accessing invalid memory
S0C1	Operation exception	Invalid instruction
S0C5	Addressing exception	Invalid address reference
S313	Program check	Program logic error

COBOL Tutorial

COBOL Debugging

What is Debugging?

Basic Debugging Techniques

1. Using DISPLAY for Tracing

2. Conditional Debug Output

3. Debugging with SPECIAL-NAMES Switch

File Status Debugging

Common File Status Codes

Debugging Common Errors

1. Uninitialized Variables

2. Data Type Mismatches

3. Loop Debugging

Abend Debugging

Common Abend Codes

Analyzing Abends

Structured Debugging Approach

Step 1: Reproduce the Problem

Step 2: Isolate the Problem

Step 3: Understand the Data

Step 4: Fix and Verify

Debugging Best Practices

Debugging Tools

Explain Like I'm 5: Debugging

Practice Exercises

Exercise 1: Add Debug Output

Exercise 2: File Status Checking

Exercise 3: Loop Debugging

Exercise 4: Error Detection

Exercise 5: Trace Program Flow

Test Your Knowledge

Related Concepts

Related Concepts

Error Handling

DISPLAY Statement

File Operations

Program Structure

Related Pages