COBOL on Linux: Debugging a Batch Processing Anomaly

COBOL on Linux: Debugging a Batch Processing Anomaly

Introduction

COBOL continues to be a vital language in many industries, driving critical systems that handle vast amounts of data and complex business logic. With the flexibility and cost-effectiveness of Linux, running COBOL applications on this platform has become an increasingly popular choice. This blog post explores the practical aspects of maintaining and enhancing COBOL applications in a Linux environment.

Debugging: Ensuring the Accuracy of Your Applications

Even well-established COBOL applications can encounter unexpected issues. Let's examine a common scenario: a batch program that suddenly begins producing incorrect results.

Example: A Batch Processing Anomaly

Consider a batch program that processes customer transactions. It reads data from an input file (TRANS.DAT), calculates a discount based on customer type and purchase amount, and writes the updated information to an output file (OUT.DAT). Suddenly, the output file contains inaccurate data. Here's a debugging strategy using gdb (the GNU Debugger):

High-Level Logic:

1. Compile with Debug Information: To leverage gdb effectively, compile your COBOL program with debugging information. This is usually achieved with a compiler flag like -g.
2. Start gdb: Launch gdb and load your compiled COBOL program.
3. Set Breakpoints: Place breakpoints in the code where the discount calculation takes place. This allows you to pause execution and examine the program's state.
4. Examine Variables: Use gdb commands to inspect the values of variables involved in the discount calculation, such as customer type, purchase amount, and the computed discount.
5. Step Through Code: Execute the code line by line, observing how variables change and identifying any logical errors.
6. Identify and Fix the Bug: Through careful examination of the code and variable values, pinpoint the source of the error and implement the necessary correction.

COBOL Code Listing (Simplified):

Code snippet

IDENTIFICATION DIVISION.
PROGRAM-ID. BATCH-PROCESS.

DATA DIVISION.
FILE SECTION.
FD  TRANS-FILE.
    01  TRANS-RECORD.
        05  CUST-TYPE    PIC X.
        05  PURCHASE-AMT PIC 9(5)V99.
FD  OUT-FILE.
    01  OUT-RECORD.
        05  CUST-TYPE    PIC X.
        05  PURCHASE-AMT PIC 9(5)V99.
        05  DISCOUNT     PIC 9(3)V99.

WORKING-STORAGE SECTION.
    01  WS-DISCOUNT-RATE PIC 9V99 VALUE 0.
    01  EOF-FLAG          PIC X VALUE 'N'.

PROCEDURE DIVISION.
MAIN-PARAGRAPH.
    OPEN INPUT TRANS-FILE
    OPEN OUTPUT OUT-FILE
    PERFORM READ-TRANS-RECORD 
    PERFORM PROCESS-TRANSACTIONS UNTIL EOF-FLAG = 'Y'
    CLOSE TRANS-FILE
    CLOSE OUT-FILE
    STOP RUN.

READ-TRANS-RECORD.
    READ TRANS-FILE AT END MOVE 'Y' TO EOF-FLAG.

PROCESS-TRANSACTIONS.
    EVALUATE CUST-TYPE
        WHEN 'A'
            MOVE 0.10 TO WS-DISCOUNT-RATE
        WHEN 'B'
            MOVE 0.05 TO WS-DISCOUNT-RATE
        WHEN OTHER
            MOVE 0.00 TO WS-DISCOUNT-RATE
    END-EVALUATE

    COMPUTE DISCOUNT = PURCHASE-AMT * WS-DISCOUNT-RATE

    MOVE CUST-TYPE TO OUT-RECORD
    MOVE PURCHASE-AMT TO OUT-RECORD
    MOVE DISCOUNT TO OUT-RECORD
    WRITE OUT-RECORD

    PERFORM READ-TRANS-RECORD.

Debugging with gdb:

1. Compile: cobc -g -o batch-process batch-process.cob
2. Start gdb: gdb batch-process
3. Set Breakpoint: break PROCESS-TRANSACTIONS
4. Run: run
5. Examine Variables: print CUST-TYPE, print PURCHASE-AMT, print WS-DISCOUNT-RATE, print DISCOUNT
6. Step Through Code: next (to execute the next line) or step (to step into a called paragraph)

A Closer Look:

As we step through the code and examine the variable values, something peculiar might catch your eye. Pay close attention to how the READ-TRANS-RECORD paragraph is being used. Can you spot a potential issue?

(Pause for dramatic effect... maybe an image of Sherlock Holmes with a magnifying glass)

The Reveal:

If you noticed that READ-TRANS-RECORD is performed twice in a row—once at the beginning of MAIN-PARAGRAPH and again at the end of PROCESS-TRANSACTIONS—you've found the bug! This seemingly innocuous duplication leads to the first transaction record being skipped entirely.

Here's why:

1. The first record is read in MAIN-PARAGRAPH.
2. PROCESS-TRANSACTIONS processes this record.
3. At the end of PROCESS-TRANSACTIONS, another READ-TRANS-RECORD is performed, reading the second record and overwriting the first.
4. The loop continues, effectively skipping the first record.

The Fix:

Remove the PERFORM READ-TRANS-RECORD from MAIN-PARAGRAPH. The read at the end of PROCESS-TRANSACTIONS will ensure that each record is read and processed correctly.

Corrected MAIN-PARAGRAPH:

Code snippet

MAIN-PARAGRAPH.
    OPEN INPUT TRANS-FILE
    OPEN OUTPUT OUT-FILE
    PERFORM PROCESS-TRANSACTIONS UNTIL EOF-FLAG = 'Y' 
    CLOSE TRANS-FILE
    CLOSE OUT-FILE
    STOP RUN.

Conclusion

This exercise highlights the importance of meticulous code review, even when dealing with seemingly straightforward logic. Subtle bugs can easily hide in plain sight, and tools like gdb can be invaluable in uncovering them. By combining careful analysis with powerful debugging techniques, we can ensure the accuracy and reliability of our COBOL applications on Linux.

Image: Mudassar Iqbal from Pixabay