Error handling is a crucial aspect of programming, and COBOL is no exception. Proper error handling ensures that your programs can gracefully handle unexpected situations and provide meaningful feedback to users. In this section, we will cover various techniques for handling errors in COBOL.
Key Concepts
-
Error Handling Basics:
- Understanding the importance of error handling.
- Types of errors: syntax errors, runtime errors, and logical errors.
-
COBOL Error Handling Mechanisms:
- Using the
FILE STATUSclause. - Implementing the
USE AFTER EXCEPTIONclause. - Handling errors with the
INVALID KEYclause. - Utilizing the
AT ENDclause.
- Using the
-
Best Practices:
- Consistent error handling strategy.
- Logging errors for debugging and auditing.
- Providing user-friendly error messages.
Error Handling Basics
Types of Errors
- Syntax Errors: These occur when the code does not conform to the syntax rules of COBOL. They are usually caught by the compiler.
- Runtime Errors: These occur during the execution of the program, such as file not found or division by zero.
- Logical Errors: These are errors in the logic of the program, leading to incorrect results. They are the hardest to detect and fix.
COBOL Error Handling Mechanisms
Using the FILE STATUS Clause
The FILE STATUS clause is used to capture the status of file operations. It helps in identifying issues like file not found, end of file, or file read errors.
Example
DATA DIVISION.
FILE SECTION.
FD INPUT-FILE
LABEL RECORDS ARE STANDARD
FILE STATUS IS WS-FILE-STATUS.
01 INPUT-RECORD.
05 FIELD1 PIC X(10).
05 FIELD2 PIC 9(5).
WORKING-STORAGE SECTION.
01 WS-FILE-STATUS PIC XX.
PROCEDURE DIVISION.
OPEN INPUT INPUT-FILE
IF WS-FILE-STATUS NOT = '00'
DISPLAY 'Error opening file: ' WS-FILE-STATUS
STOP RUN
END-IF.
READ INPUT-FILE
AT END
DISPLAY 'End of file reached.'
STOP RUN
END-READ.
CLOSE INPUT-FILE.
STOP RUN.Implementing the USE AFTER EXCEPTION Clause
The USE AFTER EXCEPTION clause is used to handle exceptions that occur during file operations.
Example
DATA DIVISION.
FILE SECTION.
FD INPUT-FILE
LABEL RECORDS ARE STANDARD
BLOCK CONTAINS 0 RECORDS
RECORDING MODE IS F
DATA RECORD IS INPUT-RECORD.
01 INPUT-RECORD.
05 FIELD1 PIC X(10).
05 FIELD2 PIC 9(5).
WORKING-STORAGE SECTION.
01 WS-ERROR-MESSAGE PIC X(50).
PROCEDURE DIVISION.
OPEN INPUT INPUT-FILE
USE AFTER EXCEPTION ON INPUT-FILE
DISPLAY 'Error occurred while opening the file.'
STOP RUN
END-USE.
READ INPUT-FILE
AT END
DISPLAY 'End of file reached.'
STOP RUN
END-READ.
CLOSE INPUT-FILE.
STOP RUN.Handling Errors with the INVALID KEY Clause
The INVALID KEY clause is used to handle errors that occur during indexed or relative file operations.
Example
DATA DIVISION.
FILE SECTION.
FD INDEXED-FILE
LABEL RECORDS ARE STANDARD
RECORDING MODE IS F
DATA RECORD IS INDEXED-RECORD.
01 INDEXED-RECORD.
05 FIELD1 PIC X(10).
05 FIELD2 PIC 9(5).
WORKING-STORAGE SECTION.
01 WS-ERROR-MESSAGE PIC X(50).
PROCEDURE DIVISION.
OPEN I-O INDEXED-FILE
IF WS-FILE-STATUS NOT = '00'
DISPLAY 'Error opening file: ' WS-FILE-STATUS
STOP RUN
END-IF.
READ INDEXED-FILE
INVALID KEY
DISPLAY 'Record not found.'
STOP RUN
END-READ.
CLOSE INDEXED-FILE.
STOP RUN.Utilizing the AT END Clause
The AT END clause is used to handle the end-of-file condition during sequential file processing.
Example
DATA DIVISION.
FILE SECTION.
FD SEQUENTIAL-FILE
LABEL RECORDS ARE STANDARD
RECORDING MODE IS F
DATA RECORD IS SEQUENTIAL-RECORD.
01 SEQUENTIAL-RECORD.
05 FIELD1 PIC X(10).
05 FIELD2 PIC 9(5).
WORKING-STORAGE SECTION.
01 WS-ERROR-MESSAGE PIC X(50).
PROCEDURE DIVISION.
OPEN INPUT SEQUENTIAL-FILE
IF WS-FILE-STATUS NOT = '00'
DISPLAY 'Error opening file: ' WS-FILE-STATUS
STOP RUN
END-IF.
READ SEQUENTIAL-FILE
AT END
DISPLAY 'End of file reached.'
STOP RUN
END-READ.
CLOSE SEQUENTIAL-FILE.
STOP RUN.Best Practices
-
Consistent Error Handling Strategy:
- Use a consistent approach to handle errors across your programs.
- Define standard error codes and messages.
-
Logging Errors:
- Implement logging to capture error details for debugging and auditing purposes.
- Ensure logs are accessible and secure.
-
User-Friendly Error Messages:
- Provide clear and concise error messages to users.
- Avoid technical jargon that may confuse end-users.
Practical Exercise
Exercise
Write a COBOL program that reads a sequential file and handles the following errors:
- File not found.
- End of file.
- Any other file read errors.
Solution
IDENTIFICATION DIVISION.
PROGRAM-ID. ErrorHandlingExample.
DATA DIVISION.
FILE SECTION.
FD INPUT-FILE
LABEL RECORDS ARE STANDARD
FILE STATUS IS WS-FILE-STATUS
DATA RECORD IS INPUT-RECORD.
01 INPUT-RECORD.
05 FIELD1 PIC X(10).
05 FIELD2 PIC 9(5).
WORKING-STORAGE SECTION.
01 WS-FILE-STATUS PIC XX.
01 WS-ERROR-MESSAGE PIC X(50).
PROCEDURE DIVISION.
OPEN INPUT INPUT-FILE
IF WS-FILE-STATUS NOT = '00'
DISPLAY 'Error opening file: ' WS-FILE-STATUS
STOP RUN
END-IF.
READ INPUT-FILE
AT END
DISPLAY 'End of file reached.'
STOP RUN
INVALID KEY
DISPLAY 'Error reading file: ' WS-FILE-STATUS
STOP RUN
END-READ.
CLOSE INPUT-FILE.
STOP RUN.Conclusion
In this section, we covered various error handling techniques in COBOL, including the use of the FILE STATUS clause, USE AFTER EXCEPTION clause, INVALID KEY clause, and AT END clause. We also discussed best practices for consistent error handling, logging, and providing user-friendly error messages. By implementing these techniques, you can ensure that your COBOL programs handle errors gracefully and provide meaningful feedback to users.