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Error Handling

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Error handling is a crucial aspect of any programming language, and RPG is no exception. Proper error handling ensures that your programs can gracefully handle unexpected situations, providing meaningful feedback to users and maintaining the integrity of your data and processes.

Key Concepts in Error Handling

  1. Types of Errors:

    • Syntax Errors: Mistakes in the code that prevent the program from compiling.
    • Runtime Errors: Errors that occur during the execution of the program.
    • Logical Errors: Errors in the logic of the program that produce incorrect results.

  2. Error Handling Techniques:

    • Built-in Error Handling: Using RPG's built-in mechanisms to handle errors.
    • Custom Error Handling: Writing your own procedures to manage errors.

  3. Common Error Handling Functions:

    • MONITOR
    • ON-ERROR
    • ENDMON

Built-in Error Handling

RPG provides several built-in constructs to handle errors. The most commonly used are MONITOR, ON-ERROR, and ENDMON.

MONITOR Block

The MONITOR block is used to monitor a section of code for errors. If an error occurs within the MONITOR block, control is transferred to the ON-ERROR block.

Dcl-S num1 Int(10);
Dcl-S num2 Int(10);
Dcl-S result Int(10);

num1 = 10;
num2 = 0;

Monitor;
   result = num1 / num2; // This will cause a divide by zero error
On-Error;
   Dsply 'An error occurred: Division by zero';
EndMon;

ON-ERROR Block

The ON-ERROR block is used to specify what should happen when an error occurs within the MONITOR block.

Monitor;
   result = num1 / num2;
On-Error;
   Dsply 'An error occurred: Division by zero';
EndMon;

ENDMON

The ENDMON keyword is used to end the MONITOR block.

Monitor;
   result = num1 / num2;
On-Error;
   Dsply 'An error occurred: Division by zero';
EndMon;

Custom Error Handling

In addition to built-in error handling, you can create custom error handling procedures to manage errors in a more sophisticated manner.

Example: Custom Error Handling Procedure

Dcl-Proc HandleError;
   Dcl-Pi HandleError;
      errorMsg Char(100);
   End-Pi;

   Dsply errorMsg;
End-Proc;

Dcl-S num1 Int(10);
Dcl-S num2 Int(10);
Dcl-S result Int(10);

num1 = 10;
num2 = 0;

Monitor;
   result = num1 / num2;
On-Error;
   CallP HandleError('An error occurred: Division by zero');
EndMon;

Practical Exercises

Exercise 1: Basic Error Handling

Task: Write a program that attempts to read a file. If the file does not exist, display an error message.

Solution:

Dcl-F myFile Disk(*Input) UsrOpn;

Monitor;
   Open myFile;
On-Error;
   Dsply 'Error: File does not exist';
EndMon;

Close myFile;

Exercise 2: Custom Error Handling

Task: Write a program that performs a division operation. If a division by zero occurs, call a custom error handling procedure.

Solution:

Dcl-Proc HandleError;
   Dcl-Pi HandleError;
      errorMsg Char(100);
   End-Pi;

   Dsply errorMsg;
End-Proc;

Dcl-S num1 Int(10);
Dcl-S num2 Int(10);
Dcl-S result Int(10);

num1 = 10;
num2 = 0;

Monitor;
   result = num1 / num2;
On-Error;
   CallP HandleError('An error occurred: Division by zero');
EndMon;

Common Mistakes and Tips

  1. Ignoring Errors: Always handle errors appropriately. Ignoring errors can lead to unexpected behavior and data corruption.
  2. Overusing MONITOR Blocks: Use MONITOR blocks judiciously. Overusing them can make your code harder to read and maintain.
  3. Not Logging Errors: Consider logging errors to a file or database for later analysis.

Conclusion

Error handling is an essential skill for any RPG programmer. By understanding and using RPG's built-in error handling constructs, as well as creating custom error handling procedures, you can create robust and reliable programs. Practice the exercises provided to reinforce your understanding and prepare for more advanced topics.

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