How do you handle errors in a CL program?

Error handling is a crucial aspect of writing robust CL programs in IBM i. Here's a breakdown of how you can effectively handle errors:

1. Understanding Messages:

Message IDs: Errors in IBM i are associated with specific message IDs (e.g., CPF0001, MCH1211). These IDs help identify the type of error that occurred.
Message Text: Each message ID has associated text that provides a description of the error.
Message Types: Messages have different types, such as:
- Escape messages: Indicate a serious error that may require program termination.
- Notify messages: Inform about a condition that might need attention but doesn't necessarily require termination.
- Status messages: Provide information about the progress of a command.

2. Using the MONMSG command:

The MONMSG command is the primary tool for handling messages in CL programs.
It allows you to monitor for specific messages and define actions to be taken when those messages are received.
You can specify:
- The message ID to monitor.
- The action to take (e.g., ignore the message, transfer control to a label, display a message, end the program).

3. Setting up MONMSG:

Program-level MONMSG: Placed at the beginning of your program to monitor messages throughout the entire program.
Command-level MONMSG: Placed immediately after a specific command to monitor messages generated by that command.

4. Actions in MONMSG:

GOTO: Transfers control to a specific label in your program, allowing you to execute error-handling routines.
EXEC: Executes a CL command, such as displaying a message or calling another program.
RTVMSG: Retrieves the message text for a specific message ID.
SNDPGMMSG: Sends a program message to the user or to a message queue.

5. Example :

PGM        /* Program to copy a file */

             CPYF FROMFILE(MYLIB/MYFILE1) TOFILE(MYLIB/MYFILE2)
             MONMSG MSGID(CPF2812) EXEC(GOTO ERROR) /* File not found */

             /* ... more processing ... */

             GOTO END

ERROR:       /* Error handling routine */
             SNDPGMMSG MSG('File not found!') MSGTYPE(*ESCAPE)
             /* ... other error handling actions ... */

END:         ENDPGM

In this example :

The MONMSG command monitors for message CPF2812 (file not found) after the CPYF command.
If the message is received, the program jumps to the ERROR label.
The ERROR routine sends an escape message to the user and can perform other error handling actions.

6. Best Practices:

Monitor for critical errors: Focus on handling errors that can cause your program to fail or produce incorrect results.
Provide informative messages: When an error occurs, provide clear and helpful messages to the user or system administrator.
Implement error recovery: If possible, try to recover from errors and allow the program to continue execution.
Use a consistent error handling strategy: Develop a standard approach for handling errors in your CL programs to make them easier to maintain and understand.

By effectively using MONMSG and following best practices, you can create robust and reliable CL programs that can handle errors gracefully and ensure the smooth operation of your IBM i system.