What is the purpose of MONMSG?

In IBM i (formerly AS400), the MONMSG command is a powerful tool used for monitoring and handling messages within Control Language (CL) programs. Its primary purpose is to allow your programs to respond to specific events or errors that occur during their execution.

Here's a breakdown of why MONMSG is essential:

1. Error Handling:

Programs often encounter errors, such as file not found, invalid data, or system issues.
MONMSG allows you to define how your program should react to these errors.
You can specify actions like:
- Ignoring the error: The program continues execution as if the error didn't occur.
- Transferring control: The program jumps to a specific label or subroutine to handle the error.
- Displaying a message: The program informs the user about the error.
- Ending the program: The program terminates gracefully.

2. Event Monitoring:

Besides errors, MONMSG can also monitor for specific events, such as a file being opened or closed, a certain condition being met, or a message arriving in a message queue.
This allows your programs to be more dynamic and responsive to different situations.

3. Program Control:

By using MONMSG strategically, you can control the flow of your program based on the messages it receives.
This can be used to implement complex logic and decision-making within your CL programs.

How it works:

MONMSG monitors for specific messages identified by their message IDs (e.g., CPF0001, MCH1211).
You can specify the message ID and the action to be taken when that message is received.
MONMSG can be placed at the beginning of a program (program-level) to monitor messages throughout the program, or it can be placed after a specific command (command-level) to monitor messages from that command only.

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, which indicates that the "from" file was not found. If this message is received, the program jumps to the ERROR label to handle the error.