Sending and Receiving Messages in CLLE

Learning Objective

By the end of this lesson, you will understand what a message queue is at a basic level, and be able to use SNDPGMMSG to send a message and RCVMSG to deliberately receive one.

Simple Explanation

You have already used SNDPGMMSG in earlier CLLE lessons to send a simple message. This lesson looks at messages more fully: where a sent message actually goes, and how a program can deliberately read a message back, rather than only reacting automatically the way MONMSG does.

Every job has a message queue, a place IBM i holds messages sent to or from that job, connecting directly to the Job Logs and Spool Files Basics lesson: many of the messages that end up in a job log arrive there by being sent to that job's message queue. SNDPGMMSG sends a message to a message queue, commonly the job's own:

DCL VAR(&MSG) TYPE(*CHAR) LEN(50)

CHGVAR VAR(&MSG) VALUE('Order processing started')
SNDPGMMSG MSG(&MSG) TOPGMQ(*EXT)

TOPGMQ(*EXT) sends the message to the external message queue of the job that called this program, a common choice for status messages meant to be visible outside the current program itself.

Where MONMSG automatically catches specific messages as they occur, a program can also deliberately read a message using RCVMSG:

DCL VAR(&RECEIVEDMSG) TYPE(*CHAR) LEN(80)

RCVMSG MSGTYPE(*ANY) MSG(&RECEIVEDMSG)

RCVMSG pulls the next available message from a message queue into a variable, letting a program inspect or respond to it directly, rather than relying only on MONMSG's automatic pattern.

Why It Matters

Understanding messages and message queues at this basic level explains where the messages you have already been sending with SNDPGMMSG throughout this lesson group actually go, and gives you a second way, RCVMSG, to work with messages deliberately rather than only reacting to them automatically through MONMSG. This becomes especially useful when a program needs to inspect a message's exact content, not just detect that some matching message occurred.

Practical Example

Imagine a CLLE program that sends a status message when it starts processing, using SNDPGMMSG with TOPGMQ(*EXT), so that whatever called it can see that status update. Separately, imagine a monitoring program that uses RCVMSG to deliberately read messages from a job's message queue, checking their content to decide what to do next, rather than only reacting to specific error messages the way MONMSG does.

This is a simplified, illustrative example rather than a specific real system, but it reflects a genuinely common, basic pattern for sending and deliberately reading messages in CLLE.

Common Confusions

"Is RCVMSG the same thing as MONMSG?" No. MONMSG automatically catches a specific message, or class of messages, immediately after a command, as covered in the MONMSG and Basic Error Handling in CLLE lesson. RCVMSG deliberately reads a message from a message queue at the point in the program where it is used, rather than reacting automatically to a specific command.

"Does every message end up in the job log?" Many messages sent to a job's message queue do become part of that job's job log, as covered in the Job Logs and Spool Files Basics lesson, though the exact details of what is logged can depend on the specific message and job settings.

"Do I need to use RCVMSG in most simple CLLE programs?" No. Many simple CLLE programs only need SNDPGMMSG to send status or error messages, and MONMSG to react automatically to specific problems. RCVMSG is most useful when a program specifically needs to inspect a message's content directly.

Quick Recap

• Every job has a message queue, where messages sent to or from that job are held; many job log entries originate this way.
• SNDPGMMSG sends a message; TOPGMQ(*EXT) is a common choice for status messages meant to be visible to whatever called the current program.
• RCVMSG deliberately reads the next available message from a message queue into a variable, unlike MONMSG's automatic reaction pattern.
• Most simple CLLE programs rely mainly on SNDPGMMSG and MONMSG; RCVMSG is most useful when a program needs to inspect a message's exact content.

Try Asking the AI Tutor

Use the AI Tutor to ask follow-up questions about this lesson. For example, try asking:

• "What is the difference between a job's message queue and a user's message queue?"
• "What does TOPGMQ(*EXT) mean compared to other TOPGMQ options?"