Messages

A message is a communication sent from one user, program, or procedure to another.

System messages can report both status information and error conditions. Most data processing systems provide communications between the system and the operator to handle errors and other conditions that occur during processing. i5/OS™ also provides message handling functions that support two-way communications between programs and system users, between programs, between procedures within a program, and between system users. Two types of messages are supported:

• Immediate messages, which are created by the program or system user when they are sent and are not permanently stored in the system.
• Predefined messages, which are created before they are used. These messages are placed in a message file when they are created, and retrieved from that file when they are used.

Because messages can be used to provide communications between programs, between procedures in a program, and between programs and users, using the i5/OS message handling functions should be considered when developing applications. The following concepts of message handling are important to application development:

• Messages can be defined in messages files, which are outside the programs that use them, and variable information can be provided in the message text when a message is sent. Because messages are defined outside the programs, the programs do not have to be changed when the messages are changed. This approach also allows the same program to be used with message files containing translations of the messages into different languages.
• Messages are sent to and received from message queues, which are separate objects on the system. A message sent to a queue can remain on the queue until it is explicitly received by a program or workstation user.
• A program can send messages to a user who requested the program regardless of what workstation that user has signed on to. Messages do not have to be sent to a specific device; one program can be used from different workstations without change.

• Message descriptions
  A message description defines a message to i5/OS.
• Message queues
  When a message is sent to a procedure, a program, or a system user, it is placed on a message queue associated with that procedure, program, or user. The procedure, program, or user sees the message by receiving it from the queue.