Writing message-driven programs

A message-driven program is similar to an MPP: it retrieves messages and processes them, and it can read and update MSDBs, DEDBs, and full-function databases.

Message-driven programs can send messages to these destinations:

• The logical terminal that sent the input message, by issuing an ISRT call referencing the I/O PCB
• A different component of the physical terminal that sent the input message, by issuing an ISRT call referencing an alternate response PCB
• A different physical terminal from the one that sent the input message, by issuing an ISRT call referencing an alternate PCB

The message processing functions available to a message-driven program have some restrictions. These restrictions apply only to messages received or sent by the I/O PCB. The input message for a message-driven program must be a single segment message. Therefore, GU is the only call you can use to obtain the input message. The response message sent by the I/O PCB also must be a single segment message.

The transactions are in the response mode. This means that you must respond before the next message can be sent. You cannot use SPAs because a message-driven program cannot be a conversational program.

Not all of the system service calls are available. These system service calls are valid in a message-driven region:

• CHKP (basic)
• DEQ
• INIT
• LOG
• SETS
• ROLB
• ROLS

However, other conditions might restrict their function in this environment. The options or calls issued using alternate terminal PCBs have no constraints.