In MCF message exchange, MCF message queues are used to store messages. In an OpenTP1 system when an MCF message is received from another system, MCF temporarily stores the received message in an input queue. MCF then activates a UAP, called a message-handling program, to process the message. The message-handling program receives the message (with the dc_mcf_receive() function) and processes it. After the message is processed, the message-handling program uses the reply function (dc_mcf_reply()) and MCF temporarily stores the reply message in the output queue. When the message-handling program terminates normally (e.g., a C program executes return), MCF sends the messages stored in the output queue to the other system.

Figure 2-3 gives an overview of the processing in sending and receiving MCF messages. The large arrow indicates the data flow of messages. The black arrow indicates how execution of return in a C program causes MCF to send the MCF message from the output queue to the other system.

Figure 2-3 Overview of MCF message-exchange processing