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Receiving Messages

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The following figure shows the method for receiving messages from the Dynamic Access Module (DMOD).

Image that shows how to receive messages using a routing procedure.
Receiving messages using a routing procedure

After DMOD initialization, the 3270 emulator registers the routing procedure by calling sepdrout. When the DMOD receives a message, it calls the 3270 emulator routing procedure, which can then process the message.

With this approach, there is no context switch between the DMOD thread and the 3270 emulator thread. However, the routing procedure must return control to the DMOD quickly. For instance, it cannot suspend waiting for a keyboard input.

The application must determine whether the received message is for this application or for another application. If the message is not for this application, the routing procedure must return, indicating that the message was not processed. If the application processes the message, it is responsible for freeing the buffer when the processing is finished.

In some cases, the routing procedure can process the message to completion. An alternative is for the routing procedure to put the message on an application queue and then clear an application semaphore. The application can then subsequently process the message.

Performance can be improved further by sending a Status-Resource message (to return credit to the local node, enabling it to send further data) from the routing procedure when a message is received, rather than waiting until the message is processed to completion. This usage is illustrated in Sample Code: Initialization and Routing Procedure. For more information about credit and flow control, see Pacing and Chunking.

After the application has received a message, the application is responsible for the buffer in which the message was received. The application must either reuse the buffer to send a message (using sbpusend) or release it (using sepdburl). If the buffer to be reused does not contain the correct number of elements for the message to be sent, the application can obtain additional elements (using sbpibegt) or release existing ones (using sbpiberl). In this case, the application must also ensure that the numelts field in the buffer header indicates the correct number of elements.