SNAP-IX CPI-C Programmer's Guide - 1.5 Changing Conversation States

1.5 Changing Conversation States

In Changing Conversation States, the conversation states appear in the left and right margins. This table shows how CPI-C calls can change the state of the conversation from Send to Receive and from Receive to Send.

Changing Conversation States

State	Invoking Program	Invoked Program	State
Reset
	Initialize_Conversation
Initialize
	Set_Sync_Level
	( sync_level=CM_CONFIRM)
	Allocate
Send
	Send_Data
	Prepare_To_Receive		Reset
		->
		Accept_Conversation
			Receive
		Receive
		( status_received=CM_CONFIRM_SEND_RECEIVED)
			Confirm-Send
		Confirmed
		<-
			Send
	( return_code=CM_OK)
Receive
		Send_Data
		Confirm
		<-
	Receive
	( status_received= CM_CONFIRM_RECEIVED )
Confirm
	Request_To_Send
	Confirmed
		->
Receive
		( return_code=CM_OK)
		( request_to_send_received= CM_REQ_TO_SEND_RECEIVED )
		Prepare_To_Receive
		<-
	Receive
	( status_received= CM_CONFIRM_SEND_RECEIVED )
Confirm-Send
	Confirmed
		->
Send
		( return_code=CM_OK)
			Receive
	Send_Data
	Deallocate
		->
		Receive
		( status_received= CM_CONFIRM_DEALLOC_RECEIVED )
			Confirm-Deallocate
		Confirmed
		<-
			Reset
	( return_code=CM_OK)
Reset

1.5.1 Initial States

Before the conversation is allocated, both programs are in Reset state.

After the conversation is allocated, the initial state is Send for the invoking program and Receive for the invoked program.

1.5.2 Changing to Receive State

The Prepare_To_Receive call enables a program to change the conversation from Send to Receive state. This call does the following:

It flushes the local LU's send buffer.
If the synchronization level is set to CM_CONFIRM, the Prepare_To_Receive call sends a CM_CONFIRM_SEND indicator to the partner program through the status_received parameter of a Receive call. This indicator tells the partner program that a Confirmed response is expected before the partner program can begin to send data.

1.5.3 Changing to Send State

The conversation state for a program changes from Receive to Send when its partner program begins to receive data (by issuing the Prepare_To_Receive call).

The local program (for which the conversation is in Receive state) can inform the partner program that it wants to send data, by issuing the Request_To_Send call. This request is communicated to the partner program through the request_to_send_received parameter. (In the previous example, this parameter is shown on the Confirm call; it is also returned to Send_Data and other calls.)

Issuing the Request_To_Send call does not change the state of the conversation, because the partner program can ignore it. When the partner program issues the Prepare_To_Receive call, the conversation state changes to Receive for the partner program. The local program receives the SEND indication on a subsequent RECEIVE verb, and can then send data.