LU 6.2 API Application Programmer's Reference Guide (30294-90008)

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6 Buffer Management
LU 6.2 API optimizes the use of the data communications line by
buffering the data that is sent and received. This chapter explains how
LU 6.2 API handles control information and how it uses the send and
receive buffers to manage the data traffic between transaction
programs.
Control Information
For every conversation allocated, LU 6.2 API establishes and maintains
a set of flags or indicators that keep track of certain control information
necessary to manage the conversation. Whenever an intrinsic is called,
LU 6.2 API checks these flags before executing the intrinsic to see if any
information must be relayed to the TP. The following flags are
associated with each conversation:
• Error flag. The error flag is used to inform the TP that some type of
error has occurred.
• RequestToSendReceived flag. The RequestToSendReceived
flag tells the TP whether a RequestToSend notification has been
received from the remote TP. A RequestToSend is the only control
information that can be received in the same intrinsic call with data.
• State indicator. The state indicator keeps track of the conversation
state of the local TP. If the state changes, the state indicator
changes. When an intrinsic is called, the state indicator is checked to
ensure that the intrinsic can be called from the current state.
• Synchronization level indicator. The synchronization level
indicator records the synchronization level that is established when
a conversation is allocated. If an intrinsic is called that can be
executed only at a certain synchronization level, the synchronization
level indicator is checked to verify that the conversation was
allocated with the appropriate synchronization level.