Pathway/iTS TCP and Terminal Programming Guide

Processing Unsolicited Messages
Compaq NonStop™ Pathway/iTS TCP and Terminal Programming Guide—426751-001
7-10
ESCAPE ON UNSOLICITED MESSAGE Design
Considerations
Applications for Intelligent Terminals
If your application is for intelligent terminals, consider the following:
•
You can use the interrupt method of responding to unsolicited messages (ESCAPE
ON UNSOLICITED MESSAGE) with SEND MESSAGE statements in IDS
requesters that communicate with front-end processes whether the front-end
processes support the use of CONTROL 26. If the front-end process supports the
use of CONTROL 26, an ESCAPE ON UNSOLICITED MESSAGE can be
performed with no loss of data. If the front-end process does not support the use of
CONTROL 26, then the TCP uses only CANCEL calls (no CONTROL 26 calls) to
terminate the underlying read operation. In the latter case, the amount of data that
could be lost is application-dependent or device-dependent.
•
If a UMP message is already queued when a SEND MESSAGE with an ESCAPE
ON UNSOLICITED MESSAGE clause is executed, the TCP still issues the SEND
MESSAGE call. To process the ESCAPE ON UNSOLICITED MESSAGE clause,
however, the TCP immediately cancels the SEND MESSAGE WRITEREAD or
issues a CONTROL 26 call against the WRITEREAD. To prevent unnecessary
processing on a SEND MESSAGE, poll the PW-UNSOLICITED-MESSAGE-
QUEUED register for a YES value to detect the arrival of UMP messages prior to
issuing the SEND MESSAGE statement.
Applications for Block-Mode Terminals
If your application is for block-mode terminals, consider the following:
•
You can also use the interrupt technique with ACCEPT statements in standard
requesters that control block-mode terminals supported by Pathway/iTS. In that
context, however, the TCP uses only CANCEL calls (no CONTROL 26 calls) to
terminate the underlying read operation; it does so with a minimal loss of data. (At
most, the loss is a single function-key stroke.)
•
See Internal Function-Key Queuing on page 3-13 for implications of the function-
key queuing capability of 6530 terminals and ESCAPE ON UNSOLICITED
MESSAGE clauses within ACCEPT verbs.
Applications for Conversational Terminals
If your application is for conversational terminals, the interrupt technique of processing
must be used judiciously with conversational-mode requesters. This is because the
arrival of an unsolicited message causes all data entered by the terminal user for all
fields being accepted by the interrupted ACCEPT verb to be discarded.