ACC Programmer's Reference Guide

50 Chapter3
ZCOM Tables and Data Structures
ZCOM Header Structure
HNDSIG - ZNODE signal number
ContainsthetypeofsignaltousewheninformingZNODEofthearrival
of new data in the ZNODE queue. This field is initialized to zero by
TTGEN and is set up by the ZNODE program when it starts up.
HNIDLE - ZNODE idle timer
This contains the time in seconds since ZNODE last issued a
Node-Status Update ioctl request to the ZCOM subsystem (LDM). The
LDM timer increments this field once every second and marks all nodes
as DOWN when it exceeds the inactivity period (currently 20 seconds).
This field is initialized to zero when the ZCOM runtime subsystem is
started up.
HNHIGH, HNLOW - ZNODE queue high and low water marks (flow
control)
ThesetwofieldsareusedbytheLDMtocontroltheflowofdatabetween
the ZNODE daemon and the LDM. When the number of pending
messages on the ZNODE queue reaches the high-water mark, programs
initiating remote requests are suspended to avoid excessive buffer usage.
Suspended programs will be allowed to resume execution when the
number of queued messages drops to or below the low-water mark.
A specia l case exists when both the high and low water marks are set to
the same value. In this situation, whenever the number of pending
messages on the ZNODE queue is at or above this mark, all remote
requestsarerejectedwithanimmediateerrorreturnvalueofZENBUSY
(-1, ZNODE is busy).
Note that this mechanism is only applicable to programs initiating
remote requests and does not limit the messages or responses generated
as a result of ZCOM subsystem operation (e.g. a terminal ZLU returns a
message to its receiver in a remote system).
ThesetwovaluesaredefinedbytheFlow-Controlkeywordinthe
TTGEN configuration file. If not specified, they will default to a
high-water mark value of t he maximum queue limit and a low-water
markvalueofzero(i.e.themechanismisdisabled).
HNWAIT - ZNODE queue high-water mark waiter
This field is maintained and used by the LDM to keep track of the
number of programs currently suspended due to the high/low water
mark flow control mechanism. It is initialized to zero by TTGEN.