STREAMS-UX Programmer's Guide (February 2007)

Messages

Message Processing and Flow Control

Chapter 3

Figure 3-3 Flow Control

When queue A uses canput () to check for available space, canput () searches through the stream. canput ()

starts searching from queue B, until it finds queue C that contains a service procedure. Then, canput ()

checks if QFULL is set on queue C. If QFULL is not set, canput () returns 1. Else, canput () marks QWANTW on

queue C, and returns 0.

Back-Enabling Mechanism

STREAMS provides a back-enabling mechanism to schedule the service procedure to processing the

messages in the message queue. This mechanism works as follows:

When the message queue reaches the high water mark, STREAMS marks the message queue as QFULL. Once

canput () detects the QFULL condition, canput () sets the QWANTW flag.

When the accumulated data drops from the high water mark to the low watermark, STREAMS clears the

QFULL flag, and checks if QWANTW is set. If QWANTW is set, STREAMS schedules the service procedure of the

nearest preceding queue to process the messages.

For example, in Figure 3-3, “Flow Control,” queue C is in the QFULL state. A canput () from queue A sets

QWANTW on queue C, and returns 0. Upon return from canput (), queue A may place the message onto the

message queue for processing when queue C becomes available at a later time.

When queue C’s state changes from QFULL to non QFULL, STREAMS checks if QWANTW is set on queue C by any

previous canput (). If QWANTW is set, STREAMS schedules the service procedure of queue A to process the

messages blocked due to the queue full condition encountered by previous canput ().

The details of how STREAMS implements the back-enabling mechanism is transparent to the stream module

and drivers. As long as the module or driver meets the three requirements listed in the “Flow Control”

section, STREAMS ensures the module or driver’s service procedure is scheduled when the QFULL state is

unset.

queue A

with service

procedure

queue B

no service

procedure

queue C

with service

procedure

QFULL

back enable queue As

service procedure

when QFULL on queue

C in not set

canput()