Defined STREAMS message types differ in their intended purposes, their treatment at the Stream head, and in their message-queueing priority.
STREAMS does not prevent a module or driver from generating any message type and sending it in any direction on the Stream. However, established processing and direction rules should be observed. Stream-head processing according to message type is fixed, although certain parameters can be altered.
The message types found in sys/stream.h are described in this appendix, classified according to their message queueing priority. Ordinary messages are described first, with high-priority messages following. In certain cases, two message types may perform similar functions, differing only in priority. Message construction is described in Chapter 3, STREAMS Application-Level Mechanisms. The use of the word module generally implies module or driver.
Ordinary messages are also called normal or nonpriority messages. Ordinary messages are subject to flow control whereas high priority messages are not.