STREAMS Programming Guide

Message Types

Several STREAMS messages differ in their purpose and queueing priority. The message types are briefly described and classified, according to their queueing priority, in Table 7-1. A detailed discussion of Message Types is in Chapter 8, Messages - Kernel Level.

Some message types are defined as high-priority types. The others can have a normal priority of 0, or a priority (also called a band) from 1 to 255.

Table 7-1 Ordinary Messages, Showing Direction of Communication Flow


Ordinary Messages (also called Normal Messages)		Direction
`M_BREAK`	Request to a Stream driver to send a "break"	Upstream
`M_CTL`	Control or status request used for intermodule communication	Bidirectional
`M_DATA`	User data message for I/O system calls	Bidirectional
`M_DELAY`	Request for a real-time delay on output	Downstream
`M_IOCTL`	Control/status request generated by a Stream head	Downstream
`M_PASSFP`	File pointer-passing message	Bidirectional
`M_PROTO`	Protocol control information	Bidirectional
`M_SETOPTS`	Sets options at the Stream head; sends upstream	Upstream
`M_SIG`	Signal sent from a module or driver	Upstream

Table 7-2 High-Priority Messages, Showing Direction of Communication Flow


High-Priority Messages:		Direction
`M_COPYIN`	Copies in data for transparent ioctl(2)s	Upstream
`M_COPYOUT`	Copies out data for transparent ioctl(2)s	Upstream
`M_ERROR`	Reports downstream error condition	Upstream
`M_FLUSH`	Flushes module queue	Bidirectional
`M_HANGUP`	Sets a Stream head hangup condition	Upstream
`M_UNHANGUP`	Reconnects line, sends upstream when hangup reverses	Upstream
`M_IOCACK`	Positiveioctl(2) acknowledgment	Upstream
`M_IOCDATA`	Data for transparent ioctl(2)s, sent downstream	Downstream
`M_IOCNAK`	Negative ioctl(2) acknowledgment	Upstream
`M_PCPROTO`	Protocol control information	Bidirectional
`M_PCSIG`	Sends signal from a module or driver	Upstream
`M_READ`	Read notification; sends downstream	Downstream
`M_START`	Restarts stopped device output	Downstream
`M_STARTI`	Restarts stopped device input	Downstream
`M_STOP`	Suspends output	Downstream
`M_STOPI`	Suspends input	Downstream