Message Types (STREAMS Programming Guide)

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 and Table 7–2. A detailed discussion of message types is in Chapter 8, STREAMS Kernel-Level Mechanisms.

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

Table 7–1 Ordinary Messages, Description of Communication Flow


Ordinary 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, Description of Communication Flow


High-Priority Messages		Direction
`M_COPYIN`	Copies in data for transparent `ioctl`s	Upstream
`M_COPYOUT`	Copies out data for transparent `ioctl`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`	Positive `ioctl` acknowledgement	Upstream
`M_IOCDATA`	Data for transparent `ioctl`s, sent downstream	Downstream
`M_IOCNAK`	Negative `ioctl` acknowledgement	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