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 ioctls Upstream M_COPYOUT Copies out data for transparent ioctls 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 ioctls, 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