V8/usr/man/man4/mesg.4

.TH MESG 4
.SH NAME
mesg, rmesg \- message line discipline modules
.SH SYNOPSIS
.B #include <sys/stream.h>
.SH DESCRIPTION
After the
.I mesg_ld
line discipline module has been pushed on a stream (see
.IR stream (4)),
all data and control blocks appearing on the stream are turned
into structured messages to be read by the user process, and 
information written on the stream is interpreted as messages in the same
format.
The
.I rmesg_ld
line discipline is exactly the opposite:
writes, and control operations, on the stream are turned into
data messages, and arriving data messages are turned into controls.
.PP
Messages written on a stream are interpreted as containing a header of
the form
.PP
.nf
.ta 5m 10m 15m 20m 25m 30m 35m 40m 45m 50m
	struct {
		short	type;
		short	size;
	};
.fi
.DT
.LP
followed by the contents of the message.
The size is the size of the message exclusive of header.
It is limited, but to a large number (at least 4KB).
Messages may be written piecemeal, and several messages may be written
at once, but messages will be read at most one at a time.
If an impossible message is written, the stream is shut down.
.PP
These message types are possible:
.TP "\w'M_HANGUP 'u"
M_DATA
(0)
Ordinary data.
.TP
M_BREAK
(01)
A line break on an RS232-style asynchronous connection.
No associated data.
.TP
M_HANGUP
(02)
When received, indicates that the other side has gone away.
It is interpreted by the message processor as well as transmitted,
and after it is read the stream is useless.
No associated data.
.TP
M_DELIM
(03)
A delimiter that introduces a record boundary in the data.
No associated data.
.TP
M_IOCTL
(06)
An
.IR ioctl (4)
request.
The associated data is an integer containing the ioctl code,
followed by the object passed with the ioctl call that generated the message.
It is mandatory to reply with
.I M_IOCACK
or
.IR M_IOCNAK .
.TP
M_DELAY
(07)
A real-time delay.
One byte of data, giving the number of sixtieths (fiftieths) of a second
delay time.
.TP
M_CTL
(010)
Device-specific control message.
.TP
M_SIGNAL
(0101)
Generate signal number given in the 1-byte message.
.TP
M_FLUSH
(0102)
Flush input and output queue if possible.
.TP
M_STOP
(0103)
Stop transmission immediately (used, for example, in conjunction with
XON-XOFF flow control.
.TP
M_START
(0104)
Restart transmission after
.IR M_STOP .
.TP
M_IOCACK
(0105)
Positive acknowledgement of
.I M_IOCTL
message.
At the other end, any included data will be stored through the
pointer given in the ioctl call.
.TP
M_IOCNAK
(0106)
Negative acknowledgement of
.IR M_IOCTL .
A 1-byte message, if present, will be used as the return code
for the failing ioctl.
.TP
M_CLOSE
(0107)
Used internally in Datakit call supervision.
.PP
.SH SEE ALSO
stream(4)
.SH BUGS
The format of arguments to
.I M_IOCTL
is machine dependent.
Sufficient unto the day is the nonportability thereof.
.br
If incorrectly formatted data is written to
.IR mesg ,
the stream may be irretrievably clogged.