MESG(4) MESG(4)
NAME
mesg, rmesg - message line discipline modules
SYNOPSIS
#include <sys/stream.h>
DESCRIPTION
After the mesg_ld line discipline module has been pushed on
a stream (see 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 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.
Messages written on a stream are interpreted as containing a
header of the form
struct {
short type;
short size;
};
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 impossi-
ble message is written, the stream is shut down.
These message types are possible:
M_DATA (0) Ordinary data.
M_BREAK (01) A line break on an RS232-style asynchronous
connection. No associated data.
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.
M_DELIM (03) A delimiter that introduces a record boundary
in the data. No associated data.
M_IOCTL (06) An ioctl(4) request. The associated data is
an integer containing the ioctl code, followed by
the object passed with the ioctl call that
MESG(4) MESG(4)
generated the message. It is mandatory to reply
with M_IOCACK or M_IOCNAK.
M_DELAY (07) A real-time delay. One byte of data, giving
the number of sixtieths (fiftieths) of a second
delay time.
M_CTL (010) Device-specific control message.
M_SIGNAL (0101) Generate signal number given in the 1-byte
message.
M_FLUSH (0102) Flush input and output queue if possible.
M_STOP (0103) Stop transmission immediately (used, for
example, in conjunction with XON-XOFF flow control.
M_START (0104) Restart transmission after M_STOP.
M_IOCACK (0105) Positive acknowledgement of M_IOCTL message.
At the other end, any included data will be stored
through the pointer given in the ioctl call.
M_IOCNAK (0106) Negative acknowledgement of M_IOCTL. A 1-
byte message, if present, will be used as the
return code for the failing ioctl.
M_CLOSE (0107) Used internally in Datakit call supervision.
SEE ALSO
stream(4)
BUGS
The format of arguments to M_IOCTL is machine dependent.
Sufficient unto the day is the nonportability thereof.
If incorrectly formatted data is written to mesg, the stream
may be irretrievably clogged.