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.TH MESGLD 4
.CT 2 comm_mach
.SH NAME
mesg_ld, rmesg_ld \- message line discipline modules
.SH SYNOPSIS
.B #include <sys/types.h>
.br
.B #include <sys/stream.h>
.SH DESCRIPTION
.I Mesg_ld
and
.I rmesg_ld
turn internal stream messages
into ordinary data and vice versa.
They supply a way to splice a stream connection together
through a process or across a network connection.
The messages represent ordinary data
and various control operations.
.PP
After
.IR mesg_ld
has been pushed on a stream,
.IR read (2)
on the stream returns encoded messages;
.I write
is expected to supply messages in the same coding.
An encoded message consists of a header
followed by zero or more bytes of associated data.
The header,
as defined in
.BR <sys/stream.h> ,
is of the form
.IP
.EX
struct mesg {
char type;
unsigned char magic;
unsigned char losize, hisize;
};
#define MSGMAGIC 0345
#define MSGHLEN 4 /* true length of struct mesg in bytes */
.EE
.LP
The header is
.B MSGHLEN
bytes long;
beware that this is not always the same as
.LR "sizeof(struct mesg)" .
The
.B magic
field contains the constant
.BR MSGMAGIC ,
to help prevent interpreting bad data as a message header.
There are
.B "losize+(hisize<<8)"
bytes of associated data.
.PP
Messages may be written in pieces,
or several messages may be written at once.
At most one message will be read at a time.
If an impossible message is written,
the stream may be shut down.
.PP
.IR Rmesg_ld
is exactly the opposite of
.IR mesg_ld .
It is intended for use with devices
that generate data containing encoded messages.
Here is a list of message types,
defined in
.BR <sys/stream.h> :
.TF M_HANGUP
.TP
.B M_DATA
(0)
Ordinary data.
.TP
.B M_BREAK
(01)
A line break on an RS232-style asynchronous connection.
No associated data.
.TP
.B M_HANGUP
(02)
When received, indicates that the other side has gone away.
Thereafter the stream is useless.
No associated data.
.TP
.B M_DELIM
(03)
A delimiter that introduces a record boundary in the data.
No associated data.
.TP
.B M_IOCTL
(06)
An
.IR ioctl (2)
request.
The associated data is a four-byte integer
containing the
function code,
least significant byte first,
followed by some amount of associated data.
An
.B M_IOCACK
or
.BR M_IOCNAK
reply is expected.
.TP
.B M_DELAY
(07)
A real-time delay.
One byte of data, giving the number of clock ticks of
delay time.
.TP
.B M_CTL
(010)
Device-specific control message.
.TP
.B M_SIGNAL
(0101)
Generate signal number given in the one-byte message.
.TP
.B M_FLUSH
(0102)
Flush input and output queue if possible.
.TP
.B M_STOP
(0103)
Stop transmission immediately.
.TP
.B M_START
(0104)
Restart transmission after
.BR M_STOP .
.TP
.B M_IOCACK
(0105)
Successful reply to
.BR M_IOCTL .
Associated data is to be written back to the caller.
.TP
.B M_IOCNAK
(0106)
Failed reply to
.BR M_IOCTL .
A single-byte message,
if present,
will be returned in
.CW errno
by the failing
.IR ioctl .
.TP
.B M_PRICTL
(0107)
High-priority
device-specific control message.
.PD
.PP
.SH SEE ALSO
.IR stream (4)
.SH BUGS
The format of arguments to
.B M_IOCTL
is machine dependent.
.br
The amount of associated data
is limited, but large (>4K).
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