MESGLD(4) MESGLD(4)
NAME
mesg_ld, rmesg_ld - message line discipline modules
SYNOPSIS
#include <sys/types.h>
#include <sys/stream.h>
DESCRIPTION
Mesg_ld and 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.
After mesg_ld has been pushed on a stream, read(2) on the
stream returns encoded messages; 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 <sys/stream.h>, is of the form
struct mesg {
char type;
unsigned char magic;
unsigned char losize, hisize;
};
#define MSGMAGIC 0345
#define MSGHLEN 4 /* true length of struct mesg in bytes */
The header is MSGHLEN bytes long; beware that this is not
always the same as `sizeof(struct mesg)'. The magic field
contains the constant MSGMAGIC, to help prevent interpreting
bad data as a message header. There are losize+(hisize<<8)
bytes of associated data.
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.
Rmesg_ld is exactly the opposite of mesg_ld. It is intended
for use with devices that generate data containing encoded
messages. Here is a list of message types, defined in
<sys/stream.h>:
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. Thereafter the stream is useless.
No associated data.
MESGLD(4) MESGLD(4)
M_DELIM (03) A delimiter that introduces a record boundary
in the data. No associated data.
M_IOCTL (06) An 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 M_IOCACK or
M_IOCNAK reply is expected.
M_DELAY (07) A real-time delay. One byte of data, giving
the number of clock ticks of delay time.
M_CTL (010) Device-specific control message.
M_SIGNAL (0101) Generate signal number given in the one-
byte message.
M_FLUSH (0102) Flush input and output queue if possible.
M_STOP (0103) Stop transmission immediately.
M_START (0104) Restart transmission after M_STOP.
M_IOCACK (0105) Successful reply to M_IOCTL. Associated
data is to be written back to the caller.
M_IOCNAK (0106) Failed reply to M_IOCTL. A single-byte
message, if present, will be returned in by the
failing ioctl.
M_PRICTL (0107) High-priority device-specific control mes-
sage.
SEE ALSO
stream(4)
BUGS
The format of arguments to M_IOCTL is machine dependent.
The amount of associated data is limited, but large (>4K).