MPXIO(5) MPXIO(5)
NAME
mpxio - multiplexed i/o
SYNOPSIS
#include <sys/mx.h>
#include <sgtty.h>
DESCRIPTION
Data transfers on mpx files (see mpx(2)) are multiplexed by
imposing a record structure on the io stream. Each record
represents data from/to a particular channel or a control
or status message associated with a particular channel.
The prototypical data record read from an mpx file is as
follows
struct input_record {
short index;
short count;
short ccount;
char data[];
};
where index identifies the channel, and count specifies the
number of characters in data. If count is zero, ccount gives
the size of data, and the record is a control or status
message. Although count or ccount might be odd, the operat-
ing system aligns records on short (i.e. 16-bit) boundaries
by skipping bytes when necessary.
Data written to an mpx file must be formatted as an array of
record structures defined as follows
struct output_record {
short index;
short count;
short ccount;
char *data;
};
where the data portion of the record is referred to indi-
rectly and the other cells have the same interpretation as
in input_record.
The control messages listed below may be read from a multi-
plexed file descriptor. They are presented as two 16-bit
integers: the first number is the message code (defined in
<sys/mx.h>), the second is an optional parameter meaningful
only with M_WATCH and M_BLK.
MPXIO(5) MPXIO(5)
M_WATCH - a process `wants to attach' on this channel.
The second parameter is the 16-bit user-id of the
process that executed the open.
M_CLOSE - the channel is closed. This message is gen-
erated when the last file descriptor referencing a
channel is closed. The detach command (see mpx(2)
should be used in response to this message.
M_EOT - indicates logical end of file on a channel. If
the channel is joined to a typewriter, EOT
(control-d) will cause the M_EOT message under the
conditions specified in tty(4) for end of file.
If the channel is attached to a process, M_EOT
will be generated whenever the process writes zero
bytes on the channel.
M_BLK - if non-blocking mode has been enabled on an mpx
file descriptor xd by executing ioctl(xd, MXNBLK,
0), write operations on the file are truncated in
the kernel when internal queues become full. This
is done on a per-channel basis: the parameter is a
count of the number of characters not transferred
to the channel on which M_BLK is received.
M_UBLK - is generated for a channel after M_BLK when
the internal queues have drained below a thresh-
old.
Two other messages may be generated by the kernel. As with
other messages, the first 16-bit quantity is the message
code.
M_OPEN - is generated in conjunction with `listener'
mode (see mpx(2)). The uid of the calling process
follows the message code as with M_WATCH. This is
followed by a null-terminated string which is the
name of the file being opened.
M_IOCTL - is generated for a channel connected to a
process when that process executes the ioctl(fd,
cmd, &vec) call on the channel file descriptor.
The M_IOCTL code is followed by the cmd argument
given to ioctl followed by the contents of the
structure vec. It is assumed, not needing a better
compromise at this time, that the length of vec is
determined by sizeof (struct sgttyb) as declared
in <sgtty.h>.
Two control messages are understood by the operating system.
M_EOT may be sent through an mpx file to a channel. It is
equivalent to propagating a zero-length record through the
channel; i.e. the channel is allowed to drain and the pro-
cess or device at the other end receives a zero-length
transfer before data starts flowing through the channel
again. M_IOCTL can also be sent through a channel. The
format is identical to that described above.