#include <sgtty.h>
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 operating 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 indirectly and the other cells have the same interpretation as in input_record.
The control messages listed below may be read from a multiplexed 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.
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 generated 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 threshold.
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 process 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.