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 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, M_BLK, and M_SIG.

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.
M_SIG - is generated instead of a normal asynchronous signal on channels that are joined to typewriters. The parameter is the signal number.

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.