2.9BSD/usr/man/cat5/mpxio.5
MPXIO(5) UNIX Programmer's Manual MPXIO(5)
NAME
mpxio - multiplexed i/o
SYNOPSIS
#include <sys/mx.h>
#include <sgtty.h>
DESCRIPTION
Data transfers on mpx files (see _m_p_x(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 _i_n_d_e_x identifies the channel, and _c_o_u_n_t specifies the
number of characters in _d_a_t_a. If _c_o_u_n_t is zero, _c_c_o_u_n_t gives
the size of _d_a_t_a, and the record is a control or status
message. Although _c_o_u_n_t or _c_c_o_u_n_t 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
indirectly and the other cells have the same interpretation
as in _i_n_p_u_t__r_e_c_o_r_d.
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
<_s_y_s/_m_x._h>), the second is an optional parameter meaningful
only with M_WATCH and M_BLK.
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MPXIO(5) UNIX Programmer's Manual 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 _d_e_t_a_c_h command (see _m_p_x(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 _t_t_y(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 _x_d by executing _i_o_c_t_l(_x_d, _M_X_N_B_L_K,
_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 thres-
hold.
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 _m_p_x(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 _i_o_c_t_l(_f_d,
_c_m_d, &_v_e_c) call on the channel file descriptor.
The M_IOCTL code is followed by the _c_m_d argument
given to _i_o_c_t_l followed by the contents of the
structure _v_e_c. It is assumed, not needing a better
compromise at this time, that the length of _v_e_c is
determined by _s_i_z_e_o_f (_s_t_r_u_c_t _s_g_t_t_y_b) as declared
in <_s_g_t_t_y._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.
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