| 1 | .\" @(#)mpxio.5 4.1 (Berkeley) %G% |
| 2 | .\" |
| 3 | .TH MPXIO 5 |
| 4 | .AT 3 |
| 5 | .SH NAME |
| 6 | mpxio \- multiplexed i/o |
| 7 | .SH SYNOPSIS |
| 8 | .B #include <sys/mx.h> |
| 9 | .PP |
| 10 | .B #include <sgtty.h> |
| 11 | .SH DESCRIPTION |
| 12 | Data transfers on |
| 13 | mpx files |
| 14 | (see |
| 15 | .IR mpx (2)) |
| 16 | are multiplexed by |
| 17 | imposing |
| 18 | a record structure on the io stream. |
| 19 | Each record represents data |
| 20 | from/to |
| 21 | a particular channel or |
| 22 | a control or status message associated with a particular channel. |
| 23 | .PP |
| 24 | The prototypical data record read from an mpx file is as follows |
| 25 | .PP |
| 26 | .in +.5i |
| 27 | .nf |
| 28 | struct input_record { |
| 29 | short index; |
| 30 | short count; |
| 31 | short ccount; |
| 32 | char data[]; |
| 33 | }; |
| 34 | .PP |
| 35 | .fi |
| 36 | where |
| 37 | .I index |
| 38 | identifies the channel, |
| 39 | and |
| 40 | .I count |
| 41 | specifies the number of characters in |
| 42 | .I data. |
| 43 | If |
| 44 | .I count |
| 45 | is zero, |
| 46 | .I ccount |
| 47 | gives the size of |
| 48 | .I data, |
| 49 | and the record is a control or status message. |
| 50 | Although |
| 51 | .I count |
| 52 | or |
| 53 | .I ccount |
| 54 | might be odd, |
| 55 | the operating system aligns records |
| 56 | on short (i.e. 16\-bit) boundaries |
| 57 | by skipping bytes when necessary. |
| 58 | .PP |
| 59 | Data written to an mpx file must be formatted as an array |
| 60 | of record structures defined as follows |
| 61 | .PP |
| 62 | .in +.5i |
| 63 | .nf |
| 64 | struct output_record { |
| 65 | short index; |
| 66 | short count; |
| 67 | short ccount; |
| 68 | char *data; |
| 69 | }; |
| 70 | .fi |
| 71 | .PP |
| 72 | where the data portion of the record is referred |
| 73 | to indirectly and the other cells have the same interpretation |
| 74 | as in |
| 75 | .I input_record. |
| 76 | .PP |
| 77 | The |
| 78 | control messages listed below may be read from |
| 79 | a multiplexed file descriptor. |
| 80 | They are presented as two 16-bit integers: |
| 81 | the first number is the message code |
| 82 | (defined in |
| 83 | .IR <sys/mx.h> ), |
| 84 | the second is an optional parameter meaningful |
| 85 | only with M_WATCH, M_BLK, and M_SIG. |
| 86 | .PP |
| 87 | .TP "\w'M_WATCH 'u" |
| 88 | M_WATCH |
| 89 | a process `wants to attach' on this channel. |
| 90 | The second parameter is the 16-bit |
| 91 | user-id of the process that executed the open. |
| 92 | .TP |
| 93 | M_CLOSE |
| 94 | the channel is closed. |
| 95 | This message is generated when the last |
| 96 | file descriptor referencing |
| 97 | a channel is closed. |
| 98 | The |
| 99 | .I detach |
| 100 | command |
| 101 | (see |
| 102 | .IR mpx (2) |
| 103 | should be used in response to this message. |
| 104 | .TP |
| 105 | M_EOT |
| 106 | indicates logical end of file on a channel. |
| 107 | If the channel is joined to a typewriter, |
| 108 | EOT (control-d) |
| 109 | will cause the M_EOT message |
| 110 | under the conditions specified in |
| 111 | .IR tty (4) |
| 112 | for end of file. |
| 113 | If the channel is attached to a process, |
| 114 | M_EOT will be generated whenever the process |
| 115 | writes zero bytes on the channel. |
| 116 | .TP |
| 117 | M_BLK |
| 118 | if non-blocking mode has been enabled on an |
| 119 | mpx file descriptor |
| 120 | .I xd |
| 121 | by executing |
| 122 | .IR "ioctl(xd, MXNBLK, 0)" , |
| 123 | write operations on the file are truncated in the kernel |
| 124 | when internal queues become full. |
| 125 | This is done on a per-channel basis: |
| 126 | the parameter |
| 127 | is a count of the number of characters |
| 128 | not transferred to the channel on which |
| 129 | M_BLK is received. |
| 130 | .TP |
| 131 | M_UBLK |
| 132 | is generated for a channel |
| 133 | after M_BLK when the internal queues have |
| 134 | drained below a threshold. |
| 135 | .TP |
| 136 | M_SIG |
| 137 | is generated instead of a normal asynchronous |
| 138 | signal on channels that are joined to typewriters. |
| 139 | The parameter is the signal number. |
| 140 | .PP |
| 141 | Two other messages may be generated by the kernel. |
| 142 | As with other messages, the first |
| 143 | 16-bit quantity is the message code. |
| 144 | .br |
| 145 | .TP "\w'M_IOCTL 'u" |
| 146 | M_OPEN |
| 147 | is generated in conjunction with |
| 148 | `listener' mode (see |
| 149 | .IR mpx (2)). |
| 150 | The uid of the calling process follows the message code |
| 151 | as with M_WATCH. |
| 152 | This is followed by a null-terminated string |
| 153 | which is the name of the file being opened. |
| 154 | .TP |
| 155 | M_IOCTL |
| 156 | is generated for a channel connected |
| 157 | to a process |
| 158 | when that process executes the |
| 159 | .I "ioctl(fd, cmd, &vec)" |
| 160 | call on the channel file descriptor. |
| 161 | The M_IOCTL code is followed by |
| 162 | the |
| 163 | .I cmd |
| 164 | argument given to |
| 165 | .I ioctl |
| 166 | followed by |
| 167 | the contents of the structure |
| 168 | .I vec. |
| 169 | It is assumed, |
| 170 | not needing a better compromise at this time, |
| 171 | that the length of |
| 172 | .I vec |
| 173 | is determined by |
| 174 | .I "sizeof (struct sgttyb)" |
| 175 | as declared in |
| 176 | .IR <sgtty.h> . |
| 177 | .in -1i |
| 178 | .PP |
| 179 | Two control messages are understood by the operating system. |
| 180 | M_EOT may be sent through an mpx file to a channel. |
| 181 | It is equivalent to propagating a zero-length record |
| 182 | through the channel; |
| 183 | i.e. the channel is allowed to drain and the process or |
| 184 | device at the other end receives a zero-length |
| 185 | transfer before data starts flowing through the channel again. |
| 186 | M_IOANS can also be sent through a channel to reply to a M_IOCTL. |
| 187 | The format is identical to that received from M_IOCTL. |
| 188 | .SH SEE ALSO |
| 189 | mpx(2) |