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1.1 root 1: .TH MESGLD 4
2: .CT 2 comm_mach
3: .SH NAME
4: mesg_ld, rmesg_ld \- message line discipline modules
5: .SH SYNOPSIS
6: .B #include <sys/types.h>
7: .br
8: .B #include <sys/stream.h>
9: .SH DESCRIPTION
10: .I Mesg_ld
11: and
12: .I rmesg_ld
13: turn internal stream messages
14: into ordinary data and vice versa.
15: They supply a way to splice a stream connection together
16: through a process or across a network connection.
17: The messages represent ordinary data
18: and various control operations.
19: .PP
20: After
21: .IR mesg_ld
22: has been pushed on a stream,
23: .IR read (2)
24: on the stream returns encoded messages;
25: .I write
26: is expected to supply messages in the same coding.
27: An encoded message consists of a header
28: followed by zero or more bytes of associated data.
29: The header,
30: as defined in
31: .BR <sys/stream.h> ,
32: is of the form
33: .IP
34: .EX
35: struct mesg {
36: char type;
37: unsigned char magic;
38: unsigned char losize, hisize;
39: };
40: #define MSGMAGIC 0345
41: #define MSGHLEN 4 /* true length of struct mesg in bytes */
42: .EE
43: .LP
44: The header is
45: .B MSGHLEN
46: bytes long;
47: beware that this is not always the same as
48: .LR "sizeof(struct mesg)" .
49: The
50: .B magic
51: field contains the constant
52: .BR MSGMAGIC ,
53: to help prevent interpreting bad data as a message header.
54: There are
55: .B "losize+(hisize<<8)"
56: bytes of associated data.
57: .PP
58: Messages may be written in pieces,
59: or several messages may be written at once.
60: At most one message will be read at a time.
61: If an impossible message is written,
62: the stream may be shut down.
63: .PP
64: .IR Rmesg_ld
65: is exactly the opposite of
66: .IR mesg_ld .
67: It is intended for use with devices
68: that generate data containing encoded messages.
69: Here is a list of message types,
70: defined in
71: .BR <sys/stream.h> :
72: .TF M_HANGUP
73: .TP
74: .B M_DATA
75: (0)
76: Ordinary data.
77: .TP
78: .B M_BREAK
79: (01)
80: A line break on an RS232-style asynchronous connection.
81: No associated data.
82: .TP
83: .B M_HANGUP
84: (02)
85: When received, indicates that the other side has gone away.
86: Thereafter the stream is useless.
87: No associated data.
88: .TP
89: .B M_DELIM
90: (03)
91: A delimiter that introduces a record boundary in the data.
92: No associated data.
93: .TP
94: .B M_IOCTL
95: (06)
96: An
97: .IR ioctl (2)
98: request.
99: The associated data is a four-byte integer
100: containing the
101: function code,
102: least significant byte first,
103: followed by some amount of associated data.
104: An
105: .B M_IOCACK
106: or
107: .BR M_IOCNAK
108: reply is expected.
109: .TP
110: .B M_DELAY
111: (07)
112: A real-time delay.
113: One byte of data, giving the number of clock ticks of
114: delay time.
115: .TP
116: .B M_CTL
117: (010)
118: Device-specific control message.
119: .TP
120: .B M_SIGNAL
121: (0101)
122: Generate signal number given in the one-byte message.
123: .TP
124: .B M_FLUSH
125: (0102)
126: Flush input and output queue if possible.
127: .TP
128: .B M_STOP
129: (0103)
130: Stop transmission immediately.
131: .TP
132: .B M_START
133: (0104)
134: Restart transmission after
135: .BR M_STOP .
136: .TP
137: .B M_IOCACK
138: (0105)
139: Successful reply to
140: .BR M_IOCTL .
141: Associated data is to be written back to the caller.
142: .TP
143: .B M_IOCNAK
144: (0106)
145: Failed reply to
146: .BR M_IOCTL .
147: A single-byte message,
148: if present,
149: will be returned in
150: .CW errno
151: by the failing
152: .IR ioctl .
153: .TP
154: .B M_PRICTL
155: (0107)
156: High-priority
157: device-specific control message.
158: .PD
159: .PP
160: .SH SEE ALSO
161: .IR stream (4)
162: .SH BUGS
163: The format of arguments to
164: .B M_IOCTL
165: is machine dependent.
166: .br
167: The amount of associated data
168: is limited, but large (>4K).
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