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1.1 root 1: .ds ZZ DEVELOPMENT PACKAGE
2: .TH MSGOP 3L "630 MTG"
3: .XE "msgsnd()"
4: .XE "msgrcv()"
5: .SH NAME
6: msgop \- message operations
7: .SH SYNOPSIS
8: .B #include <message.h>
9:
10: .B int msgsnd (msqid, msgp, msgsz, msgflg)
11: .br
12: .B long msqid;
13: .br
14: .B struct msgbuf \(**msgp;
15: .br
16: .B int msgsz, msgflg;
17: .sp
18: \fBint msgrcv (msqid, msgp, msgsz, msgtyp, msgflg)\fR
19: .br
20: .B long msqid;
21: .br
22: .B struct msgbuf [\(**]\(**msgp;
23: .br
24: .B int msgsz;
25: .br
26: .B long msgtyp;
27: .br
28: .B int msgflg;
29: .SH DESCRIPTION
30: .I Msgsnd
31: is used to send a message to the queue associated
32: with the message queue identifier specified by
33: .IR msqid .
34: .I Msgp
35: points to a structure containing the message.
36: This structure is composed of the following members:
37: .sp
38: .RS 6
39: long mtype; /* message type */
40: .br
41: char mtext[]; /* message text */
42: .sp
43: .RE
44: .I Mtype
45: is a positive integer that can be used by the
46: receiving process for message selection (see
47: .I msgrcv
48: below).
49: .I Mtext
50: is any text of length
51: .I msgsz
52: bytes.
53: .I Msgsz
54: can range from 0 to what memory will allow.
55: .PP
56: .I Msgflg
57: specifies the action to be taken if one or more of
58: the following are true:
59: .sp
60: .RS 6
61: The number of bytes already on the queue is equal to
62: .BR msg_qbytes .
63: .sp
64: There is not enough memory to put the message on the queue.
65: .sp
66: .RE
67: These actions are as follows:
68: .sp
69: .RS 6
70: If
71: .RI ( msgflg
72: &
73: .BR IPC_NOWAIT )
74: is ``true'', the message will not be sent and the
75: calling process will return immediately.
76: .sp
77: If
78: .RI ( msgflg
79: &
80: .BR IPC_NOWAIT )
81: is ``false'', the calling process will suspend
82: execution until one of the following occurs:
83: .sp
84: .RS 6
85: The condition responsible for the suspension no
86: longer exists, in which case the message is
87: sent.
88: .sp
89: .I Msqid
90: is removed from the system (see
91: .IR msgctl ).
92: .PP
93: .RE 1
94: .I Msgsnd
95: will fail and no message will be sent if one or more
96: of the following are true:
97: .RS 6
98: .sp
99: .I Msqid
100: is not a valid message queue identifier.
101: .sp
102: .I Mtype
103: is less than 1.
104: .sp
105: The message cannot be sent for one of the
106: reasons cited above and
107: .RI ( msgflg
108: &
109: .BR IPC_NOWAIT )
110: is ``true''.
111: .sp
112: .I Msgsz
113: is less than zero.
114: .sp
115: .RE
116: Upon successful completion, the following actions are taken
117: with respect to the data structure associated with
118: .IR msqid .
119: .sp
120: .RS 6
121: .B Msg_qnum
122: is incremented by 1.
123: .sp
124: .B Msg_lspid
125: is set equal to the process id of the cal\ ling process.
126: .sp
127: .B Msg_stime
128: is set equal to the current time.
129: .RE
130: .PP
131: An extra feature of
132: .I msgsnd
133: lets the user send messages without a copy being made. If
134: .RI ( msgflg
135: &
136: .BR NO_COPY )
137: is ``true'', the message pointed to by
138: .I msgp
139: is put directly
140: into the message queue. This can only be done if the
141: .I msgbuf
142: was allocated (created by a call to alloc).
143: This is because the ownership of that memory must
144: be changed. The sending process then no longer owns that memory
145: and should not try to access it or free it.
146: .PP
147: .I Msgrcv
148: reads a message from the queue associated with the
149: message queue identifier specified by
150: .I msqid
151: and places it in the structure pointed to by
152: .IR msgp .
153: This structure is composed of the following members:
154: .sp
155: .RS 6
156: long mtype; /* message type */
157: .br
158: char mtext[]; /* message text */
159: .PP
160: .RE
161: .I Mtype
162: is the received message's type as specified by the
163: sending process.
164: .I Mtext
165: is the text of the message.
166: .I Msgsz
167: specifies the size in bytes of
168: .IR mtext .
169: The received message is truncated to
170: .I msgsz
171: bytes if it is larger than
172: .I msgsz
173: and
174: .RI ( msgflg
175: &
176: .BR MSG_NOERROR )
177: is ``true''. The truncated part of
178: the message is lost and no indication of the truncation is
179: given to the calling process.
180: .PP
181: .I Msgtyp
182: specifies the type of message requested as follows:
183: .RS 6
184: .sp
185: If
186: .I msgtyp
187: is equal to 0, the first message on the queue is received.
188: .sp
189: If
190: .I msgtyp
191: is greater than 0, the first message of type
192: .I msgtyp
193: is received.
194: .sp
195: If
196: .I msgtyp
197: is less than 0, the first message of the
198: lowest type that is less than or equal to the absolute
199: value of
200: .I msgtyp
201: is received.
202: .PP
203: .RE
204: .I Msgflg
205: specifies the action to be taken if a message of the
206: desired type is not on the queue. These are as follows:
207: .RS 6
208: .sp
209: If
210: .RI ( msgflg
211: &
212: .BR IPC_NOWAIT )
213: is ``true'', the calling process will return immediately
214: with a return value of -1.
215: .sp
216: If
217: .RI ( msgflg
218: &
219: .BR IPC_NOWAIT )
220: is ``false'', the calling process will suspend execution
221: until one of the following occurs:
222: .RS 6
223: .sp
224: A message of the desired type is placed on the queue.
225: .sp
226: .I Msqid
227: is removed from the system. When this occurs,
228: a value of -1 is returned.
229: .PP
230: .RE 1
231: .I Msgrcv
232: will fail and no message will be received if one or
233: more of the following are true:
234: .RS 6
235: .sp
236: .I Msqid
237: is not a valid message queue identifier.
238: .sp
239: .I Msgsz
240: is less than 0.
241: .sp
242: .I Mtext
243: is greater than
244: .I msgsz
245: and
246: .RI ( msgflg
247: &
248: .BR MSG_NOERROR )
249: is ``false''.
250: .sp
251: The queue does not contain a message of the
252: desired type and
253: .RI ( msgtyp
254: &
255: .BR IPC_NOWAIT )
256: is ``true''.
257: .PP
258: .RE
259: Upon successful completion, the following actions are taken
260: with respect to the data structure associated with
261: .IR msqid .
262: .sp
263: .RS 6
264: .B Msg_qnum
265: is decremented by 1.
266: .sp
267: .B Msg_lrpid
268: is set equal to the process id of the calling process.
269: .sp
270: .B Msg_rtime
271: is set equal to the current time.
272: .PP
273: .RE
274: Again, the option of not copying the message is available. If
275: .RI ( msgflg
276: &
277: .BR NO_COPY )
278: is ``true'',
279: .I msgp
280: is treated as a pointer to a pointer to a message. In other words,
281: .I *msgp
282: is set to the address of the received message. The ownership of the message
283: is then set to the receiving process. The receiving process can then
284: read the message and free it.
285: .PP
286: A side effect of \fImsgrcv\fR is that it clears the MSG resource
287: ready condition used by the \fIwait\fR and \fIown\fR resource functions.
288: .PP
289: .SS Return Values
290: Upon successful completion, the return value is as follows:
291: .sp
292: .RS 6
293: .I Msgsnd
294: returns a value of 0.
295: .sp
296: .I Msgrcv
297: returns a value equal to the number of bytes
298: actually placed into
299: .IR mtext .
300: .sp
301: .RE
302: Otherwise, a value of -1 is returned.
303: .PP
304: .SH SEE ALSO
305: alloc(3R), msgctl(3L), msgget(3L), realtime(3R), resources(3R), structures(3R).
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