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1 perry 1 IPC
2 !!!IPC
3 NAME
4 SYNOPSIS
5 DESCRIPTION
6 SEE ALSO
7 ----
8 !!NAME
9
10
11 ipc - System V interprocess communication mechanisms
12 !!SYNOPSIS
13
14
15 __# include
16 __
17 !!DESCRIPTION
18
19
20 The manual page refers to the Linux implementation of the
21 System V interprocess communication mechanisms: message
22 queues, semaphore sets and shared memory segments. In the
23 following, the word __resource__ means an instantiation
24 of one among such mechanisms.
25
26
27 __Resource Access Permissions__
28
29
30 For each resource the system uses a common structure of type
31 __struct ipc_perm__ to store information needed in
32 determining permissions to perform an ipc operation. The
33 __ipc_perm__ structure, defined by the
34 '''' system header file, includes the
35 following members:
36
37
38 __ushort cuid;__ /* creator user id */__
39 ushort cgid;__ /* creator group id */__
40 ushort uid;__ /* owner user id */__
41 ushort gid;__ /* owner group id */__
42 ushort mode;__ /* r/w permissions */
43
44
45 The __mode__ member of the __ipc_perm__ structure
46 defines, with its lower 9 bits, the access permissions to
47 the resource for a process executing an ipc system call. The
48 permissions are interpreted as follows:
49
50
51 0400 Read by user.
52 0200 Write by user.
53 0040 Read by group.
54 0020 Write by group.
55 0004 Read by others.
56 0002 Write by others.
57 Bits 0100, 0010 and 0001 (the execute bits) are unused by the system. Furthermore
58
59
60 The same system header file defines also the following
61 symbolic constants:
62
63
64 __IPC_CREAT__
65
66
67 Create entry if key doesn't exists.
68
69
70 __IPC_EXCL__
71
72
73 Fail if key exists.
74
75
76 __IPC_NOWAIT__
77
78
79 Error if request must wait.
80
81
82 __IPC_PRIVATE__
83
84
85 Private key.
86
87
88 __IPC_RMID__
89
90
91 Remove resource.
92
93
94 __IPC_SET__
95
96
97 Set resource options.
98
99
100 __IPC_STAT__
101
102
103 Get resource options.
104
105
106 Note that __IPC_PRIVATE__ is a __key_t__ type, while
107 all the others symbolic constants are flag fields or-able
108 into an __int__ type variable.
109
110
111 __Message Queues__
112
113
114 A message queue is uniquely identified by a positive integer
115 (its ''msqid'') and has an associated data structure of
116 type __struct msquid_ds__, defined in
117 '''', containing the following
118 members:
119
120
121 __struct ipc_perm msg_perm;
122 ushort msg_qnum;__ /* no of messages on queue */__
123 ushort msg_qbytes;__ /* bytes max on a queue */__
124 ushort msg_lspid;__ /* pid of last msgsnd call */__
125 ushort msg_lrpid;__ /* pid of last msgrcv call */__
126 time_t msg_stime;__ /* last msgsnd time */__
127 time_t msg_rtime;__ /* last msgrcv time */__
128 time_t msg_ctime;__ /* last change time */
129
130
131 __msg_perm__
132
133
134 __ipc_perm__ structure that specifies the access
135 permissions on the message queue.
136
137
138 __msg_qnum__
139
140
141 Number of messages currently on the message
142 queue.
143
144
145 __msg_qbytes__
146
147
148 Maximum number of bytes of message text allowed on the
149 message queue.
150
151
152 __msg_lspid__
153
154
155 ID of the process that performed the last __msgsnd__
156 system call.
157
158
159 __msg_lrpid__
160
161
162 ID of the process that performed the last __msgrcv__
163 system call.
164
165
166 __msg_stime__
167
168
169 Time of the last __msgsnd__ system call.
170
171
172 __msg_rtime__
173
174
175 Time of the last __msgcv__ system call.
176
177
178 __msg_ctime__
179
180
181 Time of the last system call that changed a member of the
182 __msqid_ds__ structure.
183
184
185 __Semaphore Sets__
186
187
188 A semaphore set is uniquely identified by a positive integer
189 (its ''semid'') and has an associated data structure of
190 type __struct semid_ds__, defined in
191 '''', containing the following
192 members:
193
194
195 __struct ipc_perm sem_perm;
196 time_t sem_otime;__ /* last operation time */__
197 time_t sem_ctime;__ /* last change time */__
198 ushort sem_nsems;__ /* count of sems in set
199 */
200
201
202 __sem_perm__
203
204
205 __ipc_perm__ structure that specifies the access
206 permissions on the semaphore set.
207
208
209 __sem_otime__
210
211
212 Time of last __semop__ system call.
213
214
215 __sem_ctime__
216
217
218 Time of last __semctl__ system call that changed a member
219 of the above structure or of one semaphore belonging to the
220 set.
221
222
223 __sem_nsems__
224
225
226 Number of semaphores in the set. Each semaphore of the set
227 is referenced by a non-negative integer ranging from
228 __0__ to __sem_nsems-1__.
229
230
231 A semaphore is a data structure of type __struct sem__
232 containing the following members:
233
234
235 __ushort semval;__ /* semaphore value */__
236 short sempid;__ /* pid for last operation */__
237 ushort semncnt;__ /* no. of awaiting semval to increase
238 */__
239 ushort semzcnt;__ /* no. of awaiting semval = 0
240 */
241
242
243 __semval__
244
245
246 Semaphore value: a non-negative integer.
247
248
249 __sempid__
250
251
252 ID of the last process that performed a semaphore operation
253 on this semaphore.
254
255
256 __semncnt__
257
258
259 Number of processes suspended awaiting for __semval__ to
260 increase.
261
262
263 __semznt__
264
265
266 Number of processes suspended awaiting for __semval__ to
267 become zero.
268
269
270 __Shared Memory Segments__
271
272
273 A shared memory segment is uniquely identified by a positive
274 integer (its ''shmid'') and has an associated data
275 structure of type __struct shmid_ds__, defined in
276 '''', containing the following
277 members:
278
279
280 __struct ipc_perm shm_perm;
281 int shm_segsz;__ /* size of segment */__
282 ushort shm_cpid;__ /* pid of creator */__
283 ushort shm_lpid;__ /* pid, last operation */__
284 short shm_nattch;__ /* no. of current attaches */__
285 time_t shm_atime;__ /* time of last attach */__
286 time_t shm_dtime;__ /* time of last detach */__
287 time_t shm_ctime;__ /* time of last change */
288
289
290 __shm_perm__
291
292
293 __ipc_perm__ structure that specifies the access
294 permissions on the shared memory segment.
295
296
297 __shm_segsz__
298
299
300 Size in bytes of the shared memory segment.
301
302
303 __shm_cpid__
304
305
306 ID of the process that created the shared memory
307 segment.
308
309
310 __shm_lpid__
311
312
313 ID of the last process that executed a __shmat__ or
314 __shmdt__ system call.
315
316
317 __shm_nattch__
318
319
320 Number of current alive attaches for this shared memory
321 segment.
322
323
324 __shm_atime__
325
326
327 Time of the last __shmat__ system call.
328
329
330 __shm_dtime__
331
332
333 Time of the last __shmdt__ system call.
334
335
336 __shm_ctime__
337
338
339 Time of the last __shmctl__ system call that changed
340 __shmid_ds__.
341 !!SEE ALSO
342
343
344 ftok(3), msgctl(2), msgget(2),
345 msgrcv(2), msgsnd(2), semctl(2),
346 semget(2), semop(2), shmat(2),
347 shmctl(2), shmget(2),
348 shmdt(2)
349 ----
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