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3070730e | 1 | .\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it) |
2 | .\" | |
3 | .\" Permission is granted to make and distribute verbatim copies of this | |
4 | .\" manual provided the copyright notice and this permission notice are | |
5 | .\" preserved on all copies. | |
6 | .\" | |
7 | .\" Permission is granted to copy and distribute modified versions of this | |
8 | .\" manual under the conditions for verbatim copying, provided that the | |
9 | .\" entire resulting derived work is distributed under the terms of a | |
10 | .\" permission notice identical to this one | |
11 | .\" | |
12 | .\" Since the Linux kernel and libraries are constantly changing, this | |
13 | .\" manual page may be incorrect or out-of-date. The author(s) assume no | |
14 | .\" responsibility for errors or omissions, or for damages resulting from | |
15 | .\" the use of the information contained herein. The author(s) may not | |
16 | .\" have taken the same level of care in the production of this manual, | |
17 | .\" which is licensed free of charge, as they might when working | |
18 | .\" professionally. | |
19 | .\" | |
20 | .\" Formatted or processed versions of this manual, if unaccompanied by | |
21 | .\" the source, must acknowledge the copyright and authors of this work. | |
22 | .\" | |
23 | .\" Modified Tue Oct 22 17:54:56 1996 by Eric S. Raymond <esr@thyrsus.com> | |
24 | .\" Modified 1 Jan 2002, Martin Schulze <joey@infodrom.org> | |
25 | .\" Modified 4 Jan 2002, Michael Kerrisk <mtk16@ext.canterbury.ac.nz> | |
26 | .\" | |
27 | .TH SEMGET 2 2002-01-04 "Linux 2.5" "Linux Programmer's Manual" | |
28 | .SH NAME | |
29 | semget \- get a semaphore set identifier | |
30 | .SH SYNOPSIS | |
31 | .nf | |
32 | .B | |
33 | #include <sys/types.h> | |
34 | .B | |
35 | #include <sys/ipc.h> | |
36 | .B | |
37 | #include <sys/sem.h> | |
38 | .fi | |
39 | .sp | |
40 | .BI "int semget(key_t " key , | |
41 | .BI "int " nsems , | |
42 | .BI "int " semflg ); | |
43 | .SH DESCRIPTION | |
44 | This function returns the semaphore set identifier | |
45 | associated with the argument | |
46 | .IR key . | |
47 | A new set of | |
48 | .I nsems | |
49 | semaphores is created if | |
50 | .I key | |
51 | has the value | |
52 | .B IPC_PRIVATE | |
53 | or if no existing semaphore set is associated to | |
54 | .I key | |
55 | and | |
56 | .B IPC_CREAT | |
57 | is asserted in | |
58 | .I semflg | |
59 | (i.e. | |
60 | .IR semflg " &" | |
61 | .B IPC_CREAT | |
62 | isn't zero). | |
63 | .PP | |
64 | The presence in | |
65 | .I semflg | |
66 | of the fields | |
67 | .B IPC_CREAT | |
68 | and | |
69 | .B IPC_EXCL | |
70 | plays the same role, with respect to the existence | |
71 | of the semaphore set, as the presence | |
72 | of | |
73 | .B O_CREAT | |
74 | and | |
75 | .B O_EXCL | |
76 | in the mode argument of the | |
77 | .BR open (2) | |
78 | system call: i.e. the | |
79 | .B semget | |
80 | function fails if | |
81 | .I semflg | |
82 | asserts both | |
83 | .B IPC_CREAT | |
84 | and | |
85 | .B IPC_EXCL | |
86 | and a semaphore set already exists for | |
87 | .IR key . | |
88 | .PP | |
89 | Upon creation, the low-order 9 bits of the argument | |
90 | .I semflg | |
91 | define the access permissions (for owner, group and others) | |
92 | for the semaphore set. | |
93 | These bits have the same format, and the same | |
94 | meaning, as the mode argument in the | |
95 | .BR open (2) | |
96 | or | |
97 | .BR creat (2) | |
98 | system calls (though the execute permissions are | |
99 | not meaningful for semaphores, and write permissions mean permission | |
100 | to alter semaphore values). | |
101 | .PP | |
102 | When creating a new semaphore set, | |
103 | .B semget | |
104 | initializes the semaphore set's associated data structure | |
105 | .B semid_ds | |
106 | as follows: | |
107 | .IP | |
108 | .B sem_perm.cuid | |
109 | and | |
110 | .B sem_perm.uid | |
111 | are set to the effective user\-ID of the calling process. | |
112 | .IP | |
113 | .B sem_perm.cgid | |
114 | and | |
115 | .B sem_perm.gid | |
116 | are set to the effective group\-ID of the calling process. | |
117 | .IP | |
118 | The low-order 9 bits of | |
119 | .B sem_perm.mode | |
120 | are set to the low-order 9 bits of | |
121 | .IR semflg . | |
122 | .IP | |
123 | .B sem_nsems | |
124 | is set to the value of | |
125 | .IR nsems . | |
126 | .IP | |
127 | .B sem_otime | |
128 | is set to 0. | |
129 | .IP | |
130 | .B sem_ctime | |
131 | is set to the current time. | |
132 | .PP | |
133 | The argument | |
134 | .I nsems | |
135 | can be | |
136 | .B 0 | |
137 | (a don't care) | |
138 | when a semaphore set is not being created. | |
139 | Otherwise | |
140 | .I nsems | |
141 | must be greater than | |
142 | .B 0 | |
143 | and less than or equal to the maximum number of semaphores per semaphore set | |
144 | .RB ( SEMMSL ). | |
145 | .PP | |
146 | If the semaphore set already exists, the access permissions are | |
147 | verified. | |
148 | .\" and a check is made to see if it is marked for destruction. | |
149 | .SH "RETURN VALUE" | |
150 | If successful, the return value will be the semaphore set identifier | |
151 | (a nonnegative integer), otherwise | |
152 | .B \-1 | |
153 | is returned, with | |
154 | .I errno | |
155 | indicating the error. | |
156 | .SH ERRORS | |
157 | On failure | |
158 | .I errno | |
159 | will be set to one of the following: | |
160 | .TP 11 | |
161 | .B EACCES | |
162 | A semaphore set exists for | |
163 | .IR key , | |
164 | but the calling process does not have permission to access the set. | |
165 | .TP | |
166 | .B EEXIST | |
167 | A semaphore set exists for | |
168 | .B key | |
169 | and | |
170 | .I semflg | |
171 | was asserting both | |
172 | .B IPC_CREAT | |
173 | and | |
174 | .BR IPC_EXCL . | |
175 | .\" .TP | |
176 | .\" .B EIDRM | |
177 | .\" The semaphore set is marked to be deleted. | |
178 | .TP | |
179 | .B ENOENT | |
180 | No semaphore set exists for | |
181 | .I key | |
182 | and | |
183 | .I semflg | |
184 | wasn't asserting | |
185 | .BR IPC_CREAT . | |
186 | .TP | |
187 | .B EINVAL | |
188 | .IR nsems | |
189 | is less than 0 or greater than the limit on the number | |
190 | of semaphores per semaphore set | |
191 | .RB ( SEMMSL ), | |
192 | or a semaphore set corresponding to | |
193 | .I key | |
194 | already exists, and | |
195 | .I nsems | |
196 | is larger than the number of semaphores in that set. | |
197 | .TP | |
198 | .B ENOMEM | |
199 | A semaphore set has to be created but the system has not enough memory for | |
200 | the new data structure. | |
201 | .TP | |
202 | .B ENOSPC | |
203 | A semaphore set has to be created but the system limit for the maximum | |
204 | number of semaphore sets | |
205 | .RB ( SEMMNI ), | |
206 | or the system wide maximum number of semaphores | |
207 | .RB ( SEMMNS ), | |
208 | would be exceeded. | |
209 | .SH NOTES | |
210 | .B IPC_PRIVATE | |
211 | isn't a flag field but a | |
212 | .B key_t | |
213 | type. | |
214 | If this special value is used for | |
215 | .IR key , | |
216 | the system call ignores everything but the low-order 9 bits of | |
217 | .I semflg | |
218 | and creates a new semaphore set (on success). | |
219 | .PP | |
220 | The followings are limits on semaphore set resources affecting a | |
221 | .B semget | |
222 | call: | |
223 | .TP 11 | |
224 | .B SEMMNI | |
225 | System wide maximum number of semaphore sets: policy dependent. | |
226 | .TP | |
227 | .B SEMMSL | |
228 | Maximum number of semaphores per semid: implementation dependent | |
229 | (500 currently). | |
230 | .TP | |
231 | .B SEMMNS | |
232 | System wide maximum number of semaphores: policy dependent. | |
233 | Values greater than | |
234 | .B SEMMSL * SEMMNI | |
235 | makes it irrelevant. | |
236 | .SH BUGS | |
237 | Use of | |
238 | .B IPC_PRIVATE | |
239 | doesn't inhibit to other processes the access to the allocated | |
240 | semaphore set. | |
241 | .PP | |
242 | As for the files, there is currently no intrinsic way for a process to ensure | |
243 | exclusive access to a semaphore set. | |
244 | Asserting both | |
245 | .B IPC_CREAT | |
246 | and | |
247 | .B IPC_EXCL | |
248 | in | |
249 | .I semflg | |
250 | only ensures (on success) that a new semaphore set will be created, | |
251 | it doesn't imply exclusive access to the semaphore set. | |
252 | .PP | |
253 | The data structure associated with each semaphore in the set | |
254 | isn't initialized by the system call. | |
255 | In order to initialize those data structures, one has to execute a | |
256 | subsequent call to | |
257 | .BR semctl (2) | |
258 | to perform a | |
259 | .B SETVAL | |
260 | or a | |
261 | .B SETALL | |
262 | command on the semaphore set. | |
263 | .SH "CONFORMING TO" | |
264 | SVr4, SVID. | |
265 | SVr4 documents additional error conditions EFBIG, E2BIG, EAGAIN, | |
266 | ERANGE, EFAULT. | |
267 | .SH "SEE ALSO" | |
268 | .BR ftok (3), | |
269 | .BR ipc (5), | |
270 | .BR semctl (2), | |
271 | .BR semop (2) |