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1 | .\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it) |
2 | .\" | |
93015253 | 3 | .\" %%%LICENSE_START(VERBATIM) |
fea681da MK |
4 | .\" Permission is granted to make and distribute verbatim copies of this |
5 | .\" manual provided the copyright notice and this permission notice are | |
6 | .\" preserved on all copies. | |
7 | .\" | |
8 | .\" Permission is granted to copy and distribute modified versions of this | |
9 | .\" manual under the conditions for verbatim copying, provided that the | |
10 | .\" entire resulting derived work is distributed under the terms of a | |
11 | .\" permission notice identical to this one. | |
c13182ef | 12 | .\" |
fea681da MK |
13 | .\" Since the Linux kernel and libraries are constantly changing, this |
14 | .\" manual page may be incorrect or out-of-date. The author(s) assume no | |
15 | .\" responsibility for errors or omissions, or for damages resulting from | |
16 | .\" the use of the information contained herein. The author(s) may not | |
17 | .\" have taken the same level of care in the production of this manual, | |
18 | .\" which is licensed free of charge, as they might when working | |
19 | .\" professionally. | |
c13182ef | 20 | .\" |
fea681da MK |
21 | .\" Formatted or processed versions of this manual, if unaccompanied by |
22 | .\" the source, must acknowledge the copyright and authors of this work. | |
4b72fb64 | 23 | .\" %%%LICENSE_END |
fea681da MK |
24 | .\" |
25 | .\" Modified Tue Oct 22 17:54:56 1996 by Eric S. Raymond <esr@thyrsus.com> | |
26 | .\" Modified 1 Jan 2002, Martin Schulze <joey@infodrom.org> | |
c11b1abf MK |
27 | .\" Modified 4 Jan 2002, Michael Kerrisk <mtk.manpages@gmail.com> |
28 | .\" Modified, 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com> | |
fea681da | 29 | .\" Added notes on capability requirements |
c11b1abf | 30 | .\" Modified, 11 Nov 2004, Michael Kerrisk <mtk.manpages@gmail.com> |
c952e226 MK |
31 | .\" Language and formatting clean-ups |
32 | .\" Added notes on /proc files | |
d9bfdb9c | 33 | .\" Rewrote BUGS note about semget()'s failure to initialize |
c952e226 | 34 | .\" semaphore values |
fea681da | 35 | .\" |
63121bd4 | 36 | .TH SEMGET 2 2019-08-02 "Linux" "Linux Programmer's Manual" |
fea681da | 37 | .SH NAME |
47f065d6 | 38 | semget \- get a System V semaphore set identifier |
fea681da MK |
39 | .SH SYNOPSIS |
40 | .nf | |
2fadbfb5 MK |
41 | .B #include <sys/types.h> |
42 | .B #include <sys/ipc.h> | |
43 | .B #include <sys/sem.h> | |
fea681da | 44 | .fi |
68e4db0a | 45 | .PP |
fea681da MK |
46 | .BI "int semget(key_t " key , |
47 | .BI "int " nsems , | |
48 | .BI "int " semflg ); | |
49 | .SH DESCRIPTION | |
c952e226 MK |
50 | The |
51 | .BR semget () | |
efbfd7ec | 52 | system call returns the System\ V semaphore set identifier |
fea681da MK |
53 | associated with the argument |
54 | .IR key . | |
f8ac3de3 MK |
55 | It may be used either to obtain the identifier of a previously created |
56 | semaphore set (when | |
57 | .I semflg | |
58 | is zero and | |
59 | .I key | |
60 | does not have the value | |
61 | .BR IPC_PRIVATE ), | |
62 | or to create a new set. | |
63 | .PP | |
fea681da MK |
64 | A new set of |
65 | .I nsems | |
66 | semaphores is created if | |
67 | .I key | |
68 | has the value | |
69 | .B IPC_PRIVATE | |
540036b2 | 70 | or if no existing semaphore set is associated with |
fea681da MK |
71 | .I key |
72 | and | |
73 | .B IPC_CREAT | |
c952e226 MK |
74 | is specified in |
75 | .IR semflg . | |
fea681da | 76 | .PP |
c952e226 | 77 | If |
fea681da | 78 | .I semflg |
c952e226 | 79 | specifies both |
fea681da MK |
80 | .B IPC_CREAT |
81 | and | |
82 | .B IPC_EXCL | |
83 | and a semaphore set already exists for | |
c952e226 MK |
84 | .IR key , |
85 | then | |
86 | .BR semget () | |
c13182ef | 87 | fails with |
c952e226 MK |
88 | .I errno |
89 | set to | |
90 | .BR EEXIST . | |
c13182ef | 91 | (This is analogous to the effect of the combination |
c952e226 MK |
92 | .B O_CREAT | O_EXCL |
93 | for | |
94 | .BR open (2).) | |
fea681da | 95 | .PP |
c952e226 | 96 | Upon creation, the least significant 9 bits of the argument |
fea681da | 97 | .I semflg |
c952e226 | 98 | define the permissions (for owner, group and others) |
fea681da MK |
99 | for the semaphore set. |
100 | These bits have the same format, and the same | |
c952e226 MK |
101 | meaning, as the |
102 | .I mode | |
103 | argument of | |
fea681da | 104 | .BR open (2) |
c952e226 | 105 | (though the execute permissions are |
fea681da MK |
106 | not meaningful for semaphores, and write permissions mean permission |
107 | to alter semaphore values). | |
108 | .PP | |
109 | When creating a new semaphore set, | |
c952e226 | 110 | .BR semget () |
d9bfdb9c | 111 | initializes the set's associated data structure, |
c952e226 | 112 | .I semid_ds |
c13182ef | 113 | (see |
2c5e151c | 114 | .BR semctl (2)), |
fea681da MK |
115 | as follows: |
116 | .IP | |
c952e226 | 117 | .I sem_perm.cuid |
fea681da | 118 | and |
c952e226 | 119 | .I sem_perm.uid |
499d62f2 | 120 | are set to the effective user ID of the calling process. |
fea681da | 121 | .IP |
c952e226 | 122 | .I sem_perm.cgid |
fea681da | 123 | and |
c952e226 | 124 | .I sem_perm.gid |
499d62f2 | 125 | are set to the effective group ID of the calling process. |
fea681da | 126 | .IP |
c952e226 MK |
127 | The least significant 9 bits of |
128 | .I sem_perm.mode | |
129 | are set to the least significant 9 bits of | |
fea681da MK |
130 | .IR semflg . |
131 | .IP | |
c952e226 | 132 | .I sem_nsems |
fea681da MK |
133 | is set to the value of |
134 | .IR nsems . | |
135 | .IP | |
c952e226 | 136 | .I sem_otime |
fea681da MK |
137 | is set to 0. |
138 | .IP | |
c952e226 | 139 | .I sem_ctime |
fea681da MK |
140 | is set to the current time. |
141 | .PP | |
142 | The argument | |
143 | .I nsems | |
c952e226 | 144 | can be 0 |
fea681da MK |
145 | (a don't care) |
146 | when a semaphore set is not being created. | |
e9001b56 | 147 | Otherwise, |
fea681da | 148 | .I nsems |
c952e226 | 149 | must be greater than 0 |
fea681da MK |
150 | and less than or equal to the maximum number of semaphores per semaphore set |
151 | .RB ( SEMMSL ). | |
152 | .PP | |
c952e226 | 153 | If the semaphore set already exists, the permissions are |
fea681da MK |
154 | verified. |
155 | .\" and a check is made to see if it is marked for destruction. | |
47297adb | 156 | .SH RETURN VALUE |
fea681da | 157 | If successful, the return value will be the semaphore set identifier |
32cdf39c | 158 | (a nonnegative integer), otherwise, \-1 |
fea681da MK |
159 | is returned, with |
160 | .I errno | |
161 | indicating the error. | |
162 | .SH ERRORS | |
90db854a | 163 | On failure, |
fea681da MK |
164 | .I errno |
165 | will be set to one of the following: | |
89b3c6b8 | 166 | .TP |
fea681da MK |
167 | .B EACCES |
168 | A semaphore set exists for | |
169 | .IR key , | |
170 | but the calling process does not have permission to access the set, | |
171 | and does not have the | |
0daa9e92 | 172 | .B CAP_IPC_OWNER |
3294109d | 173 | capability in the user namespace that governs its IPC namespace. |
fea681da MK |
174 | .TP |
175 | .B EEXIST | |
fea681da MK |
176 | .B IPC_CREAT |
177 | and | |
df036ba6 MK |
178 | .BR IPC_EXCL |
179 | were specified in | |
3fcc7a7a | 180 | .IR semflg , |
df036ba6 MK |
181 | but a semaphore set already exists for |
182 | .IR key . | |
fea681da MK |
183 | .\" .TP |
184 | .\" .B EIDRM | |
185 | .\" The semaphore set is marked to be deleted. | |
186 | .TP | |
187 | .B EINVAL | |
0daa9e92 | 188 | .I nsems |
fea681da MK |
189 | is less than 0 or greater than the limit on the number |
190 | of semaphores per semaphore set | |
a6cad8ff MK |
191 | .RB ( SEMMSL ). |
192 | .TP | |
193 | .B EINVAL | |
194 | A semaphore set corresponding to | |
fea681da | 195 | .I key |
a6cad8ff | 196 | already exists, but |
fea681da MK |
197 | .I nsems |
198 | is larger than the number of semaphores in that set. | |
199 | .TP | |
200 | .B ENOENT | |
201 | No semaphore set exists for | |
202 | .I key | |
203 | and | |
204 | .I semflg | |
c952e226 | 205 | did not specify |
fea681da MK |
206 | .BR IPC_CREAT . |
207 | .TP | |
208 | .B ENOMEM | |
c952e226 MK |
209 | A semaphore set has to be created but the system does not have |
210 | enough memory for the new data structure. | |
fea681da MK |
211 | .TP |
212 | .B ENOSPC | |
213 | A semaphore set has to be created but the system limit for the maximum | |
214 | number of semaphore sets | |
215 | .RB ( SEMMNI ), | |
216 | or the system wide maximum number of semaphores | |
217 | .RB ( SEMMNS ), | |
218 | would be exceeded. | |
47297adb | 219 | .SH CONFORMING TO |
a1d5f77c MK |
220 | SVr4, POSIX.1-2001. |
221 | .\" SVr4 documents additional error conditions EFBIG, E2BIG, EAGAIN, | |
222 | .\" ERANGE, EFAULT. | |
fea681da | 223 | .SH NOTES |
b93dd595 MK |
224 | The inclusion of |
225 | .I <sys/types.h> | |
226 | and | |
227 | .I <sys/ipc.h> | |
228 | isn't required on Linux or by any version of POSIX. | |
229 | However, | |
230 | some old implementations required the inclusion of these header files, | |
231 | and the SVID also documented their inclusion. | |
232 | Applications intended to be portable to such old systems may need | |
233 | to include these header files. | |
234 | .\" Like Linux, the FreeBSD man pages still document | |
235 | .\" the inclusion of these header files. | |
efeece04 | 236 | .PP |
b93dd595 MK |
237 | .B IPC_PRIVATE |
238 | isn't a flag field but a | |
239 | .I key_t | |
240 | type. | |
241 | If this special value is used for | |
242 | .IR key , | |
243 | the system call ignores all but the least significant 9 bits of | |
244 | .I semflg | |
245 | and creates a new semaphore set (on success). | |
246 | .\" | |
247 | .SS Semaphore initialization | |
143f4ec3 MK |
248 | The values of the semaphores in a newly created set are indeterminate. |
249 | (POSIX.1-2001 and POSIX.1-2008 are explicit on this point, | |
250 | although POSIX.1-2008 notes that a future version of the standard | |
251 | may require an implementation to initialize the semaphores to 0.) | |
252 | Although Linux, like many other implementations, | |
253 | initializes the semaphore values to 0, | |
254 | a portable application cannot rely on this: | |
255 | it should explicitly initialize the semaphores to the desired values. | |
256 | .\" In truth, every one of the many implementations that I've tested sets | |
257 | .\" the values to zero, but I suppose there is/was some obscure | |
258 | .\" implementation out there that does not. | |
efeece04 | 259 | .PP |
143f4ec3 MK |
260 | Initialization can be done using |
261 | .BR semctl (2) | |
262 | .B SETVAL | |
263 | or | |
264 | .B SETALL | |
265 | operation. | |
0e94942d | 266 | Where multiple peers do not know who will be the first to |
143f4ec3 MK |
267 | initialize the set, checking for a nonzero |
268 | .I sem_otime | |
269 | in the associated data structure retrieved by a | |
270 | .BR semctl (2) | |
271 | .B IPC_STAT | |
0e94942d | 272 | operation can be used to avoid races. |
098c4f64 MK |
273 | .\" |
274 | .SS Semaphore limits | |
540036b2 | 275 | The following limits on semaphore set resources affect the |
c952e226 | 276 | .BR semget () |
fea681da | 277 | call: |
89b3c6b8 | 278 | .TP |
fea681da | 279 | .B SEMMNI |
8a3e6dc7 MK |
280 | System-wide limit on the number of semaphore sets. |
281 | On Linux systems before version 3.19, | |
282 | the default value for this limit was 128. | |
283 | Since Linux 3.19, | |
284 | .\" commit e843e7d2c88b7db107a86bd2c7145dc715c058f4 | |
285 | the default value is 32,000. | |
286 | On Linux, this limit can be read and modified via the fourth field of | |
287 | .IR /proc/sys/kernel/sem . | |
c952e226 | 288 | .\" This /proc file is not available in Linux 2.2 and earlier -- MTK |
fea681da MK |
289 | .TP |
290 | .B SEMMSL | |
8a3e6dc7 MK |
291 | Maximum number of semaphores per semaphore ID. |
292 | On Linux systems before version 3.19, | |
293 | the default value for this limit was 250. | |
294 | Since Linux 3.19, | |
295 | .\" commit e843e7d2c88b7db107a86bd2c7145dc715c058f4 | |
296 | the default value is 32,000. | |
297 | On Linux, this limit can be read and modified via the first field of | |
298 | .IR /proc/sys/kernel/sem . | |
fea681da MK |
299 | .TP |
300 | .B SEMMNS | |
80816b0b | 301 | System-wide limit on the number of semaphores: policy dependent |
c952e226 MK |
302 | (on Linux, this limit can be read and modified via the second field of |
303 | .IR /proc/sys/kernel/sem ). | |
8f891eb0 MK |
304 | Note that the number of semaphores system-wide |
305 | is also limited by the product of | |
96c1bf31 MK |
306 | .B SEMMSL |
307 | and | |
308 | .BR SEMMNI . | |
fea681da | 309 | .SH BUGS |
682edefb MK |
310 | The name choice |
311 | .B IPC_PRIVATE | |
9091595d MK |
312 | was perhaps unfortunate, |
313 | .B IPC_NEW | |
fea681da | 314 | would more clearly show its function. |
47297adb | 315 | .SH SEE ALSO |
fea681da MK |
316 | .BR semctl (2), |
317 | .BR semop (2), | |
318 | .BR ftok (3), | |
f675ea37 | 319 | .BR capabilities (7), |
2c5e151c | 320 | .BR sem_overview (7), |
343cdc5a | 321 | .BR sysvipc (7) |