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1 .\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it)
2 .\"
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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>
27 .\" Modified 4 Jan 2002, Michael Kerrisk <mtk.manpages@gmail.com>
28 .\" Modified, 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com>
29 .\" Added notes on capability requirements
30 .\" Modified, 11 Nov 2004, Michael Kerrisk <mtk.manpages@gmail.com>
31 .\" Language and formatting clean-ups
32 .\" Added notes on /proc files
33 .\" Rewrote BUGS note about semget()'s failure to initialize
34 .\" semaphore values
35 .\"
36 .TH SEMGET 2 2018-04-30 "Linux" "Linux Programmer's Manual"
37 .SH NAME
38 semget \- get a System V semaphore set identifier
39 .SH SYNOPSIS
40 .nf
41 .B #include <sys/types.h>
42 .B #include <sys/ipc.h>
43 .B #include <sys/sem.h>
44 .fi
45 .PP
46 .BI "int semget(key_t " key ,
47 .BI "int " nsems ,
48 .BI "int " semflg );
49 .SH DESCRIPTION
50 The
51 .BR semget ()
52 system call returns the System\ V semaphore set identifier
53 associated with the argument
54 .IR key .
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
64 A new set of
65 .I nsems
66 semaphores is created if
67 .I key
68 has the value
69 .B IPC_PRIVATE
70 or if no existing semaphore set is associated with
71 .I key
72 and
73 .B IPC_CREAT
74 is specified in
75 .IR semflg .
76 .PP
77 If
78 .I semflg
79 specifies both
80 .B IPC_CREAT
81 and
82 .B IPC_EXCL
83 and a semaphore set already exists for
84 .IR key ,
85 then
86 .BR semget ()
87 fails with
88 .I errno
89 set to
90 .BR EEXIST .
91 (This is analogous to the effect of the combination
92 .B O_CREAT | O_EXCL
93 for
94 .BR open (2).)
95 .PP
96 Upon creation, the least significant 9 bits of the argument
97 .I semflg
98 define the permissions (for owner, group and others)
99 for the semaphore set.
100 These bits have the same format, and the same
101 meaning, as the
102 .I mode
103 argument of
104 .BR open (2)
105 (though the execute permissions are
106 not meaningful for semaphores, and write permissions mean permission
107 to alter semaphore values).
108 .PP
109 When creating a new semaphore set,
110 .BR semget ()
111 initializes the set's associated data structure,
112 .I semid_ds
113 (see
114 .BR semctl (2)),
115 as follows:
116 .IP
117 .I sem_perm.cuid
118 and
119 .I sem_perm.uid
120 are set to the effective user ID of the calling process.
121 .IP
122 .I sem_perm.cgid
123 and
124 .I sem_perm.gid
125 are set to the effective group ID of the calling process.
126 .IP
127 The least significant 9 bits of
128 .I sem_perm.mode
129 are set to the least significant 9 bits of
130 .IR semflg .
131 .IP
132 .I sem_nsems
133 is set to the value of
134 .IR nsems .
135 .IP
136 .I sem_otime
137 is set to 0.
138 .IP
139 .I sem_ctime
140 is set to the current time.
141 .PP
142 The argument
143 .I nsems
144 can be 0
145 (a don't care)
146 when a semaphore set is not being created.
147 Otherwise,
148 .I nsems
149 must be greater than 0
150 and less than or equal to the maximum number of semaphores per semaphore set
151 .RB ( SEMMSL ).
152 .PP
153 If the semaphore set already exists, the permissions are
154 verified.
155 .\" and a check is made to see if it is marked for destruction.
156 .SH RETURN VALUE
157 If successful, the return value will be the semaphore set identifier
158 (a nonnegative integer), otherwise, \-1
159 is returned, with
160 .I errno
161 indicating the error.
162 .SH ERRORS
163 On failure,
164 .I errno
165 will be set to one of the following:
166 .TP
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
172 .B CAP_IPC_OWNER
173 capability in the user namespace that governs its IPC namespace.
174 .TP
175 .B EEXIST
176 .B IPC_CREAT
177 and
178 .BR IPC_EXCL
179 were specified in
180 .IR semflg ,
181 but a semaphore set already exists for
182 .IR key .
183 .\" .TP
184 .\" .B EIDRM
185 .\" The semaphore set is marked to be deleted.
186 .TP
187 .B EINVAL
188 .I nsems
189 is less than 0 or greater than the limit on the number
190 of semaphores per semaphore set
191 .RB ( SEMMSL ).
192 .TP
193 .B EINVAL
194 A semaphore set corresponding to
195 .I key
196 already exists, but
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
205 did not specify
206 .BR IPC_CREAT .
207 .TP
208 .B ENOMEM
209 A semaphore set has to be created but the system does not have
210 enough memory for the new data structure.
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.
219 .SH CONFORMING TO
220 SVr4, POSIX.1-2001.
221 .\" SVr4 documents additional error conditions EFBIG, E2BIG, EAGAIN,
222 .\" ERANGE, EFAULT.
223 .SH NOTES
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.
236 .PP
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
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.
259 .PP
260 Initialization can be done using
261 .BR semctl (2)
262 .B SETVAL
263 or
264 .B SETALL
265 operation.
266 Where multiple peers do not know who will be the first to
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
272 operation can be used to avoid races.
273 .\"
274 .SS Semaphore limits
275 The following limits on semaphore set resources affect the
276 .BR semget ()
277 call:
278 .TP
279 .B SEMMNI
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 .
288 .\" This /proc file is not available in Linux 2.2 and earlier -- MTK
289 .TP
290 .B SEMMSL
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 .
299 .TP
300 .B SEMMNS
301 System-wide limit on the number of semaphores: policy dependent
302 (on Linux, this limit can be read and modified via the second field of
303 .IR /proc/sys/kernel/sem ).
304 Note that the number of semaphores system-wide
305 is also limited by the product of
306 .B SEMMSL
307 and
308 .BR SEMMNI .
309 .SH BUGS
310 The name choice
311 .B IPC_PRIVATE
312 was perhaps unfortunate,
313 .B IPC_NEW
314 would more clearly show its function.
315 .SH SEE ALSO
316 .BR semctl (2),
317 .BR semop (2),
318 .BR ftok (3),
319 .BR capabilities (7),
320 .BR sem_overview (7),
321 .BR svipc (7)