1 .\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it)
3 .\" %%%LICENSE_START(VERBATIM)
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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
36 .TH SEMGET 2 2014-05-21 "Linux" "Linux Programmer's Manual"
38 semget \- get a System V semaphore set identifier
41 .B #include <sys/types.h>
42 .B #include <sys/ipc.h>
43 .B #include <sys/sem.h>
46 .BI "int semget(key_t " key ,
52 system call returns the System\ V semaphore set identifier
53 associated with the argument
57 semaphores is created if
61 or if no existing semaphore set is associated with
74 and a semaphore set already exists for
82 (This is analogous to the effect of the combination
87 Upon creation, the least significant 9 bits of the argument
89 define the permissions (for owner, group and others)
90 for the semaphore set.
91 These bits have the same format, and the same
96 (though the execute permissions are
97 not meaningful for semaphores, and write permissions mean permission
98 to alter semaphore values).
100 When creating a new semaphore set,
102 initializes the set's associated data structure,
111 are set to the effective user ID of the calling process.
116 are set to the effective group ID of the calling process.
118 The least significant 9 bits of
120 are set to the least significant 9 bits of
124 is set to the value of
131 is set to the current time.
137 when a semaphore set is not being created.
140 must be greater than 0
141 and less than or equal to the maximum number of semaphores per semaphore set
144 If the semaphore set already exists, the permissions are
146 .\" and a check is made to see if it is marked for destruction.
148 If successful, the return value will be the semaphore set identifier
149 (a nonnegative integer), otherwise, \-1
152 indicating the error.
156 will be set to one of the following:
159 A semaphore set exists for
161 but the calling process does not have permission to access the set,
162 and does not have the
172 but a semaphore set already exists for
176 .\" The semaphore set is marked to be deleted.
180 is less than 0 or greater than the limit on the number
181 of semaphores per semaphore set
185 A semaphore set corresponding to
189 is larger than the number of semaphores in that set.
192 No semaphore set exists for
200 A semaphore set has to be created but the system does not have
201 enough memory for the new data structure.
204 A semaphore set has to be created but the system limit for the maximum
205 number of semaphore sets
207 or the system wide maximum number of semaphores
212 .\" SVr4 documents additional error conditions EFBIG, E2BIG, EAGAIN,
219 isn't required on Linux or by any version of POSIX.
221 some old implementations required the inclusion of these header files,
222 and the SVID also documented their inclusion.
223 Applications intended to be portable to such old systems may need
224 to include these header files.
225 .\" Like Linux, the FreeBSD man pages still document
226 .\" the inclusion of these header files.
229 isn't a flag field but a
232 If this special value is used for
234 the system call ignores all but the least significant 9 bits of
236 and creates a new semaphore set (on success).
238 .SS Semaphore initialization
239 The values of the semaphores in a newly created set are indeterminate.
240 (POSIX.1-2001 and POSIX.1-2008 are explicit on this point,
241 although POSIX.1-2008 notes that a future version of the standard
242 may require an implementation to initialize the semaphores to 0.)
243 Although Linux, like many other implementations,
244 initializes the semaphore values to 0,
245 a portable application cannot rely on this:
246 it should explicitly initialize the semaphores to the desired values.
247 .\" In truth, every one of the many implementations that I've tested sets
248 .\" the values to zero, but I suppose there is/was some obscure
249 .\" implementation out there that does not.
251 Initialization can be done using
257 Where multiple peers do not know who will be the first to
258 initialize the set, checking for a nonzero
260 in the associated data structure retrieved by a
263 operation can be used to avoid races.
266 The following limits on semaphore set resources affect the
271 System-wide limit on the number of semaphore sets: policy dependent
272 (on Linux, this limit can be read and modified via the fourth field of
273 .IR /proc/sys/kernel/sem ).
274 .\" This /proc file is not available in Linux 2.2 and earlier -- MTK
277 Maximum number of semaphores per semaphore ID: implementation dependent
278 (on Linux, this limit can be read and modified via the first field of
279 .IR /proc/sys/kernel/sem ).
282 System-wide limit on the number of semaphores: policy dependent
283 (on Linux, this limit can be read and modified via the second field of
284 .IR /proc/sys/kernel/sem ).
285 Note that the number of semaphores system-wide
286 is also limited by the product of
293 was perhaps unfortunate,
295 would more clearly show its function.
300 .BR capabilities (7),
301 .BR sem_overview (7),
304 This page is part of release 3.79 of the Linux
307 A description of the project,
308 information about reporting bugs,
309 and the latest version of this page,
311 \%http://www.kernel.org/doc/man\-pages/.