1 .\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it)
2 .\" and Copyright 2004, 2005 Michael Kerrisk <mtk.manpages@gmail.com>
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.
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.
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14 .\" manual page may be incorrect or out-of-date.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
21 .\" Formatted or processed versions of this manual, if unaccompanied by
22 .\" the source, must acknowledge the copyright and authors of this work.
24 .\" Modified Tue Oct 22 17:53:56 1996 by Eric S. Raymond <esr@thyrsus.com>
25 .\" Modified Fri Jun 19 10:59:15 1998 by Andries Brouwer <aeb@cwi.nl>
26 .\" Modified Sun Feb 18 01:59:29 2001 by Andries Brouwer <aeb@cwi.nl>
27 .\" Modified 20 Dec 2001, Michael Kerrisk <mtk.manpages@gmail.com>
28 .\" Modified 21 Dec 2001, aeb
29 .\" Modified 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com>
30 .\" Added notes on CAP_IPC_OWNER requirement
31 .\" Modified 17 Jun 2004, Michael Kerrisk <mtk.manpages@gmail.com>
32 .\" Added notes on CAP_SYS_ADMIN requirement for IPC_SET and IPC_RMID
33 .\" Modified, 11 Nov 2004, Michael Kerrisk <mtk.manpages@gmail.com>
34 .\" Language and formatting clean-ups
35 .\" Rewrote semun text
36 .\" Added semid_ds and ipc_perm structure definitions
37 .\" 2005-08-02, mtk: Added IPC_INFO, SEM_INFO, SEM_STAT descriptions.
39 .TH SEMCTL 2 2004-11-10 "Linux" "Linux Programmer's Manual"
41 semctl \- semaphore control operations
44 .B #include <sys/types.h>
45 .B #include <sys/ipc.h>
46 .B #include <sys/sem.h>
48 .BI "int semctl(int " semid ", int " semnum ", int " cmd ", ...);"
52 performs the control operation specified by
54 on the semaphore set identified by
58 semaphore of that set.
59 (The semaphores in a set are numbered starting at 0.)
61 This function has three or four arguments, depending on
63 When there are four, the fourth has the type
65 The \fIcalling program\fP must define this union as follows:
70 int val; /* Value for SETVAL */
71 struct semid_ds *buf; /* Buffer for IPC_STAT, IPC_SET */
72 unsigned short *array; /* Array for GETALL, SETALL */
73 struct seminfo *__buf; /* Buffer for IPC_INFO
81 data structure is defined in \fI<sys/sem.h>\fP as follows:
86 struct ipc_perm sem_perm; /* Ownership and permissions */
87 time_t sem_otime; /* Last semop time */
88 time_t sem_ctime; /* Last change time */
89 unsigned short sem_nsems; /* No. of semaphores in set */
96 structure is defined in \fI<sys/ipc.h>\fP as follows
97 (the highlighted fields are settable using
103 key_t key; /* Key supplied to semget(2) */
104 uid_t \fBuid\fP; /* Effective UID of owner */
105 gid_t \fBgid\fP; /* Effective GID of owner */
106 uid_t cuid; /* Effective UID of creator */
107 gid_t cgid; /* Effective GID of creator */
108 unsigned short \fBmode\fP; /* Permissions */
109 unsigned short seq; /* Sequence number */
119 Copy information from the kernel data structure associated with
123 structure pointed to by
128 The calling process must have read permission on the semaphore set.
131 Write the values of some members of the
133 structure pointed to by
135 to the kernel data structure associated with this semaphore set,
139 The following members of the structure are updated:
142 and (the least significant 9 bits of)
144 The effective UID of the calling process must match the owner
147 .RI ( sem_perm.cuid )
148 of the semaphore set, or the caller must be privileged.
154 Immediately remove the semaphore set,
155 awakening all processes blocked in
157 calls on the set (with an error return and
161 The effective user ID of the calling process must
162 match the creator or owner of the semaphore set,
163 or the caller must be privileged.
168 .BR IPC_INFO " (Linux specific)"
169 Returns information about system-wide semaphore limits and
170 parameters in the structure pointed to by
172 This structure is of type
178 feature test macro is defined:
183 int semmap; /* # of entries in semaphore map;
185 int semmni; /* Max. # of semaphore sets */
186 int semmns; /* Max. # of semaphores in all
188 int semmnu; /* System-wide max. # of undo
189 structures; unused */
190 int semmsl; /* Max. # of semaphores in a set */
191 int semopm; /* Max. # of operations for semop(2) */
192 int semume; /* Max. # of undo entries per
194 int semusz; /* size of struct sem_undo */
195 int semvmx; /* Maximum semaphore value */
196 int semaem; /* Max. value that can be recorded for
197 semaphore adjustment (SEM_UNDO) */
208 settings can be changed via
209 .IR /proc/sys/kernel/sem ;
214 .BR SEM_INFO " (Linux specific)"
217 structure containing the same information as for
219 except that the following fields are returned with information
220 about system resources consumed by semaphores: the
222 field returns the number of semaphore sets that currently exist
223 on the system; and the
225 field returns the total number of semaphores in all semaphore sets
228 .BR SEM_STAT " (Linux specific)"
235 argument is not a semaphore identifier, but instead an index into
236 the kernel's internal array that maintains information about
237 all semaphore sets on the system.
242 (i.e., the current value)
243 for all semaphores of the set into
248 The calling process must have read permission on the semaphore set.
251 The system call returns the value of
253 (i.e., the number of processes waiting for the value of
254 this semaphore to increase) for the
257 (i.e., the number of processes waiting for an increase of
261 semaphore of the set).
262 The calling process must have read permission on the semaphore set.
265 The system call returns the value of
270 (i.e. the PID of the process that executed the last
274 semaphore of the set).
275 The calling process must have read permission on the semaphore set.
278 The system call returns the value of
282 semaphore of the set.
283 The calling process must have read permission on the semaphore set.
286 The system call returns the value of
288 (i.e., the number of processes waiting for the value of
289 this semaphore to become zero) for the
292 (i.e. the number of processes waiting for
296 semaphore of the set to become 0).
297 The calling process must have read permission on the semaphore set.
302 for all semaphores of the set using
308 structure associated with the set.
311 are cleared for altered semaphores in all processes.
312 If the changes to semaphore values would permit blocked
314 calls in other processes to proceed, then those processes are woken up.
318 The calling process must have alter (write) permission on
328 semaphore of the set, updating also the
332 structure associated with the set.
333 Undo entries are cleared for altered semaphores in all processes.
334 If the changes to semaphore values would permit blocked
336 calls in other processes to proceed, then those processes are woken up.
337 The calling process must have alter permission on the semaphore set.
344 indicating the error.
346 Otherwise the system call returns a non-negative value depending on
367 the index of the highest used entry in the
368 kernel's internal array recording information about all
370 (This information can be used with repeated
372 operations to obtain information about all semaphore sets on the system.)
379 the identifier of the semaphore set whose index was given in
384 values return 0 on success.
388 will be set to one of the following:
393 has one of the values
404 and the calling process does not have the required
405 permissions on the semaphore set and does not have the
410 The address pointed to by
417 The semaphore set was removed.
426 operation, the index value specified in
428 referred to an array slot that is currently unused.
437 but the effective user ID of the calling process is not the creator
443 of the semaphore set,
444 and the process does not have the
455 and the value to which
457 is to be set (for some semaphore of the set) is less than 0
458 or greater than the implementation limit
462 .\" SVr4 documents more error conditions EINVAL and EOVERFLOW.
469 operations are used by the
471 program to provide information on allocated resources.
472 In the future these may modified or moved to a /proc file system
475 Various fields in a \fIstruct semid_ds\fP were typed as
481 To take advantage of this,
482 a recompilation under glibc-2.1.91 or later should suffice.
483 (The kernel distinguishes old and new calls by an
488 In some earlier versions of glibc, the
490 union was defined in \fI<sys/sem.h>\fP, but POSIX.1-2001 requires
491 that the caller define this union.
492 On versions of glibc where this union is \fInot\fP defined,
494 .B _SEM_SEMUN_UNDEFINED
495 is defined in \fI<sys/sem.h>\fP.
497 The following system limit on semaphore sets affects a
504 implementation dependent (32767).
506 For greater portability it is best to always call
513 .BR capabilities (7),
514 .BR sem_overview (7),