1 .\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it)
2 .\" and Copyright 2004, 2005 Michael Kerrisk <mtk-manpages@gmx.net>
4 .\" Permission is granted to make and distribute verbatim copies of this
5 .\" manual provided the copyright notice and this permission notice are
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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14 .\" manual page may be incorrect or out-of-date.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@gmx.net>
28 .\" Modified 21 Dec 2001, aeb
29 .\" Modified 27 May 2004, Michael Kerrisk <mtk-manpages@gmx.net>
30 .\" Added notes on CAP_IPC_OWNER requirement
31 .\" Modified 17 Jun 2004, Michael Kerrisk <mtk-manpages@gmx.net>
32 .\" Added notes on CAP_SYS_ADMIN requirement for IPC_SET and IPC_RMID
33 .\" Modified, 11 Nov 2004, Michael Kerrisk <mtk-manpages@gmx.net>
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 2.6.9" "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 <sys/sem.h> 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 <sys/ipc.h> as follows
97 (the highlighted fields are settable using
103 key_t key; /* Key supplied to semget() */
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
176 if the _GNU_SOURCE feature test macro is defined:
181 int semmap; /* # of entries in semaphore map;
183 int semmni; /* Max. # of semaphore sets */
184 int semmns; /* Max. # of semaphores in all
186 int semmnu; /* System-wide max. # of undo
187 structures; unused */
188 int semmsl; /* Max. # of semaphores in a set */
189 int semopm; /* Max. # of operations for semop() */
190 int semume; /* Max. # of undo entries per
192 int semusz; /* size of struct sem_undo */
193 int semvmx; /* Maximum semaphore value */
194 int semaem; /* Max. value that can be recorded for
195 semaphore adjustment (SEM_UNDO) */
206 settings can be changed via
207 .IR /proc/sys/kernel/sem ;
212 .BR SEM_INFO " (Linux specific)"
215 structure containing the same information as for
217 except that the following fields are returned with information
218 about system resources consumed by semaphores: the
220 field returns the number of semaphore sets that currently exist
221 on the system; and the
223 field returns the total number of semaphores in all semaphore sets
226 .BR SEM_STAT " (Linux specific)"
233 argument is not a semaphore identifier, but instead an index into
234 the kernel's internal array that maintains information about
235 all semaphore sets on the system.
240 (i.e., the current value)
241 for all semaphores of the set into
246 The calling process must have read permission on the semaphore set.
249 The system call returns the value of
251 (i.e., the number of processes waiting for the value of
252 this semaphore to increase) for the
255 (i.e. the number of processes waiting for an increase of
259 semaphore of the set).
260 The calling process must have read permission on the semaphore set.
263 The system call returns the value of
268 (i.e. the PID of the process that executed the last
272 semaphore of the set).
273 The calling process must have read permission on the semaphore set.
276 The system call returns the value of
280 semaphore of the set.
281 The calling process must have read permission on the semaphore set.
284 The system call returns the value of
286 (i.e., the number of processes waiting for the value of
287 this semaphore to become zero) for the
290 (i.e. the number of processes waiting for
294 semaphore of the set to become 0).
295 The calling process must have read permission on the semaphore set.
300 for all semaphores of the set using
306 structure associated with the set.
309 are cleared for altered semaphores in all processes.
310 If the changes to semaphore values would permit blocked
312 calls in other processes to proceed, then those processes are woken up.
316 The calling process must have alter (write) permission on
326 semaphore of the set, updating also the
330 structure associated with the set.
331 Undo entries are cleared for altered semaphores in all processes.
332 If the changes to semaphore values would permit blocked
334 calls in other processes to proceed, then those processes are woken up.
335 The calling process must have alter permission on the semaphore set.
342 indicating the error.
344 Otherwise the system call returns a nonnegative value depending on
365 the index of the highest used entry in the
366 kernel's internal array recording information about all
368 (This information can be used with repeated
370 operations to obtain information about all semaphore sets on the system.)
377 the identifier of the semaphore set whose index was given in
382 values return 0 on success.
386 will be set to one of the following:
391 has one of the values
402 and the calling process does not have the required
403 permissions on the semaphore set and does not have the
408 The address pointed to by
415 The semaphore set was removed.
424 operation, the index value specified in
426 referred to an array slot that is currently unused.
435 but the effective user ID of the calling process is not the creator
441 of the semaphore set,
442 and the process does not have the
453 and the value to which
455 is to be set (for some semaphore of the set) is less than 0
456 or greater than the implementation limit
464 operations are used by the
466 program to provide information on allocated resources.
467 In the future these may modified or moved to a /proc file system
470 Various fields in a \fIstruct semid_ds\fP were shorts under Linux 2.2
471 and have become longs under Linux 2.4. To take advantage of this,
472 a recompilation under glibc-2.1.91 or later should suffice.
473 (The kernel distinguishes old and new calls by an IPC_64 flag in
476 In some earlier versions of glibc, the
478 union was defined in <sys/sem.h>, but POSIX.1-2001 requires
479 that the caller define this union.
480 On versions of glibc where this union is \fInot\fP defined,
482 .B _SEM_SEMUN_UNDEFINED
483 is defined in <sys/sem.h>.
485 The following system limit on semaphore sets affects a
492 implementation dependent (32767).
494 For greater portability it is best to always call
500 is not a system call, but is implemented via the system call
503 SVr4, SVID. SVr4 documents more error conditions EINVAL and EOVERFLOW.
509 .BR capabilities (7),