1 .\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it)
2 .\" and Copyright 2004, 2005 Michael Kerrisk <mtk.manpages@gmail.com>
4 .\" %%%LICENSE_START(VERBATIM)
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10 .\" manual under the conditions for verbatim copying, provided that the
11 .\" entire resulting derived work is distributed under the terms of a
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14 .\" Since the Linux kernel and libraries are constantly changing, this
15 .\" manual page may be incorrect or out-of-date. The author(s) assume no
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19 .\" which is licensed free of charge, as they might when working
22 .\" Formatted or processed versions of this manual, if unaccompanied by
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26 .\" Modified Tue Oct 22 17:53:56 1996 by Eric S. Raymond <esr@thyrsus.com>
27 .\" Modified Fri Jun 19 10:59:15 1998 by Andries Brouwer <aeb@cwi.nl>
28 .\" Modified Sun Feb 18 01:59:29 2001 by Andries Brouwer <aeb@cwi.nl>
29 .\" Modified 20 Dec 2001, Michael Kerrisk <mtk.manpages@gmail.com>
30 .\" Modified 21 Dec 2001, aeb
31 .\" Modified 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com>
32 .\" Added notes on CAP_IPC_OWNER requirement
33 .\" Modified 17 Jun 2004, Michael Kerrisk <mtk.manpages@gmail.com>
34 .\" Added notes on CAP_SYS_ADMIN requirement for IPC_SET and IPC_RMID
35 .\" Modified, 11 Nov 2004, Michael Kerrisk <mtk.manpages@gmail.com>
36 .\" Language and formatting clean-ups
37 .\" Rewrote semun text
38 .\" Added semid_ds and ipc_perm structure definitions
39 .\" 2005-08-02, mtk: Added IPC_INFO, SEM_INFO, SEM_STAT descriptions.
41 .TH SEMCTL 2 2013-06-03 "Linux" "Linux Programmer's Manual"
43 semctl \- System V semaphore control operations
46 .B #include <sys/types.h>
47 .B #include <sys/ipc.h>
48 .B #include <sys/sem.h>
50 .BI "int semctl(int " semid ", int " semnum ", int " cmd ", ...);"
54 performs the control operation specified by
56 on the System V semaphore set identified by
60 semaphore of that set.
61 (The semaphores in a set are numbered starting at 0.)
63 This function has three or four arguments, depending on
65 When there are four, the fourth has the type
67 The \fIcalling program\fP must define this union as follows:
72 int val; /* Value for SETVAL */
73 struct semid_ds *buf; /* Buffer for IPC_STAT, IPC_SET */
74 unsigned short *array; /* Array for GETALL, SETALL */
75 struct seminfo *__buf; /* Buffer for IPC_INFO
83 data structure is defined in \fI<sys/sem.h>\fP as follows:
88 struct ipc_perm sem_perm; /* Ownership and permissions */
89 time_t sem_otime; /* Last semop time */
90 time_t sem_ctime; /* Last change time */
91 unsigned long sem_nsems; /* No. of semaphores in set */
98 structure is defined as follows
99 (the highlighted fields are settable using
105 key_t __key; /* Key supplied to semget(2) */
106 uid_t \fBuid\fP; /* Effective UID of owner */
107 gid_t \fBgid\fP; /* Effective GID of owner */
108 uid_t cuid; /* Effective UID of creator */
109 gid_t cgid; /* Effective GID of creator */
110 unsigned short \fBmode\fP; /* Permissions */
111 unsigned short __seq; /* Sequence number */
121 Copy information from the kernel data structure associated with
125 structure pointed to by
130 The calling process must have read permission on the semaphore set.
133 Write the values of some members of the
135 structure pointed to by
137 to the kernel data structure associated with this semaphore set,
141 The following members of the structure are updated:
144 and (the least significant 9 bits of)
146 The effective UID of the calling process must match the owner
149 .RI ( sem_perm.cuid )
150 of the semaphore set, or the caller must be privileged.
156 Immediately remove the semaphore set,
157 awakening all processes blocked in
159 calls on the set (with an error return and
163 The effective user ID of the calling process must
164 match the creator or owner of the semaphore set,
165 or the caller must be privileged.
170 .BR IPC_INFO " (Linux-specific)"
171 Return information about system-wide semaphore limits and
172 parameters in the structure pointed to by
174 This structure is of type
180 feature test macro is defined:
185 int semmap; /* Number of entries in semaphore
186 map; unused within kernel */
187 int semmni; /* Maximum number of semaphore sets */
188 int semmns; /* Maximum number of semaphores in all
190 int semmnu; /* System-wide maximum number of undo
191 structures; unused within kernel */
192 int semmsl; /* Maximum number of semaphores in a
194 int semopm; /* Maximum number of operations for
196 int semume; /* Maximum number of undo entries per
197 process; unused within kernel */
198 int semusz; /* Size of struct sem_undo */
199 int semvmx; /* Maximum semaphore value */
200 int semaem; /* Max. value that can be recorded for
201 semaphore adjustment (SEM_UNDO) */
212 settings can be changed via
213 .IR /proc/sys/kernel/sem ;
218 .BR SEM_INFO " (Linux-specific)"
221 structure containing the same information as for
223 except that the following fields are returned with information
224 about system resources consumed by semaphores: the
226 field returns the number of semaphore sets that currently exist
227 on the system; and the
229 field returns the total number of semaphores in all semaphore sets
232 .BR SEM_STAT " (Linux-specific)"
239 argument is not a semaphore identifier, but instead an index into
240 the kernel's internal array that maintains information about
241 all semaphore sets on the system.
246 (i.e., the current value)
247 for all semaphores of the set into
252 The calling process must have read permission on the semaphore set.
260 (i.e., the number of processes waiting for an increase of
264 semaphore of the set).
265 The calling process must have read permission on the semaphore set.
273 (i.e., the PID of the process that executed the last
277 semaphore of the set).
278 The calling process must have read permission on the semaphore set.
285 semaphore of the set.
286 The calling process must have read permission on the semaphore set.
294 (i.e., the number of processes waiting for
298 semaphore of the set to become 0).
299 The calling process must have read permission on the semaphore set.
304 for all semaphores of the set using
310 structure associated with the set.
313 are cleared for altered semaphores in all processes.
314 If the changes to semaphore values would permit blocked
316 calls in other processes to proceed, then those processes are woken up.
320 The calling process must have alter (write) permission on
330 semaphore of the set, updating also the
334 structure associated with the set.
335 Undo entries are cleared for altered semaphores in all processes.
336 If the changes to semaphore values would permit blocked
338 calls in other processes to proceed, then those processes are woken up.
339 The calling process must have alter permission on the semaphore set.
346 indicating the error.
348 Otherwise the system call returns a nonnegative value depending on
369 the index of the highest used entry in the
370 kernel's internal array recording information about all
372 (This information can be used with repeated
374 operations to obtain information about all semaphore sets on the system.)
381 the identifier of the semaphore set whose index was given in
386 values return 0 on success.
390 will be set to one of the following:
395 has one of the values
406 and the calling process does not have the required
407 permissions on the semaphore set and does not have the
412 The address pointed to by
419 The semaphore set was removed.
428 operation, the index value specified in
430 referred to an array slot that is currently unused.
439 but the effective user ID of the calling process is not the creator
445 of the semaphore set,
446 and the process does not have the
457 and the value to which
459 is to be set (for some semaphore of the set) is less than 0
460 or greater than the implementation limit
464 .\" SVr4 documents more error conditions EINVAL and EOVERFLOW.
466 POSIX.1-2001 specifies the
470 structure as having the type
471 .IR "unsigned\ short" ,
472 and the field is so defined on most other systems.
473 It was also so defined on Linux 2.2 and earlier,
474 but, since Linux 2.4, the field has the type
475 .IR "unsigned\ long" .
481 isn't required on Linux or by any version of POSIX.
483 some old implementations required the inclusion of these header files,
484 and the SVID also documented their inclusion.
485 Applications intended to be portable to such old systems may need
486 to include these header files.
487 .\" Like Linux, the FreeBSD man pages still document
488 .\" the inclusion of these header files.
495 operations are used by the
497 program to provide information on allocated resources.
498 In the future these may modified or moved to a
500 filesystem interface.
502 Various fields in a \fIstruct semid_ds\fP were typed as
508 To take advantage of this,
509 a recompilation under glibc-2.1.91 or later should suffice.
510 (The kernel distinguishes old and new calls by an
515 In some earlier versions of glibc, the
517 union was defined in \fI<sys/sem.h>\fP, but POSIX.1-2001 requires
518 that the caller define this union.
519 On versions of glibc where this union is \fInot\fP defined,
521 .B _SEM_SEMUN_UNDEFINED
522 is defined in \fI<sys/sem.h>\fP.
524 The following system limit on semaphore sets affects a
531 implementation dependent (32767).
533 For greater portability it is best to always call
540 .BR capabilities (7),
541 .BR sem_overview (7),