Section (2) semctl
Name
semctl — System V semaphore control operations
Synopsis
#include <sys/types.h> #include <sys/ipc.h> #include <sys/sem.h>
int
semctl( |
int semid, |
int semnum, | |
int cmd, | |
...) ; |
DESCRIPTION
semctl
() performs the
control operation specified by cmd
on the System V semaphore
set identified by semid
, or on the semnum
-th semaphore of that
set. (The semaphores in a set are numbered starting at
0.)
This function has three or four arguments, depending on
cmd
. When there are
four, the fourth has the type union semun. The calling program must define this
union as follows:
union semun { int val
; /* Value for SETVAL */struct semid_ds * buf
; /* Buffer for IPC_STAT, IPC_SET */unsigned short * array
; /* Array for GETALL, SETALL */struct seminfo * __buf
; /* Buffer for IPC_INFO
(Linux-specific) */};
The semid_ds
data structure is defined in <
sys/sem.h
>
as follows:
struct semid_ds { struct ipc_perm sem_perm
; /* Ownership and permissions */time_t sem_otime
; /* Last semop time */time_t sem_ctime
; /* Last change time */unsigned long sem_nsems
; /* No. of semaphores in set */};
The ipc_perm
structure is defined as follows (the highlighted fields are
settable using IPC_SET
):
struct ipc_perm { key_t __key
; /* Key supplied to semget(2) */uid_t uid
; /* Effective UID of owner */gid_t gid
; /* Effective GID of owner */uid_t cuid
; /* Effective UID of creator */gid_t cgid
; /* Effective GID of creator */unsigned short mode
; /* Permissions */unsigned short __seq
; /* Sequence number */};
Valid values for cmd
are:
IPC_STAT
-
Copy information from the kernel data structure associated with
semid
into thesemid_ds
structure pointed to byarg.buf
. The argumentsemnum
is ignored. The calling process must have read permission on the semaphore set. IPC_SET
-
Write the values of some members of the
semid_ds
structure pointed to byarg.buf
to the kernel data structure associated with this semaphore set, updating also itssem_ctime
member. The following members of the structure are updated:sem_perm.uid
,sem_perm.gid
, and (the least significant 9 bits of)sem_perm.mode
. The effective UID of the calling process must match the owner (sem_perm.uid
) or creator (sem_perm.cuid
) of the semaphore set, or the caller must be privileged. The argumentsemnum
is ignored. IPC_RMID
-
Immediately remove the semaphore set, awakening all processes blocked in semop(2) calls on the set (with an error return and
errno
set to EIDRM). The effective user ID of the calling process must match the creator or owner of the semaphore set, or the caller must be privileged. The argumentsemnum
is ignored. IPC_INFO
(Linux-specific)-
Return information about system-wide semaphore limits and parameters in the structure pointed to by
arg.__buf
. This structure is of typeseminfo
, defined in<
sys/sem.h
>
if the_GNU_SOURCE
feature test macro is defined:struct seminfo { int semmap
;/* Number of entries in semaphore
map; unused within kernel */int semmni
;/* Maximum number of semaphore sets */int semmns
;/* Maximum number of semaphores in all
semaphore sets */int semmnu
;/* System-wide maximum number of undo
structures; unused within kernel */int semmsl
;/* Maximum number of semaphores in a
set */int semopm
;/* Maximum number of operations for
semop(2) */int semume
;/* Maximum number of undo entries per
process; unused within kernel */int semusz
;/* Size of struct sem_undo */int semvmx
;/* Maximum semaphore value */int semaem
;/* Max. value that can be recorded for
semaphore adjustment (SEM_UNDO) */}; The
semmsl
,semmns
,semopm
, andsemmni
settings can be changed via/proc/sys/kernel/sem
; see proc(5) for details. SEM_INFO
(Linux-specific)-
Return a
seminfo
structure containing the same information as forIPC_INFO
, except that the following fields are returned with information about system resources consumed by semaphores: thesemusz
field returns the number of semaphore sets that currently exist on the system; and thesemaem
field returns the total number of semaphores in all semaphore sets on the system. SEM_STAT
(Linux-specific)-
Return a
semid_ds
structure as forIPC_STAT
. However, thesemid
argument is not a semaphore identifier, but instead an index into the kernel_zsingle_quotesz_s internal array that maintains information about all semaphore sets on the system. SEM_STAT_ANY
(Linux-specific, since Linux 4.17)-
Return a
seminfo
structure containing the same information as forSEM_STAT
. However,sem_perm.mode
is not checked for read access forsemid
meaning that any user can employ this operation (just as any user may read/proc/sysvipc/sem
to obtain the same information). GETALL
-
Return
semval
(i.e., the current value) for all semaphores of the set intoarg.array
. The argumentsemnum
is ignored. The calling process must have read permission on the semaphore set. GETNCNT
-
Return the value of
semncnt
for thesemnum
−th semaphore of the set (i.e., the number of processes waiting for an increase ofsemval
for thesemnum
−th semaphore of the set). The calling process must have read permission on the semaphore set. GETPID
-
Return the value of
sempid
for thesemnum
−th semaphore of the set. This is the PID of the process that last performed an operation on that semaphore (but see NOTES). The calling process must have read permission on the semaphore set. GETVAL
-
Return the value of
semval
for thesemnum
−th semaphore of the set. The calling process must have read permission on the semaphore set. GETZCNT
-
Return the value of
semzcnt
for thesemnum
−th semaphore of the set (i.e., the number of processes waiting forsemval
of thesemnum
−th semaphore of the set to become 0). The calling process must have read permission on the semaphore set. SETALL
-
Set
semval
for all semaphores of the set usingarg.array
, updating also thesem_ctime
member of thesemid_ds
structure associated with the set. Undo entries (see semop(2)) are cleared for altered semaphores in all processes. If the changes to semaphore values would permit blocked semop(2) calls in other processes to proceed, then those processes are woken up. The argumentsemnum
is ignored. The calling process must have alter (write) permission on the semaphore set. SETVAL
-
Set the value of
semval
toarg.val
for thesemnum
−th semaphore of the set, updating also thesem_ctime
member of thesemid_ds
structure associated with the set. Undo entries are cleared for altered semaphores in all processes. If the changes to semaphore values would permit blocked semop(2) calls in other processes to proceed, then those processes are woken up. The calling process must have alter permission on the semaphore set.
RETURN VALUE
On failure, semctl
() returns
−1 with errno
indicating
the error.
Otherwise, the system call returns a nonnegative value
depending on cmd
as
follows:
GETNCNT
-
the value of
semncnt
. GETPID
-
the value of
sempid
. GETVAL
-
the value of
semval
. GETZCNT
-
the value of
semzcnt
. IPC_INFO
-
the index of the highest used entry in the kernel_zsingle_quotesz_s internal array recording information about all semaphore sets. (This information can be used with repeated
SEM_STAT
orSEM_STAT_ANY
operations to obtain information about all semaphore sets on the system.) SEM_INFO
-
as for
IPC_INFO
. SEM_STAT
-
the identifier of the semaphore set whose index was given in
semid
. SEM_STAT_ANY
-
as for
SEM_STAT
.
All other cmd
values return 0 on success.
ERRORS
On failure, errno
will be set
to one of the following:
- EACCES
-
The argument
cmd
has one of the valuesGETALL
,GETPID
,GETVAL
,GETNCNT
,GETZCNT
,IPC_STAT
,SEM_STAT
,SEM_STAT_ANY
,SETALL
, orSETVAL
and the calling process does not have the required permissions on the semaphore set and does not have theCAP_IPC_OWNER
capability in the user namespace that governs its IPC namespace. - EFAULT
-
The address pointed to by
arg.buf
orarg.array
isn_zsingle_quotesz_t accessible. - EIDRM
-
The semaphore set was removed.
- EINVAL
-
Invalid value for
cmd
orsemid
. Or: for aSEM_STAT
operation, the index value specified insemid
referred to an array slot that is currently unused. - EPERM
-
The argument
cmd
has the valueIPC_SET
orIPC_RMID
but the effective user ID of the calling process is not the creator (as found insem_perm.cuid
) or the owner (as found insem_perm.uid
) of the semaphore set, and the process does not have theCAP_SYS_ADMIN
capability. - ERANGE
-
The argument
cmd
has the valueSETALL
orSETVAL
and the value to whichsemval
is to be set (for some semaphore of the set) is less than 0 or greater than the implementation limitSEMVMX
.
CONFORMING TO
POSIX.1-2001, POSIX.1-2008, SVr4.
POSIX.1 specifies the sem_nsems
field of the
semid_ds
structure
as having the type unsigned short,
and the field is so defined on most other systems. It was
also so defined on Linux 2.2 and earlier, but, since Linux
2.4, the field has the type unsigned
long.
NOTES
The inclusion of <
sys/types.h
>
and <
sys/ipc.h
>
isn_zsingle_quotesz_t required on Linux or by any version of POSIX. However,
some old implementations required the inclusion of these
header files, and the SVID also documented their inclusion.
Applications intended to be portable to such old systems may
need to include these header files.
The IPC_INFO
, SEM_STAT
and SEM_INFO
operations are used by the
ipcs(1) program to provide
information on allocated resources. In the future these may
modified or moved to a /proc
filesystem interface.
Various fields in a struct
semid_ds were typed as short under Linux 2.2 and have become
long under Linux 2.4. To take
advantage of this, a recompilation under glibc-2.1.91 or
later should suffice. (The kernel distinguishes old and new
calls by an IPC_64
flag in
cmd
.)
In some earlier versions of glibc, the semun
union was defined in
<
sys/sem.h
>
but POSIX.1 requires that the caller define this union. On
versions of glibc where this union is not
defined, the macro
_SEM_SEMUN_UNDEFINED
is defined
in <
sys/sem.h
>
The following system limit on semaphore sets affects a
semctl
() call:
SEMVMX
-
Maximum value for
semval
: implementation dependent (32767).
For greater portability, it is best to always call
semctl
() with four
arguments.
The sempid value
POSIX.1 defines sempid
as the process ID
of [the] last operation on a semaphore, and explicitly
notes that this value is set by a successful semop(2) call, with the
implication that no other interface affects the sempid
value.
While some implementations conform to the behavior
specified in POSIX.1, others do not. (The fault here
probably lies with POSIX.1 inasmuch as it likely failed to
capture the full range of existing implementation
behaviors.) Various other implementations also update
sempid
for the
other operations that update the value of a semaphore: the
SETVAL
and SETALL
operations, as well as the
semaphore adjustments performed on process termination as a
consequence of the use of the SEM_UNDO
flag (see semop(2)).
Linux also updates sempid
for SETVAL
operations and semaphore
adjustments. However, somewhat inconsistently, up to and
including 4.5, Linux did not update sempid
for SETALL
operations. This was rectified in
Linux 4.6.
COLOPHON
This page is part of release 5.04 of the Linux man-pages
project. A
description of the project, information about reporting bugs,
and the latest version of this page, can be found at
https://www.kernel.org/doc/man−pages/.
Copyright 1993 Giorgio Ciucci (giorgiocrcc.it) and Copyright 2004, 2005 Michael Kerrisk <mtk.manpagesgmail.com> %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. The author(s) may not have taken the same level of care in the production of this manual, which is licensed free of charge, as they might when working professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. %%%LICENSE_END Modified Tue Oct 22 17:53:56 1996 by Eric S. Raymond <esrthyrsus.com> Modified Fri Jun 19 10:59:15 1998 by Andries Brouwer <aebcwi.nl> Modified Sun Feb 18 01:59:29 2001 by Andries Brouwer <aebcwi.nl> Modified 20 Dec 2001, Michael Kerrisk <mtk.manpagesgmail.com> Modified 21 Dec 2001, aeb Modified 27 May 2004, Michael Kerrisk <mtk.manpagesgmail.com> Added notes on CAP_IPC_OWNER requirement Modified 17 Jun 2004, Michael Kerrisk <mtk.manpagesgmail.com> Added notes on CAP_SYS_ADMIN requirement for IPC_SET and IPC_RMID Modified, 11 Nov 2004, Michael Kerrisk <mtk.manpagesgmail.com> Language and formatting clean-ups Rewrote semun text Added semid_ds and ipc_perm structure definitions 2005-08-02, mtk: Added IPC_INFO, SEM_INFO, SEM_STAT descriptions. 2018-03-20, dbueso: Added SEM_STAT_ANY description. |