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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | STANDARDS | HISTORY | NOTES | EXAMPLES | SEE ALSO | COLOPHON |
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semctl(2) System Calls Manual semctl(2)
semctl - System V semaphore control operations
Standard C library (libc, -lc)
#include <sys/sem.h>
int semctl(int semid, int semnum, int op, ...);
semctl() performs the control operation specified by op 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 op. 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; /* Creation time/time of last
modification via semctl() */
unsigned long sem_nsems; /* No. of semaphores in set */
};
The fields of the semid_ds structure are as follows:
sem_perm
This is an ipc_perm structure (see below) that specifies
the access permissions on the semaphore set.
sem_otime
Time of last semop(2) system call.
sem_ctime
Time of creation of semaphore set or time of last semctl()
IPCSET, SETVAL, or SETALL operation.
sem_nsems
Number of semaphores in the set. Each semaphore of the set
is referenced by a nonnegative integer ranging from 0 to
sem_nsems-1.
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 */
};
The least significant 9 bits of the mode field of the ipc_perm
structure define the access permissions for the shared memory
segment. The permission bits are as follows:
0400 Read by user
0200 Write by user
0040 Read by group
0020 Write by group
0004 Read by others
0002 Write by others
In effect, "write" means "alter" for a semaphore set. Bits 0100,
0010, and 0001 (the execute bits) are unused by the system.
Valid values for op are:
IPC_STAT
Copy information from the kernel data structure associated
with semid into the semid_ds structure pointed to by
arg.buf. The argument semnum 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 by arg.buf to the kernel data structure
associated with this semaphore set, updating also its
sem_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
argument semnum 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 argument semnum 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 type seminfo, 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, and semmni 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 for IPC_INFO, except that the following fields are
returned with information about system resources consumed
by semaphores: the semusz field returns the number of
semaphore sets that currently exist on the system; and the
semaem field returns the total number of semaphores in all
semaphore sets on the system.
SEM_STAT (Linux-specific)
Return a semid_ds structure as for IPC_STAT. However, the
semid argument is not a semaphore identifier, but instead
an index into the kernel's internal array that maintains
information about all semaphore sets on the system.
SEM_STAT_ANY (Linux-specific, since Linux 4.17)
Return a semid_ds structure as for SEM_STAT. However,
sem_perm.mode is not checked for read access for semid
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 into arg.array. The argument semnum is ignored.
The calling process must have read permission on the
semaphore set.
GETNCNT
Return the semncnt value for the semnum-th semaphore of the
set (i.e., the number of processes waiting for the
semaphore's value to increase). The calling process must
have read permission on the semaphore set.
GETPID Return the sempid value for the semnum-th semaphore of the
set. This is the PID of the process that last performed an
operation on that semaphore (but see VERSIONS). The
calling process must have read permission on the semaphore
set.
GETVAL Return semval (i.e., the semaphore value) for the semnum-th
semaphore of the set. The calling process must have read
permission on the semaphore set.
GETZCNT
Return the semzcnt value for the semnum-th semaphore of the
set (i.e., the number of processes waiting for the
semaphore value to become 0). The calling process must
have read permission on the semaphore set.
SETALL Set the semval values for all semaphores of the set using
arg.array, updating also the sem_ctime member of the
semid_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 argument semnum is
ignored. The calling process must have alter (write)
permission on the semaphore set.
SETVAL Set the semaphore value (semval) to arg.val for the
semnum-th semaphore of the set, updating also the sem_ctime
member of the semid_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.
On success, semctl() returns a nonnegative value depending on op
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's
internal array recording information about all semaphore
sets. (This information can be used with repeated SEM_STAT
or SEM_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 op values return 0 on success.
On failure, semctl() returns -1 and sets errno to indicate the
error.
EACCES The argument op has one of the values GETALL, GETPID,
GETVAL, GETNCNT, GETZCNT, IPC_STAT, SEM_STAT, SEM_STAT_ANY,
SETALL, or SETVAL and the calling process does not have the
required permissions on the semaphore set and does not have
the CAP_IPC_OWNER capability in the user namespace that
governs its IPC namespace.
EFAULT The address pointed to by arg.buf or arg.array isn't
accessible.
EIDRM The semaphore set was removed.
EINVAL Invalid value for op or semid. Or: for a SEM_STAT
operation, the index value specified in semid referred to
an array slot that is currently unused.
EPERM The argument op has the value IPC_SET or IPC_RMID but the
effective user ID of the calling process is not the creator
(as found in sem_perm.cuid) or the owner (as found in
sem_perm.uid) of the semaphore set, and the process does
not have the CAP_SYS_ADMIN capability.
ERANGE The argument op has the value SETALL or SETVAL and the
value to which semval is to be set (for some semaphore of
the set) is less than 0 or greater than the implementation
limit SEMVMX.
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.
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 Linux 4.5, the kernel did not update sempid for SETALL
operations. This was rectified in Linux 4.6.
POSIX.1-2008.
POSIX.1-2001, SVr4.
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 op.)
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 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.
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.
See shmop(2).
ipc(2), semget(2), semop(2), capabilities(7), sem_overview(7),
sysvipc(7)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
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[email protected]
Linux man-pages 6.15 2025-05-17 semctl(2)
Pages that refer to this page: ipcrm(1), ipcs(1), ipc(2), semget(2), semop(2), syscalls(2), sysvipc(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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