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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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SEMCTL(3P) POSIX Programmer's Manual SEMCTL(3P)
This manual page is part of the POSIX Programmer's Manual. The
Linux implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.
semctl — XSI semaphore control operations
#include <sys/sem.h>
int semctl(int semid, int semnum, int cmd, ...);
The semctl() function operates on XSI semaphores (see the Base
Definitions volume of POSIX.1‐2017, Section 4.17, Semaphore). It
is unspecified whether this function interoperates with the
realtime interprocess communication facilities defined in Section
2.8, Realtime.
The semctl() function provides a variety of semaphore control
operations as specified by cmd. The fourth argument is optional
and depends upon the operation requested. If required, it is of
type union semun, which the application shall explicitly declare:
union semun {
int val;
struct semid_ds *buf;
unsigned short *array;
} arg;
Each operation shall be performed atomically.
The following semaphore control operations as specified by cmd are
executed with respect to the semaphore specified by semid and
semnum. The level of permission required for each operation is
shown with each command; see Section 2.7, XSI Interprocess
Communication. The symbolic names for the values of cmd are
defined in the <sys/sem.h> header:
GETVAL Return the value of semval; see <sys/sem.h>. Requires
read permission.
SETVAL Set the value of semval to arg.val, where arg is the
value of the fourth argument to semctl(). When this
command is successfully executed, the semadj value
corresponding to the specified semaphore in all
processes is cleared. Also, the sem_ctime timestamp
shall be set to the current time, as described in
Section 2.7.1, IPC General Description. Requires
alter permission; see Section 2.7, XSI Interprocess
Communication.
GETPID Return the value of sempid. Requires read permission.
GETNCNT Return the value of semncnt. Requires read
permission.
GETZCNT Return the value of semzcnt. Requires read
permission.
The following values of cmd operate on each semval in the set of
semaphores:
GETALL Return the value of semval for each semaphore in the
semaphore set and place into the array pointed to by
arg.array, where arg is the fourth argument to
semctl(). Requires read permission.
SETALL Set the value of semval for each semaphore in the
semaphore set according to the array pointed to by
arg.array, where arg is the fourth argument to
semctl(). When this command is successfully executed,
the semadj values corresponding to each specified
semaphore in all processes are cleared. Also, the
sem_ctime timestamp shall be set to the current time,
as described in Section 2.7.1, IPC General
Description. Requires alter permission.
The following values of cmd are also available:
IPC_STAT Place the current value of each member of the semid_ds
data structure associated with semid into the
structure pointed to by arg.buf, where arg is the
fourth argument to semctl(). The contents of this
structure are defined in <sys/sem.h>. Requires read
permission.
IPC_SET Set the value of the following members of the semid_ds
data structure associated with semid to the
corresponding value found in the structure pointed to
by arg.buf, where arg is the fourth argument to
semctl():
sem_perm.uid
sem_perm.gid
sem_perm.mode
The mode bits specified in Section 2.7.1, IPC General
Description are copied into the corresponding bits of
the sem_perm.mode associated with semid. The stored
values of any other bits are unspecified. The
sem_ctime timestamp shall be set to the current time,
as described in Section 2.7.1, IPC General
Description.
This command can only be executed by a process that
has an effective user ID equal to either that of a
process with appropriate privileges or to the value of
sem_perm.cuid or sem_perm.uid in the semid_ds data
structure associated with semid.
IPC_RMID Remove the semaphore identifier specified by semid
from the system and destroy the set of semaphores and
semid_ds data structure associated with it. This
command can only be executed by a process that has an
effective user ID equal to either that of a process
with appropriate privileges or to the value of
sem_perm.cuid or sem_perm.uid in the semid_ds data
structure associated with semid.
If successful, the value returned by semctl() depends on cmd as
follows:
GETVAL The value of semval.
GETPID The value of sempid.
GETNCNT The value of semncnt.
GETZCNT The value of semzcnt.
All others 0.
Otherwise, semctl() shall return -1 and set errno to indicate the
error.
The semctl() function shall fail if:
EACCES Operation permission is denied to the calling process; see
Section 2.7, XSI Interprocess Communication.
EINVAL The value of semid is not a valid semaphore identifier, or
the value of semnum is less than 0 or greater than or equal
to sem_nsems, or the value of cmd is not a valid command.
EPERM The argument cmd is equal to IPC_RMID or IPC_SET and the
effective user ID of the calling process is not equal to
that of a process with appropriate privileges and it is not
equal to the value of sem_perm.cuid or sem_perm.uid in the
data structure associated with semid.
ERANGE The argument cmd is equal to SETVAL or SETALL and the value
to which semval is to be set is greater than the system-
imposed maximum.
The following sections are informative.
Refer to semop(3p).
The fourth parameter in the SYNOPSIS section is now specified as
"..." in order to avoid a clash with the ISO C standard when
referring to the union semun (as defined in Issue 3) and for
backwards-compatibility.
The POSIX Realtime Extension defines alternative interfaces for
interprocess communication. Application developers who need to use
IPC should design their applications so that modules using the IPC
routines described in Section 2.7, XSI Interprocess Communication
can be easily modified to use the alternative interfaces.
None.
None.
Section 2.7, XSI Interprocess Communication, Section 2.8,
Realtime, semget(3p), semop(3p), sem_close(3p), sem_destroy(3p),
sem_getvalue(3p), sem_init(3p), sem_open(3p), sem_post(3p),
sem_trywait(3p), sem_unlink(3p)
The Base Definitions volume of POSIX.1‐2017, Section 4.17,
Semaphore, sys_sem.h(0p)
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 by the Institute of Electrical and Electronics Engineers,
Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard,
the original IEEE and The Open Group Standard is the referee
document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear in this page
are most likely to have been introduced during the conversion of
the source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2017 SEMCTL(3P)
Pages that refer to this page: sys_sem.h(0p), ipcrm(1p), sem_close(3p), sem_destroy(3p), semget(3p), sem_getvalue(3p), semop(3p), sem_open(3p), sem_post(3p), sem_timedwait(3p), sem_trywait(3p), sem_unlink(3p)
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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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