SEMCTL(3P) POSIX Programmer's Manual SEMCTL(3P)PROLOG
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.
NAMEsemctl — XSI semaphore control operations
SYNOPSIS
#include <sys/sem.h>
int semctl(int semid, int semnum, int cmd, ...);
DESCRIPTION
The semctl() function operates on XSI semaphores (see the Base Defini‐
tions volume of POSIX.1‐2008, Section 4.16, Semaphore). It is unspeci‐
fied 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 opera‐
tions 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;
The following semaphore control operations as specified by cmd are exe‐
cuted with respect to the semaphore specified by semid and semnum. The
level of permission required for each operation is shown with each com‐
mand; 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 Interpro‐
cess 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 sema‐
phores:
GETALL Return the value of semval for each semaphore in the sema‐
phore 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 Descrip‐
tion. 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 sem‐
ctl(). 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 exe‐
cuted 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.
RETURN VALUE
If successful, the value returned by semctl() depends on cmd as fol‐
lows:
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.
ERRORS
The semctl() function shall fail if:
EACCES Operation permission is denied to the calling process; see Sec‐
tion 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 effec‐
tive 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.
EXAMPLES
Refer to semop().
APPLICATION USAGE
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 inter‐
process 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.
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSO
Section 2.7, XSI Interprocess Communication, Section 2.8, Realtime,
semget(), semop(), sem_close(), sem_destroy(), sem_getvalue(),
sem_init(), sem_open(), sem_post(), sem_trywait(), sem_unlink()
The Base Definitions volume of POSIX.1‐2008, Section 4.16, Semaphore,
<sys_sem.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
cal and Electronics Engineers, Inc and The Open Group. (This is
POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
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IEEE/The Open Group 2013 SEMCTL(3P)
