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NAME
semctl — 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 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.
RETURN VALUE
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.
ERRORS
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.
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 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.
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.1‐2017, Section 4.17, Semaphore, <sys_sem.h>
COPYRIGHT
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 .
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