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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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