SEMCTL
Section: POSIX Programmer's Manual (3P)
Updated: 2013
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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.
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.1-2008,
Section 4.16,
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;
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
dq...dq
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.1-2008,
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
Electrical 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 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 .
Index
- PROLOG
-
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- EXAMPLES
-
- APPLICATION USAGE
-
- RATIONALE
-
- FUTURE DIRECTIONS
-
- SEE ALSO
-
- COPYRIGHT
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