|
PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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.1‐2008, 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.
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.1‐2008, Section 4.16, Semaphore,
sys_sem.h(0p)
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 .
IEEE/The Open Group 2013 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)