The semctl() function operates on XSI semaphores (see the Base
Definitions volume of IEEE Std 1003.1-2001, Section 4.15, Semaphore).
It is unspecified whether this function interoperates
with the realtime interprocess communication facilities defined in
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 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. Requires alter permission; see 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.
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 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.
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 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
None.
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 XSI Interprocess Communication can
be easily modified to use the alternative
interfaces.
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSO
XSI Interprocess Communication , Realtime , semget()
, semop() , sem_close() , sem_destroy() , sem_getvalue()
, sem_init() , sem_open() , sem_post() , sem_unlink()
, sem_wait() , the Base Definitions volume of IEEE Std 1003.1-2001,
<sys/sem.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 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 .
