SEMCTL(P) POSIX Programmer's Manual SEMCTL(P)NAMEsemctl - 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 Defini‐
tions 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 opera‐
tions 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 exe‐
cuted with respect to the semaphore specified by semid and semnum. The
level of permission required for each operation is shown with each com‐
mand; 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 per‐
mission.
SETVAL Set the value of semval to arg.val, where arg is the value of
the fourth argument to semctl(). When this command is success‐
fully executed, the semadj value corresponding to the specified
semaphore in all processes is cleared. Requires alter permis‐
sion; 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 sema‐
phores:
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 sem‐
aphore 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 con‐
tents 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 sys‐
tem and destroy the set of semaphores and semid_ds data struc‐
ture 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 fol‐
lows:
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 effec‐
tive 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 inter‐
process 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 .
IEEE/The Open Group 2003 SEMCTL(P)
