243 lines
6.3 KiB
Groff
243 lines
6.3 KiB
Groff
.\" $OpenBSD: semctl.2,v 1.18 2021/11/21 23:44:55 jan Exp $
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.\" $NetBSD: semctl.2,v 1.2 1997/03/27 08:20:40 mikel Exp $
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.\"
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.\" Copyright (c) 1995 Frank van der Linden
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.\" All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\" This product includes software developed for the NetBSD Project
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.\" by Frank van der Linden
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.\" 4. The name of the author may not be used to endorse or promote products
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.\" derived from this software without specific prior written permission
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
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.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
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.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
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.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
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.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
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.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
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.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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.\"/
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.Dd $Mdocdate: November 21 2021 $
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.Dt SEMCTL 2
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.Os
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.Sh NAME
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.Nm semctl
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.Nd semaphore control operations
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.Sh SYNOPSIS
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.In sys/sem.h
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.Ft int
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.Fn semctl "int semid" "int semnum" "int cmd" "union semun arg"
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.Sh DESCRIPTION
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The
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.Fn semctl
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system call provides a number of control operations on the semaphore specified
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by
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.Fa semnum
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and
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.Fa semid .
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The operation to be performed is specified in
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.Fa cmd
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(see below).
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.Fa arg
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is a union of the following fields:
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.Bd -literal
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int val; /* value for SETVAL */
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struct semid_ds *buf; /* buffer for IPC_{STAT,SET} */
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u_short *array; /* array for GETALL & SETALL */
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.Ed
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.Pp
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The
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.Bf -literal
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semid_ds
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.Ef
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structure used in the
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.Dv IPC_SET
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and
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.Dv IPC_STAT
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commands is defined as follows in
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.In sys/sem.h :
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.Bd -literal
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struct semid_ds {
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struct ipc_perm sem_perm; /* operation permissions */
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struct sem *sem_base; /* semaphore set */
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u_short sem_nsems; /* number of sems in set */
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time_t sem_otime; /* last operation time */
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time_t sem_ctime; /* last change time */
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};
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.Ed
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.Pp
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The
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.Bf -literal
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ipc_perm
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.Ef
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structure used inside the
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.Bf -literal
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semid_ds
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.Ef
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structure is defined in
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.In sys/ipc.h
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and looks like this:
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.Bd -literal
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struct ipc_perm {
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uid_t cuid; /* creator user id */
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gid_t cgid; /* creator group id */
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uid_t uid; /* user id */
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gid_t gid; /* group id */
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mode_t mode; /* r/w permission (see chmod(2)) */
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u_short seq; /* sequence # */
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/* (to generate unique msg/sem/shm id) */
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key_t key; /* user specified msg/sem/shm key */
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};
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.Ed
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.Pp
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.Fn semctl
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provides the following operations:
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.Bl -tag -width IPC_RMIDX
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.It Dv GETVAL
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Return the value of the semaphore.
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.It Dv SETVAL
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Set the value of the semaphore to
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.Va arg.val .
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.It Dv GETPID
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Return the pid of the last process that did an operation on this semaphore.
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.It Dv GETNCNT
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Return the number of processes waiting to acquire the semaphore.
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.It Dv GETZCNT
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Return the number of processes waiting for the value of the semaphore to
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reach 0.
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.It Dv GETALL
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Return the values for all the semaphores associated with
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.Fa semid .
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.It Dv SETALL
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Set the values for all the semaphores that are associated with the semaphore
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identifier
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.Fa semid
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to the corresponding values in
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.Va arg.array .
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.It Dv IPC_STAT
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Gather statistics about a semaphore and place the information in the
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.Bf -literal
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semid_ds
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.Ef
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structure pointed to by
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.Fa arg.buf
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(see above).
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.It Dv IPC_SET
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Set the value of the
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.Va sem_perm.uid ,
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.Va sem_perm.gid
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and
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.Va sem_perm.mode
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fields in the structure associated with the semaphore.
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The values are taken from the corresponding fields in the structure
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pointed to by
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.Fa arg.buf .
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This operation can only be executed by the superuser, or a process that
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has an effective user ID equal to either
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.Va sem_perm.cuid
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or
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.Va sem_perm.uid
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in the data structure associated with the message queue.
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.It Dv IPC_RMID
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Remove the semaphores associated with
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.Fa semid
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from the system and destroy the data structures associated with it.
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Only the superuser or a process with an effective UID equal to the
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.Va sem_perm.cuid
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or
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.Va sem_perm.uid
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values in the data structure associated with the semaphore can do this.
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.El
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.Pp
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The permission to read or change a message queue (see
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.Xr semop 2 )
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is determined by the
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.Va sem_perm.mode
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field in the same way as is
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done with files (see
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.Xr chmod 2 ) ,
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but the effective UID can match either the
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.Va sem_perm.cuid
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field or the
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.Va sem_perm.uid
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field, and the
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effective GID can match either
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.Va sem_perm.cgid
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or
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.Va sem_perm.gid .
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.Sh RETURN VALUES
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For the
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.Dv GETVAL ,
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.Dv GETPID ,
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.Dv GETNCNT ,
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and
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.Dv GETZCNT
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operations,
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.Fn semctl
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returns one of the values described above if successful.
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All other operations will make
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.Fn semctl
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return 0 if no errors occur.
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Otherwise \-1 is returned and
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.Va errno
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set to reflect the error.
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.Sh ERRORS
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.Fn semctl
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will fail if:
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.Bl -tag -width Er
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.It Bq Er EPERM
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.Fa cmd
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is equal to
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.Dv IPC_SET
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or
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.Dv IPC_RMID
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and the caller is not the superuser, nor does
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the effective UID match either the
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.Va sem_perm.uid
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or
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.Va sem_perm.cuid
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fields of the data structure associated with the message queue.
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.It Bq Er EACCES
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The caller has no operation permission for this semaphore.
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.It Bq Er EINVAL
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.Fa semid
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is not a valid message semaphore identifier.
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.Pp
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.Va cmd
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is not a valid command.
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.It Bq Er EFAULT
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.Fa arg.buf
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or
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.Fa arg.array
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specify an invalid address.
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.It Bq Er ERANGE
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.Fa cmd
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is equal to
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.Dv SETVAL
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or
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.Dv SETALL
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and
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.Va arg.val
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or the values in
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.Va arg.array
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are greater than the system-imposed limit.
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.El
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.Sh SEE ALSO
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.Xr ipcrm 1 ,
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.Xr ipcs 1 ,
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.Xr semget 2 ,
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.Xr semop 2
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