f4f16af1d3
While this behaviour is harmless, it is really just an artifact of the fact that the msgctl(2) implementation uses a user-visible structure as part of the internal implementation, so it is not deliberate and these pointers are not useful to userspace. Thus, NULL them out before copying out, and remove references to them from the manual page. Reported by: Jeffball <jeffball@grimm-co.com> Reviewed by: emaste, kib MFC after: 1 week Sponsored by: The FreeBSD Foundation Differential Revision: https://reviews.freebsd.org/D25600
209 lines
5.7 KiB
Groff
209 lines
5.7 KiB
Groff
.\" $NetBSD: msgctl.2,v 1.1 1995/10/16 23:49:15 jtc 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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.\" $FreeBSD$
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.\"/
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.Dd July 9, 2020
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.Dt MSGCTL 2
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.Os
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.Sh NAME
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.Nm msgctl
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.Nd message control operations
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In sys/types.h
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.In sys/ipc.h
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.In sys/msg.h
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.Ft int
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.Fn msgctl "int msqid" "int cmd" "struct msqid_ds *buf"
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.Sh DESCRIPTION
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The
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.Fn msgctl
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system call performs some control operations on the message queue specified
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by
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.Fa msqid .
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.Pp
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Each message queue has a data structure associated with it, parts of which
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may be altered by
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.Fn msgctl
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and parts of which determine the actions of
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.Fn msgctl .
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The data structure is defined in
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.In sys/msg.h
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and contains (amongst others) the following members:
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.Bd -literal
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struct msqid_ds {
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struct ipc_perm msg_perm; /* msg queue permission bits */
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msglen_t msg_cbytes; /* number of bytes in use on the queue */
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msgqnum_t msg_qnum; /* number of msgs in the queue */
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msglen_t msg_qbytes; /* max # of bytes on the queue */
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pid_t msg_lspid; /* pid of last msgsnd() */
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pid_t msg_lrpid; /* pid of last msgrcv() */
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time_t msg_stime; /* time of last msgsnd() */
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time_t msg_rtime; /* time of last msgrcv() */
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time_t msg_ctime; /* time of last msgctl() */
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};
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.Ed
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.Pp
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The
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.Vt ipc_perm
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structure used inside the
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.Vt msqid_ds
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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 */
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unsigned short seq; /* sequence # (to generate unique ipcid) */
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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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The operation to be performed by
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.Fn msgctl
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is specified in
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.Fa cmd
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and is one of:
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.Bl -tag -width IPC_RMIDX
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.It Dv IPC_STAT
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Gather information about the message queue and place it in the
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structure pointed to by
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.Fa buf .
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.It Dv IPC_SET
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Set the value of the
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.Va msg_perm.uid ,
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.Va msg_perm.gid ,
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.Va msg_perm.mode
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and
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.Va msg_qbytes
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fields in the structure associated with
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.Fa msqid .
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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 buf .
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This operation can only be executed by the super-user, or a process that
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has an effective user id equal to either
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.Va msg_perm.cuid
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or
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.Va msg_perm.uid
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in the data structure associated with the message queue.
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The value of
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.Va msg_qbytes
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can only be increased by the super-user.
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Values for
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.Va msg_qbytes
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that exceed the system limit (MSGMNB from
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.In sys/msg.h )
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are silently truncated to that limit.
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.It Dv IPC_RMID
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Remove the message queue specified by
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.Fa msqid
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and destroy the data associated with it.
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Only the super-user or a process
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with an effective uid equal to the
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.Va msg_perm.cuid
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or
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.Va msg_perm.uid
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values in the data structure associated with the queue can do this.
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.El
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.Pp
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The permission to read from or write to a message queue (see
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.Xr msgsnd 2
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and
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.Xr msgrcv 2 )
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is determined by the
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.Va msg_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 msg_perm.cuid
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field or the
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.Va msg_perm.uid
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field, and the
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effective gid can match either
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.Va msg_perm.cgid
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or
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.Va msg_perm.gid .
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.Sh RETURN VALUES
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.Rv -std msgctl
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.Sh ERRORS
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The
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.Fn msgctl
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function
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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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The
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.Fa cmd
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argument
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is equal to IPC_SET or IPC_RMID and the caller is not the super-user, nor does
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the effective uid match either the
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.Va msg_perm.uid
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or
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.Va msg_perm.cuid
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fields of the data structure associated with the message queue.
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.Pp
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An attempt is made to increase the value of
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.Va msg_qbytes
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through IPC_SET
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but the caller is not the super-user.
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.It Bq Er EACCES
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The command is IPC_STAT
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and the caller has no read permission for this message queue.
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.It Bq Er EINVAL
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The
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.Fa msqid
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argument
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is not a valid message queue 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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The
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.Fa buf
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argument
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specifies an invalid address.
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.El
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.Sh SEE ALSO
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.Xr msgget 2 ,
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.Xr msgrcv 2 ,
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.Xr msgsnd 2
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.Sh HISTORY
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Message queues appeared in the first release of
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.At V .
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