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.\" $OpenBSD: msgctl.2,v 1.18 2019/07/18 13:32:40 schwarze Exp $
.\" $NetBSD: msgctl.2,v 1.2 1997/03/27 08:20:35 mikel Exp $
.\"
.\" Copyright (c) 1995 Frank van der Linden
.\" All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\" notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\" notice, this list of conditions and the following disclaimer in the
.\" documentation and/or other materials provided with the distribution.
.\" 3. All advertising materials mentioning features or use of this software
.\" must display the following acknowledgement:
.\" This product includes software developed for the NetBSD Project
.\" by Frank van der Linden
.\" 4. The name of the author may not be used to endorse or promote products
.\" derived from this software without specific prior written permission
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
.\"/
.Dd $Mdocdate: July 18 2019 $
.Dt MSGCTL 2
.Os
.Sh NAME
.Nm msgctl
.Nd message control operations
.Sh SYNOPSIS
.In sys/msg.h
.Ft int
.Fn msgctl "int msqid" "int cmd" "struct msqid_ds *buf"
.Sh DESCRIPTION
The
.Fn msgctl
system call performs some control operations on the message queue specified
by
.Fa msqid .
.Pp
Each message queue has a data structure associated with it, parts of which
may be altered by
.Fn msgctl
and parts of which determine the actions of
.Fn msgctl .
The data structure is defined in
.In sys/msg.h
and contains (amongst others) the following members:
.Bd -literal
struct msqid_ds {
struct ipc_perm msg_perm; /* msg queue permission bits */
u_long msg_cbytes; /* # of bytes in use on the queue */
u_long msg_qnum; /* # of msgs in the queue */
u_long msg_qbytes; /* max # of bytes on the queue */
pid_t msg_lspid; /* pid of last msgsnd() */
pid_t msg_lrpid; /* pid of last msgrcv() */
time_t msg_stime; /* time of last msgsnd() */
time_t msg_rtime; /* time of last msgrcv() */
time_t msg_ctime; /* time of last msgctl() */
};
.Ed
.Pp
The
.Vt ipc_perm
structure used inside the
.Vt msqid_ds
structure is defined in
.In sys/ipc.h
and looks like this:
.Bd -literal
struct ipc_perm {
uid_t cuid; /* creator user id */
gid_t cgid; /* creator group id */
uid_t uid; /* user id */
gid_t gid; /* group id */
mode_t mode; /* permission (9 bits, see chmod(2)) */
u_short seq; /* sequence # (to generate unique id) */
key_t key; /* user specified msg/sem/shm key */
};
.Ed
.Pp
The operation to be performed by
.Fn msgctl
is specified in
.Fa cmd
and is one of:
.Bl -tag -width IPC_RMIDX
.It Dv IPC_STAT
Gather information about the message queue and place it in the
structure pointed to by
.Fa buf .
.It Dv IPC_SET
Set the value of the
.Fa msg_perm.uid ,
.Fa msg_perm.gid ,
.Fa msg_perm.mode
and
.Fa msg_qbytes
fields in the structure associated with
.Fa msqid .
The values are taken from the corresponding fields in the structure
pointed to by
.Fa buf .
This operation can only be executed by the superuser, or a process that
has an effective user ID equal to either
.Fa msg_perm.cuid
or
.Fa msg_perm.uid
in the data structure associated with the message queue.
The value of
.Fa msg_qbytes
can only be increased by the superuser.
Values for
.Fa msg_qbytes
that exceed the system limit
.Pf ( Dv MSGMNB
from
.In sys/msg.h )
are silently truncated to that limit.
.It Dv IPC_RMID
Remove the message queue specified by
.Fa msqid
and destroy the data associated with it.
Only the superuser or a process with an effective UID equal to the
.Fa msg_perm.cuid
or
.Fa msg_perm.uid
values in the data structure associated with the queue can do this.
.El
.Pp
The permission to read from or write to a message queue (see
.Xr msgsnd 2
and
.Xr msgrcv 2 )
is determined by the
.Fa msg_perm.mode
field in the same way as is
done with files (see
.Xr chmod 2 ) ,
but the effective UID can match either the
.Fa msg_perm.cuid
field or the
.Fa msg_perm.uid
field, and the
effective GID can match either
.Fa msg_perm.cgid
or
.Fa msg_perm.gid .
.Sh RETURN VALUES
.Rv -std
.Sh ERRORS
.Fn msgctl
will fail if:
.Bl -tag -width Er
.It Bq Er EPERM
.Fa cmd
is equal to
.Dv IPC_SET
or
.Dv IPC_RMID
and the caller is not the superuser, nor does
the effective UID match either the
.Fa msg_perm.uid
or
.Fa msg_perm.cuid
fields of the data structure associated with the message queue.
.Pp
An attempt is made to increase the value of
.Fa msg_qbytes
through
.Dv IPC_SET
but the caller is not the superuser.
.It Bq Er EACCES
The command is
.Dv IPC_STAT
and the caller has no read permission for this message queue.
.It Bq Er EINVAL
.Fa msqid
is not a valid message queue identifier.
.Pp
.Fa cmd
is not a valid command.
.It Bq Er EFAULT
.Fa buf
specifies an invalid address.
.El
.Sh SEE ALSO
.Xr msgget 2 ,
.Xr msgrcv 2 ,
.Xr msgsnd 2
.Sh STANDARDS
The
.Fn msgctl
function conforms to the X/Open System Interfaces option of
.St -p1003.1-2008 .
.Sh HISTORY
Message queues first appeared in
.At V.1
and have been available since
.Nx 1.0 .
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