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.\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it)
.\"
.\" Permission is granted to make and distribute verbatim copies of this
.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\"
.\" Permission is granted to copy and distribute modified versions of this
.\" manual under the conditions for verbatim copying, provided that the
.\" entire resulting derived work is distributed under the terms of a
.\" permission notice identical to this one.
.\"
.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date. The author(s) assume no
.\" responsibility for errors or omissions, or for damages resulting from
.\" the use of the information contained herein. The author(s) may not
.\" have taken the same level of care in the production of this manual,
.\" which is licensed free of charge, as they might when working
.\" professionally.
.\"
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
.\"
.\" Modified Tue Oct 22 08:11:14 EDT 1996 by Eric S. Raymond <esr@thyrsus.com>
.\" Modified Sun Feb 18 01:59:29 2001 by Andries E. Brouwer <aeb@cwi.nl>
.\" Modified, 27 May 2004, Michael Kerrisk <mtk16@ext.canterbury.ac.nz>
.\" Added notes on CAP_IPC_OWNER requirement
.\" Modified, 17 Jun 2004, Michael Kerrisk <mtk16@ext.canterbury.ac.nz>
.\" Added notes on CAP_SYS_ADMIN requirement for IPC_SET and IPC_RMID
.\"
.TH MSGCTL 2 2004-06-17 "Linux 2.6.7" "Linux Programmer's Manual"
.SH NAME
msgctl \- message control operations
.SH SYNOPSIS
.nf
.B
#include <sys/types.h>
.B
#include <sys/ipc.h>
.B
#include <sys/msg.h>
.fi
.sp
.BI "int msgctl(int " msqid ,
.BI "int " cmd ,
.BI "struct msqid_ds *" buf );
.SH DESCRIPTION
This function performs the control operation specified by
.I cmd
on the message queue with identifier
.IR msqid .
Legal values for
.I cmd
are:
.TP
.B IPC_STAT
Copy info from the message queue data structure associated with
.I msqid
into the structure pointed to by
.IR buf .
The caller must have read permission on the message queue.
.TP
.B IPC_SET
Write the values of some members of the
.B msqid_ds
structure pointed to by
.I buf
to the message queue data structure, updating also its
.B msg_ctime
member.
The following members of the structure can be updated:
.nf
.sp
.ft B
msg_perm.uid
msg_perm.gid
msg_perm.mode \fR/* only lowest 9-bits */\fP
msg_qbytes
.fi
.ft R
.sp
The calling process must have appropriate (probably, root) privileges
or its effective user\-ID must be that of the creator
.RI ( msg_perm.cuid )
or owner
.RI ( msg_perm.uid )
of the message queue.
Appropriate privilege (Linux: the
.B CAP_IPC_RESOURCE
capability) is required to raise the
.B msg_qbytes
value beyond the system parameter
.BR MSGMNB .
.TP
.B IPC_RMID
Immediately remove the message queue and its associated data structure,
awakening all waiting reader and writer processes (with an error
return and
.B errno
set to
.BR EIDRM ).
The calling process must have appropriate (probably, root) privileges
or its effective user\-ID must be either that of the creator or owner
of the message queue.
.SH "RETURN VALUE"
On success, the return value will be
.BR 0 ,
otherwise
.B \-1
with
.B errno
indicating the error.
.SH ERRORS
On failure,
.B errno
is set to one of the following:
.TP 11
.B EACCES
The argument
.I cmd
is equal to
.B IPC_STAT
or
.BR MSG_STAT ,
but the calling process does not have read permission on the message queue
.IR msqid ,
and does not have the
.B CAP_IPC_OWNER
capability.
.TP
.B EFAULT
The argument
.I cmd
has the value
.B IPC_SET
or
.BR IPC_STAT ,
but the address pointed to by
.I buf
isn't accessible.
.TP
.B EIDRM
The message queue was removed.
.TP
.B EINVAL
Invalid value for
.I cmd
or
.IR msqid .
.TP
.B EPERM
The argument
.I cmd
has the value
.B IPC_SET
or
.BR IPC_RMID ,
but the effective user ID of the calling process is not the creator
(as found in
.IR msg_perm.cuid )
or the owner
(as found in
.IR msg_perm.uid )
of the message queue,
and the process is not privileged (Linux: it does not have the
.B CAP_SYS_ADMIN
capability).
.SH NOTES
Various fields in a \fIstruct msqid_ds\fP were shorts under Linux 2.2
and have become longs under Linux 2.4. To take advantage of this,
a recompilation under glibc-2.1.91 or later should suffice.
(The kernel distinguishes old and new calls by a IPC_64 flag in
.IR cmd .)
.SH "CONFORMING TO"
SVr4, SVID. SVID does not document the EIDRM error condition.
.SH "SEE ALSO"
.BR msgget (2),
.BR msgrcv (2),
.BR msgsnd (2),
.BR ipc (5),
.BR capabilities (7)
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