.\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it)
.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date.  The author(s) assume no
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.\" 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)