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'\" t
.\" Hey Emacs! This file is -*- nroff -*- source.
.\"
.\" Copyright (C) 2006 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" 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.\" manual page may be incorrect or out-of-date.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.
.\"
.TH MQ_GETATTR 3 2008-09-29 "Linux" "Linux Programmer's Manual"
.SH NAME
mq_getattr, mq_setattr \- get/set message queue attributes
.SH SYNOPSIS
.nf
.B #include <mqueue.h>
.sp
.BI "mqd_t mq_getattr(mqd_t " mqdes ", struct mq_attr *" attr );
.BI "mqd_t mq_setattr(mqd_t " mqdes ", struct mq_attr *" newattr ","
.BI " struct mq_attr *" oldattr );
.fi
.sp
Link with \fI\-lrt\fP.
.SH DESCRIPTION
.BR mq_getattr ()
and
.BR mq_setattr ()
respectively retrieve and modify attributes of the message queue
referred to by the descriptor
.IR mqdes .
.BR mq_getattr ()
returns an
.I mq_attr
structure in the buffer pointed by
.IR attr .
This structure is defined as:
.in +4n
.nf
struct mq_attr {
long mq_flags; /* Flags: 0 or O_NONBLOCK */
long mq_maxmsg; /* Max. # of messages on queue */
long mq_msgsize; /* Max. message size (bytes) */
long mq_curmsgs; /* # of messages currently in queue */
};
.fi
.in
.PP
The
.I mq_flags
field contains flags associated with the open message queue description.
This field is initialized when the queue is created by
.BR mq_open (3).
The only flag that can appear in this field is
.BR O_NONBLOCK .
The
.I mq_maxmsg
and
.I mq_msgsize
fields are set when the message queue is created by
.BR mq_open (3).
The
.I mq_maxmsg
field is an upper limit on the number of messages
that may be placed on the queue using
.BR mq_send (3).
The
.I mq_msgsize
field is an upper limit on the size of messages
that may be placed on the queue.
Both of these fields must have a value greater than zero.
Two
.I /proc
files that place ceilings on the values for these fields are described in
.BR mq_open (3).
The
.I mq_curmsgs
field returns the number of messages currently held in the queue.
.BR mq_setattr ()
sets message queue attributes using information supplied in the
.I mq_attr
structure pointed to by
.IR newattr .
The only attribute that can be modified is the setting of the
.B O_NONBLOCK
flag in
.IR mq_flags .
The other fields in
.I newattr
are ignored.
If the
.I oldattr
field is not NULL,
then the buffer that it points to is used to return an
.I mq_attr
structure that contains the same information that is returned by
.BR mq_getattr ().
.SH RETURN VALUE
On success
.BR mq_getattr ()
and
.BR mq_setattr ()
return 0; on error, \-1 is returned, with
.I errno
set to indicate the error.
.SH ERRORS
.TP
.B EBADF
The descriptor specified in
.I mqdes
is invalid.
.TP
.B EINVAL
.I newattr\->mq_flags
contained set bits other than
.BR O_NONBLOCK .
.SH CONFORMING TO
POSIX.1-2001.
.SH NOTES
On Linux,
.BR mq_getattr ()
and
.BR mq_setattr ()
are library functions layered on top of the
.BR mq_getsetattr (2)
system call.
.SH "SEE ALSO"
.BR mq_close (3),
.BR mq_notify (3),
.BR mq_open (3),
.BR mq_receive (3),
.BR mq_send (3),
.BR mq_unlink (3),
.BR mq_overview (7)
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