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.TH MQ_GETATTR 3 2010-08-29 "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 "int mq_getattr(mqd_t " mqdes ", struct mq_attr *" attr );

.BI "int 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)