man3/mq_getattr.3

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.TH MQ_GETATTR 3 2021-03-22 "Linux" "Linux Programmer's Manual"
.SH NAME
mq_getattr, mq_setattr \- get/set message queue attributes
.SH SYNOPSIS
.nf
.B #include <mqueue.h>
.PP
.BI "int mq_getattr(mqd_t " mqdes ", struct mq_attr *" attr );
.BI "int mq_setattr(mqd_t " mqdes ", const struct mq_attr *restrict " newattr ,
.BI "               struct mq_attr *restrict " oldattr );
.fi
.PP
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 message queue descriptor
.IR mqdes .
.PP
.BR mq_getattr ()
returns an
.I mq_attr
structure in the buffer pointed by
.IR attr .
This structure is defined as:
.PP
.in +4n
.EX
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 */
};
.EE
.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 .
.PP
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_overview (7).
.PP
The
.I mq_curmsgs
field returns the number of messages currently held in the queue.
.PP
.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 message queue descriptor specified in
.I mqdes
is invalid.
.TP
.B EINVAL
.I newattr\->mq_flags
contained set bits other than
.BR O_NONBLOCK .
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.ad l
.nh
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.BR mq_getattr (),
.BR mq_setattr ()
T}	Thread safety	MT-Safe
.TE
.hy
.ad
.sp 1
.SH CONFORMING TO
POSIX.1-2001, POSIX.1-2008.
.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 EXAMPLES
The program below can be used to show the default
.I mq_maxmsg
and
.I mq_msgsize
values that are assigned to a message queue that is created with a call to
.BR mq_open (3)
in which the
.I attr
argument is NULL.
Here is an example run of the program:
.PP
.in +4n
.EX
$ \fB./a.out /testq\fP
Maximum # of messages on queue:   10
Maximum message size:             8192
.EE
.in
.PP
Since Linux 3.5, the following
.I /proc
files (described in
.BR mq_overview (7))
can be used to control the defaults:
.PP
.in +4n
.EX
$ \fBuname \-sr\fP
Linux 3.8.0
$ \fBcat /proc/sys/fs/mqueue/msg_default\fP
10
$ \fBcat /proc/sys/fs/mqueue/msgsize_default\fP
8192
.EE
.in
.SS Program source
\&
.EX
#include <mqueue.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

#define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \e
                        } while (0)

int
main(int argc, char *argv[])
{
    mqd_t mqd;
    struct mq_attr attr;

    if (argc != 2) {
        fprintf(stderr, "Usage: %s mq\-name\en", argv[0]);
        exit(EXIT_FAILURE);
    }

    mqd = mq_open(argv[1], O_CREAT | O_EXCL, S_IRUSR | S_IWUSR, NULL);
    if (mqd == (mqd_t) \-1)
        errExit("mq_open");

    if (mq_getattr(mqd, &attr) == \-1)
        errExit("mq_getattr");

    printf("Maximum # of messages on queue:   %ld\en", attr.mq_maxmsg);
    printf("Maximum message size:             %ld\en", attr.mq_msgsize);

    if (mq_unlink(argv[1]) == \-1)
        errExit("mq_unlink");

    exit(EXIT_SUCCESS);
}
.EE
.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)