mirror of https://github.com/mkerrisk/man-pages
238 lines
5.5 KiB
Groff
238 lines
5.5 KiB
Groff
'\" t
|
|
.\" Copyright (C) 2006 Michael Kerrisk <mtk.manpages@gmail.com>
|
|
.\"
|
|
.\" %%%LICENSE_START(VERBATIM)
|
|
.\" 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.\" manual page may be incorrect or out-of-date.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.
|
|
.\" %%%LICENSE_END
|
|
.\"
|
|
.TH MQ_GETATTR 3 2014-10-02 "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 ", const 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_overview (7).
|
|
|
|
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 ATTRIBUTES
|
|
.SS Multithreading (see pthreads(7))
|
|
The
|
|
.BR mq_getattr ()
|
|
and
|
|
.BR mq_setattr ()
|
|
functions are thread-safe.
|
|
.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 EXAMPLE
|
|
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:
|
|
|
|
.in +4n
|
|
.nf
|
|
$ \fB./a.out /testq\fP
|
|
Maximum # of messages on queue: 10
|
|
Maximum message size: 8192
|
|
.fi
|
|
.in
|
|
|
|
Since Linux 3.5, the following
|
|
.I /proc
|
|
files (described in
|
|
.BR mq_overview (7))
|
|
can be used to control the defaults:
|
|
|
|
.in +4n
|
|
.nf
|
|
$ \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
|
|
.fi
|
|
.in
|
|
.SS Program source
|
|
\&
|
|
.nf
|
|
#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); \\
|
|
} while (0)
|
|
|
|
int
|
|
main(int argc, char *argv[])
|
|
{
|
|
mqd_t mqd;
|
|
struct mq_attr attr;
|
|
|
|
if (argc != 2) {
|
|
fprintf(stderr, "Usage: %s mq\-name\\n", 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\\n", attr.mq_maxmsg);
|
|
printf("Maximum message size: %ld\\n", attr.mq_msgsize);
|
|
|
|
if (mq_unlink(argv[1]) == \-1)
|
|
errExit("mq_unlink");
|
|
|
|
exit(EXIT_SUCCESS);
|
|
}
|
|
.fi
|
|
.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)
|