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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | VERSIONS | STANDARDS | HISTORY | EXAMPLES | SEE ALSO | COLOPHON |
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mq_getattr(3) Library Functions Manual mq_getattr(3)
mq_getattr, mq_setattr - get/set message queue attributes
Real-time library (librt, -lrt)
#include <mqueue.h>
int mq_getattr(mqd_t mqdes, struct mq_attr *attr);
int mq_setattr(mqd_t mqdes, const struct mq_attr *restrict newattr,
struct mq_attr *restrict oldattr);
mq_getattr() and mq_setattr() respectively retrieve and modify
attributes of the message queue referred to by the message queue
descriptor mqdes.
mq_getattr() returns an mq_attr structure in the buffer pointed by
attr. This structure is defined as:
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 */
};
The mq_flags field contains flags associated with the open message
queue description. This field is initialized when the queue is
created by mq_open(3). The only flag that can appear in this
field is O_NONBLOCK.
The mq_maxmsg and mq_msgsize fields are set when the message queue
is created by mq_open(3). The mq_maxmsg field is an upper limit
on the number of messages that may be placed on the queue using
mq_send(3). The 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 /proc files that place
ceilings on the values for these fields are described in
mq_overview(7).
The mq_curmsgs field returns the number of messages currently held
in the queue.
mq_setattr() sets message queue attributes using information
supplied in the mq_attr structure pointed to by newattr. The only
attribute that can be modified is the setting of the O_NONBLOCK
flag in mq_flags. The other fields in newattr are ignored. If
the oldattr field is not NULL, then the buffer that it points to
is used to return an mq_attr structure that contains the same
information that is returned by mq_getattr().
On success mq_getattr() and mq_setattr() return 0; on error, -1 is
returned, with errno set to indicate the error.
EBADF The message queue descriptor specified in mqdes is invalid.
EINVAL newattr->mq_flags contained set bits other than O_NONBLOCK.
For an explanation of the terms used in this section, see
attributes(7).
┌──────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├──────────────────────────────────────┼───────────────┼─────────┤
│ mq_getattr(), mq_setattr() │ Thread safety │ MT-Safe │
└──────────────────────────────────────┴───────────────┴─────────┘
On Linux, mq_getattr() and mq_setattr() are library functions
layered on top of the mq_getsetattr(2) system call.
POSIX.1-2008.
POSIX.1-2001.
The program below can be used to show the default mq_maxmsg and
mq_msgsize values that are assigned to a message queue that is
created with a call to mq_open(3) in which the attr argument is
NULL. Here is an example run of the program:
$ ./a.out /testq;
Maximum # of messages on queue: 10
Maximum message size: 8192
Since Linux 3.5, the following /proc files (described in
mq_overview(7)) can be used to control the defaults:
$ uname -sr;
Linux 3.8.0
$ cat /proc/sys/fs/mqueue/msg_default;
10
$ cat /proc/sys/fs/mqueue/msgsize_default;
8192
Program source
#include <fcntl.h>
#include <mqueue.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.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, 0600, 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);
}
mq_close(3), mq_notify(3), mq_open(3), mq_receive(3), mq_send(3),
mq_unlink(3), mq_overview(7)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-05-17 mq_getattr(3)
Pages that refer to this page: mq_getsetattr(2), mq_close(3), mq_notify(3), mq_open(3), mq_receive(3), mq_send(3), mq_unlink(3), systemd.socket(5), mq_overview(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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