mq_getattr(3) Library Functions Manual mq_getattr(3)
NAME
mq_getattr, mq_setattr - get/set message queue attributes
LIBRARY
Real-time library (librt, -lrt)
SYNOPSIS
#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);
DESCRIPTION
mq_getattr() and mq_setattr() respectively retrieve and modify attrib-
utes 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_NON-
BLOCK.
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().
RETURN VALUE
On success mq_getattr() and mq_setattr() return 0; on error, -1 is re-
turned, with errno set to indicate the error.
ERRORS
EBADF The message queue descriptor specified in mqdes is invalid.
EINVAL newattr->mq_flags contained set bits other than O_NONBLOCK.
ATTRIBUTES
For an explanation of the terms used in this section, see attrib-
utes(7).
┌───────────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├───────────────────────────────────────────┼───────────────┼─────────┤
│ mq_getattr(), mq_setattr() │ Thread safety │ MT-Safe │
└───────────────────────────────────────────┴───────────────┴─────────┘
VERSIONS
On Linux, mq_getattr() and mq_setattr() are library functions layered
on top of the mq_getsetattr(2) system call.
STANDARDS
POSIX.1-2008.
HISTORY
POSIX.1-2001.
EXAMPLES
The program below can be used to show the default mq_maxmsg and mq_msg-
size 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 ex-
ample 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);
}
SEE ALSO
mq_close(3), mq_notify(3), mq_open(3), mq_receive(3), mq_send(3),
mq_unlink(3), mq_overview(7)
Linux man-pages 6.7 2023-10-31 mq_getattr(3)
Generated by dwww version 1.16 on Tue Dec 16 11:09:34 CET 2025.