MQ_GETATTR(3) Linux Programmer's Manual MQ_GETATTR(3)

mq_getattr, mq_setattr - get/set message queue attributes

#include <mqueue.h>
int mq_getattr(mqd_t mqdes, struct mq_attr *attr);
int mq_setattr(mqd_t mqdes, const struct mq_attr *newattr,
                 struct mq_attr *oldattr);
Link with -lrt.

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.

The message queue descriptor specified in mqdes is invalid.
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

POSIX.1-2001, POSIX.1-2008.

On Linux, mq_getattr() and mq_setattr() are library functions layered on top of the mq_getsetattr(2) system call.

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
$  cat /proc/sys/fs/mqueue/msgsize_default

#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");

mq_close(3), mq_notify(3), mq_open(3), mq_receive(3), mq_send(3), mq_unlink(3), mq_overview(7)
2019-03-06 Linux