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 \nmqdes\n, struct mq_attr *\nattr\n);\n
\nint mq_setattr(mqd_t \nmqdes\n, const struct mq_attr *restrict \nnewattr\n,\n
\n               struct mq_attr *restrict \noldattr\n);

DESCRIPTION#

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 {
\n
    long mq_flags;       /* Flags: 0 or O_NONBLOCK */
\n
    long mq_maxmsg;      /* Max. # of messages on queue */
\n
    long mq_msgsize;     /* Max. message size (bytes) */
\n
    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().

RETURN VALUE#

On success mq_getattr() and mq_setattr() return 0; on error, -1 is returned, 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 attributes(7).

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_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:

$ \n./a.out /testq\n
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:

$ \nuname -sr\n
Linux 3.8.0
$ \ncat /proc/sys/fs/mqueue/msg_default\n
10
$ \ncat /proc/sys/fs/mqueue/msgsize_default\n
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); \
\n
                        } while (0)
int
main(int argc, char *argv[])
{
\n
    mqd_t mqd;
\n
    struct mq_attr attr;
\n
    if (argc != 2) {
\n
        fprintf(stderr, "Usage: %s mq-name\n", argv[0]);
\n
        exit(EXIT_FAILURE);
\n
    }
\n
    mqd = mq_open(argv[1], O_CREAT | O_EXCL, 0600, NULL);
\n
    if (mqd == (mqd_t) -1)
\n
        errExit("mq_open");
\n
    if (mq_getattr(mqd, &attr) == -1)
\n
        errExit("mq_getattr");
\n
    printf("Maximum # of messages on queue:   %ld\n", attr.mq_maxmsg);
\n
    printf("Maximum message size:             %ld\n", attr.mq_msgsize);
\n
    if (mq_unlink(argv[1]) == -1)
\n
        errExit("mq_unlink");
\n
    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)