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Name

Library

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() 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()

Return Value

Errors

EBADF The message queue descriptor specified in mqdes is invalid.

EINVAL newattr->mq_flags contained set bits other than O_NONBLOCK .

Attributes

Versions

Standards

History

Examples

$ ./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); \e
                        } while (0)

int
main(int argc, char *argv[])
{
    mqd_t mqd;
    struct mq_attr attr;

    if (argc != 2) {
        fprintf(stderr, "Usage: %s mq-name\en", 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\en", attr.mq_maxmsg);
    printf("Maximum message size:             %ld\en", attr.mq_msgsize);

    if (mq_unlink(argv[1]) == -1)
        errExit("mq_unlink");

    exit(EXIT_SUCCESS);
}

See Also

1. mq_close(3),
2. mq_notify(3),
3. mq_open(3),
4. mq_receive(3),
5. mq_send(3),
6. mq_unlink(3),
7. mq_overview(7)