无法设置POSIX消息队列属性

我的环境：

• CentOS 6.5（64位内核）
• gcc 4.4.7 20120313

我正在尝试为POSIX消息队列设置属性，但是代码不会更改属性。我仅获得默认属性值。

您能否指出我的代码出了什么问题？

我以用户（不是root）的身份执行a.out。

#include <stdio.h>
#include <mqueue.h> // for message queue
#include <sys/stat.h>
#include <stdlib.h> // for EXIT_FAILURE
#include <string.h>

/*
gcc [file] -lrt
*/

static void showAttr(mqd_t mqd)
{
    struct mq_attr attr;

    mq_getattr(mqd, &attr);

    printf("maxmsg = %d\n", attr.mq_maxmsg);
    printf("msgsize = %d\n", attr.mq_msgsize);
    printf("curmsgs = %d\n", attr.mq_curmsgs);

}

int main()
{
    mqd_t mqd;
    int flags;
    int ret;
    struct mq_attr attr;

    flags = O_RDWR | O_CREAT;

    attr.mq_flags = 0; // or O_NONBLOCK
    attr.mq_maxmsg = 60;
    attr.mq_msgsize = 120;
    attr.mq_curmsgs = 0;

    // POSIX IPC name should start with "/"
    mqd = mq_open("/mq", flags,
//      (S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH),
        0644,
        &attr );

    if (mqd < 0) {
        printf("open failed\n");
        exit(EXIT_FAILURE);
    }
    printf("open ok\n");

    sleep(1);

    showAttr(mqd);

    ret = mq_close(mqd);
    if (ret != 0) {
        printf("open failed\n");
        exit(EXIT_FAILURE);     
    }
    printf("close ok\n");

    return 0;
}

我发现以下代码有效。但是，当我尝试设置attr.mq_maxmsg（= 60）时，mq_open失败。

#include <stdio.h>
#include <mqueue.h> // for message queue
#include <sys/stat.h>
#include <stdlib.h> // for EXIT_FAILURE
#include <string.h>
#include <errno.h>

/*
gcc [file] -lrt
*/

static void showAttr(mqd_t mqd)
{
    struct mq_attr attr;

    mq_getattr(mqd, &attr);

    printf("maxmsg = %d\n", attr.mq_maxmsg);
    printf("msgsize = %d\n", attr.mq_msgsize);
    printf("curmsgs = %d\n", attr.mq_curmsgs);

}

int main()
{
    mqd_t mqd;
    int flags;
    int ret;
    struct mq_attr attr;

    flags = O_RDWR | O_CREAT;

    // POSIX IPC name should start with "/"

    // 1. once open without attribute setting
    mqd = mq_open("/mq", flags,
        (S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH) );
    mq_getattr(mqd, &attr);
    mq_unlink("/mq");
    mq_close(mqd);

    // 2. set values of attribute
    // attr.mq_maxmsg = 10;
    attr.mq_msgsize = 120;

    // 3. allocate attribute
    mqd = mq_open("/mq", flags,
        (S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH),
    //   0644,
        &attr );

    if (mqd < 0) {
        printf("open failed %d\n", mqd);
        exit(EXIT_FAILURE);
    }
    printf("open ok\n");

    sleep(1);

    showAttr(mqd);


    ret = mq_close(mqd);
    if (ret != 0) {
        printf("open failed\n");
        exit(EXIT_FAILURE);     
    }
    printf("close ok\n");

    return 0;
}