无法设置POSIX消息队列属性 [英] cannot set POSIX message queue attribute

问题描述

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. 我只得到默认属性值.

I am trying to set attribute for POSIX message queue, but the code does not change the attribute. I only get default attribute values.

您能指出我的代码出了什么问题吗?

Could you point out what the problem is with my code?

我以用户(不是root)的身份执行a.out.

I execute the a.out as an user (not root).

#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.

I found that following code works. However, when I try to set attr.mq_maxmsg (=60), the mq_open fails.

#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;
}

推荐答案

mq_open()返回一个已经存在的队列，其属性是在创建队列时设置的.因此，O_CREAT标志无效，并且您指定的属性也将被忽略.

mq_open() is returning an already existing queue whose attributes were set at the time it was created. The O_CREAT flag thus has no effect, and the attributes you specify are ignored as well.

在打开之前调用mq_unlink()，也许还设置O_EXCL，看看有什么区别.

Call mq_unlink() before your open, and perhaps set O_EXCL as well, and see the difference.

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