无法以读取模式打开消息队列 (mq_open())答案

【问题标题】：Not able to open a message queue in read mode (mq_open())无法以读取模式打开消息队列 (mq_open())
【发布时间】：2015-05-09 12:40:47
【问题描述】：

我有一个程序可以创建一个消息队列并向该队列发送一条消息（带有mq_send()）。然后我尝试打开相同的消息队列以从另一个进程读取消息。但是mq_open() 返回 -1。

open_and_write_MQ.c

#include <stdio.h>
#include <mqueue.h>
#include <stdlib.h>
#include <string.h>

#define LEN 50


int main(int argc, char * argv[])
{

    struct mq_attr attr;
    mqd_t fd;
    char buff[LEN];
    attr.mq_flags = 0;
    attr.mq_maxmsg = 10;
    attr.mq_msgsize = LEN;
    attr.mq_curmsgs = 0;
    memset(buff,0,LEN);
    strcpy(buff,"This is just a test message");
    if(argc < 2)
    {
        printf("\nPlease enter at least one argument\n");
        exit(0);
    }   
    else
    {
        fd = mq_open(argv[1], O_CREAT | O_RDWR, 7777, &attr);  
        if(fd == -1)
        {
            printf("\nCould not create a message queue\n");
            exit(0);
        }
        else
        {
            if(mq_send(fd, buff, sizeof(buff), 1) == -1)
            {
                printf("\nCouldn't write message to the queue\n");
                mq_close(fd);
                mq_unlink(argv[1]);
                exit(0);
            } 
            else
            {
                printf("\n Message written to the queue sucussfully\n");
            }
        }
    }
    mq_close(fd);
    printf("\n waiting for other process to read the MQ\n");
    getchar();
    mq_unlink(argv[1]);
    exit(0);
}

这将创建 MQ。下面的程序尝试读取相同的 MQ。 open_and_read_MQ.c

#include <stdio.h>
#include <stdlib.h>
#include <mqueue.h>
#include <string.h>
#include <errno.h>

#define LEN 50

int main(int argc , char *argv[])
{
    mqd_t fd;
    char buff[LEN];
    int sterr;
    memset(buff,0,LEN);
    if(argc < 2)
    {
        printf("\nPlease enter the name of message queue\n");
        exit(0);
    }
    else
    {
        fd = mq_open(argv[1], O_RDONLY);
        if(fd == -1)
        {
            sterr = errno;
            printf("\nCouldn't open the message queue. Error : %s\n",strerror(sterr));
            exit(0);
        }
        else
        {
            if(mq_receive(fd, buff, sizeof(buff), NULL) == -1)
            {
                printf("\nMessage could not be received\n");
                mq_close(fd);
                exit(0);
            }   
            else
            {
                printf("Received Message : %s",buff);
            }
        }
    }
    exit(0);
}

编译步骤：

$ gcc open_and_read_MQ.c -lrt -o open_and_read_MQ
$ gcc open_and_write_MQ.c -lrt -o open_and_write_MQ

执行步骤：

$ ./open_and_write_MQ /new
Message written to the queue sucussfully

waiting for other process to read the MQ

然后在其他终端运行下面的程序。

$ ./open_and_read_MQ /new
Couldn't open the message queue. Error : Permission denied

如何设置进程的权限，以便它可以从消息队列中读取消息？

【问题讨论】：

标签： c linux gcc message-queue

【解决方案1】：

问题是线

fd = mq_open(argv[1], O_CREAT | O_RDWR, 7777, &attr);

7777 不是以八进制指定的数字。使用例如0777 代替（或open(2) 中描述的S_I* 常量）。不过，这些权限可能过于宽泛。

【讨论】：