无法在读取模式下打开消息队列(mq_open())

Question

我有一个程序，它创建一个消息队列并向这个队列发送一条消息(用mq_send())。然后，我尝试打开相同的消息队列来读取来自另一个进程的消息。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);
}

汇编步骤：

$ 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？

Answer 1

问题是这条线

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 (或open(2)中描述的S_I*常量)。不过，这些权限可能过于宽泛。