Ardhan Madras wrote:
> I'm using POSIX message queue to exchange data as follow:
>
> struct data {
> unsigned char cmd;
> long value;
> };
>
> Creating a queue in program B:
>
> struct mq_attr attr = { .mq_maxmsg = 1, .mq_msgsize = sizeof(struct data) };
> mqd_t mqd = open("/somename", O_RDWR | O_CREAT, 0777, &attr);
>
> Open that queue in program A:
>
> mqd_t mqd = open("/somename", O_RDWR); /* O_RDONLY should works here */
I presume that you mean mq_open() rather than open(), right?
> With given queue, i can't open this queue in program A with O_RDWR or
> O_WRONLY flag (O_RDONLY should works), no matter how i set the flag
> mode in program B's mq_open() i got EACCESS, i need both program can
> send and receive queues.
>
> Did i miss something here?
The requested permissions have the process' umask subtracted from
them. Unless you've changed it, the umask probably removes at least
world-write permission, and possibly others. Either clear the umask
before calling mq_open(), or set the permissions afterwards with
fchmod() (although I don't know whether the latter is portable).
Also, if the queue already exists, mq_open(O_CREAT) will open the
existing queue; it won't replace it or change the permissions. Add
O_EXCL if you want it to fail if a queue with the same name already
exists.
--
Glynn Clements <glynn@xxxxxxxxxxxxxxxxxx>
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