Create message queue with mqueue.h

I am trying to create a posix message queue using mq_open from <mqueue.h>.

MAX_MSG and MAX_MSG_LGTH are macros currently defined as 256 and name is never longer than MAX_MSG_LGTH.

This is the code I currently have:

mqd_t func(char* name)
{
    mqd_t queueId;
    char localbuffer[MAX_MSG_LGTH + 1];
    localbuffer[0] = '/';
    strncpy(&localbuffer[1], name, MAX_MSG_LGTH);
    localbuffer[MAX_MSG_LGTH] = '\0';

    struct mq_attr mqAttr = {0, MAX_MSG, MAX_MSG_LGTH, 0};

    queueId = mq_open(localbuffer, O_RDWR | O_CREAT, 007, &mqAttr);

    return queueId;
}

• input: "task1"
• output: -1
• errno: 22 (EINVAL)

I have checked localbuffer and the queue name is being stored correctly ("/task1").

Your MSG_MAX value is [may be] too large.

I ran your code and I reduced MSG_MAX and [eventually] it worked.

To see the system-wide values you can use, try:

head -1000 /proc/sys/fs/mqueue/*

On my system, I got:

==> /proc/sys/fs/mqueue/msg_default <==
10

==> /proc/sys/fs/mqueue/msg_max <==
10

==> /proc/sys/fs/mqueue/msgsize_default <==
8192

==> /proc/sys/fs/mqueue/msgsize_max <==
8192

==> /proc/sys/fs/mqueue/queues_max <==
256

Note that you can [as root] write to these files to change/increase the values.

Also, MSG_MAX_LGTH has nothing to do with the max size for the filename.

The max length for the filename is always 256.

Using 007 for a permission is a little screwy. It says that other users have full access, but your user has none.

I think you want 0700 or [better] 0600. Note that the leading 0 is needed to interpret the value as octal.