theoretical limit on message queue sizes

[aryan_]

Hi All,

Following function obtains the system-maintained structure for a message queue:

bool getMessageQueueStats(int mqId, struct msqid_ds* buf)
{
    if(msgctl(mqId, IPC_STAT, buf) == -1)
    {
        return false;
    }

    return true;
}

And following function simply resizes a message queue to a user-defined value, namely, "size":

bool resizeMessageQueue(int mqId, struct msqid_ds* buf, int size)
{
    /* Modify the message queue features */
    buf->msg_perm.uid = geteuid();
    buf->msg_perm.gid = getegid();
    buf->msg_perm.mode = 0660;
    buf->msg_qbytes = size; // The new size for the message queue

    if(msgctl(mqId, IPC_SET, buf) == -1)
    {
        return false;
    }

    return true;
}

My question is if there is a theoretical limit on user-defined message queue sizes.

To put in other words, is it possible to increase the default message queue size to any user-defined value as long as that certain value does not exceed the size of system's physical memory ?

By the way, I use 64-bit Ubuntu 9.04.

Thanks.

[Rubberman]

From the msgctl man page:

       IPC_SET
              Write  the  values of some members of the msqid_ds structure pointed to by
              buf to the kernel data  structure  associated  with  this  message  queue,
              updating  also  its msg_ctime member.  The following members of the struc-
              ture are updated: msg_qbytes, msg_perm.uid, msg_perm.gid, and  (the  least
              significant  9  bits  of) msg_perm.mode.  The effective UID of the calling
              process must match the owner (msg_perm.uid) or creator (msg_perm.cuid)  of
              the  message  queue, or the caller must be privileged.  Appropriate privi-
              lege (Linux: the CAP_IPC_RESOURCE capability) is  required  to  raise  the
              msg_qbytes value beyond the system parameter MSGMNB.

Note the last couple of lines. In the header files there is a defined value (probably a #define, but can be a const variable) MSGMNB which is the largest size a user application can set msg_qbytes, unless the application has the CAP_IPC_RESOURCE capability enabled.