mq_notify not notifying about an event

[arunalapati]

Hi,

I'm using mq_notify to be notified about events on a message queue.
But my registered notifier function is not being called.
I'm pasting my code snippet below:

-----------------------------------------------------------------------

static void sigNotifier(union sigval sv)
{

printf ("I'm called.\n");

}


int main()
{
mqd_t queueID = 0;
message_t msg;
int retval;
struct mq_attr attr;
struct sigevent sev;

attr.mq_msgsize = MSG_SIZE;
attr.mq_maxmsg = 30;

errno = 0;

queueID = mq_open(MSG_QUEUE_NAME, O_RDONLY, 0666, &attr);

if (queueID == -1) {
printf ("Message queue open failed: %d\n", errno);
}

sev.sigev_notify = SIGEV_THREAD;
sev.sigev_notify_function = sigNotifier;
sev.sigev_notify_attributes = NULL;
sev.sigev_value.sival_ptr = &queueID;

retval = mq_notify(queueID, &sev);
if (retval < 0) {
printf ("Notification failed: %d\n", errno);
}

while (1);

}

Did I miss something, Please help me.

[Rubberman]

From the mq_notify manpage:

       SIGEV_THREAD
              Deliver  notification by invoking notification->sigev_thread_function as the start function of
              a new thread.  The function is invoked with notification->sigev_value as  its  sole  argument.
              If notification->sigev_notify_attributes is not NULL, then it should point to a pthread_attr_t
              structure that defines attributes for the thread.

       Only one process can be registered to receive notification from a message queue.

So, you are using SIGEV_THREAD as the notification method. That means your function is invoked in a new pthread. The mq_notify() method only registers the function. It wil not be invoked until a message is received in the queue. Where are you being sent a message?

Another problem is that you are passing the address of a function-local (automatic) variable, &queueID, in the sigev_value field. This is invalid since there is no way the thread-dispatched notification method can know if the address is still valid. You either should use a translation-unit static variable, a function-unit static variable, a global variable, or a heap-allocated pointer.

[arunalapati]

Thanks for your reply.
I was sending message from another process. mq_send() was a success there. I verified it by capturing the message using mq_receive().

and about the sigev_value field, you are right about not using the pointer from heap memory. But, I guess it has nothing to do with the problem I'm facing. I'm not using that value anyway.

[arunalapati]

man page says-
"Message notification only occurs when a new message arrives and the queue was previously empty"

I missed this point.

[Rubberman]

man page says-
"Message notification only occurs when a new message arrives and the queue was previously empty"

I missed this point.

Glad to hear it! Sometimes things just function as designed, not how we think they should!