c - mq_notify not notifying about an event (Linux programming) -

i'm using mq_notify notified events on message queue, registered notifier function not being called. did miss something?

i'm pasting code snippet below:

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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);     } 

because code copies man page ask:

1. are sure sending message queue correctly?
2. try first read using blocking read , see if something.

under linux can use select/poll/epoll wait notifications queue mqd_t ordinary file descriptor.