---

static int clock_thread(void *arg)
{
    pj_timestamp now;
    pjmedia_clock *clock = (pjmedia_clock*) arg;

    /* Set thread priority to maximum unless not wanted. */
    if ((clock->options & PJMEDIA_CLOCK_NO_HIGHEST_PRIO) == 0) {
        int max = pj_thread_get_prio_max(pj_thread_this());
        if (max > 0)
            pj_thread_set_prio(pj_thread_this(), max);
    }
    /* Get the first tick */
    pj_get_timestamp(&clock->next_tick);
    clock->next_tick.u64 += clock->interval.u64;

printf("pjmedia_clock_size=%u pj_timestamp=%u\n",sizeof(pjmedia_clock),sizeof(now));
    while (!clock->quitting) {
        pj_get_timestamp(&now);

        /* Wait for the next tick to happen */
        if (now.u64 < clock->next_tick.u64) {
            unsigned msec;
            msec = pj_elapsed_msec(&now, &clock->next_tick);
            pj_thread_sleep(msec);
        }

        /* Skip if not running */
        if (!clock->running) {
            /* Calculate next tick */
            clock_calc_next_tick(clock, &now);
            continue;
        }

        pj_lock_acquire(clock->lock);

        /* Call callback, if any */
        if (clock->cb)
            (*clock->cb)(&clock->timestamp, clock->user_data);

        /* Best effort way to detect if we've been destroyed in the callback */
        if (clock->quitting) {
            pj_lock_release(clock->lock);
            break;
        }

        /* Increment timestamp */
        clock->timestamp.u64 += clock->timestamp_inc;

        /* Calculate next tick */
        clock_calc_next_tick(clock, &now);

        pj_lock_release(clock->lock);
    }

    return 0;
}

Если я правильно понимаю, тут реализация выпонения callbacks по clock