The usleep function suspends the current process from execution
    for the number of microseconds specified by the mseconds
    argument. This argument must be less than 1,000,000. However,
    if its value is 0, then the call has no effect.

    Be aware that usleep time specifications are rounded up
    approximately to the next millisecond because that is the finest
    time interval granularity possible on OpenVMS systems.

    There is one real-time interval timer for each process. The
    usleep function does not interfere with a previous setting of
    this timer. If the process set this timer before calling usleep
    and if the time specified by mseconds equals or exceeds the
    interval timer's prior setting, then the process is awakened
    shortly before the timer was set to expire.