The nanosleep function suspends a process or thread until one of
    the following conditions is met:

    o  The time interval specified by the rqtp argument has elapsed.

    o  A signal is delivered to the calling process and the action
       is to invoke a signal-catching function or to terminate the
       process.

    The suspension time may be longer than requested because the
    argument value is rounded up to an integer multiple of the sleep
    resolution or because of the scheduling of other activity by the
    system. Except when interrupted by a signal, the suspension time
    is not less than the time specified by the rqtp argument (as
    measured by the system clock, CLOCK_REALTIME).

    The use of the nanosleep function has no effect on the action or
    blockage of any signal.

    If the requested time has elapsed, the call was successful and
    the nanosleep function returns zero.

    On failure, the nanosleep function returns -1 and sets errno
    to indicate the failure. The function fails if it has been
    interrupted by a signal, or if the rqtp argument specified
    a nanosecond value less than 0 or greater than or equal to 1
    billion.

    If the rmtp argument is non-NULL, the timespec structure it
    references is updated to contain the amount of time remaining in
    the interval (the requested time minus the time actually slept).

    If the rmtp argument is NULL, the remaining time is not returned.

    See also clock_getres, clock_gettime, clock_settime, and sleep.