rtapi_clock_set_period - set the basic time interval for realtime tasks

nsec   The desired basic time interval for realtime tasks.

rtapi_clock_set_period  sets  the basic time interval for realtime tasks.  All periodic tasks will run at
       an integer multiple of this period.  The first call to rtapi_clock_set_period with nsec greater than zero
       will start the clock, using nsec as  the  clock  period  in  nano-seconds.   Due  to  hardware  and  RTOS
       limitations,  the  actual  period  may  not be exactly what was requested.  On success, the function will
       return the actual clock period if it is available, otherwise it returns the  requested  period.   If  the
       requested  period  is outside the limits imposed by the hardware or RTOS, it returns -EINVAL and does not
       start the clock.  Once the clock is started, subsequent calls with non-zero nsec return -EINVAL and  have
       no effect.  Calling rtapi_clock_set_period with nsec set to zero queries the clock, returning the current
       clock period, or zero if the clock has not yet been started.

       rtapi_clock_set_period - set the basic time interval for realtime tasks

       Call  only  from  within init/cleanup code, not from realtime tasks.  This function is not available from
       non-realtime code.

       The actual period provided by the RTOS, which may be different than the  requested  period,  or  a  RTAPI
       status code.

LinuxCNC Documentation                             2006-10-12                     rtapi_clock_set_period(3rtapi)

               rtapi_clock_set_period(long int nsec)