As we've been saying throughout this chapter, the timing resolution of everything in the system is going to be no more accurate than the base timing resolution coming into the system. So the obvious question is, how do you set the base timing resolution? You can use the ClockPeriod() function for this:
int
ClockPeriod (clockid_t id,
const struct _clockperiod *new,
struct _clockperiod *old,
int reserved);
As with the ClockAdjust() function described above, the new and the old parameters are how you get and/or set the values of the base timing resolution.
The new and old parameters are pointers to structures of struct _clockperiod, which contains two members, nsec and fract. Currently, the fract member must be set to zero (it's the number of femtoseconds; we probably won't use this kind of resolution for a little while yet!) The nsec member indicates how many nanoseconds elapse between ticks of the base timing clock. The default is 10 milliseconds (1 millisecond on machines with CPU speeds of greater than 40 MHz), so the nsec member (if you use the "get" form of the call by specifying the old parameter) will show approximately 10 million nanoseconds. (As we discussed above, in "Clock interrupt sources," it's not going to be exactly 10 millisecond.)
While you can certainly feel free to try to set the base timing resolution on your system to something ridiculously small, the kernel will step in and prevent you from doing that. Generally, you can set most systems in the 1 millisecond to hundreds of microseconds range.