Getting and setting the realtime clock and more

Apart from using timers, you can also get and set the current realtime clock, and adjust it gradually. The following functions can be used for these purposes:

Function	Type?	Description
ClockAdjust()	Neutrino	Gradually adjust the time
ClockCycles()	Neutrino	High-resolution snapshot
clock_getres()	POSIX	Fetch the base timing resolution
clock_gettime()	POSIX	Get the current time of day
ClockPeriod()	Neutrino	Get or set the base timing resolution
clock_settime()	POSIX	Set the current time of day
ClockTime()	Neutrino	Get or set the current time of day

Getting and setting
The functions clock_gettime() and clock_settime() are the POSIX functions based on the kernel function ClockTime(). These functions can be used to get or set the current time of day. Unfortunately, setting this is a "hard" adjustment, meaning that whatever time you specify in the buffer is immediately taken as the current time. This can have startling consequences, especially when time appears to move "backwards" because the time was ahead of the "real" time. Generally, setting a clock using this method should be done only during power up or when the time is very much out of synchronization with the real time.
Adjusting the timebase
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:
An accurate timestamp
There is one timebase that might be available on your processor that doesn't obey the rules of "base timing resolution" we just described. Some processors have a high-frequency (high-accuracy) counter built right into them, which Neutrino can let you have access to via the ClockCycles() call. For example, on a Pentium processor running at 200 MHz, this counter increments at 200 MHz as well, so it can give you timing samples right down to 5 nanoseconds. This is particularly useful if you want to figure out exactly how long a piece of code takes to execute (assuming of course, that you don't get preempted). You'd call ClockCycles() before your code and after your code, and then compute the delta. See the QNX Neutrino C Library Reference for more details.