Clock, rdtsc and CLOCKS_PER_SEC

Question

I am trying to implement my own version of clock() using asm and rdtsc. However I am quite unsure about its return value. Is it cycles? Oder is it micro seconds? I am also confused about CLOCKS_PER_SEC. How can this be constant?

Is there any kind of formula which sets these values into relation?

Answer 1

You can find a rdtsc reference implementation here:

https://github.com/LITMUS-RT/liblitmus/blob/master/arch/x86/include/asm/cycles.h

TSC counts the number of cycles since reset. If you need a time value unit in seconds, you also need to read the CPU clock frequency and divide TSC value by frequency. However, this may not be accurate if the CPU frequency scaling is enabled. Recent Intel processors include a constant rate TSC (identified by the "constant_tsc" flag in Linux's /proc/cpuinfo). With these processors, the TSC ticks at the processor's nominal frequency, regardless of the actual CPU clock frequency due to turbo or power saving states.

https://en.wikipedia.org/wiki/Time_Stamp_Counter