Is clock-based timing reliable when CPU frequency is variable?

Question

A common way to measure elapsed time is:

const clock_t START = clock();
// ...
const clock_t END = clock();
double T_ELAPSED = (double)(END - START) / CLOCKS_PER_SEC;

I know this is not the best way to measure real time, but I wonder if it works on a system with a variable frequency CPU. Is it just wrong?

Answer 1

There are system architectures that change the frequency of the CPU but have a separate and constant frequency to drive a system clock. One would think that a clock() function would return a time independent of the CPU frequency but this would have to be verified on each system the code is intended to run on.

Answer 2

It is not good to use on a variable clock speed CPU.

http://support.ntp.org/bin/view/Support/KnownHardwareIssues

NTP (network time prototcol) daemon on linux had issues with it.

Most OS's have some API calls for more accurate values, an example being on windows, QueryPerformanceCounter

http://msdn.microsoft.com/en-us/library/ms644904%28VS.85%29.aspx