why the clock() execution so time-consuming on EVM board

Hi, all

I encounter a really strange problem. I want to profile our algorithm on EVM C6424, and when I use the clock() function for the profiler of several functions, the execution takes really a lot of time. But when I just read the TSCH and TSCL cycle counter instead of clock() function calls, the execution seems reasonable, about one tenth of the time consumed with clock() profilers. So can anyone explain to me why the time consumption  differs so much?

By the way, I may call the clock() or read the TSCH, TSCL cycle counter 100 timers per frame, one frame corresponds to ~30ms playback.

Thanks for any reply.

BR

Touse