Record execution time in nios ii

Hi,

I have a custom component in my qsys project and I want to record the processing time in the component by using 'clock()' in the nios C code.
The problem is whenever I add these lines:

'#include<time.h>',
'begin = clock()',
'end = clock()',
'time = (end-begin) / CLOCKS_PER_SEC'

to the code, the ELF downloading will fail.
Any suggestion why this happens and any alternative way I can record the execution time for my component?

Thanks!