I've got something like this:

clock_t start, end;
start=clock();

something_else();

end=clock();
printf("\nClock cycles are: %d - %d\n",start,end);

and I always get as an output "Clock cycles are: 0 - 0"

Any idea why this happens?

(Just to give little detail, the something_else() function performs a left-to-right exponentiation using montgomery represantation, moreover I don't know for certain that the something_else() function does indeed take some not negligible time.)

This is on Linux. The result of uname -a is:

Linux snowy.*****.ac.uk 2.6.32-71.el6.x86_64 #1 SMP Fri May 20 03:51:51 BST 2011 x86_64 x86_64 x86_64 GNU/Linux