Calculates elapsed cpu time. From: http://www.cs.utah.edu/dept/old/texinfo/glibc-manual-0.02/library_19.html

CPU time

#include <time.h>

clock_t start, end;
double elapsed;

start = clock();
... /* Do the work. */
end = clock();
elapsed = ((double) (end - start)) / CLOCKS_PER_SEC;