Time function calculation efficiency of C

#include<time.h>
clock_t t;
t=clock()-t;// Find the time difference  
printf(" when ：%lf second \n",((float)t)/CLOCKS_PER_SEC); // Convert to seconds  
// CLOCKS_PER_SEC  The number of milliseconds in a second  

log10(n)/log10(2) //log2(n)