Measuring performance

How do we measure erformance? What is wrong with this code to time a loop?

  clock_t start, finish;
  start = clock();
  for (int j = 0; j < i; j++) {
    a[j] = b[j]+b[j]*c[j];
  }
  finish = clock();
  double timeused = (double) (finish - start)/(CLOCKS_PER_SEC );