C++ Timers

The following code illustrates the usage of various APIs to time execution in C++ code. Some of these are applicable in C as well.

>
#include <chrono>
#include <cstdint>
#include <math.h>  /* sqrt */
#include <stdio.h> /* printf */
#include <sys/time.h>
#include <time.h> /* clock_t, clock, CLOCKS_PER_SEC */

using namespace std::chrono;

// THe function to be timed.
int frequency_of_primes(int n) {
  int i, j;
  int freq = n - 1;
  for (i = 2; i <= n; ++i)
    for (j = sqrt(i); j > 1; --j)
      if (i % j == 0) {
        --freq;
        break;
      }
  return freq;
}

int main() {
  clock_t start1, end1;
  time_t start2, end2;
  timeval start3, end3;
  std::chrono::time_point<std::chrono::system_clock> start4, end4;
  std::chrono::high_resolution_clock::time_point start5, end5;

  /* Collect the start times using different methods. */
  // Method 1
  start1 = clock();
  // Method 2
  time(&start2);
  // Method 3
  gettimeofday(&start3, NULL);
  // Method 4
  start4 = std::chrono::system_clock::now();
  // Method 5
  start5 = std::chrono::high_resolution_clock::now();

  // The computaton to be timed.
  printf("Calculating...\n");
  int f = frequency_of_primes(999999);
  printf("The number of primes lower than 100,000 is: %d\n\n", f);

  /* Collect the end times using different methods. */
  // Method 1
  end1 = clock();
  // Method 2
  time(&end2);
  // Method 3
  gettimeofday(&end3, NULL);
  // Method 4
  end4 = std::chrono::system_clock::now();
  // Method 5
  end5 = std::chrono::high_resolution_clock::now();

  /* Compute elapsed times using different methods. */
  // Method 1
  long int elapsed_seconds1 = end1 - start1;
  // Method 2
  double elapsed_seconds2 = difftime(end2, start2);
  // Method 3
  double elapsed_seconds3 = (double(end3.tv_sec - start3.tv_sec)) * 1000000.00 +
                            double(end3.tv_usec - start3.tv_usec);
  // Method 4
  std::chrono::duration<double> elapsed_seconds4 = end4 - start4;
  // Method 5
  std::chrono::duration<double> elapsed_seconds5 = (end5 - start5);
  double elapsed_seconds6 =
      double(duration_cast<nanoseconds>(end5 - start5).count());

  /* Display the elapsed times */
  printf("Clock: %ld clicks (%f us).\n", elapsed_seconds1,
         (double(elapsed_seconds1)) * 1000000.00 / CLOCKS_PER_SEC);
  printf("Time: %f us).\n", elapsed_seconds2 * 1000000.00);
  printf("gettimeofday: %f us).\n", elapsed_seconds3);
  printf("chrono::system_clock %f us).\n",
         elapsed_seconds4.count() * 1000000.00);
  printf("chrono::high_resolution_clock %f us).\n",
         elapsed_seconds5.count() * 1000000.00);
  printf("chrono::high_resolution_clock %f us).\n", elapsed_seconds6);

  return 0;
}