demonstrate that unlike clock() the chrono::steady_clock doesn't ignore sleep(), children time :c++11:chrono:clock:

test_chrono.cpp

// $ sudo apt-get install libboost-{system,chrono}1.48-dev
// $ g++ *.cpp -lboost_{system,chrono} && time ./a.out
#include <boost/chrono.hpp> // <chrono> in C++11
#include <iostream>
#include <string>

#include <stdint.h> // uintmax_t
#include <stdlib.h> // system()
#include <unistd.h> // sleep()

namespace {
  namespace chrono = boost::chrono;
  typedef chrono::steady_clock Clock; //

  class Timeit {
    std::string msg;
    Clock::time_point start;
  public:
    explicit Timeit(std::string msg_) : msg(msg_), start(Clock::now()) {}
    ~Timeit() {
      Clock::duration dur = Clock::now() - start;
      std::cout << "'" << msg << "' took us " << dur << std::endl;
      // to get float number of seconds:
      //      << std::setprecision(3)
      //      << chrono::duration_cast<chrono::duration<double> >(dur).count()
      // to get integer number of milliseconds:
      //      << chrono::duration_cast<chrono::milliseconds >(dur).count()
    }
  };
}

int main(int argc, char* argv[]) {
  { Timeit _("sleep");
    sleep(1);
  }

  { Timeit _("system sleep");
    system("sleep 1");
  }

  { Timeit _("loop");
    int n = (argc == 2) ? atoi(argv[argc-1]) : 500000000;
    uintmax_t s = 0;
    for (int i = 0; i < n; ++i)
      s += i;
    if (s != (uintmax_t)(n-1)*(n/2)) return 1;
  }
}

test_clock.c

// $ gcc -std=c99 *.c && time ./a.out
#include <stdint.h> // uintmax_t
#include <stdio.h>
#include <stdlib.h> // system()
#include <time.h>
#include <unistd.h> // sleep()

static double double_clock() {
  return clock() / (double) CLOCKS_PER_SEC;
}

int main(int argc, char* argv[]) {
  double start = double_clock();
  sleep(1);
  printf("after sleep %.2g\n", double_clock() - start);

  start = double_clock();
  system("sleep 1");
  printf("after sleep %.2g\n", double_clock() - start);

  start = double_clock();
  int n = (argc == 2) ? atoi(argv[argc-1]) : 500000000;
  uintmax_t s = 0;
  for (int i = 0; i < n; ++i)
    s += i;
  if (s != (uintmax_t)(n-1)*(n/2)) return 1;
  printf("after loop %.2g\n", double_clock() - start);

  return 0;
}