Cost of flushing files

flush.cpp

#include <chrono>
#include <cstdio>
#include <iostream>

void write_file(std::FILE* fd)
{
  for (int i=0; i<1000000; i++)
  {
    std::fprintf(fd, "iteration no %d (last bit=%d)\n", i, i % 2);
  }
}

void write_file_and_flush(std::FILE* fd)
{
  for (int i=0; i<1000000; i++)
  {
    std::fprintf(fd, "iteration no %d (last bit=%d)\n", i, i % 2);
    std::fflush(fd);
  }
}

int main()
{
  auto f1 = std::fopen("test_1.txt", "a");
  auto f2 = std::fopen("test_2.txt", "a");

  auto start1 = std::chrono::system_clock::now();
  write_file(f1);
  auto end1 = std::chrono::system_clock::now();

  auto start2 = std::chrono::system_clock::now();
  write_file_and_flush(f2);
  auto end2 = std::chrono::system_clock::now();

  std::fclose(f1);
  std::fclose(f2);

  std::cout << "without flushing : " << (end1 - start1).count() << " ns\n";
  std::cout << "with flushing :    " << (end2 - start2).count() << " ns\n";
}

/*
results:
without flushing : 297908807 ns
with flushing :    945402154 ns
*/