fopen vs fstream, C++ (clang)

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#include <iostream>
#include <fstream>
#include <chrono>
#include <memory>
#include <stdio.h>
#ifdef __linux__
#include <unistd.h>
#endif
using namespace std;
using namespace std::chrono;
const unsigned long long sz = 512ULL * 1024ULL * 1024ULL;
int main(int argc, char**argv)
{
  unique_ptr<char> a(new char[sz]);
  unlink("file.binary");
  int64_t total = 0;
  if (argc < 2 || strcmp(argv[1], "fopen")) {
    cout << "fstream mode\n";
    fstream myfile;
    myfile = fstream("file.binary", ios::out | ios::binary);
    if (!myfile) {
      cerr << "open failed\n"; return 1;
    }
    for (int i = 0; i < 20; ++i) {
      auto tm1 = high_resolution_clock::now();
      myfile.write(a.get(), sz);
      auto tm = (duration_cast<milliseconds>(high_resolution_clock::now() - tm1).count());
      cout << tm << " ms\n";
      total += tm;
    }
    myfile.close();
  }
  else {
    cout << "fopen mode\n";
    FILE* pFile;
    pFile = fopen("file.binary", "wb");
    if (!pFile) {
      cerr << "open failed\n"; return 1;
    }
    auto tm1 = high_resolution_clock::now();
    for (int i = 0; i < 20; ++i) {
      auto tm1 = high_resolution_clock::now();
      fwrite(a.get(), 1, sz, pFile);
      auto tm = (duration_cast<milliseconds>(high_resolution_clock::now() - tm1).count());
      cout << tm << " ms\n";
      total += tm;
    }
    fclose(pFile);
    auto tm2 = high_resolution_clock::now();
  }
  cout << "Total: " << total << " ms\n";
}

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Compilation time: 0.72 sec, absolute running time: 0.13 sec, cpu time: 0 sec, memory peak: 3 Mb, absolute service time: 0,9 sec