std::fstream mFile;
---
mFile.open(filename, std::ios::in | std::ios::out | std::ios::binary | std::ios::app);
---
  mFile.seekp(0, mFile.beg);
  std::cout << std::to_string(mFile.tellg()) + " " + std::to_string(mFile.tellp()) + "\n";
  mFile.write(bin.dataAsConstChar(), bin.size());
  std::cout << std::to_string(mFile.tellg()) + " " + std::to_string(mFile.tellp()) + "\n";

output:
0 0
3522791 3522791