I'm not able to see why downloading is sometimes fails in asio asyncread().
And I appriciate any suggestion.

When 'comment===>> //' and 'insert ===>>' lines are valid, download file size is
sometimes short than the original size from internet server(with big files).
But if I put 'comment===>> //' back to no comment and remove 'insert ===>>' lines,
the downloading result is perfect.

class ftp_client_dtp {
...
void start()
{
  work_resolver_.reset(new boost::asio::ip::tcp::resolver(work_io_service_));
  work_socket_.reset(new boost::asio::ip::tcp::socket(work_io_service_));
  work_.reset(new boost::asio::io_service::work(work_io_service_));
  work_thread_.reset(new boost::thread(
   boost::bind(&boost::asio::io_service::run, &work_io_service_)));
  ...
comment===>> // std::string spath = scurrentpath_ + "\\" + sfilename_;
comment===>> // f.open(spath.c_str(), std::ios::binary | std::ios_base::out);
  ...
}
...
void handle_read_content(const boost::system::error_code& err)
{
  if (!err) {
insert ===>>  std::string spath = scurrentpath_ + "\\" + sfilename_;
insert ===>>  f.open(spath.c_str(), std::ios::binary | std::ios_base::out | std::ios_base::app);
   if (response_.size() > 0) {
    f << &response_;
   }
insert ===>>  f.close();

   // Continue reading remaining data until EOF.
   boost::asio::async_read(*work_socket_, response_,
     boost::asio::transfer_at_least(1),
     boost::bind(&ftp_client_dtp::handle_read_content, shared_from_this(),
     boost::asio::placeholders::error));
  } else if (err == boost::asio::error::eof) {
   // eof
   std::cout << err;
insert ===>>  std::string spath = scurrentpath_ + "\\" + sfilename_;
insert ===>>  f.open(spath.c_str(), std::ios::binary | std::ios_base::out | std::ios_base::app);
   if (response_.size() > 0) {
    f << &response_;
   }
insert ===>>  f.close();

   do_close();
  } else {
   std::cout << err;
   do_close();
  }
}
void do_close()
{
comment===>> // f.close();
  ...
}
std::ofstream f;
std::string sip_, sport_, scurrentpath_, sfilename_;
...
boost::asio::streambuf response_;
...
};