Boost C++ Libraries of the most highly regarded and expertly designed C++ library projects in the world. Herb Sutter and Andrei Alexandrescu, C++ Coding Standards


// blocking_token_tcp_client.cpp
// ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
// Copyright (c) 2003-2022 Christopher M. Kohlhoff (chris at kohlhoff dot com)
// Distributed under the Boost Software License, Version 1.0. (See accompanying
// file LICENSE_1_0.txt or copy at

#include <boost/asio/connect.hpp>
#include <boost/asio/io_context.hpp>
#include <boost/asio/ip/tcp.hpp>
#include <boost/asio/read_until.hpp>
#include <boost/asio/streambuf.hpp>
#include <boost/system/system_error.hpp>
#include <boost/asio/write.hpp>
#include <cstdlib>
#include <iostream>
#include <memory>
#include <string>

using boost::asio::ip::tcp;

// NOTE: This example uses the new form of the boost::asio::async_result trait.
// For an example that works with the Networking TS style of completion tokens,
// please see an older version of asio.

// We will use our sockets only with an io_context.
using tcp_socket = boost::asio::basic_stream_socket<
    tcp, boost::asio::io_context::executor_type>;


// A custom completion token that makes asynchronous operations behave as
// though they are blocking calls with a timeout.
struct close_after
  close_after(std::chrono::steady_clock::duration t, tcp_socket& s)
    : timeout_(t), socket_(s)

  // The maximum time to wait for an asynchronous operation to complete.
  std::chrono::steady_clock::duration timeout_;

  // The socket to be closed if the operation does not complete in time.
  tcp_socket& socket_;

namespace boost {
namespace asio {

// The async_result template is specialised to allow the close_after token to
// be used with asynchronous operations that have a completion signature of
// void(error_code, T). Generalising this for all completion signature forms is
// left as an exercise for the reader.
template <typename T>
class async_result<close_after, void(boost::system::error_code, T)>
  // The initiate() function is used to launch the asynchronous operation by
  // calling the operation's initiation function object. For the close_after
  // completion token, we use this function to run the io_context until the
  // operation is complete.
  template <typename Init, typename... Args>
  static T initiate(Init init, close_after token, Args&&... args)
    boost::asio::io_context& io_context = boost::asio::query(
        token.socket_.get_executor(), boost::asio::execution::context);

    // Call the operation's initiation function object to start the operation.
    // A lambda is supplied as the completion handler, to be called when the
    // operation completes.
    boost::system::error_code error;
    T result;
    init([&](boost::system::error_code e, T t)
          error = e;
          result = t;
        }, std::forward<Args>(args)...);

    // Restart the io_context, as it may have been left in the "stopped" state
    // by a previous operation.

    // Block until the asynchronous operation has completed, or timed out. If
    // the pending asynchronous operation is a composed operation, the deadline
    // applies to the entire operation, rather than individual operations on
    // the socket.

    // If the asynchronous operation completed successfully then the io_context
    // would have been stopped due to running out of work. If it was not
    // stopped, then the io_context::run_for call must have timed out and the
    // operation is still incomplete.
    if (!io_context.stopped())
      // Close the socket to cancel the outstanding asynchronous operation.

      // Run the io_context again until the operation completes.;

    // If the operation failed, throw an exception. Otherwise return the result.
    return error ? throw std::system_error(error) : result;

} // namespace asio
} // namespace boost


int main(int argc, char* argv[])
    if (argc != 4)
      std::cerr << "Usage: blocking_tcp_client <host> <port> <message>\n";
      return 1;

    boost::asio::io_context io_context;

    // Resolve the host name and service to a list of endpoints.
    auto endpoints = tcp::resolver(io_context).resolve(argv[1], argv[2]);

    tcp_socket socket(io_context);

    // Run an asynchronous connect operation with a timeout.
    boost::asio::async_connect(socket, endpoints,
        close_after(std::chrono::seconds(10), socket));

    auto time_sent = std::chrono::steady_clock::now();

    // Run an asynchronous write operation with a timeout.
    std::string msg = argv[3] + std::string("\n");
    boost::asio::async_write(socket, boost::asio::buffer(msg),
        close_after(std::chrono::seconds(10), socket));

    for (std::string input_buffer;;)
      // Run an asynchronous read operation with a timeout.
      std::size_t n = boost::asio::async_read_until(socket,
          boost::asio::dynamic_buffer(input_buffer), '\n',
          close_after(std::chrono::seconds(10), socket));

      std::string line(input_buffer.substr(0, n - 1));
      input_buffer.erase(0, n);

      // Keep going until we get back the line that was sent.
      if (line == argv[3])

    auto time_received = std::chrono::steady_clock::now();

    std::cout << "Round trip time: ";
    std::cout << std::chrono::duration_cast<
        time_received - time_sent).count();
    std::cout << " microseconds\n";
  catch (std::exception& e)
    std::cerr << "Exception: " << e.what() << "\n";

  return 0;