C++ boost.asio server and client connection undersanding

Question

i started learning boost.asio and i have some problems with undersanding tcp connections. There is example from official boost site:

  #include <ctime>
#include <iostream>
#include <string>
#include <boost/asio.hpp>

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

std::string make_daytime_string()
{
  using namespace std; // For time_t, time and ctime;
  time_t now = time(0);
  return ctime(&now);
}

int main()
{
  try
  {
    boost::asio::io_service io_service;

    tcp::acceptor acceptor(io_service, tcp::endpoint(tcp::v4(), 13));

    for (;;)
    {
      tcp::socket socket(io_service);
      acceptor.accept(socket);

      std::string message = make_daytime_string();

      boost::system::error_code ignored_error;
      boost::asio::write(socket, boost::asio::buffer(message),
          boost::asio::transfer_all(), ignored_error);
    }
  }
  catch (std::exception& e)
  {
    std::cerr << e.what() << std::endl;
  }

  return 0;
}

there is question, why if i want to connet to this server via client i have t write:

    boost::asio::io_service io_service;

tcp::resolver resolver(io_service);
tcp::resolver::query query(host_ip, "daytime"); //why daytime?
tcp::resolver::iterator endpoint_iterator = resolver.resolve(query);
tcp::resolver::iterator end;

why daytime?, what it meant and where it is inicialized in server, or i just doesn't missed somefing?

there is full client code : www.boost.org/doc/libs/1_39_0/doc/html/boost_asio/tutorial/tutdaytime1.html thanks for explanation in advance

Answer 1

Daytime is simply another protocol (like FTP, etc) and it uses port 13. If you want to connect to the server on a specific port number, then your code would look like this:

tcp::resolver::query query(host_ip, "5678"); // 5678 is the port number

Answer 2

daytime is the service name, this is well described in the tcp::resolver::query documentation

service_name

A string identifying the requested service. This may be a descriptive name or a numeric string corresponding to a port number. May be an empty string, in which case all resolved endpoints will have a port number of 0.