int main()
{
	for(;;)
	{
		switch(getch())
		{
		case 's': // der server
			try
			{
				boost::asio::io_service io_service;

				tcp::acceptor acceptor(io_service, tcp::endpoint(tcp::v4(), 8357));

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

					std::string message = "vom server";

					boost::system::error_code ignored_error;
					boost::asio::write(socket, boost::asio::buffer(message), ignored_error);
				}
			}
			catch(std::exception& e)
			{
				std::cerr << e.what() << std::endl;
			}
			return 0;
		case 'c': // der client
			try
			{
				boost::asio::io_service io_service;

				tcp::resolver resolver(io_service);
				tcp::resolver::query query("127.0.0.1", "8357");
				tcp::resolver::iterator endpoint_iterator = resolver.resolve(query);

				tcp::socket socket(io_service);
				boost::asio::connect(socket, endpoint_iterator);

				boost::asio::streambuf buf;
				boost::system::error_code error;

				size_t len = boost::asio::read(socket, buf, error);

				if(error == boost::asio::error::eof)
				{
					cout << boost::asio::buffer_cast<const char *>(buf.data()), buf.size();
				}
				else if(error)
				{
					throw boost::system::system_error(error);
				}
			}
			catch(std::exception& e)
			{
				std::cerr << e.what() << std::endl;
			}
			return 0;
		case VK_ESCAPE:
			return 0;
		}
	}

	getch();
	return 0;
}

Aber das hier nicht:

int main()
{
	for(;;)
	{
		switch(getch())
		{
		case 's': // der server
			try
			{
				boost::asio::io_service io_service;

				tcp::acceptor acceptor(io_service, tcp::endpoint(tcp::v4(), 8357));

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

					std::string message = "vom server";

					boost::system::error_code ignored_error;
					boost::asio::write(socket, boost::asio::buffer(message), ignored_error);

					// neuer Teil
					boost::asio::streambuf buf;
					boost::system::error_code error;

					// hier hängt es
					size_t len = boost::asio::read(socket, buf, error);

					if(error == boost::asio::error::eof)
					{
						cout << boost::asio::buffer_cast<const char *>(buf.data()), buf.size();
					}
					else if(error)
					{
						throw boost::system::system_error(error);
					}
				}
			}
			catch(std::exception& e)
			{
				std::cerr << e.what() << std::endl;
			}
			return 0;
		case 'c': // der client
			try
			{
				boost::asio::io_service io_service;

				tcp::resolver resolver(io_service);
				tcp::resolver::query query("127.0.0.1", "8357");
				tcp::resolver::iterator endpoint_iterator = resolver.resolve(query);

				tcp::socket socket(io_service);
				boost::asio::connect(socket, endpoint_iterator);

				boost::asio::streambuf buf;
				boost::system::error_code error;

				// hier hängt es jetzt auch
				size_t len = boost::asio::read(socket, buf, error);

				if(error == boost::asio::error::eof)
				{
					cout << boost::asio::buffer_cast<const char *>(buf.data()), buf.size();
				}
				else if(error)
				{
					throw boost::system::system_error(error);
				}

				// neuer Teil
				std::string message = "vom client";

				boost::system::error_code ignored_error;
				boost::asio::write(socket, boost::asio::buffer(message), ignored_error);
			}
			catch(std::exception& e)
			{
				std::cerr << e.what() << std::endl;
			}
			return 0;
		case VK_ESCAPE:
			return 0;
		}
	}

	getch();
	return 0;
}