1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>Daytime.1 - A synchronous TCP daytime client</title> 5<link rel="stylesheet" href="../../../../doc/src/boostbook.css" type="text/css"> 6<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 7<link rel="home" href="../../boost_asio.html" title="Boost.Asio"> 8<link rel="up" href="../tutorial.html" title="Tutorial"> 9<link rel="prev" href="tuttimer5/src.html" title="Source listing for Timer.5"> 10<link rel="next" href="tutdaytime1/src.html" title="Source listing for Daytime.1"> 11</head> 12<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 13<table cellpadding="2" width="100%"><tr> 14<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../boost.png"></td> 15<td align="center"><a href="../../../../index.html">Home</a></td> 16<td align="center"><a href="../../../../libs/libraries.htm">Libraries</a></td> 17<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 18<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 19<td align="center"><a href="../../../../more/index.htm">More</a></td> 20</tr></table> 21<hr> 22<div class="spirit-nav"> 23<a accesskey="p" href="tuttimer5/src.html"><img src="../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../tutorial.html"><img src="../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../boost_asio.html"><img src="../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="tutdaytime1/src.html"><img src="../../../../doc/src/images/next.png" alt="Next"></a> 24</div> 25<div class="section"> 26<div class="titlepage"><div><div><h3 class="title"> 27<a name="boost_asio.tutorial.tutdaytime1"></a><a class="link" href="tutdaytime1.html" title="Daytime.1 - A synchronous TCP daytime client">Daytime.1 - A synchronous 28 TCP daytime client</a> 29</h3></div></div></div> 30<p> 31 This tutorial program shows how to use asio to implement a client application 32 with TCP. 33 </p> 34<p> 35 We start by including the necessary header files. 36 </p> 37<pre class="programlisting">#include <iostream> 38#include <boost/array.hpp> 39#include <boost/asio.hpp> 40</pre> 41<p> 42 The purpose of this application is to access a daytime service, so we need 43 the user to specify the server. 44 </p> 45<pre class="programlisting">using boost::asio::ip::tcp; 46 47int main(int argc, char* argv[]) 48{ 49 try 50 { 51 if (argc != 2) 52 { 53 std::cerr << "Usage: client <host>" << std::endl; 54 return 1; 55 } 56</pre> 57<p> 58 All programs that use asio need to have at least one I/O execution context, 59 such as an <a class="link" href="../reference/io_context.html" title="io_context">io_context</a> 60 object. 61 </p> 62<pre class="programlisting"> boost::asio::io_context io_context; 63</pre> 64<p> 65 We need to turn the server name that was specified as a parameter to the 66 application, into a TCP endpoint. To do this we use an <a class="link" href="../reference/ip__tcp/resolver.html" title="ip::tcp::resolver">ip::tcp::resolver</a> 67 object. 68 </p> 69<pre class="programlisting"> tcp::resolver resolver(io_context); 70</pre> 71<p> 72 A resolver takes a host name and service name and turns them into a list 73 of endpoints. We perform a resolve call using the name of the server, specified 74 in <code class="computeroutput">argv[1]</code>, and the name of the service, in this case <code class="computeroutput">"daytime"</code>. 75 </p> 76<p> 77 The list of endpoints is returned using an object of type <a class="link" href="../reference/ip__basic_resolver/results_type.html" title="ip::basic_resolver::results_type">ip::tcp::resolver::results_type</a>. 78 This object is a range, with begin() and end() member functions that may 79 be used for iterating over the results. 80 </p> 81<pre class="programlisting"> tcp::resolver::results_type endpoints = 82 resolver.resolve(argv[1], "daytime"); 83</pre> 84<p> 85 Now we create and connect the socket. The list of endpoints obtained above 86 may contain both IPv4 and IPv6 endpoints, so we need to try each of them 87 until we find one that works. This keeps the client program independent of 88 a specific IP version. The boost::asio::connect() function does this for 89 us automatically. 90 </p> 91<pre class="programlisting"> tcp::socket socket(io_context); 92 boost::asio::connect(socket, endpoints); 93</pre> 94<p> 95 The connection is open. All we need to do now is read the response from the 96 daytime service. 97 </p> 98<p> 99 We use a <code class="computeroutput">boost::array</code> to hold the received data. The boost::asio::buffer() 100 function automatically determines the size of the array to help prevent buffer 101 overruns. Instead of a <code class="computeroutput">boost::array</code>, we could have used a <code class="computeroutput">char 102 []</code> or <code class="computeroutput">std::vector</code>. 103 </p> 104<pre class="programlisting"> for (;;) 105 { 106 boost::array<char, 128> buf; 107 boost::system::error_code error; 108 109 size_t len = socket.read_some(boost::asio::buffer(buf), error); 110</pre> 111<p> 112 When the server closes the connection, the <a class="link" href="../reference/basic_stream_socket/read_some.html" title="basic_stream_socket::read_some">ip::tcp::socket::read_some()</a> 113 function will exit with the boost::asio::error::eof error, which is how we 114 know to exit the loop. 115 </p> 116<pre class="programlisting"> if (error == boost::asio::error::eof) 117 break; // Connection closed cleanly by peer. 118 else if (error) 119 throw boost::system::system_error(error); // Some other error. 120 121 std::cout.write(buf.data(), len); 122 } 123</pre> 124<p> 125 Finally, handle any exceptions that may have been thrown. 126 </p> 127<pre class="programlisting"> } 128 catch (std::exception& e) 129 { 130 std::cerr << e.what() << std::endl; 131 } 132</pre> 133<p> 134 See the <a class="link" href="tutdaytime1/src.html" title="Source listing for Daytime.1">full source listing</a> 135 </p> 136<p> 137 Return to the <a class="link" href="../tutorial.html" title="Tutorial">tutorial index</a> 138 </p> 139<p> 140 Next: <a class="link" href="tutdaytime2.html" title="Daytime.2 - A synchronous TCP daytime server">Daytime.2 - A synchronous 141 TCP daytime server</a> 142 </p> 143</div> 144<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 145<td align="left"></td> 146<td align="right"><div class="copyright-footer">Copyright © 2003-2020 Christopher M. 147 Kohlhoff<p> 148 Distributed under the Boost Software License, Version 1.0. (See accompanying 149 file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>) 150 </p> 151</div></td> 152</tr></table> 153<hr> 154<div class="spirit-nav"> 155<a accesskey="p" href="tuttimer5/src.html"><img src="../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../tutorial.html"><img src="../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../boost_asio.html"><img src="../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="tutdaytime1/src.html"><img src="../../../../doc/src/images/next.png" alt="Next"></a> 156</div> 157</body> 158</html> 159
