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149 lines
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<title>Daytime.1 - A synchronous TCP daytime client</title>
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<link rel="next" href="tutdaytime1/src.html" title="Source listing for Daytime.1">
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
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<table cellpadding="2" width="100%"><tr><td valign="top"><img alt="asio C++ library" width="250" height="60" src="../../asio.png"></td></tr></table>
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<hr>
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<div class="spirit-nav">
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<a accesskey="p" href="tuttimer5/src.html"><img src="../../prev.png" alt="Prev"></a><a accesskey="u" href="../tutorial.html"><img src="../../up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../home.png" alt="Home"></a><a accesskey="n" href="tutdaytime1/src.html"><img src="../../next.png" alt="Next"></a>
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<div class="section">
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<div class="titlepage"><div><div><h3 class="title">
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<a name="asio.tutorial.tutdaytime1"></a><a class="link" href="tutdaytime1.html" title="Daytime.1 - A synchronous TCP daytime client">Daytime.1 - A synchronous
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TCP daytime client</a>
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</h3></div></div></div>
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<p>
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This tutorial program shows how to use asio to implement a client application
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with TCP.
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</p>
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<p>
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We start by including the necessary header files.
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</p>
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<pre class="programlisting"><span class="special">#</span><span class="identifier">include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span>
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<span class="special">#</span><span class="identifier">include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">array</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
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<span class="special">#</span><span class="identifier">include</span> <span class="special"><</span><span class="identifier">asio</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
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</pre>
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<p>
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The purpose of this application is to access a daytime service, so we need
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the user to specify the server.
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</p>
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<pre class="programlisting"><span class="keyword">using</span> <span class="identifier">asio</span><span class="special">::</span><span class="identifier">ip</span><span class="special">::</span><span class="identifier">tcp</span><span class="special">;</span>
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<span class="keyword">int</span> <span class="identifier">main</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">argc</span><span class="special">,</span> <span class="keyword">char</span><span class="special">*</span> <span class="identifier">argv</span><span class="special">[])</span>
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<span class="special">{</span>
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<span class="keyword">try</span>
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<span class="special">{</span>
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<span class="keyword">if</span> <span class="special">(</span><span class="identifier">argc</span> <span class="special">!=</span> <span class="number">2</span><span class="special">)</span>
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<span class="special">{</span>
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<span class="identifier">std</span><span class="special">::</span><span class="identifier">cerr</span> <span class="special"><<</span> <span class="string">"Usage: client <host>"</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
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<span class="keyword">return</span> <span class="number">1</span><span class="special">;</span>
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<span class="special">}</span>
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</pre>
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<p>
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All programs that use asio need to have at least one I/O execution context,
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such as an <a class="link" href="../reference/io_context.html" title="io_context">io_context</a> object.
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</p>
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<pre class="programlisting"> <span class="identifier">asio</span><span class="special">::</span><span class="identifier">io_context</span> <span class="identifier">io_context</span><span class="special">;</span>
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</pre>
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<p>
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We need to turn the server name that was specified as a parameter to the
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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>
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object.
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</p>
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<pre class="programlisting"> <span class="identifier">tcp</span><span class="special">::</span><span class="identifier">resolver</span> <span class="identifier">resolver</span><span class="special">(</span><span class="identifier">io_context</span><span class="special">);</span>
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</pre>
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<p>
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A resolver takes a host name and service name and turns them into a list
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of endpoints. We perform a resolve call using the name of the server, specified
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in <code class="computeroutput"><span class="identifier">argv</span><span class="special">[</span><span class="number">1</span><span class="special">]</span></code>, and the
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name of the service, in this case <code class="computeroutput"><span class="string">"daytime"</span></code>.
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</p>
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<p>
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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>.
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This object is a range, with begin() and end() member functions that may
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be used for iterating over the results.
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</p>
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<pre class="programlisting"> <span class="identifier">tcp</span><span class="special">::</span><span class="identifier">resolver</span><span class="special">::</span><span class="identifier">results_type</span> <span class="identifier">endpoints</span> <span class="special">=</span>
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<span class="identifier">resolver</span><span class="special">.</span><span class="identifier">resolve</span><span class="special">(</span><span class="identifier">argv</span><span class="special">[</span><span class="number">1</span><span class="special">],</span> <span class="string">"daytime"</span><span class="special">);</span>
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</pre>
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<p>
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Now we create and connect the socket. The list of endpoints obtained above
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may contain both IPv4 and IPv6 endpoints, so we need to try each of them
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until we find one that works. This keeps the client program independent of
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a specific IP version. The asio::connect() function does this for us automatically.
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</p>
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<pre class="programlisting"> <span class="identifier">tcp</span><span class="special">::</span><span class="identifier">socket</span> <span class="identifier">socket</span><span class="special">(</span><span class="identifier">io_context</span><span class="special">);</span>
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<span class="identifier">asio</span><span class="special">::</span><span class="identifier">connect</span><span class="special">(</span><span class="identifier">socket</span><span class="special">,</span> <span class="identifier">endpoints</span><span class="special">);</span>
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</pre>
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<p>
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The connection is open. All we need to do now is read the response from the
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daytime service.
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</p>
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<p>
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We use a <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">array</span></code> to hold the received data. The asio::buffer()
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function automatically determines the size of the array to help prevent buffer
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overruns. Instead of a <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">array</span></code>,
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we could have used a <code class="computeroutput"><span class="keyword">char</span> <span class="special">[]</span></code>
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or <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span></code>.
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</p>
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<pre class="programlisting"> <span class="keyword">for</span> <span class="special">(;;)</span>
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<span class="special">{</span>
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<span class="identifier">boost</span><span class="special">::</span><span class="identifier">array</span><span class="special"><</span><span class="keyword">char</span><span class="special">,</span> <span class="number">128</span><span class="special">></span> <span class="identifier">buf</span><span class="special">;</span>
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<span class="identifier">asio</span><span class="special">::</span><span class="identifier">error_code</span> <span class="identifier">error</span><span class="special">;</span>
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<span class="identifier">size_t</span> <span class="identifier">len</span> <span class="special">=</span> <span class="identifier">socket</span><span class="special">.</span><span class="identifier">read_some</span><span class="special">(</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">buffer</span><span class="special">(</span><span class="identifier">buf</span><span class="special">),</span> <span class="identifier">error</span><span class="special">);</span>
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</pre>
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<p>
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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>
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function will exit with the asio::error::eof error, which is how we know
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to exit the loop.
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</p>
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<pre class="programlisting"> <span class="keyword">if</span> <span class="special">(</span><span class="identifier">error</span> <span class="special">==</span> <span class="identifier">asio</span><span class="special">::</span><span class="identifier">error</span><span class="special">::</span><span class="identifier">eof</span><span class="special">)</span>
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<span class="keyword">break</span><span class="special">;</span> <span class="comment">// Connection closed cleanly by peer.</span>
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<span class="keyword">else</span> <span class="keyword">if</span> <span class="special">(</span><span class="identifier">error</span><span class="special">)</span>
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<span class="keyword">throw</span> <span class="identifier">asio</span><span class="special">::</span><span class="identifier">system_error</span><span class="special">(</span><span class="identifier">error</span><span class="special">);</span> <span class="comment">// Some other error.</span>
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<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special">.</span><span class="identifier">write</span><span class="special">(</span><span class="identifier">buf</span><span class="special">.</span><span class="identifier">data</span><span class="special">(),</span> <span class="identifier">len</span><span class="special">);</span>
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<span class="special">}</span>
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</pre>
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<p>
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Finally, handle any exceptions that may have been thrown.
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</p>
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<pre class="programlisting"> <span class="special">}</span>
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<span class="keyword">catch</span> <span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">exception</span><span class="special">&</span> <span class="identifier">e</span><span class="special">)</span>
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<span class="special">{</span>
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<span class="identifier">std</span><span class="special">::</span><span class="identifier">cerr</span> <span class="special"><<</span> <span class="identifier">e</span><span class="special">.</span><span class="identifier">what</span><span class="special">()</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
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<span class="special">}</span>
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</pre>
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<p>
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See the <a class="link" href="tutdaytime1/src.html" title="Source listing for Daytime.1">full source listing</a>
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</p>
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<p>
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Return to the <a class="link" href="../tutorial.html" title="Tutorial">tutorial index</a>
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</p>
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<p>
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Next: <a class="link" href="tutdaytime2.html" title="Daytime.2 - A synchronous TCP daytime server">Daytime.2 - A synchronous
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TCP daytime server</a>
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</p>
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</div>
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<div class="copyright-footer">Copyright © 2003-2023 Christopher M. Kohlhoff<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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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>)
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</p>
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</div>
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<hr>
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<div class="spirit-nav">
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<a accesskey="p" href="tuttimer5/src.html"><img src="../../prev.png" alt="Prev"></a><a accesskey="u" href="../tutorial.html"><img src="../../up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../home.png" alt="Home"></a><a accesskey="n" href="tutdaytime1/src.html"><img src="../../next.png" alt="Next"></a>
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