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<div class="titlepage"><div><div><h3 class="title">
<a name="asio.tutorial.tutdaytime1"></a><a class="link" href="tutdaytime1.html" title="Daytime.1 - A synchronous TCP daytime client">Daytime.1 - A synchronous
TCP daytime client</a>
</h3></div></div></div>
<p>
This tutorial program shows how to use asio to implement a client application
with TCP.
</p>
<p>
We start by including the necessary header files.
</p>
<pre class="programlisting"><span class="special">#</span><span class="identifier">include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
<span class="special">#</span><span class="identifier">include</span> <span class="special">&lt;</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">&gt;</span>
<span class="special">#</span><span class="identifier">include</span> <span class="special">&lt;</span><span class="identifier">asio</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
</pre>
<p>
The purpose of this application is to access a daytime service, so we need
the user to specify the server.
</p>
<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>
<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>
<span class="special">{</span>
<span class="keyword">try</span>
<span class="special">{</span>
<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>
<span class="special">{</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cerr</span> <span class="special">&lt;&lt;</span> <span class="string">"Usage: client &lt;host&gt;"</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="keyword">return</span> <span class="number">1</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
All programs that use asio need to have at least one I/O execution context,
such as an <a class="link" href="../reference/io_context.html" title="io_context">io_context</a> object.
</p>
<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>
</pre>
<p>
We need to turn the server name that was specified as a parameter to the
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>
object.
</p>
<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>
</pre>
<p>
A resolver takes a host name and service name and turns them into a list
of endpoints. We perform a resolve call using the name of the server, specified
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
name of the service, in this case <code class="computeroutput"><span class="string">"daytime"</span></code>.
</p>
<p>
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>.
This object is a range, with begin() and end() member functions that may
be used for iterating over the results.
</p>
<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>
<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>
</pre>
<p>
Now we create and connect the socket. The list of endpoints obtained above
may contain both IPv4 and IPv6 endpoints, so we need to try each of them
until we find one that works. This keeps the client program independent of
a specific IP version. The asio::connect() function does this for us automatically.
</p>
<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>
<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>
</pre>
<p>
The connection is open. All we need to do now is read the response from the
daytime service.
</p>
<p>
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()
function automatically determines the size of the array to help prevent buffer
overruns. Instead of a <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">array</span></code>,
we could have used a <code class="computeroutput"><span class="keyword">char</span> <span class="special">[]</span></code>
or <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span></code>.
</p>
<pre class="programlisting"> <span class="keyword">for</span> <span class="special">(;;)</span>
<span class="special">{</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">array</span><span class="special">&lt;</span><span class="keyword">char</span><span class="special">,</span> <span class="number">128</span><span class="special">&gt;</span> <span class="identifier">buf</span><span class="special">;</span>
<span class="identifier">asio</span><span class="special">::</span><span class="identifier">error_code</span> <span class="identifier">error</span><span class="special">;</span>
<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>
</pre>
<p>
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>
function will exit with the asio::error::eof error, which is how we know
to exit the loop.
</p>
<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>
<span class="keyword">break</span><span class="special">;</span> <span class="comment">// Connection closed cleanly by peer.</span>
<span class="keyword">else</span> <span class="keyword">if</span> <span class="special">(</span><span class="identifier">error</span><span class="special">)</span>
<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>
<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>
<span class="special">}</span>
</pre>
<p>
Finally, handle any exceptions that may have been thrown.
</p>
<pre class="programlisting"> <span class="special">}</span>
<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">&amp;</span> <span class="identifier">e</span><span class="special">)</span>
<span class="special">{</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cerr</span> <span class="special">&lt;&lt;</span> <span class="identifier">e</span><span class="special">.</span><span class="identifier">what</span><span class="special">()</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
See the <a class="link" href="tutdaytime1/src.html" title="Source listing for Daytime.1">full source listing</a>
</p>
<p>
Return to the <a class="link" href="../tutorial.html" title="Tutorial">tutorial index</a>
</p>
<p>
Next: <a class="link" href="tutdaytime2.html" title="Daytime.2 - A synchronous TCP daytime server">Daytime.2 - A synchronous
TCP daytime server</a>
</p>
</div>
<div class="copyright-footer">Copyright © 2003-2023 Christopher M. Kohlhoff<p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
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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