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<title>Daytime.4 - A synchronous UDP daytime client</title>
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<div class="titlepage"><div><div><h3 class="title">
<a name="asio.tutorial.tutdaytime4"></a><a class="link" href="tutdaytime4.html" title="Daytime.4 - A synchronous UDP daytime client">Daytime.4 - A synchronous
UDP daytime client</a>
</h3></div></div></div>
<p>
This tutorial program shows how to use asio to implement a client application
with UDP.
</p>
<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>
<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>
<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>
<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">udp</span><span class="special">;</span>
</pre>
<p>
The start of the application is essentially the same as for the TCP daytime
client.
</p>
<pre class="programlisting"><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"><<</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>
<span class="keyword">return</span> <span class="number">1</span><span class="special">;</span>
<span class="special">}</span>
<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 use an <a class="link" href="../reference/ip__udp/resolver.html" title="ip::udp::resolver">ip::udp::resolver</a>
object to find the correct remote endpoint to use based on the host and service
names. The query is restricted to return only IPv4 endpoints by the <a class="link" href="../reference/ip__udp/v4.html" title="ip::udp::v4">ip::udp::v4()</a> argument.
</p>
<pre class="programlisting"> <span class="identifier">udp</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>
<span class="identifier">udp</span><span class="special">::</span><span class="identifier">endpoint</span> <span class="identifier">receiver_endpoint</span> <span class="special">=</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">udp</span><span class="special">::</span><span class="identifier">v4</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><span class="identifier">begin</span><span class="special">();</span>
</pre>
<p>
The <a class="link" href="../reference/ip__basic_resolver/resolve.html" title="ip::basic_resolver::resolve">ip::udp::resolver::resolve()</a>
function is guaranteed to return at least one endpoint in the list if it
does not fail. This means it is safe to dereference the return value directly.
</p>
<p>
Since UDP is datagram-oriented, we will not be using a stream socket. Create
an <a class="link" href="../reference/ip__udp/socket.html" title="ip::udp::socket">ip::udp::socket</a> and
initiate contact with the remote endpoint.
</p>
<pre class="programlisting"> <span class="identifier">udp</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">socket</span><span class="special">.</span><span class="identifier">open</span><span class="special">(</span><span class="identifier">udp</span><span class="special">::</span><span class="identifier">v4</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="keyword">char</span><span class="special">,</span> <span class="number">1</span><span class="special">></span> <span class="identifier">send_buf</span> <span class="special">=</span> <span class="special">{{</span> <span class="number">0</span> <span class="special">}};</span>
<span class="identifier">socket</span><span class="special">.</span><span class="identifier">send_to</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">send_buf</span><span class="special">),</span> <span class="identifier">receiver_endpoint</span><span class="special">);</span>
</pre>
<p>
Now we need to be ready to accept whatever the server sends back to us. The
endpoint on our side that receives the server's response will be initialised
by <a class="link" href="../reference/basic_datagram_socket/receive_from.html" title="basic_datagram_socket::receive_from">ip::udp::socket::receive_from()</a>.
</p>
<pre class="programlisting"> <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">recv_buf</span><span class="special">;</span>
<span class="identifier">udp</span><span class="special">::</span><span class="identifier">endpoint</span> <span class="identifier">sender_endpoint</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">receive_from</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">recv_buf</span><span class="special">),</span> <span class="identifier">sender_endpoint</span><span class="special">);</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">recv_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="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>
<span class="special">{</span>
<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>
<span class="special">}</span>
<span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
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Next: <a class="link" href="tutdaytime5.html" title="Daytime.5 - A synchronous UDP daytime server">Daytime.5 - A synchronous
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<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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