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<title>Daytime.1 - A synchronous TCP daytime client</title>
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<div class="titlepage"><div><div><h3 class="title">
<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
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">#include <iostream>
#include <boost/array.hpp>
#include <boost/asio.hpp>
</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">using boost::asio::ip::tcp;
int main(int argc, char* argv[])
{
try
{
if (argc != 2)
{
std::cerr << "Usage: client <host>" << std::endl;
return 1;
}
</pre>
<p>
All programs that use asio need to have at least one <a class="link" href="../reference/io_context.html" title="io_context">io_context</a>
object.
</p>
<pre class="programlisting"> boost::asio::io_context io_context;
</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"> tcp::resolver resolver(io_context);
</pre>
<p>
A resolver takes a query object and turns it into a list of endpoints. We
construct a query using the name of the server, specified in <code class="computeroutput">argv[1]</code>,
and the name of the service, in this case <code class="computeroutput">"daytime"</code>.
</p>
<p>
The list of endpoints is returned using an iterator of type <a class="link" href="../reference/ip__basic_resolver/iterator.html" title="ip::basic_resolver::iterator">ip::tcp::resolver::iterator</a>.
(Note that a default constructed <a class="link" href="../reference/ip__basic_resolver/iterator.html" title="ip::basic_resolver::iterator">ip::tcp::resolver::iterator</a>
object can be used as an end iterator.)
</p>
<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 boost::asio::connect() function does this for
us automatically.
</p>
<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">boost::array</code> to hold the received data. The boost::asio::buffer()
function automatically determines the size of the array to help prevent buffer
overruns. Instead of a <code class="computeroutput">boost::array</code>, we could have used a <code class="computeroutput">char
[]</code> or <code class="computeroutput">std::vector</code>.
</p>
<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 boost::asio::error::eof error, which is how we
know to exit the loop.
</p>
<p>
Finally, handle any exceptions that may have been thrown.
</p>
<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>
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<td align="right"><div class="copyright-footer">Copyright © 2003-2018 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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