159 lines
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159 lines
7.4 KiB
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<title>Daytime.1 - A synchronous TCP daytime client</title>
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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="boost_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">#include <iostream>
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#include <boost/array.hpp>
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#include <boost/asio.hpp>
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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">using boost::asio::ip::tcp;
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int main(int argc, char* argv[])
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{
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try
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{
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if (argc != 2)
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{
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std::cerr << "Usage: client <host>" << std::endl;
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return 1;
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}
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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>
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object.
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</p>
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<pre class="programlisting"> boost::asio::io_context io_context;
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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"> tcp::resolver resolver(io_context);
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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">argv[1]</code>, and the name of the service, in this case <code class="computeroutput">"daytime"</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"> tcp::resolver::results_type endpoints =
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resolver.resolve(argv[1], "daytime");
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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 boost::asio::connect() function does this for
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us automatically.
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</p>
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<pre class="programlisting"> tcp::socket socket(io_context);
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boost::asio::connect(socket, endpoints);
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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">boost::array</code> to hold the received data. The boost::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">boost::array</code>, we could have used a <code class="computeroutput">char
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[]</code> or <code class="computeroutput">std::vector</code>.
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</p>
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<pre class="programlisting"> for (;;)
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{
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boost::array<char, 128> buf;
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boost::system::error_code error;
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size_t len = socket.read_some(boost::asio::buffer(buf), error);
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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 boost::asio::error::eof error, which is how we
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know to exit the loop.
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</p>
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<pre class="programlisting"> if (error == boost::asio::error::eof)
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break; // Connection closed cleanly by peer.
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else if (error)
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throw boost::system::system_error(error); // Some other error.
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std::cout.write(buf.data(), len);
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}
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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"> }
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catch (std::exception& e)
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{
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std::cerr << e.what() << std::endl;
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}
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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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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="left"></td>
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<td align="right"><div class="copyright-footer">Copyright © 2003-2021 Christopher
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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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