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<html>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
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<title>async_read (2 of 8 overloads)</title>
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<link rel="prev" href="overload1.html" title="async_read (1 of 8 overloads)">
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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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<a accesskey="p" href="overload1.html"><img src="../../../prev.png" alt="Prev"></a><a accesskey="u" href="../async_read.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="overload3.html"><img src="../../../next.png" alt="Next"></a>
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<div class="section">
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<div class="titlepage"><div><div><h4 class="title">
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<a name="asio.reference.async_read.overload2"></a><a class="link" href="overload2.html" title="async_read (2 of 8 overloads)">async_read (2
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of 8 overloads)</a>
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</h4></div></div></div>
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<p>
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Start an asynchronous operation to read a certain amount of data from a
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stream.
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</p>
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<pre class="programlisting">template<
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typename <a class="link" href="../AsyncReadStream.html" title="Buffer-oriented asynchronous read stream requirements">AsyncReadStream</a>,
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typename <a class="link" href="../MutableBufferSequence.html" title="Mutable buffer sequence requirements">MutableBufferSequence</a>,
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typename <a class="link" href="../CompletionCondition.html" title="Completion condition requirements">CompletionCondition</a>,
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typename <a class="link" href="../ReadHandler.html" title="Read handler requirements">ReadHandler</a> = <a class="link" href="../asynchronous_operations.html#asio.reference.asynchronous_operations.default_completion_tokens"><span class="emphasis"><em>DEFAULT</em></span></a>>
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<a class="link" href="../asynchronous_operations.html#asio.reference.asynchronous_operations.automatic_deduction_of_initiating_function_return_type"><span class="emphasis"><em>DEDUCED</em></span></a> async_read(
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AsyncReadStream & s,
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const MutableBufferSequence & buffers,
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CompletionCondition completion_condition,
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ReadHandler && handler = <a class="link" href="../asynchronous_operations.html#asio.reference.asynchronous_operations.default_completion_tokens"><span class="emphasis"><em>DEFAULT</em></span></a>,
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typename enable_if< is_mutable_buffer_sequence< MutableBufferSequence >::value >::type * = 0);
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</pre>
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<p>
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This function is used to asynchronously read a certain number of bytes
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of data from a stream. The function call always returns immediately. The
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asynchronous operation will continue until one of the following conditions
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is true:
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</p>
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<div class="itemizedlist"><ul class="itemizedlist" type="disc">
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<li class="listitem">
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The supplied buffers are full. That is, the bytes transferred is equal
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to the sum of the buffer sizes.
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</li>
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<li class="listitem">
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The completion_condition function object returns 0.
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</li>
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</ul></div>
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<h6>
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<a name="asio.reference.async_read.overload2.h0"></a>
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<span><a name="asio.reference.async_read.overload2.parameters"></a></span><a class="link" href="overload2.html#asio.reference.async_read.overload2.parameters">Parameters</a>
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</h6>
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<div class="variablelist">
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<p class="title"><b></b></p>
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<dl>
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<dt><span class="term">s</span></dt>
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<dd><p>
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The stream from which the data is to be read. The type must support
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the AsyncReadStream concept.
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</p></dd>
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<dt><span class="term">buffers</span></dt>
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<dd><p>
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One or more buffers into which the data will be read. The sum of
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the buffer sizes indicates the maximum number of bytes to read from
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the stream. Although the buffers object may be copied as necessary,
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ownership of the underlying memory blocks is retained by the caller,
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which must guarantee that they remain valid until the handler is
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called.
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</p></dd>
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<dt><span class="term">completion_condition</span></dt>
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<dd>
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<p>
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The function object to be called to determine whether the read operation
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is complete. The signature of the function object must be:
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</p>
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<pre class="programlisting">std::size_t completion_condition(
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// Result of latest async_read_some operation.
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const asio::error_code& error,
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// Number of bytes transferred so far.
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std::size_t bytes_transferred
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);
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</pre>
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<p>
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A return value of 0 indicates that the read operation is complete.
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A non-zero return value indicates the maximum number of bytes to
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be read on the next call to the stream's async_read_some function.
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</p>
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</dd>
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<dt><span class="term">handler</span></dt>
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<dd>
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<p>
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The handler to be called when the read operation completes. Copies
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will be made of the handler as required. The function signature of
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the handler must be:
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</p>
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<pre class="programlisting">void handler(
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const asio::error_code& error, // Result of operation.
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std::size_t bytes_transferred // Number of bytes copied into the
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// buffers. If an error occurred,
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// this will be the number of
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// bytes successfully transferred
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// prior to the error.
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);
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</pre>
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<p>
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Regardless of whether the asynchronous operation completes immediately
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or not, the handler will not be invoked from within this function.
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On immediate completion, invocation of the handler will be performed
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in a manner equivalent to using <a class="link" href="../post.html" title="post"><code class="computeroutput">post</code></a>.
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</p>
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</dd>
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</dl>
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</div>
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<h6>
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<a name="asio.reference.async_read.overload2.h1"></a>
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<span><a name="asio.reference.async_read.overload2.example"></a></span><a class="link" href="overload2.html#asio.reference.async_read.overload2.example">Example</a>
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</h6>
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<p>
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To read into a single data buffer use the <a class="link" href="../buffer.html" title="buffer"><code class="computeroutput">buffer</code></a>
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function as follows:
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</p>
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<pre class="programlisting">asio::async_read(s,
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asio::buffer(data, size),
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asio::transfer_at_least(32),
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handler);
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</pre>
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<p>
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See the <a class="link" href="../buffer.html" title="buffer"><code class="computeroutput">buffer</code></a>
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documentation for information on reading into multiple buffers in one go,
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and how to use it with arrays, boost::array or std::vector.
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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 © 2003-2020 Christopher M.
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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></td>
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</tr></table>
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<hr>
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