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164 lines
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<title>async_read (3 of 8 overloads)</title>
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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="overload2.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="overload4.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.overload3"></a><a class="link" href="overload3.html" title="async_read (3 of 8 overloads)">async_read (3
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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"><span class="keyword">template</span><span class="special"><</span>
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<span class="keyword">typename</span> <a class="link" href="../AsyncReadStream.html" title="Buffer-oriented asynchronous read stream requirements">AsyncReadStream</a><span class="special">,</span>
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<span class="keyword">typename</span> <a class="link" href="../DynamicBuffer_v1.html" title="Dynamic buffer requirements (version 1)">DynamicBuffer_v1</a><span class="special">,</span>
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<span class="keyword">typename</span> <a class="link" href="../ReadToken.html" title="Read token requirements">ReadToken</a> <span class="special">=</span> <a class="link" href="../asynchronous_operations.html#asio.reference.asynchronous_operations.default_completion_tokens"><span class="emphasis"><em>DEFAULT</em></span></a><span class="special">></span>
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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> <span class="identifier">async_read</span><span class="special">(</span>
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<span class="identifier">AsyncReadStream</span> <span class="special">&</span> <span class="identifier">s</span><span class="special">,</span>
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<span class="identifier">DynamicBuffer_v1</span> <span class="special">&&</span> <span class="identifier">buffers</span><span class="special">,</span>
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<span class="identifier">ReadToken</span> <span class="special">&&</span> <span class="identifier">token</span> <span class="special">=</span> <a class="link" href="../asynchronous_operations.html#asio.reference.asynchronous_operations.default_completion_tokens"><span class="emphasis"><em>DEFAULT</em></span></a><span class="special">,</span>
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<span class="keyword">typename</span> <span class="identifier">constraint</span><span class="special"><</span> <span class="identifier">is_dynamic_buffer_v1</span><span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">decay</span><span class="special"><</span> <span class="identifier">DynamicBuffer_v1</span> <span class="special">>::</span><span class="identifier">type</span> <span class="special">>::</span><span class="identifier">value</span> <span class="special">>::</span><span class="identifier">type</span> <span class="special">=</span> <span class="number">0</span><span class="special">,</span>
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<span class="keyword">typename</span> <span class="identifier">constraint</span><span class="special"><</span> <span class="special">!</span><span class="identifier">is_dynamic_buffer_v2</span><span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">decay</span><span class="special"><</span> <span class="identifier">DynamicBuffer_v1</span> <span class="special">>::</span><span class="identifier">type</span> <span class="special">>::</span><span class="identifier">value</span> <span class="special">>::</span><span class="identifier">type</span> <span class="special">=</span> <span class="number">0</span><span class="special">);</span>
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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. It is an initiating function for an <a class="link" href="../../overview/model/async_ops.html" title="Asynchronous Operations">asynchronous
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operation</a>, and always returns immediately. The asynchronous operation
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will continue until one of the following conditions 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 specified dynamic buffer sequence is full (that is, it has reached
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maximum size).
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</li>
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<li class="listitem">
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An error occurred.
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</li>
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</ul></div>
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<p>
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This operation is implemented in terms of zero or more calls to the stream's
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async_read_some function, and is known as a <span class="emphasis"><em>composed operation</em></span>.
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The program must ensure that the stream performs no other read operations
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(such as async_read, the stream's async_read_some function, or any other
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composed operations that perform reads) until this operation completes.
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</p>
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<h6>
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<a name="asio.reference.async_read.overload3.h0"></a>
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<span><a name="asio.reference.async_read.overload3.parameters"></a></span><a class="link" href="overload3.html#asio.reference.async_read.overload3.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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The dynamic buffer sequence into which the data will be read. Although
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the buffers object may be copied as necessary, ownership of the underlying
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memory blocks is retained by the caller, which must guarantee that
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they remain valid until the completion handler is called.
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</p></dd>
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<dt><span class="term">token</span></dt>
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<dd>
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<p>
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The <a class="link" href="../../overview/model/completion_tokens.html" title="Completion Tokens">completion
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token</a> that will be used to produce a completion handler, which
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will be called when the read completes. Potential completion tokens
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include <a class="link" href="../use_future.html" title="use_future"><code class="computeroutput"><span class="identifier">use_future</span></code></a>,
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<a class="link" href="../use_awaitable.html" title="use_awaitable"><code class="computeroutput"><span class="identifier">use_awaitable</span></code></a>,
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<a class="link" href="../yield_context.html" title="yield_context"><code class="computeroutput"><span class="identifier">yield_context</span></code></a>,
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or a function object with the correct completion signature. The function
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signature of the completion handler must be:
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</p>
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<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">handler</span><span class="special">(</span>
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<span class="comment">// Result of operation.</span>
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<span class="keyword">const</span> <span class="identifier">asio</span><span class="special">::</span><span class="identifier">error_code</span><span class="special">&</span> <span class="identifier">error</span><span class="special">,</span>
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<span class="comment">// Number of bytes copied into the buffers. If an error</span>
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<span class="comment">// occurred, this will be the number of bytes successfully</span>
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<span class="comment">// transferred prior to the error.</span>
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<span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">bytes_transferred</span>
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<span class="special">);</span>
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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 completion handler will not be invoked from within this
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function. On immediate completion, invocation of the handler will
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be performed in a manner equivalent to using <a class="link" href="../post.html" title="post"><code class="computeroutput"><span class="identifier">post</span></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.overload3.h1"></a>
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<span><a name="asio.reference.async_read.overload3.completion_signature"></a></span><a class="link" href="overload3.html#asio.reference.async_read.overload3.completion_signature">Completion
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Signature</a>
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</h6>
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<pre class="programlisting"><span class="keyword">void</span><span class="special">(</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">error_code</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span><span class="special">)</span>
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</pre>
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<h6>
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<a name="asio.reference.async_read.overload3.h2"></a>
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<span><a name="asio.reference.async_read.overload3.remarks"></a></span><a class="link" href="overload3.html#asio.reference.async_read.overload3.remarks">Remarks</a>
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</h6>
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<p>
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This overload is equivalent to calling:
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</p>
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<pre class="programlisting"><span class="identifier">asio</span><span class="special">::</span><span class="identifier">async_read</span><span class="special">(</span>
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<span class="identifier">s</span><span class="special">,</span> <span class="identifier">buffers</span><span class="special">,</span>
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<span class="identifier">asio</span><span class="special">::</span><span class="identifier">transfer_all</span><span class="special">(),</span>
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<span class="identifier">handler</span><span class="special">);</span>
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</pre>
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<h6>
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<a name="asio.reference.async_read.overload3.h3"></a>
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<span><a name="asio.reference.async_read.overload3.per_operation_cancellation"></a></span><a class="link" href="overload3.html#asio.reference.async_read.overload3.per_operation_cancellation">Per-Operation
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Cancellation</a>
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</h6>
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<p>
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This asynchronous operation supports cancellation for the following <a class="link" href="../cancellation_type.html" title="cancellation_type"><code class="computeroutput"><span class="identifier">cancellation_type</span></code></a>
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values:
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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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<code class="computeroutput"><span class="identifier">cancellation_type</span><span class="special">::</span><span class="identifier">terminal</span></code>
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</li>
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<li class="listitem">
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<code class="computeroutput"><span class="identifier">cancellation_type</span><span class="special">::</span><span class="identifier">partial</span></code>
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</li>
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</ul></div>
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<p>
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if they are also supported by the <code class="computeroutput"><span class="identifier">AsyncReadStream</span></code>
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type's <code class="computeroutput"><span class="identifier">async_read_some</span></code>
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operation.
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</p>
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</div>
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<div class="copyright-footer">Copyright © 2003-2023 Christopher 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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</p>
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</div>
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