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<div class="titlepage"><div><div><h4 class="title">
<a name="asio.reference.async_read.overload3"></a><a class="link" href="overload3.html" title="async_read (3 of 8 overloads)">async_read (3
of 8 overloads)</a>
</h4></div></div></div>
<p>
Start an asynchronous operation to read a certain amount of data from a
stream.
</p>
<pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span>
<span class="keyword">typename</span> <a class="link" href="../AsyncReadStream.html" title="Buffer-oriented asynchronous read stream requirements">AsyncReadStream</a><span class="special">,</span>
<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>
<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">&gt;</span>
<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>
<span class="identifier">AsyncReadStream</span> <span class="special">&amp;</span> <span class="identifier">s</span><span class="special">,</span>
<span class="identifier">DynamicBuffer_v1</span> <span class="special">&amp;&amp;</span> <span class="identifier">buffers</span><span class="special">,</span>
<span class="identifier">ReadToken</span> <span class="special">&amp;&amp;</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>
<span class="keyword">typename</span> <span class="identifier">constraint</span><span class="special">&lt;</span> <span class="identifier">is_dynamic_buffer_v1</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">decay</span><span class="special">&lt;</span> <span class="identifier">DynamicBuffer_v1</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="special">&gt;::</span><span class="identifier">value</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="special">=</span> <span class="number">0</span><span class="special">,</span>
<span class="keyword">typename</span> <span class="identifier">constraint</span><span class="special">&lt;</span> <span class="special">!</span><span class="identifier">is_dynamic_buffer_v2</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">decay</span><span class="special">&lt;</span> <span class="identifier">DynamicBuffer_v1</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="special">&gt;::</span><span class="identifier">value</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="special">=</span> <span class="number">0</span><span class="special">);</span>
</pre>
<p>
This function is used to asynchronously read a certain number of bytes
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
operation</a>, and always returns immediately. The asynchronous operation
will continue until one of the following conditions is true:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
The specified dynamic buffer sequence is full (that is, it has reached
maximum size).
</li>
<li class="listitem">
An error occurred.
</li>
</ul></div>
<p>
This operation is implemented in terms of zero or more calls to the stream's
async_read_some function, and is known as a <span class="emphasis"><em>composed operation</em></span>.
The program must ensure that the stream performs no other read operations
(such as async_read, the stream's async_read_some function, or any other
composed operations that perform reads) until this operation completes.
</p>
<h6>
<a name="asio.reference.async_read.overload3.h0"></a>
<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>
</h6>
<div class="variablelist">
<p class="title"><b></b></p>
<dl>
<dt><span class="term">s</span></dt>
<dd><p>
The stream from which the data is to be read. The type must support
the AsyncReadStream concept.
</p></dd>
<dt><span class="term">buffers</span></dt>
<dd><p>
The dynamic buffer sequence into which the data will be read. Although
the buffers object may be copied as necessary, ownership of the underlying
memory blocks is retained by the caller, which must guarantee that
they remain valid until the completion handler is called.
</p></dd>
<dt><span class="term">token</span></dt>
<dd>
<p>
The <a class="link" href="../../overview/model/completion_tokens.html" title="Completion Tokens">completion
token</a> that will be used to produce a completion handler, which
will be called when the read completes. Potential completion tokens
include <a class="link" href="../use_future.html" title="use_future"><code class="computeroutput"><span class="identifier">use_future</span></code></a>,
<a class="link" href="../use_awaitable.html" title="use_awaitable"><code class="computeroutput"><span class="identifier">use_awaitable</span></code></a>,
<a class="link" href="../yield_context.html" title="yield_context"><code class="computeroutput"><span class="identifier">yield_context</span></code></a>,
or a function object with the correct completion signature. The function
signature of the completion handler must be:
</p>
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">handler</span><span class="special">(</span>
<span class="comment">// Result of operation.</span>
<span class="keyword">const</span> <span class="identifier">asio</span><span class="special">::</span><span class="identifier">error_code</span><span class="special">&amp;</span> <span class="identifier">error</span><span class="special">,</span>
<span class="comment">// Number of bytes copied into the buffers. If an error</span>
<span class="comment">// occurred, this will be the number of bytes successfully</span>
<span class="comment">// transferred prior to the error.</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">bytes_transferred</span>
<span class="special">);</span>
</pre>
<p>
Regardless of whether the asynchronous operation completes immediately
or not, the completion handler will not be invoked from within this
function. On immediate completion, invocation of the handler will
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>.
</p>
</dd>
</dl>
</div>
<h6>
<a name="asio.reference.async_read.overload3.h1"></a>
<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
Signature</a>
</h6>
<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>
</pre>
<h6>
<a name="asio.reference.async_read.overload3.h2"></a>
<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>
</h6>
<p>
This overload is equivalent to calling:
</p>
<pre class="programlisting"><span class="identifier">asio</span><span class="special">::</span><span class="identifier">async_read</span><span class="special">(</span>
<span class="identifier">s</span><span class="special">,</span> <span class="identifier">buffers</span><span class="special">,</span>
<span class="identifier">asio</span><span class="special">::</span><span class="identifier">transfer_all</span><span class="special">(),</span>
<span class="identifier">handler</span><span class="special">);</span>
</pre>
<h6>
<a name="asio.reference.async_read.overload3.h3"></a>
<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
Cancellation</a>
</h6>
<p>
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>
values:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
<code class="computeroutput"><span class="identifier">cancellation_type</span><span class="special">::</span><span class="identifier">terminal</span></code>
</li>
<li class="listitem">
<code class="computeroutput"><span class="identifier">cancellation_type</span><span class="special">::</span><span class="identifier">partial</span></code>
</li>
</ul></div>
<p>
if they are also supported by the <code class="computeroutput"><span class="identifier">AsyncReadStream</span></code>
type's <code class="computeroutput"><span class="identifier">async_read_some</span></code>
operation.
</p>
</div>
<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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