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<div class="titlepage"><div><div><h4 class="title">
<a name="asio.reference.async_read.overload2"></a><a class="link" href="overload2.html" title="async_read (2 of 8 overloads)">async_read (2
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="../MutableBufferSequence.html" title="Mutable buffer sequence requirements">MutableBufferSequence</a><span class="special">,</span>
<span class="keyword">typename</span> <a class="link" href="../CompletionCondition.html" title="Completion condition requirements">CompletionCondition</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="keyword">const</span> <span class="identifier">MutableBufferSequence</span> <span class="special">&amp;</span> <span class="identifier">buffers</span><span class="special">,</span>
<span class="identifier">CompletionCondition</span> <span class="identifier">completion_condition</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_mutable_buffer_sequence</span><span class="special">&lt;</span> <span class="identifier">MutableBufferSequence</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 supplied buffers are full. That is, the bytes transferred is equal
to the sum of the buffer sizes.
</li>
<li class="listitem">
The completion_condition function object returns 0.
</li>
</ul></div>
<h6>
<a name="asio.reference.async_read.overload2.h0"></a>
<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>
</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>
One or more buffers into which the data will be read. The sum of
the buffer sizes indicates the maximum number of bytes to read from
the stream. 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">completion_condition</span></dt>
<dd>
<p>
The function object to be called to determine whether the read operation
is complete. The signature of the function object must be:
</p>
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">completion_condition</span><span class="special">(</span>
<span class="comment">// Result of latest async_read_some 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 transferred so far.</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>
A return value of 0 indicates that the read operation is complete.
A non-zero return value indicates the maximum number of bytes to
be read on the next call to the stream's async_read_some function.
</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.overload2.h1"></a>
<span><a name="asio.reference.async_read.overload2.completion_signature"></a></span><a class="link" href="overload2.html#asio.reference.async_read.overload2.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.overload2.h2"></a>
<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>
</h6>
<p>
To read into a single data buffer use the <a class="link" href="../buffer.html" title="buffer"><code class="computeroutput"><span class="identifier">buffer</span></code></a> function as follows:
</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">asio</span><span class="special">::</span><span class="identifier">buffer</span><span class="special">(</span><span class="identifier">data</span><span class="special">,</span> <span class="identifier">size</span><span class="special">),</span>
<span class="identifier">asio</span><span class="special">::</span><span class="identifier">transfer_at_least</span><span class="special">(</span><span class="number">32</span><span class="special">),</span>
<span class="identifier">handler</span><span class="special">);</span>
</pre>
<p>
See the <a class="link" href="../buffer.html" title="buffer"><code class="computeroutput"><span class="identifier">buffer</span></code></a>
documentation for information on reading into multiple buffers in one go,
and how to use it with arrays, boost::array or std::vector.
</p>
<h6>
<a name="asio.reference.async_read.overload2.h3"></a>
<span><a name="asio.reference.async_read.overload2.per_operation_cancellation"></a></span><a class="link" href="overload2.html#asio.reference.async_read.overload2.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>)
</p>
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