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<div class="titlepage"><div><div><h4 class="title">
<a name="boost_asio.reference.async_read_at.overload1"></a><a class="link" href="overload1.html" title="async_read_at (1 of 4 overloads)">async_read_at
(1 of 4 overloads)</a>
</h4></div></div></div>
<p>
Start an asynchronous operation to read a certain amount of data at the
specified offset.
</p>
<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span>
<span class="keyword">typename</span> <a class="link" href="../AsyncRandomAccessReadDevice.html" title="Buffer-oriented asynchronous random-access read device requirements">AsyncRandomAccessReadDevice</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="../ReadHandler.html" title="Read handler requirements">ReadHandler</a><span class="special">></span>
<a class="link" href="../asynchronous_operations.html#boost_asio.reference.asynchronous_operations.return_type_of_an_initiating_function"><span class="emphasis"><em>void-or-deduced</em></span></a> <span class="identifier">async_read_at</span><span class="special">(</span>
<span class="identifier">AsyncRandomAccessReadDevice</span> <span class="special">&</span> <span class="identifier">d</span><span class="special">,</span>
<span class="identifier">uint64_t</span> <span class="identifier">offset</span><span class="special">,</span>
<span class="keyword">const</span> <span class="identifier">MutableBufferSequence</span> <span class="special">&</span> <span class="identifier">buffers</span><span class="special">,</span>
<span class="identifier">ReadHandler</span> <span class="identifier">handler</span><span class="special">);</span>
</pre>
<p>
This function is used to asynchronously read a certain number of bytes
of data from a random access device at the specified offset. The function
call always returns immediately. The asynchronous operation will continue
until one of the following conditions is true:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-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">
An error occurred.
</li>
</ul></div>
<p>
This operation is implemented in terms of zero or more calls to the device's
async_read_some_at function.
</p>
<h6>
<a name="boost_asio.reference.async_read_at.overload1.h0"></a>
<span class="phrase"><a name="boost_asio.reference.async_read_at.overload1.parameters"></a></span><a class="link" href="overload1.html#boost_asio.reference.async_read_at.overload1.parameters">Parameters</a>
</h6>
<div class="variablelist">
<p class="title"><b></b></p>
<dl class="variablelist">
<dt><span class="term">d</span></dt>
<dd><p>
The device from which the data is to be read. The type must support
the AsyncRandomAccessReadDevice concept.
</p></dd>
<dt><span class="term">offset</span></dt>
<dd><p>
The offset at which the data will be read.
</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 device. 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 handler is
called.
</p></dd>
<dt><span class="term">handler</span></dt>
<dd>
<p>
The handler to be called when the read operation completes. Copies
will be made of the handler as required. The function signature of
the 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">boost</span><span class="special">::</span><span class="identifier">system</span><span class="special">::</span><span class="identifier">error_code</span><span class="special">&</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 handler will not be invoked from within this function.
Invocation of the handler will be performed in a manner equivalent
to using <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">post</span><span class="special">()</span></code>.
</p>
</dd>
</dl>
</div>
<h6>
<a name="boost_asio.reference.async_read_at.overload1.h1"></a>
<span class="phrase"><a name="boost_asio.reference.async_read_at.overload1.example"></a></span><a class="link" href="overload1.html#boost_asio.reference.async_read_at.overload1.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">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">async_read_at</span><span class="special">(</span><span class="identifier">d</span><span class="special">,</span> <span class="number">42</span><span class="special">,</span> <span class="identifier">boost</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">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="boost_asio.reference.async_read_at.overload1.h2"></a>
<span class="phrase"><a name="boost_asio.reference.async_read_at.overload1.remarks"></a></span><a class="link" href="overload1.html#boost_asio.reference.async_read_at.overload1.remarks">Remarks</a>
</h6>
<p>
This overload is equivalent to calling:
</p>
<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">async_read_at</span><span class="special">(</span>
<span class="identifier">d</span><span class="special">,</span> <span class="number">42</span><span class="special">,</span> <span class="identifier">buffers</span><span class="special">,</span>
<span class="identifier">boost</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>
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<td align="right"><div class="copyright-footer">Copyright © 2003-2015 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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