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<title>async_read_at (3 of 4 overloads)</title>
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<div class="titlepage"><div><div><h4 class="title">
<a name="boost_asio.reference.async_read_at.overload3"></a><a class="link" href="overload3.html" title="async_read_at (3 of 4 overloads)">async_read_at
(3 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">template<
typename <a class="link" href="../AsyncRandomAccessReadDevice.html" title="Buffer-oriented asynchronous random-access read device requirements">AsyncRandomAccessReadDevice</a>,
typename Allocator,
typename <a class="link" href="../ReadHandler.html" title="Read handler requirements">ReadHandler</a>>
<a class="link" href="../asynchronous_operations.html#boost_asio.reference.asynchronous_operations.automatic_deduction_of_initiating_function_return_type"><span class="emphasis"><em>DEDUCED</em></span></a> async_read_at(
AsyncRandomAccessReadDevice & d,
uint64_t offset,
basic_streambuf< Allocator > & b,
ReadHandler && handler);
</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">
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.overload3.h0"></a>
<span class="phrase"><a name="boost_asio.reference.async_read_at.overload3.parameters"></a></span><a class="link" href="overload3.html#boost_asio.reference.async_read_at.overload3.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">b</span></dt>
<dd><p>
A <a class="link" href="../basic_streambuf.html" title="basic_streambuf"><code class="computeroutput">basic_streambuf</code></a>
object into which the data will be read. Ownership of the streambuf
is retained by the caller, which must guarantee that it remains 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">void handler(
// Result of operation.
const boost::system::error_code& error,
// Number of bytes copied into the buffers. If an error
// occurred, this will be the number of bytes successfully
// transferred prior to the error.
std::size_t bytes_transferred
);
</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">boost::asio::io_context::post()</code>.
</p>
</dd>
</dl>
</div>
<h6>
<a name="boost_asio.reference.async_read_at.overload3.h1"></a>
<span class="phrase"><a name="boost_asio.reference.async_read_at.overload3.remarks"></a></span><a class="link" href="overload3.html#boost_asio.reference.async_read_at.overload3.remarks">Remarks</a>
</h6>
<p>
This overload is equivalent to calling:
</p>
<pre class="programlisting">boost::asio::async_read_at(
d, 42, b,
boost::asio::transfer_all(),
handler);
</pre>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright © 2003-2018 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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