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<title>async_read_at (2 of 4 overloads)</title>
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<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="asio.reference.async_read_at.overload2"></a><a class="link" href="overload2.html" title="async_read_at (2 of 4 overloads)">async_read_at
(2 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 <a class="link" href="../MutableBufferSequence.html" title="Mutable buffer sequence requirements">MutableBufferSequence</a>,
typename <a class="link" href="../CompletionCondition.html" title="Completion condition requirements">CompletionCondition</a>,
typename <a class="link" href="../ReadHandler.html" title="Read handler requirements">ReadHandler</a> = <a class="link" href="../asynchronous_operations.html#asio.reference.asynchronous_operations.default_completion_tokens"><span class="emphasis"><em>DEFAULT</em></span></a>>
<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> async_read_at(
AsyncRandomAccessReadDevice & d,
uint64_t offset,
const MutableBufferSequence & buffers,
CompletionCondition completion_condition,
ReadHandler && handler = <a class="link" href="../asynchronous_operations.html#asio.reference.asynchronous_operations.default_completion_tokens"><span class="emphasis"><em>DEFAULT</em></span></a>);
</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" 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_at.overload2.h0"></a>
<span><a name="asio.reference.async_read_at.overload2.parameters"></a></span><a class="link" href="overload2.html#asio.reference.async_read_at.overload2.parameters">Parameters</a>
</h6>
<div class="variablelist">
<p class="title"><b></b></p>
<dl>
<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">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">std::size_t completion_condition(
// Result of latest async_read_some_at operation.
const asio::error_code& error,
// Number of bytes transferred so far.
std::size_t bytes_transferred
);
</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 device's async_read_some_at function.
</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 asio::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.
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">post</code></a>.
</p>
</dd>
</dl>
</div>
<h6>
<a name="asio.reference.async_read_at.overload2.h1"></a>
<span><a name="asio.reference.async_read_at.overload2.example"></a></span><a class="link" href="overload2.html#asio.reference.async_read_at.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">buffer</code></a>
function as follows:
</p>
<pre class="programlisting">asio::async_read_at(d, 42,
asio::buffer(data, size),
asio::transfer_at_least(32),
handler);
</pre>
<p>
See the <a class="link" href="../buffer.html" title="buffer"><code class="computeroutput">buffer</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>
</div>
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<td align="right"><div class="copyright-footer">Copyright © 2003-2020 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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