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134 lines
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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">
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<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
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(3 of 4 overloads)</a>
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</h4></div></div></div>
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<p>
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Start an asynchronous operation to read a certain amount of data at the
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specified offset.
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</p>
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<pre class="programlisting">template<
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typename <a class="link" href="../AsyncRandomAccessReadDevice.html" title="Buffer-oriented asynchronous random-access read device requirements">AsyncRandomAccessReadDevice</a>,
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typename Allocator,
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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.default_completion_tokens"><span class="emphasis"><em>DEFAULT</em></span></a>>
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<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(
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AsyncRandomAccessReadDevice & d,
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uint64_t offset,
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basic_streambuf< Allocator > & b,
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ReadHandler && handler = <a class="link" href="../asynchronous_operations.html#boost_asio.reference.asynchronous_operations.default_completion_tokens"><span class="emphasis"><em>DEFAULT</em></span></a>);
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</pre>
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<p>
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This function is used to asynchronously read a certain number of bytes
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of data from a random access device at the specified offset. The function
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call always returns immediately. The asynchronous operation will continue
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until one of the following conditions is true:
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</p>
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<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
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An error occurred.
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</li></ul></div>
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<p>
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This operation is implemented in terms of zero or more calls to the device's
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async_read_some_at function.
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</p>
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<h6>
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<a name="boost_asio.reference.async_read_at.overload3.h0"></a>
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<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>
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</h6>
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<div class="variablelist">
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<p class="title"><b></b></p>
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<dl class="variablelist">
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<dt><span class="term">d</span></dt>
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<dd><p>
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The device from which the data is to be read. The type must support
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the AsyncRandomAccessReadDevice concept.
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</p></dd>
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<dt><span class="term">offset</span></dt>
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<dd><p>
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The offset at which the data will be read.
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</p></dd>
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<dt><span class="term">b</span></dt>
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<dd><p>
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A <a class="link" href="../basic_streambuf.html" title="basic_streambuf"><code class="computeroutput">basic_streambuf</code></a>
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object into which the data will be read. Ownership of the streambuf
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is retained by the caller, which must guarantee that it remains valid
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until the handler is called.
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</p></dd>
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<dt><span class="term">handler</span></dt>
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<dd>
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<p>
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The handler to be called when the read operation completes. Copies
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will be made of the handler as required. The function signature of
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the handler must be:
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</p>
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<pre class="programlisting">void handler(
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// Result of operation.
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const boost::system::error_code& error,
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// Number of bytes copied into the buffers. If an error
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// occurred, this will be the number of bytes successfully
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// transferred prior to the error.
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std::size_t bytes_transferred
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);
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</pre>
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<p>
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Regardless of whether the asynchronous operation completes immediately
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or not, the handler will not be invoked from within this function.
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On immediate completion, invocation of the handler will be performed
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in a manner equivalent to using <a class="link" href="../post.html" title="post"><code class="computeroutput">post</code></a>.
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</p>
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</dd>
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</dl>
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</div>
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<h6>
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<a name="boost_asio.reference.async_read_at.overload3.h1"></a>
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<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>
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</h6>
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<p>
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This overload is equivalent to calling:
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</p>
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<pre class="programlisting">boost::asio::async_read_at(
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d, 42, b,
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boost::asio::transfer_all(),
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handler);
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</pre>
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</div>
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="left"></td>
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<td align="right"><div class="copyright-footer">Copyright © 2003-2021 Christopher
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M. Kohlhoff<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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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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