139 lines
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139 lines
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<title>read_at (2 of 8 overloads)</title>
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<div class="titlepage"><div><div><h4 class="title">
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<a name="boost_asio.reference.read_at.overload2"></a><a class="link" href="overload2.html" title="read_at (2 of 8 overloads)">read_at (2
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of 8 overloads)</a>
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</h4></div></div></div>
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<p>
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Attempt to read a certain amount of data at the specified offset before
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returning.
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</p>
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<pre class="programlisting">template<
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typename <a class="link" href="../SyncRandomAccessReadDevice.html" title="Buffer-oriented synchronous random-access read device requirements">SyncRandomAccessReadDevice</a>,
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typename <a class="link" href="../MutableBufferSequence.html" title="Mutable buffer sequence requirements">MutableBufferSequence</a>>
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std::size_t read_at(
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SyncRandomAccessReadDevice & d,
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uint64_t offset,
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const MutableBufferSequence & buffers,
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boost::system::error_code & ec);
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</pre>
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<p>
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This function is used to read a certain number of bytes of data from a
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random access device at the specified offset. The call will block until
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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; ">
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<li class="listitem">
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The supplied buffers are full. That is, the bytes transferred is equal
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to the sum of the buffer sizes.
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</li>
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<li class="listitem">
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An error occurred.
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</li>
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</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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read_some_at function.
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</p>
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<h6>
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<a name="boost_asio.reference.read_at.overload2.h0"></a>
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<span class="phrase"><a name="boost_asio.reference.read_at.overload2.parameters"></a></span><a class="link" href="overload2.html#boost_asio.reference.read_at.overload2.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 SyncRandomAccessReadDevice 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">buffers</span></dt>
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<dd><p>
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One or more buffers into which the data will be read. The sum of
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the buffer sizes indicates the maximum number of bytes to read from
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the device.
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</p></dd>
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<dt><span class="term">ec</span></dt>
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<dd><p>
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Set to indicate what error occurred, if any.
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</p></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.read_at.overload2.h1"></a>
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<span class="phrase"><a name="boost_asio.reference.read_at.overload2.return_value"></a></span><a class="link" href="overload2.html#boost_asio.reference.read_at.overload2.return_value">Return Value</a>
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</h6>
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<p>
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The number of bytes transferred.
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</p>
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<h6>
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<a name="boost_asio.reference.read_at.overload2.h2"></a>
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<span class="phrase"><a name="boost_asio.reference.read_at.overload2.example"></a></span><a class="link" href="overload2.html#boost_asio.reference.read_at.overload2.example">Example</a>
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</h6>
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<p>
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To read into a single data buffer use the <a class="link" href="../buffer.html" title="buffer"><code class="computeroutput">buffer</code></a>
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function as follows:
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</p>
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<pre class="programlisting">boost::asio::read_at(d, 42,
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boost::asio::buffer(data, size), ec);
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</pre>
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<p>
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See the <a class="link" href="../buffer.html" title="buffer"><code class="computeroutput">buffer</code></a>
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documentation for information on reading into multiple buffers in one go,
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and how to use it with arrays, boost::array or std::vector.
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</p>
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<h6>
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<a name="boost_asio.reference.read_at.overload2.h3"></a>
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<span class="phrase"><a name="boost_asio.reference.read_at.overload2.remarks"></a></span><a class="link" href="overload2.html#boost_asio.reference.read_at.overload2.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::read_at(
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d, 42, buffers,
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boost::asio::transfer_all(), ec);
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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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</p>
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</div></td>
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