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128 lines
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<title>read_at (8 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.overload8"></a><a class="link" href="overload8.html" title="read_at (8 of 8 overloads)">read_at (8
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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 Allocator,
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typename <a class="link" href="../CompletionCondition.html" title="Completion condition requirements">CompletionCondition</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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basic_streambuf< Allocator > & b,
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CompletionCondition completion_condition,
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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; "><li class="listitem">
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The completion_condition function object returns 0.
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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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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.overload8.h0"></a>
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<span class="phrase"><a name="boost_asio.reference.read_at.overload8.parameters"></a></span><a class="link" href="overload8.html#boost_asio.reference.read_at.overload8.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">b</span></dt>
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<dd><p>
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The <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.
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</p></dd>
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<dt><span class="term">completion_condition</span></dt>
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<dd>
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<p>
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The function object to be called to determine whether the read operation
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is complete. The signature of the function object must be:
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</p>
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<pre class="programlisting">std::size_t completion_condition(
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// Result of latest read_some_at operation.
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const boost::system::error_code& error,
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// Number of bytes transferred so far.
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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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A return value of 0 indicates that the read operation is complete.
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A non-zero return value indicates the maximum number of bytes to
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be read on the next call to the device's read_some_at function.
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</p>
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</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.overload8.h1"></a>
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<span class="phrase"><a name="boost_asio.reference.read_at.overload8.return_value"></a></span><a class="link" href="overload8.html#boost_asio.reference.read_at.overload8.return_value">Return Value</a>
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</h6>
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<p>
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The number of bytes read. If an error occurs, returns the total number
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of bytes successfully transferred prior to the error.
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</p>
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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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