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<title>read_at (4 of 8 overloads)</title>
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<div class="titlepage"><div><div><h4 class="title">
<a name="boost_asio.reference.read_at.overload4"></a><a class="link" href="overload4.html" title="read_at (4 of 8 overloads)">read_at (4
        of 8 overloads)</a>
</h4></div></div></div>
<p>
          Attempt to read a certain amount of data at the specified offset before
          returning.
        </p>
<pre class="programlisting">template&lt;
    typename <a class="link" href="../SyncRandomAccessReadDevice.html" title="Buffer-oriented synchronous random-access read device requirements">SyncRandomAccessReadDevice</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>&gt;
std::size_t read_at(
    SyncRandomAccessReadDevice &amp; d,
    uint64_t offset,
    const MutableBufferSequence &amp; buffers,
    CompletionCondition completion_condition,
    boost::system::error_code &amp; ec);
</pre>
<p>
          This function is used to read a certain number of bytes of data from a
          random access device at the specified offset. The call will block until
          one of the following conditions is true:
        </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-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>
<p>
          This operation is implemented in terms of zero or more calls to the device's
          read_some_at function.
        </p>
<h6>
<a name="boost_asio.reference.read_at.overload4.h0"></a>
          <span class="phrase"><a name="boost_asio.reference.read_at.overload4.parameters"></a></span><a class="link" href="overload4.html#boost_asio.reference.read_at.overload4.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 SyncRandomAccessReadDevice 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.
              </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 read_some_at operation.
  const boost::system::error_code&amp; 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 read_some_at function.
              </p>
</dd>
<dt><span class="term">ec</span></dt>
<dd><p>
                Set to indicate what error occurred, if any.
              </p></dd>
</dl>
</div>
<h6>
<a name="boost_asio.reference.read_at.overload4.h1"></a>
          <span class="phrase"><a name="boost_asio.reference.read_at.overload4.return_value"></a></span><a class="link" href="overload4.html#boost_asio.reference.read_at.overload4.return_value">Return Value</a>
        </h6>
<p>
          The number of bytes read. If an error occurs, returns the total number
          of bytes successfully transferred prior to the error.
        </p>
</div>
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<td align="right"><div class="copyright-footer">Copyright &#169; 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>)
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