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  26 <div class="titlepage"><div><div><h4 class="title">
  27 <a name="boost_asio.reference.async_read_at.overload1"></a><a class="link" href="overload1.html" title="async_read_at (1 of 4 overloads)">async_read_at
  28         (1 of 4 overloads)</a>
  29 </h4></div></div></div>
  30 <p>
  31           Start an asynchronous operation to read a certain amount of data at the
  32           specified offset.
  33         </p>
  34 <pre class="programlisting">template&lt;
  35     typename <a class="link" href="../AsyncRandomAccessReadDevice.html" title="Buffer-oriented asynchronous random-access read device requirements">AsyncRandomAccessReadDevice</a>,
  36     typename <a class="link" href="../MutableBufferSequence.html" title="Mutable buffer sequence requirements">MutableBufferSequence</a>,
  37     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>&gt;
  38 <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(
  39     AsyncRandomAccessReadDevice &amp; d,
  40     uint64_t offset,
  41     const MutableBufferSequence &amp; buffers,
  42     ReadHandler &amp;&amp; handler = <a class="link" href="../asynchronous_operations.html#boost_asio.reference.asynchronous_operations.default_completion_tokens"><span class="emphasis"><em>DEFAULT</em></span></a>);
  43 </pre>
  44 <p>
  45           This function is used to asynchronously read a certain number of bytes
  46           of data from a random access device at the specified offset. The function
  47           call always returns immediately. The asynchronous operation will continue
  48           until one of the following conditions is true:
  49         </p>
  50 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
  51 <li class="listitem">
  52               The supplied buffers are full. That is, the bytes transferred is equal
  53               to the sum of the buffer sizes.
  54             </li>
  55 <li class="listitem">
  56               An error occurred.
  57             </li>
  58 </ul></div>
  59 <p>
  60           This operation is implemented in terms of zero or more calls to the device's
  61           async_read_some_at function.
  62         </p>
  63 <h6>
  64 <a name="boost_asio.reference.async_read_at.overload1.h0"></a>
  65           <span class="phrase"><a name="boost_asio.reference.async_read_at.overload1.parameters"></a></span><a class="link" href="overload1.html#boost_asio.reference.async_read_at.overload1.parameters">Parameters</a>
  66         </h6>
  67 <div class="variablelist">
  68 <p class="title"><b></b></p>
  69 <dl class="variablelist">
  70 <dt><span class="term">d</span></dt>
  71 <dd><p>
  72                 The device from which the data is to be read. The type must support
  73                 the AsyncRandomAccessReadDevice concept.
  74               </p></dd>
  75 <dt><span class="term">offset</span></dt>
  76 <dd><p>
  77                 The offset at which the data will be read.
  78               </p></dd>
  79 <dt><span class="term">buffers</span></dt>
  80 <dd><p>
  81                 One or more buffers into which the data will be read. The sum of
  82                 the buffer sizes indicates the maximum number of bytes to read from
  83                 the device. Although the buffers object may be copied as necessary,
  84                 ownership of the underlying memory blocks is retained by the caller,
  85                 which must guarantee that they remain valid until the handler is
  86                 called.
  87               </p></dd>
  88 <dt><span class="term">handler</span></dt>
  89 <dd>
  90 <p>
  91                 The handler to be called when the read operation completes. Copies
  92                 will be made of the handler as required. The function signature of
  93                 the handler must be:
  94 </p>
  95 <pre class="programlisting">void handler(
  96   // Result of operation.
  97   const boost::system::error_code&amp; error,
  98
  99   // Number of bytes copied into the buffers. If an error
 100   // occurred, this will be the number of bytes successfully
 101   // transferred prior to the error.
 102   std::size_t bytes_transferred
 103 );
 104 </pre>
 105 <p>
 106                 Regardless of whether the asynchronous operation completes immediately
 107                 or not, the handler will not be invoked from within this function.
 108                 On immediate completion, invocation of the handler will be performed
 109                 in a manner equivalent to using <a class="link" href="../post.html" title="post"><code class="computeroutput">post</code></a>.
 110               </p>
 111 </dd>
 112 </dl>
 113 </div>
 114 <h6>
 115 <a name="boost_asio.reference.async_read_at.overload1.h1"></a>
 116           <span class="phrase"><a name="boost_asio.reference.async_read_at.overload1.example"></a></span><a class="link" href="overload1.html#boost_asio.reference.async_read_at.overload1.example">Example</a>
 117         </h6>
 118 <p>
 119           To read into a single data buffer use the <a class="link" href="../buffer.html" title="buffer"><code class="computeroutput">buffer</code></a>
 120           function as follows:
 121         </p>
 122 <pre class="programlisting">boost::asio::async_read_at(d, 42, boost::asio::buffer(data, size), handler);
 123 </pre>
 124 <p>
 125           See the <a class="link" href="../buffer.html" title="buffer"><code class="computeroutput">buffer</code></a>
 126           documentation for information on reading into multiple buffers in one go,
 127           and how to use it with arrays, boost::array or std::vector.
 128         </p>
 129 <h6>
 130 <a name="boost_asio.reference.async_read_at.overload1.h2"></a>
 131           <span class="phrase"><a name="boost_asio.reference.async_read_at.overload1.remarks"></a></span><a class="link" href="overload1.html#boost_asio.reference.async_read_at.overload1.remarks">Remarks</a>
 132         </h6>
 133 <p>
 134           This overload is equivalent to calling:
 135         </p>
 136 <pre class="programlisting">boost::asio::async_read_at(
 137     d, 42, buffers,
 138     boost::asio::transfer_all(),
 139     handler);
 140 </pre>
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