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  26 <div class="titlepage"><div><div><h4 class="title">
  27 <a name="boost_asio.reference.async_read_at.overload2"></a><a class="link" href="overload2.html" title="async_read_at (2 of 4 overloads)">async_read_at
  28         (2 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="../CompletionCondition.html" title="Completion condition requirements">CompletionCondition</a>,
  38     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;
  39 <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(
  40     AsyncRandomAccessReadDevice &amp; d,
  41     uint64_t offset,
  42     const MutableBufferSequence &amp; buffers,
  43     CompletionCondition completion_condition,
  44     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>);
  45 </pre>
  46 <p>
  47           This function is used to asynchronously read a certain number of bytes
  48           of data from a random access device at the specified offset. The function
  49           call always returns immediately. The asynchronous operation will continue
  50           until one of the following conditions is true:
  51         </p>
  52 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
  53 <li class="listitem">
  54               The supplied buffers are full. That is, the bytes transferred is equal
  55               to the sum of the buffer sizes.
  56             </li>
  57 <li class="listitem">
  58               The completion_condition function object returns 0.
  59             </li>
  60 </ul></div>
  61 <h6>
  62 <a name="boost_asio.reference.async_read_at.overload2.h0"></a>
  63           <span class="phrase"><a name="boost_asio.reference.async_read_at.overload2.parameters"></a></span><a class="link" href="overload2.html#boost_asio.reference.async_read_at.overload2.parameters">Parameters</a>
  64         </h6>
  65 <div class="variablelist">
  66 <p class="title"><b></b></p>
  67 <dl class="variablelist">
  68 <dt><span class="term">d</span></dt>
  69 <dd><p>
  70                 The device from which the data is to be read. The type must support
  71                 the AsyncRandomAccessReadDevice concept.
  72               </p></dd>
  73 <dt><span class="term">offset</span></dt>
  74 <dd><p>
  75                 The offset at which the data will be read.
  76               </p></dd>
  77 <dt><span class="term">buffers</span></dt>
  78 <dd><p>
  79                 One or more buffers into which the data will be read. The sum of
  80                 the buffer sizes indicates the maximum number of bytes to read from
  81                 the device. Although the buffers object may be copied as necessary,
  82                 ownership of the underlying memory blocks is retained by the caller,
  83                 which must guarantee that they remain valid until the handler is
  84                 called.
  85               </p></dd>
  86 <dt><span class="term">completion_condition</span></dt>
  87 <dd>
  88 <p>
  89                 The function object to be called to determine whether the read operation
  90                 is complete. The signature of the function object must be:
  91 </p>
  92 <pre class="programlisting">std::size_t completion_condition(
  93   // Result of latest async_read_some_at operation.
  94   const boost::system::error_code&amp; error,
  95
  96   // Number of bytes transferred so far.
  97   std::size_t bytes_transferred
  98 );
  99 </pre>
 100 <p>
 101                 A return value of 0 indicates that the read operation is complete.
 102                 A non-zero return value indicates the maximum number of bytes to
 103                 be read on the next call to the device's async_read_some_at function.
 104               </p>
 105 </dd>
 106 <dt><span class="term">handler</span></dt>
 107 <dd>
 108 <p>
 109                 The handler to be called when the read operation completes. Copies
 110                 will be made of the handler as required. The function signature of
 111                 the handler must be:
 112 </p>
 113 <pre class="programlisting">void handler(
 114   // Result of operation.
 115   const boost::system::error_code&amp; error,
 116
 117   // Number of bytes copied into the buffers. If an error
 118   // occurred, this will be the number of bytes successfully
 119   // transferred prior to the error.
 120   std::size_t bytes_transferred
 121 );
 122 </pre>
 123 <p>
 124                 Regardless of whether the asynchronous operation completes immediately
 125                 or not, the handler will not be invoked from within this function.
 126                 On immediate completion, invocation of the handler will be performed
 127                 in a manner equivalent to using <a class="link" href="../post.html" title="post"><code class="computeroutput">post</code></a>.
 128               </p>
 129 </dd>
 130 </dl>
 131 </div>
 132 <h6>
 133 <a name="boost_asio.reference.async_read_at.overload2.h1"></a>
 134           <span class="phrase"><a name="boost_asio.reference.async_read_at.overload2.example"></a></span><a class="link" href="overload2.html#boost_asio.reference.async_read_at.overload2.example">Example</a>
 135         </h6>
 136 <p>
 137           To read into a single data buffer use the <a class="link" href="../buffer.html" title="buffer"><code class="computeroutput">buffer</code></a>
 138           function as follows:
 139         </p>
 140 <pre class="programlisting">boost::asio::async_read_at(d, 42,
 141     boost::asio::buffer(data, size),
 142     boost::asio::transfer_at_least(32),
 143     handler);
 144 </pre>
 145 <p>
 146           See the <a class="link" href="../buffer.html" title="buffer"><code class="computeroutput">buffer</code></a>
 147           documentation for information on reading into multiple buffers in one go,
 148           and how to use it with arrays, boost::array or std::vector.
 149         </p>
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