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  26 <div class="titlepage"><div><div><h4 class="title">
  27 <a name="boost_asio.reference.async_read_at.overload4"></a><a class="link" href="overload4.html" title="async_read_at (4 of 4 overloads)">async_read_at
  28         (4 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 Allocator,
  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     basic_streambuf&lt; Allocator &gt; &amp; b,
  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; "><li class="listitem">
  53               The completion_condition function object returns 0.
  54             </li></ul></div>
  55 <p>
  56           This operation is implemented in terms of zero or more calls to the device's
  57           async_read_some_at function.
  58         </p>
  59 <h6>
  60 <a name="boost_asio.reference.async_read_at.overload4.h0"></a>
  61           <span class="phrase"><a name="boost_asio.reference.async_read_at.overload4.parameters"></a></span><a class="link" href="overload4.html#boost_asio.reference.async_read_at.overload4.parameters">Parameters</a>
  62         </h6>
  63 <div class="variablelist">
  64 <p class="title"><b></b></p>
  65 <dl class="variablelist">
  66 <dt><span class="term">d</span></dt>
  67 <dd><p>
  68                 The device from which the data is to be read. The type must support
  69                 the AsyncRandomAccessReadDevice concept.
  70               </p></dd>
  71 <dt><span class="term">offset</span></dt>
  72 <dd><p>
  73                 The offset at which the data will be read.
  74               </p></dd>
  75 <dt><span class="term">b</span></dt>
  76 <dd><p>
  77                 A <a class="link" href="../basic_streambuf.html" title="basic_streambuf"><code class="computeroutput">basic_streambuf</code></a>
  78                 object into which the data will be read. Ownership of the streambuf
  79                 is retained by the caller, which must guarantee that it remains valid
  80                 until the handler is called.
  81               </p></dd>
  82 <dt><span class="term">completion_condition</span></dt>
  83 <dd>
  84 <p>
  85                 The function object to be called to determine whether the read operation
  86                 is complete. The signature of the function object must be:
  87 </p>
  88 <pre class="programlisting">std::size_t completion_condition(
  89   // Result of latest async_read_some_at operation.
  90   const boost::system::error_code&amp; error,
  91
  92   // Number of bytes transferred so far.
  93   std::size_t bytes_transferred
  94 );
  95 </pre>
  96 <p>
  97                 A return value of 0 indicates that the read operation is complete.
  98                 A non-zero return value indicates the maximum number of bytes to
  99                 be read on the next call to the device's async_read_some_at function.
 100               </p>
 101 </dd>
 102 <dt><span class="term">handler</span></dt>
 103 <dd>
 104 <p>
 105                 The handler to be called when the read operation completes. Copies
 106                 will be made of the handler as required. The function signature of
 107                 the handler must be:
 108 </p>
 109 <pre class="programlisting">void handler(
 110   // Result of operation.
 111   const boost::system::error_code&amp; error,
 112
 113   // Number of bytes copied into the buffers. If an error
 114   // occurred, this will be the number of bytes successfully
 115   // transferred prior to the error.
 116   std::size_t bytes_transferred
 117 );
 118 </pre>
 119 <p>
 120                 Regardless of whether the asynchronous operation completes immediately
 121                 or not, the handler will not be invoked from within this function.
 122                 On immediate completion, invocation of the handler will be performed
 123                 in a manner equivalent to using <a class="link" href="../post.html" title="post"><code class="computeroutput">post</code></a>.
 124               </p>
 125 </dd>
 126 </dl>
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