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  26 <div class="titlepage"><div><div><h4 class="title">
  27 <a name="boost_asio.reference.async_read_at.overload3"></a><a class="link" href="overload3.html" title="async_read_at (3 of 4 overloads)">async_read_at
  28         (3 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"><span class="keyword">template</span><span class="special">&lt;</span>
  35     <span class="keyword">typename</span> <a class="link" href="../AsyncRandomAccessReadDevice.html" title="Buffer-oriented asynchronous random-access read device requirements">AsyncRandomAccessReadDevice</a><span class="special">,</span>
  36     <span class="keyword">typename</span> <span class="identifier">Allocator</span><span class="special">,</span>
  37     <span class="keyword">typename</span> <a class="link" href="../ReadHandler.html" title="Read handler requirements">ReadHandler</a><span class="special">&gt;</span>
  38 <a class="link" href="../asynchronous_operations.html#boost_asio.reference.asynchronous_operations.return_type_of_an_initiating_function"><span class="emphasis"><em>void-or-deduced</em></span></a> <span class="identifier">async_read_at</span><span class="special">(</span>
  39     <span class="identifier">AsyncRandomAccessReadDevice</span> <span class="special">&amp;</span> <span class="identifier">d</span><span class="special">,</span>
  40     <span class="identifier">uint64_t</span> <span class="identifier">offset</span><span class="special">,</span>
  41     <span class="identifier">basic_streambuf</span><span class="special">&lt;</span> <span class="identifier">Allocator</span> <span class="special">&gt;</span> <span class="special">&amp;</span> <span class="identifier">b</span><span class="special">,</span>
  42     <span class="identifier">ReadHandler</span> <span class="identifier">handler</span><span class="special">);</span>
  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; "><li class="listitem">
  51               An error occurred.
  52             </li></ul></div>
  53 <p>
  54           This operation is implemented in terms of zero or more calls to the device's
  55           async_read_some_at function.
  56         </p>
  57 <h6>
  58 <a name="boost_asio.reference.async_read_at.overload3.h0"></a>
  59           <span class="phrase"><a name="boost_asio.reference.async_read_at.overload3.parameters"></a></span><a class="link" href="overload3.html#boost_asio.reference.async_read_at.overload3.parameters">Parameters</a>
  60         </h6>
  61 <div class="variablelist">
  62 <p class="title"><b></b></p>
  63 <dl class="variablelist">
  64 <dt><span class="term">d</span></dt>
  65 <dd><p>
  66                 The device from which the data is to be read. The type must support
  67                 the AsyncRandomAccessReadDevice concept.
  68               </p></dd>
  69 <dt><span class="term">offset</span></dt>
  70 <dd><p>
  71                 The offset at which the data will be read.
  72               </p></dd>
  73 <dt><span class="term">b</span></dt>
  74 <dd><p>
  75                 A <a class="link" href="../basic_streambuf.html" title="basic_streambuf"><code class="computeroutput"><span class="identifier">basic_streambuf</span></code></a> object into
  76                 which the data will be read. Ownership of the streambuf is retained
  77                 by the caller, which must guarantee that it remains valid until the
  78                 handler is called.
  79               </p></dd>
  80 <dt><span class="term">handler</span></dt>
  81 <dd>
  82 <p>
  83                 The handler to be called when the read operation completes. Copies
  84                 will be made of the handler as required. The function signature of
  85                 the handler must be:
  86 </p>
  87 <pre class="programlisting"><span class="keyword">void</span> <span class="identifier">handler</span><span class="special">(</span>
  88   <span class="comment">// Result of operation.</span>
  89   <span class="keyword">const</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">system</span><span class="special">::</span><span class="identifier">error_code</span><span class="special">&amp;</span> <span class="identifier">error</span><span class="special">,</span>
  90
  91   <span class="comment">// Number of bytes copied into the buffers. If an error</span>
  92   <span class="comment">// occurred, this will be the number of bytes successfully</span>
  93   <span class="comment">// transferred prior to the error.</span>
  94   <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">bytes_transferred</span>
  95 <span class="special">);</span>
  96 </pre>
  97 <p>
  98                 Regardless of whether the asynchronous operation completes immediately
  99                 or not, the handler will not be invoked from within this function.
 100                 Invocation of the handler will be performed in a manner equivalent
 101                 to using <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">io_service</span><span class="special">::</span><span class="identifier">post</span><span class="special">()</span></code>.
 102               </p>
 103 </dd>
 104 </dl>
 105 </div>
 106 <h6>
 107 <a name="boost_asio.reference.async_read_at.overload3.h1"></a>
 108           <span class="phrase"><a name="boost_asio.reference.async_read_at.overload3.remarks"></a></span><a class="link" href="overload3.html#boost_asio.reference.async_read_at.overload3.remarks">Remarks</a>
 109         </h6>
 110 <p>
 111           This overload is equivalent to calling:
 112         </p>
 113 <pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">async_read_at</span><span class="special">(</span>
 114     <span class="identifier">d</span><span class="special">,</span> <span class="number">42</span><span class="special">,</span> <span class="identifier">b</span><span class="special">,</span>
 115     <span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">transfer_all</span><span class="special">(),</span>
 116     <span class="identifier">handler</span><span class="special">);</span>
 117 </pre>
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 121 <td align="right"><div class="copyright-footer">Copyright &#169; 2003-2017 Christopher M. Kohlhoff<p>
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 123         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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