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  26 <div class="titlepage"><div><div><h4 class="title">
  27 <a name="boost_asio.reference.async_read.overload2"></a><a class="link" href="overload2.html" title="async_read (2 of 8 overloads)">async_read
  28         (2 of 8 overloads)</a>
  29 </h4></div></div></div>
  30 <p>
  31           Start an asynchronous operation to read a certain amount of data from a
  32           stream.
  33         </p>
  34 <pre class="programlisting">template&lt;
  35     typename <a class="link" href="../AsyncReadStream.html" title="Buffer-oriented asynchronous read stream requirements">AsyncReadStream</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(
  40     AsyncReadStream &amp; s,
  41     const MutableBufferSequence &amp; buffers,
  42     CompletionCondition completion_condition,
  43     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>,
  44     typename enable_if&lt; is_mutable_buffer_sequence&lt; MutableBufferSequence &gt;::value &gt;::type *  = 0);
  45 </pre>
  46 <p>
  47           This function is used to asynchronously read a certain number of bytes
  48           of data from a stream. The function call always returns immediately. The
  49           asynchronous operation will continue until one of the following conditions
  50           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.overload2.h0"></a>
  63           <span class="phrase"><a name="boost_asio.reference.async_read.overload2.parameters"></a></span><a class="link" href="overload2.html#boost_asio.reference.async_read.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">s</span></dt>
  69 <dd><p>
  70                 The stream from which the data is to be read. The type must support
  71                 the AsyncReadStream concept.
  72               </p></dd>
  73 <dt><span class="term">buffers</span></dt>
  74 <dd><p>
  75                 One or more buffers into which the data will be read. The sum of
  76                 the buffer sizes indicates the maximum number of bytes to read from
  77                 the stream. Although the buffers object may be copied as necessary,
  78                 ownership of the underlying memory blocks is retained by the caller,
  79                 which must guarantee that they remain valid until the handler is
  80                 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 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 stream's async_read_some 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   const boost::system::error_code&amp; error, // Result of operation.
 111
 112   std::size_t bytes_transferred           // Number of bytes copied into the
 113                                           // buffers. If an error occurred,
 114                                           // this will be the  number of
 115                                           // bytes successfully transferred
 116                                           // prior to the error.
 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>
 127 </div>
 128 <h6>
 129 <a name="boost_asio.reference.async_read.overload2.h1"></a>
 130           <span class="phrase"><a name="boost_asio.reference.async_read.overload2.example"></a></span><a class="link" href="overload2.html#boost_asio.reference.async_read.overload2.example">Example</a>
 131         </h6>
 132 <p>
 133           To read into a single data buffer use the <a class="link" href="../buffer.html" title="buffer"><code class="computeroutput">buffer</code></a>
 134           function as follows:
 135         </p>
 136 <pre class="programlisting">boost::asio::async_read(s,
 137     boost::asio::buffer(data, size),
 138     boost::asio::transfer_at_least(32),
 139     handler);
 140 </pre>
 141 <p>
 142           See the <a class="link" href="../buffer.html" title="buffer"><code class="computeroutput">buffer</code></a>
 143           documentation for information on reading into multiple buffers in one go,
 144           and how to use it with arrays, boost::array or std::vector.
 145         </p>
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