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  26 <div class="titlepage"><div><div><h4 class="title">
  27 <a name="boost_asio.reference.async_read.overload8"></a><a class="link" href="overload8.html" title="async_read (8 of 8 overloads)">async_read
  28         (8 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="../DynamicBuffer_v2.html" title="Dynamic buffer requirements (version 2)">DynamicBuffer_v2</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     DynamicBuffer_v2 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_dynamic_buffer_v2&lt; DynamicBuffer_v2 &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 specified dynamic buffer sequence is full (that is, it has reached
  55               maximum size).
  56             </li>
  57 <li class="listitem">
  58               The completion_condition function object returns 0.
  59             </li>
  60 </ul></div>
  61 <p>
  62           This operation is implemented in terms of zero or more calls to the stream's
  63           async_read_some function, and is known as a <span class="emphasis"><em>composed operation</em></span>.
  64           The program must ensure that the stream performs no other read operations
  65           (such as async_read, the stream's async_read_some function, or any other
  66           composed operations that perform reads) until this operation completes.
  67         </p>
  68 <h6>
  69 <a name="boost_asio.reference.async_read.overload8.h0"></a>
  70           <span class="phrase"><a name="boost_asio.reference.async_read.overload8.parameters"></a></span><a class="link" href="overload8.html#boost_asio.reference.async_read.overload8.parameters">Parameters</a>
  71         </h6>
  72 <div class="variablelist">
  73 <p class="title"><b></b></p>
  74 <dl class="variablelist">
  75 <dt><span class="term">s</span></dt>
  76 <dd><p>
  77                 The stream from which the data is to be read. The type must support
  78                 the AsyncReadStream concept.
  79               </p></dd>
  80 <dt><span class="term">buffers</span></dt>
  81 <dd><p>
  82                 The dynamic buffer sequence into which the data will be read. Although
  83                 the buffers object may be copied as necessary, ownership of the underlying
  84                 memory blocks is retained by the caller, which must guarantee that
  85                 they remain valid until the handler is called.
  86               </p></dd>
  87 <dt><span class="term">completion_condition</span></dt>
  88 <dd>
  89 <p>
  90                 The function object to be called to determine whether the read operation
  91                 is complete. The signature of the function object must be:
  92 </p>
  93 <pre class="programlisting">std::size_t completion_condition(
  94   // Result of latest async_read_some operation.
  95   const boost::system::error_code&amp; error,
  96
  97   // Number of bytes transferred so far.
  98   std::size_t bytes_transferred
  99 );
 100 </pre>
 101 <p>
 102                 A return value of 0 indicates that the read operation is complete.
 103                 A non-zero return value indicates the maximum number of bytes to
 104                 be read on the next call to the stream's async_read_some function.
 105               </p>
 106 </dd>
 107 <dt><span class="term">handler</span></dt>
 108 <dd>
 109 <p>
 110                 The handler to be called when the read operation completes. Copies
 111                 will be made of the handler as required. The function signature of
 112                 the handler must be:
 113 </p>
 114 <pre class="programlisting">void handler(
 115   const boost::system::error_code&amp; error, // Result of operation.
 116
 117   std::size_t bytes_transferred           // Number of bytes copied into the
 118                                           // buffers. If an error occurred,
 119                                           // this will be the  number of
 120                                           // bytes successfully transferred
 121                                           // prior to the error.
 122 );
 123 </pre>
 124 <p>
 125                 Regardless of whether the asynchronous operation completes immediately
 126                 or not, the handler will not be invoked from within this function.
 127                 On immediate completion, invocation of the handler will be performed
 128                 in a manner equivalent to using <a class="link" href="../post.html" title="post"><code class="computeroutput">post</code></a>.
 129               </p>
 130 </dd>
 131 </dl>
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