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  26 <div class="titlepage"><div><div><h4 class="title">
  27 <a name="boost_asio.reference.async_write_at.overload4"></a><a class="link" href="overload4.html" title="async_write_at (4 of 4 overloads)">async_write_at
  28         (4 of 4 overloads)</a>
  29 </h4></div></div></div>
  30 <p>
  31           Start an asynchronous operation to write a certain amount of data at the
  32           specified offset.
  33         </p>
  34 <pre class="programlisting">template&lt;
  35     typename <a class="link" href="../AsyncRandomAccessWriteDevice.html" title="Buffer-oriented asynchronous random-access write device requirements">AsyncRandomAccessWriteDevice</a>,
  36     typename Allocator,
  37     typename <a class="link" href="../CompletionCondition.html" title="Completion condition requirements">CompletionCondition</a>,
  38     typename <a class="link" href="../WriteHandler.html" title="Write handler requirements">WriteHandler</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_write_at(
  40     AsyncRandomAccessWriteDevice &amp; d,
  41     uint64_t offset,
  42     basic_streambuf&lt; Allocator &gt; &amp; b,
  43     CompletionCondition completion_condition,
  44     WriteHandler &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 write a certain number of bytes
  48           of data to a random access device at a specified offset. The function call
  49           always returns immediately. The asynchronous operation will continue until
  50           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               All of the data in the supplied <a class="link" href="../basic_streambuf.html" title="basic_streambuf"><code class="computeroutput">basic_streambuf</code></a>
  55               has been written.
  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 device's
  63           async_write_some_at function, and is known as a <span class="emphasis"><em>composed operation</em></span>.
  64           The program must ensure that the device performs no <span class="emphasis"><em>overlapping</em></span>
  65           write operations (such as async_write_at, the device's async_write_some_at
  66           function, or any other composed operations that perform writes) until this
  67           operation completes. Operations are overlapping if the regions defined
  68           by their offsets, and the numbers of bytes to write, intersect.
  69         </p>
  70 <h6>
  71 <a name="boost_asio.reference.async_write_at.overload4.h0"></a>
  72           <span class="phrase"><a name="boost_asio.reference.async_write_at.overload4.parameters"></a></span><a class="link" href="overload4.html#boost_asio.reference.async_write_at.overload4.parameters">Parameters</a>
  73         </h6>
  74 <div class="variablelist">
  75 <p class="title"><b></b></p>
  76 <dl class="variablelist">
  77 <dt><span class="term">d</span></dt>
  78 <dd><p>
  79                 The device to which the data is to be written. The type must support
  80                 the AsyncRandomAccessWriteDevice concept.
  81               </p></dd>
  82 <dt><span class="term">offset</span></dt>
  83 <dd><p>
  84                 The offset at which the data will be written.
  85               </p></dd>
  86 <dt><span class="term">b</span></dt>
  87 <dd><p>
  88                 A <a class="link" href="../basic_streambuf.html" title="basic_streambuf"><code class="computeroutput">basic_streambuf</code></a>
  89                 object from which data will be written. Ownership of the streambuf
  90                 is retained by the caller, which must guarantee that it remains valid
  91                 until the handler is called.
  92               </p></dd>
  93 <dt><span class="term">completion_condition</span></dt>
  94 <dd>
  95 <p>
  96                 The function object to be called to determine whether the write operation
  97                 is complete. The signature of the function object must be:
  98 </p>
  99 <pre class="programlisting">std::size_t completion_condition(
 100   // Result of latest async_write_some_at operation.
 101   const boost::system::error_code&amp; error,
 102
 103   // Number of bytes transferred so far.
 104   std::size_t bytes_transferred
 105 );
 106 </pre>
 107 <p>
 108                 A return value of 0 indicates that the write operation is complete.
 109                 A non-zero return value indicates the maximum number of bytes to
 110                 be written on the next call to the device's async_write_some_at function.
 111               </p>
 112 </dd>
 113 <dt><span class="term">handler</span></dt>
 114 <dd>
 115 <p>
 116                 The handler to be called when the write operation completes. Copies
 117                 will be made of the handler as required. The function signature of
 118                 the handler must be:
 119 </p>
 120 <pre class="programlisting">void handler(
 121   // Result of operation.
 122   const boost::system::error_code&amp; error,
 123
 124   // Number of bytes written from the buffers. If an error
 125   // occurred, this will be less than the sum of the buffer sizes.
 126   std::size_t bytes_transferred
 127 );
 128 </pre>
 129 <p>
 130                 Regardless of whether the asynchronous operation completes immediately
 131                 or not, the handler will not be invoked from within this function.
 132                 On immediate completion, invocation of the handler will be performed
 133                 in a manner equivalent to using <a class="link" href="../post.html" title="post"><code class="computeroutput">post</code></a>.
 134               </p>
 135 </dd>
 136 </dl>
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