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4<title>read_at (7 of 8 overloads)</title>
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26<div class="titlepage"><div><div><h4 class="title">
27<a name="boost_asio.reference.read_at.overload7"></a><a class="link" href="overload7.html" title="read_at (7 of 8 overloads)">read_at (7
28        of 8 overloads)</a>
29</h4></div></div></div>
30<p>
31          Attempt to read a certain amount of data at the specified offset before
32          returning.
33        </p>
34<pre class="programlisting">template&lt;
35    typename <a class="link" href="../SyncRandomAccessReadDevice.html" title="Buffer-oriented synchronous random-access read device requirements">SyncRandomAccessReadDevice</a>,
36    typename Allocator,
37    typename <a class="link" href="../CompletionCondition.html" title="Completion condition requirements">CompletionCondition</a>&gt;
38std::size_t read_at(
39    SyncRandomAccessReadDevice &amp; d,
40    uint64_t offset,
41    basic_streambuf&lt; Allocator &gt; &amp; b,
42    CompletionCondition completion_condition);
43</pre>
44<p>
45          This function is used to read a certain number of bytes of data from a
46          random access device at the specified offset. The call will block until
47          one of the following conditions is true:
48        </p>
49<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
50              The completion_condition function object returns 0.
51            </li></ul></div>
52<p>
53          This operation is implemented in terms of zero or more calls to the device's
54          read_some_at function.
55        </p>
56<h6>
57<a name="boost_asio.reference.read_at.overload7.h0"></a>
58          <span class="phrase"><a name="boost_asio.reference.read_at.overload7.parameters"></a></span><a class="link" href="overload7.html#boost_asio.reference.read_at.overload7.parameters">Parameters</a>
59        </h6>
60<div class="variablelist">
61<p class="title"><b></b></p>
62<dl class="variablelist">
63<dt><span class="term">d</span></dt>
64<dd><p>
65                The device from which the data is to be read. The type must support
66                the SyncRandomAccessReadDevice concept.
67              </p></dd>
68<dt><span class="term">offset</span></dt>
69<dd><p>
70                The offset at which the data will be read.
71              </p></dd>
72<dt><span class="term">b</span></dt>
73<dd><p>
74                The <a class="link" href="../basic_streambuf.html" title="basic_streambuf"><code class="computeroutput">basic_streambuf</code></a>
75                object into which the data will be read.
76              </p></dd>
77<dt><span class="term">completion_condition</span></dt>
78<dd>
79<p>
80                The function object to be called to determine whether the read operation
81                is complete. The signature of the function object must be:
82</p>
83<pre class="programlisting">std::size_t completion_condition(
84  // Result of latest read_some_at operation.
85  const boost::system::error_code&amp; error,
86
87  // Number of bytes transferred so far.
88  std::size_t bytes_transferred
89);
90</pre>
91<p>
92                A return value of 0 indicates that the read operation is complete.
93                A non-zero return value indicates the maximum number of bytes to
94                be read on the next call to the device's read_some_at function.
95              </p>
96</dd>
97</dl>
98</div>
99<h6>
100<a name="boost_asio.reference.read_at.overload7.h1"></a>
101          <span class="phrase"><a name="boost_asio.reference.read_at.overload7.return_value"></a></span><a class="link" href="overload7.html#boost_asio.reference.read_at.overload7.return_value">Return Value</a>
102        </h6>
103<p>
104          The number of bytes transferred.
105        </p>
106<h6>
107<a name="boost_asio.reference.read_at.overload7.h2"></a>
108          <span class="phrase"><a name="boost_asio.reference.read_at.overload7.exceptions"></a></span><a class="link" href="overload7.html#boost_asio.reference.read_at.overload7.exceptions">Exceptions</a>
109        </h6>
110<div class="variablelist">
111<p class="title"><b></b></p>
112<dl class="variablelist">
113<dt><span class="term">boost::system::system_error</span></dt>
114<dd><p>
115                Thrown on failure.
116              </p></dd>
117</dl>
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123      Kohlhoff<p>
124        Distributed under the Boost Software License, Version 1.0. (See accompanying
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