1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>async_read_at (2 of 4 overloads)</title> 5<link rel="stylesheet" href="../../../../../doc/src/boostbook.css" type="text/css"> 6<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 7<link rel="home" href="../../../boost_asio.html" title="Boost.Asio"> 8<link rel="up" href="../async_read_at.html" title="async_read_at"> 9<link rel="prev" href="overload1.html" title="async_read_at (1 of 4 overloads)"> 10<link rel="next" href="overload3.html" title="async_read_at (3 of 4 overloads)"> 11</head> 12<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 13<table cellpadding="2" width="100%"><tr> 14<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../boost.png"></td> 15<td align="center"><a href="../../../../../index.html">Home</a></td> 16<td align="center"><a href="../../../../../libs/libraries.htm">Libraries</a></td> 17<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 18<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 19<td align="center"><a href="../../../../../more/index.htm">More</a></td> 20</tr></table> 21<hr> 22<div class="spirit-nav"> 23<a accesskey="p" href="overload1.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../async_read_at.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../boost_asio.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="overload3.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> 24</div> 25<div class="section"> 26<div class="titlepage"><div><div><h4 class="title"> 27<a name="boost_asio.reference.async_read_at.overload2"></a><a class="link" href="overload2.html" title="async_read_at (2 of 4 overloads)">async_read_at 28 (2 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">template< 35 typename <a class="link" href="../AsyncRandomAccessReadDevice.html" title="Buffer-oriented asynchronous random-access read device requirements">AsyncRandomAccessReadDevice</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>> 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_at( 40 AsyncRandomAccessReadDevice & d, 41 uint64_t offset, 42 const MutableBufferSequence & buffers, 43 CompletionCondition completion_condition, 44 ReadHandler && 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 read a certain number of bytes 48 of data from a random access device at the specified offset. The function 49 call always returns immediately. The asynchronous operation will continue 50 until 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 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_at.overload2.h0"></a> 63 <span class="phrase"><a name="boost_asio.reference.async_read_at.overload2.parameters"></a></span><a class="link" href="overload2.html#boost_asio.reference.async_read_at.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">d</span></dt> 69<dd><p> 70 The device from which the data is to be read. The type must support 71 the AsyncRandomAccessReadDevice concept. 72 </p></dd> 73<dt><span class="term">offset</span></dt> 74<dd><p> 75 The offset at which the data will be read. 76 </p></dd> 77<dt><span class="term">buffers</span></dt> 78<dd><p> 79 One or more buffers into which the data will be read. The sum of 80 the buffer sizes indicates the maximum number of bytes to read from 81 the device. Although the buffers object may be copied as necessary, 82 ownership of the underlying memory blocks is retained by the caller, 83 which must guarantee that they remain valid until the handler is 84 called. 85 </p></dd> 86<dt><span class="term">completion_condition</span></dt> 87<dd> 88<p> 89 The function object to be called to determine whether the read operation 90 is complete. The signature of the function object must be: 91</p> 92<pre class="programlisting">std::size_t completion_condition( 93 // Result of latest async_read_some_at operation. 94 const boost::system::error_code& error, 95 96 // Number of bytes transferred so far. 97 std::size_t bytes_transferred 98); 99</pre> 100<p> 101 A return value of 0 indicates that the read operation is complete. 102 A non-zero return value indicates the maximum number of bytes to 103 be read on the next call to the device's async_read_some_at function. 104 </p> 105</dd> 106<dt><span class="term">handler</span></dt> 107<dd> 108<p> 109 The handler to be called when the read operation completes. Copies 110 will be made of the handler as required. The function signature of 111 the handler must be: 112</p> 113<pre class="programlisting">void handler( 114 // Result of operation. 115 const boost::system::error_code& error, 116 117 // Number of bytes copied into the buffers. If an error 118 // occurred, this will be the number of bytes successfully 119 // transferred prior to the error. 120 std::size_t bytes_transferred 121); 122</pre> 123<p> 124 Regardless of whether the asynchronous operation completes immediately 125 or not, the handler will not be invoked from within this function. 126 On immediate completion, invocation of the handler will be performed 127 in a manner equivalent to using <a class="link" href="../post.html" title="post"><code class="computeroutput">post</code></a>. 128 </p> 129</dd> 130</dl> 131</div> 132<h6> 133<a name="boost_asio.reference.async_read_at.overload2.h1"></a> 134 <span class="phrase"><a name="boost_asio.reference.async_read_at.overload2.example"></a></span><a class="link" href="overload2.html#boost_asio.reference.async_read_at.overload2.example">Example</a> 135 </h6> 136<p> 137 To read into a single data buffer use the <a class="link" href="../buffer.html" title="buffer"><code class="computeroutput">buffer</code></a> 138 function as follows: 139 </p> 140<pre class="programlisting">boost::asio::async_read_at(d, 42, 141 boost::asio::buffer(data, size), 142 boost::asio::transfer_at_least(32), 143 handler); 144</pre> 145<p> 146 See the <a class="link" href="../buffer.html" title="buffer"><code class="computeroutput">buffer</code></a> 147 documentation for information on reading into multiple buffers in one go, 148 and how to use it with arrays, boost::array or std::vector. 149 </p> 150</div> 151<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 152<td align="left"></td> 153<td align="right"><div class="copyright-footer">Copyright © 2003-2020 Christopher M. 154 Kohlhoff<p> 155 Distributed under the Boost Software License, Version 1.0. (See accompanying 156 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>) 157 </p> 158</div></td> 159</tr></table> 160<hr> 161<div class="spirit-nav"> 162<a accesskey="p" href="overload1.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../async_read_at.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../boost_asio.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="overload3.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> 163</div> 164</body> 165</html> 166
